Inmr Issue 95

VOLUME 20 NUMBER 1 • QUARTER ONE - 2012

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Issue 95 • Quarter 1 - 2012 • Volume 20 - Number 1

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TRUSTED INSULATORS

Xinjiang Xinneng TIANNING Electrical Engineering Isolating Materials Co., Ltd. No. 45, Taishan Road, Economic & Technical Development Zone Urumqi, Xinjiang, China 830026 Tel: ++86-991-2928153 路 Telefax: ++86-991-2928143 www.tenet-insulator.com路email: tenet_tn@163.com

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TRUSTED SUPPLIER

As one of China’s most respected and qualified insulator manufacturers, we understand that changing suppliers is not something to be done lightly.

That’s why we offer our new customers throughout the world the peace of mind that we have been supplying insulators to the highest voltage levels as well as highest quality standards for almost 20 years.

Our insulators have provided safe and reliable service on many of China’s most important lines, including critical UHV lines such as the Xiangjiaba-Shanghai ±800kV UHV DC Transmission Demonstration Project.

We have supplied as much as 80% of the new overhead network in Western China, including all the key lines for Xinjiang and Northwest Power Grid 750kV Interconnection Project.

Look into all the advantages we can offer at all voltages in terms of our unique solid state silastic manufacturing, timely delivery and reduced cost.

We hope you will agree that Tenet should be included among your preferred insulator suppliers.

A SUPPLIER OF INSULATORS YOU CAN TRUST. INMR Issue 95 (A).indd 3

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INMR 2012 www.inmr.com This issue marks the start of Volume 20 – our 20th year of producing INMR. Since its introduction as a modest industry newsletter in September of 1993, we have grown up quite a bit by following one unchanging rule: every successive issue would have to be at least as good as and ideally better than the previous.

Today, we still follow this same basic rule to assure that our readers always receive a growing body of valuable technical information supported by compelling graphic design and the best on-site photos in the industry. That has become the tradition of INMR over the years and it is a tradition that will continue into the future.

For 2012, we have made some important changes, particularly at our web site: www.inmr.com, which we are proud to say ranks among the best in the business.

1. Visitors can access past issues of INMR going back 7 years and read the entire issue on-line – for free; 2. Visitors can enter any key words and search our archive of hundreds of past articles to retrieve any that deal with their specific topic of interest; 3. There is now a library of hundreds of high quality photos covering virtually every aspect of overhead lines and substations that readers can scan for their interest or even purchase for any commercial or professional use; 4. Visitors can read the Papers presented at past INMR WORLD CONGRESSES on-line – for free.

In our print version of INMR for 2012, we have added a new column dealing with cable accessories, which is written by Prof.

Dr-Ing. K-D Haim, a prominent academic and industry specialist from the University of Zittau in Germany. Just as with insulators, surge arresters and bushings, INMR will now increasingly cover this important product group and report on changing technologies, interesting suppliers, industry standards and testing requirements.

For 2012, we have made some important changes, particularly at our web site: www.inmr.com

Our commitment to you in 2012 will be to provide you with more information on more technical areas of T&D. And, as over the past 20 years, INMR is dedicated to remaining independent and objective – a journal that readers can always depend on to give an unbiased and authoritative view of what's going on in the field of electrical power supply.

Thank you for your valued readership over these past two decades, Marvin L. Zimmerman Publisher

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FAREWELLS & WELCOME

It is indeed with much sadness that INMR announces the departure of Prof. Ravi S. Gorur from our team of expert columnists.

I first met Ravi at the IEEE T&D Conference in Chicago way back in 1994 and it was here that our long and happy collaboration first began. INMR was still a fledgling journal at the time, seeking to establish credibility. As Assistant Professor at Arizona State University, Ravi was already a ‘rising star’ in the insulator world yet looking for new ways to expand the reach of his ideas and opinions. Each of us, it seemed, could offer something to the other and so began a partnership that would last nearly two decades.

Ravi’s cutting edge scientific knowledge soon helped put INMR on the right course and over the years contributed to making it the trusted publication it has since become.

There is no need to go into all the contributions Ravi has made to the field of insulators, as most readers know this well. His years of research, instruction, technical papers and participation in standards formulation have helped move the entire field forward and the industry owes him a great debt. This debt was acknowledged in small measure when Ravi received the Claude de Tourreil Memorial Award for Lifetime Achievement at the 2011 INMR WORLD CONGRESS in Seoul.

As if this alone were not sufficient, early in 1995 Ravi made a proposal that we cooperate on a seminar to instruct engineers about the still relatively new and ‘unproven’ technology behind composite insulators. His suggestion

Thank you so much, Ravi. INMR, its staff and columnists and especially our many readers worldwide express deep appreciation and wish you continued success in a field to which you still have a great deal to contribute.

INMR wishes to acknowledge the upcoming retirement of Dr. Hans-Jörg Winter after a distinguished career spanning a quarter of a century at Wacker-Chemie in Germany.

true. Then, he would find just the right way to explain what really was correct yet in a way that did not diminish the dignity of the person involved (often me).

I first met Jörg in New York City at an IEEE event during the mid 1990s and I can still remember my first sight of him at Wacker’s booth, busily explaining to those gathered around all the virtues of silicone rubber for electrical insulation. Well, for the next two decades that task became very much his single-minded goal and one can say that, given the popularity of silicone as the material of choice in composite insulators, arresters and cable accessories he has clearly succeeded.

One of the things that most endeared me to Jörg over the years is the fact that he has always been the consummate gentleman, patiently listening, never interrupting – even when he knew that what he was hearing was not always

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stimulated an idea for a much broader conference that eventually resulted in our successful SYMPOSIUM ON NON-CERAMIC INSULATOR TECHNOLOGY that took place in Zurich, Switzerland in November 1995. I recall that we ran into each other in the conference room at six o’clock that first morning (apparently neither of us could sleep from the excitement) and Ravi and I looked out at the sea of chairs with a mix of awe and satisfaction that so many people had registered. This event then morphed into our bi-annual INMR WORLD CONGRESS and Ravi was Chairman of many of these as they took place across the globe.

INMR is truly delighted to welcome to our team of expert columnists Professor Dr. Klaus-Dieter Haim of the University of Zittau in Germany.

Dr. Haim completed his studies in electrical engineering, earning his degree in 1985 with specialization in the field of MV network design

Oh yes – one more important item. Dr. Winter is among only three people who have attended every single of INMR’s 10 WORLD CONGRESS events since 1995. Such loyalty and consistency is indeed a good measure of him as a person and as a professional.

Danke, Jörg and best wishes in your new life away from silicone.

and optimization. He has worked abroad as a university professor as well as participated in an important project to compare the networks in France and Germany and he has become a Senior Fellow for electrical power systems and networks His expertise in the field of cable accessories stems largely from his role as Head of Production for medium voltage cable accessories at a large European supplier and it is this topic that will form the focus of his future columns.

Wilkommen zum INMR, Prof. Haim.

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PREVIEW

of ofthis thisissue issue

Issue 95 0 Quarter Quarter01- 2000 − 2012

8 INMR in 2012

Utility Practice & Experience

10 Farewells & Welcome 16 Editorial

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Line Insulators Will Inevitably Fail The Only Question: At What Rate?

20 Inside Track on Smart Grid Communication Between Equipment and Grid is Vital

22 From the World of Testing Path of Least Resistance

24 Silicone Technology Review Much Progress in Standards & Testing

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26 Reporting from Cigre Investigating Polymeric Housings Using the Inclined Plane Test

28 Transient Thoughts

Using Arresters to Meet Learning Objectives

30 Scene from China

Coating Line Insulators with RTV Silicone

32 Pigini Commentary

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DC Insulator Market Poised for Explosive Growth

34 From the Research View Increasing Transmission Capacity: The View From a New CIGRE Brochure

36 Woodworth on Arresters 10 Things to Always Remember About Arrester Applications

104 Advertisers Advertisers in in This This Issue Issue

38 Focus On Cable Accessories

ABB Components & Insulation Materials 119 Alstom 33 Balestro 109 CESI Group 47 China EPower 135 Comem 35 CSL Silicones - SiCoat Outside Back Cover DTR Corp.

63 Dalian Composite Insulator DCI 4 Dalian Insulator Group 14, 15 Dalian Reliable Industrial 109 Dekuma Rubber & Plastic 79 Dextra Power 13 EGU HV Laboratory 99 Glasforms 13

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Database to Make PD Measurements of MV Cable Systems More Effective

Hidro Jet 75 Huayi Machinery Group 11 Hubbell Power Systems Inside Back Cover Hübers Verfahrenstechnik 59 Huntsman Advanced Materials 115 Jinan Meide Casting 59 KEMA 23 Maxwell Technologies 81 Meister International 4 Nikdim 85 Ofil 63 PingGao Group 40, 41 PK Insulators 9 PPC Insulators 57

Volume Volume 2000 − Number − Number 10

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Turkish Transmission Utility Looks to Silicone Insulators to Reduce Construction & Maintenance Costs

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RTV Coatings Play Key Role at Chinese Substations

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Application of Composite Interphase Spacers to Prevent Conductor Galloping

GIS Technology

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Supplier of GIS Equipment & Components Looks to Rising Demand

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Growth Expected in Portable SF6 Handling Equipment

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GIS Supplier Relocates Insulator Production to New Plant

Insulators

82 90 94 100

Modern Pollution Monitoring Principles Allow Better Selection of Insulators for Polluted Service Conditions (Part 1 of 2) Improved Insulator Design for Performance Under Icing Porcelain Insulator Manufacturer Undergoes Factory Expansion Insulator Manufacturer Implements New Production Scheme

Arresters 104

Utility Reduces Lightning Outages With Line Arrester Investment Program

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Supplier Adjusts Strategy and Design For Arresters & Cutouts

Bushings 116

Experience in Mexico With Non-Ceramic Bushings in Contaminated & Seismic Areas

Cable Accessories 124

Overview of Testing Requirements for Cable Accessories

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A City Built on Insulators

Primtech 75 Qingdao Highton Machinery 99 Reinhausen Power Composites 1 Rugao Dasheng Line Material 5 SGD La Granja 45 STRI 89 Shaanxi Taporel Electrical Insulation 21 Shandong Peiport Electric Power 53 Shanxi Century Metal Industries 85 Shenma Electric Power 6, 7 Sichuan YiBin Global Group SYGG 73 Siemens, Arresters Div. Inside Front Cover TE Connectivity 39 Tianning Electrical Isolating Materials 2

Tridelta Überspannungsableiter 37 Uvirco Technologies 85 Vogel moulds and machines 131 Volani Metais 63 Wacker Chemie 25 Wellwin Precision Moulds 11 Wenzhou Yikun Electric 29, 75 W. S. Test Systems 121 Xi’an Gaoqiang Insulation 27 Yizumi Rubber Machinery 18, 19 Zhengzhou Jingwei Electric 87 Zhengzhou Xianghe Group Electric Equipment 17 Zibo Taiguang Electrical Equipment 31

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Line Insulators Will Inevitably Fail The Only Question: At What Rate?

In the last issue of INMR there was an article about the different possible failure modes of porcelain, glass and composite line insulators. In this column, I’d like to focus on a related issue – namely the reasons insulators will fail.

EDITORIAL EDITORIAL EDITORIAL

To begin with, it must be stated that, viewed against their relatively low acquisition price, insulators clearly rank among the most reliable and cost-effective components in a power network. However, a proportion of them inevitably fail each year. Here’s why:

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1. Hostile Environment Insulators are probably the most stressed components in the power grid. Not only do they live under continuous high electrical and mechanical tension but they are also fully exposed to unpredictable forces, including sharp temperature and humidity swings, storms and vandalism. Add to this the gradual accumulation of conductive pollutants on their surface (whether from natural or human sources) and one can conclude that, even if perfectly made and properly specified, a proportion of them will be overwhelmed by all these hostile factors.

2. Explosion of New Suppliers Over the past 20 years, there has been a significant jump in the number of insulator suppliers worldwide. While the majority may have been careful to master the correct production technology and exercise proper quality control at each step, one cannot say this applies to all. That means that there are at least some suppliers who sell insulators of inferior design or non-uniform quality, with potential defects that could lead to premature ageing or outright failure.

3. Focus on Price Over Quality Every user naturally expects that all the insulators they purchase will perform without problem. But the reality is that failure rate is influenced mainly by quality and that quality is influenced mainly by price. With the emphasis on lowering costs that has become the power industry’s mantra over the past two decades, suppliers have had to find ways to deliver the best possible quality at the going market price. That invariably has meant optimizing design with more emphasis on cost reduction than on quality enhancement.

Given these reasons for inevitable insulator failures on lines, power supply companies can still do something to minimize the rate of such problems. For one, it is important to select the insulator technology best suited for the needs of the application. In this regard, it is critical to know all the service factors that will affect insulators and work with suppliers to identify the design that will perform best under these stresses.

Secondly, it is also important to find suppliers who exercise all the required standards for production methods and quality control procedures. Thanks to intense competition in this industry, today there are numerous good supplier choices among established as well as comparatively new manufacturers who have invested in the latest production know-how and facilities. And finally, it is critical to regularly monitor insulators for any abnormalities. Incipient problems can frequently be anticipated by diagnostics such as visual inspection, leakage current, corona or abnormal temperature profiles.

Another aspect of this same issue is that some suppliers seem to be climbing the ‘voltage ladder’ at unprecedented speeds.

It used to be that it would take years before a manufacturer moved from offering insulators for one voltage level up to the next higher voltage. Today, one finds relatively new suppliers who already produce insulators for EHV or even

So, while some rate of line insulator failure will always be a fact of life, it can at least be kept to manageable levels, with lowest adverse impact on reliability and maintenance costs.





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Photo 1: This single circuit tower in China, where vandalism is not an issue, has 3 shattered stubs out of the 48 glass discs on its 6 tension strings (two showing in the photo). The odds of this occurring by self-shattering of high quality glass should be less than one in a hundred million.

Photo 2: This naturally polluted silicone rubber insulator, removed for laboratory testing after only a handful of years of service, has already lost most of its hydrophobicity, leaving it far less able to perform effectively.

Marvin L. Zimmerman mzimmerman@inmr.com



Photo 3: Porcelain long rods have long enjoyed an excellent reputation for reliability yet can prove especially vulnerable to design or production deficiencies.

Photo 4: Poorly manufactured porcelain cap & pin insulators suffer high rates of internal puncture or radial cracking.

INMR Issue 95 • www.inmr.com

ISSN 1198-7332 • P.O. Box 95, Westmount (Montréal), Québec, Canada H3Z 2T1 • Telephone: (514) 939-9540 • Telefax: (514) 939-6151 E-mail: info@inmr.com • Editor & Advertising Sales: Marvin L. Zimmerman 中国地区联系方式：余娟女士 电话: 135 1001 6825 / juan.inmr@yahoo.cn Magazine Design: Cusmano Design and Communication Inc • (514) 509-0888 • E-mail: corrado@cusmanodesign.com Contents of this publication are protected by international copyrights and treaties. Reproduction of the publication, in whole or in part, without express written permission of the publishers is prohibited. While every effort is made to verify the data and information contained in this publication, the publishers accept no liability, direct or implied, for the accuracy of all information presented.

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UHV applications but who in some cases have not yet had sufficient field experience on which to base these designs.

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Communication Between Equipment and Grid is Vital Many companies are currently investing in the development and manufacture of Smart Grid equipment for buildings, homes, distributed energy resources, power storage apparatus, EVs and so forth. That’s fine. But something important is missing.

Inside Track on Smart Grid

That missing element are the common requirements of all these types of key demand side objects. Moreover, such common requirements have to take into account all the characteristics of residential, commercial and industrial users.

In other words, to guarantee safe and cost-efficient operation as well as interoperability within the power grid, it has now become indispensable that the demand side interface with the Smart Grid be standardized.

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In addition, such new standards will have to address the function and performance requirements of demand side systems (including power management systems) as well as the information exchange interfaces between systems/ equipment and the grid. In this respect, they will also need to include: terminology; general interface requirements; specifications of function and modes of interaction; data format; information and communication models; as well as all security and protection protocols.

To achieve all this, the IEC has put in place PC (Project Committee) 118 Smart Grid User Interface, with its first meeting from February 8 to 10 in Tianjin, China. This meeting was the result of a proposal from the IEC Chinese National Committee that was subsequently voted positively by the Standard Management Board.

During this high-level international exchange, participants, representing all Smart Grid stakeholders reviewed and discussed current standardization activities for smart grid user interfaces. For the first time ever, all major global Smart Grid players were invited to formulate and coordinate the strategies and standards needed by the entire industry.

To guarantee safe, cost-efficient operation and interoperability within the power grid, it has become indispensable that the demand side interface with the Smart Grid be standardized.

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Without standardization there cannot be open communication between equipment and the grid and therefore no real long-term growth in this market.

Following a presentation on the latest status of smart grid user interface standardization activities in all IEC participant countries, the PC presented an overview of relevant published IEC International Standards. Standards of other standardization bodies and Consortia that must be taken into consideration were also reviewed.

At the end of the meeting, participants were invited to draft a strategic business plan that will guide the activities of the two Working Groups (WGs) tasked with developing the relevant standards. A technical visit to the Smart Grid Comprehensive Pilot Project in Sino-Singapore Tianjin Eco-City concluded the meeting.

The scope of the two PC 118 WGs has been outlined as follows: WG 1 will focus on providing the definition and description of the interoperable interface between the Smart Grid and demand-side equipment from the perspective of the power grid. This will include smart buildings and homes, distributed energy resources, EVs, and more. It will also establish a model for exchange of information between the two, taking into account all of the parameters mentioned above.

WG 2 will define and describe the communication model for the interaction between smart systems and the power grid, covering the same technologies and defining general functional requirements, performance indexes as well as security and protection of power demand response.

Without standardization there cannot be open communication between equipment and the grid and this will clearly hamper long-term growth in this market.

Richard Schomberg IEC Chairman of the Smart Grid Strategic Group Chairman, TC 8 – Systems aspects for energy delivery Chairman, PC 118 - Smart Grid user interface Responsible for Smart Energy Standards at EDF-Group

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Path of Least Resistance

from the world of TESTING

Regularly, it seems, electrical currents decide to suddenly exit the highways that power system designers have so carefully laid-out for them. In fact, this path of least resistance is so enjoyable for the currents that their magnitude can increase tremendously and there is yet another short-circuit event for us to cope with. While much has been done to limit this problem, the fact remains that each year roughly one short circuit has to be reckoned with for every 100 km of overhead transmission line.

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Moreover, if all this were not enough, short-circuit current levels are on the rise on electrical networks worldwide. For example, on the Dutch 400 kV system, the calculated median value of maximum possible three-phase, short-circuit currents at all substations is trending upwards – from 32 kA in 2003 to a projected 37 kA in 2013 to an expected 43 kA by 2020. Fortunately, as CIGRE studies demonstrate, fault currents that actually occur are often much lower than their possible maximums.

The basic reason behind the increase in transmission system short-circuit levels is the growing concentration of largescale conventional power generation at certain favourable locations. In the case of distribution grids, it is the addition of more renewable power sources. We, at KEMA, regularly see evidence of this trend in the rapidly increasing demand for tests on all kinds of substation equipment at fault currents of as high as 80 kA and sometimes even more.

Circuit breakers are there, of course, to limit the duration of any fault. But before the ‘evil’ has been fully eliminated, each such short-circuit event has the potential to cause serious damage. This is the case because of the extremely high electro-dynamic (mechanical) forces that act upon various key system components and also because of the thermal effects of the fault arc itself, i.e. the path of low resistance so eagerly strived for in nature.

In a recent series of tests at KEMA laboratories, the essential elements of a typical high voltage substation were erected in order to verify withstand against an 80 kA short-circuit. Both dynamic effects (i.e. those focussed around the asymmetrical current peak of 200 kA) and thermal effects (in a short-time current test of relatively long duration) were included. The disconnectors must remain closed under the passage of such short-circuit currents since, if not, even slight arcing between their contacts can increase their resistance leading to unwanted heating during operation.

The forces resulting from short-circuits on busbar supports can be tremendous. While it is often thought that breakers are the last line of defence, it must be remembered that they have higher level ‘brothers’ to back them up in case they fail to interrupt. However busbars (supports) do not. Indeed, several cases of total busbar collapse were observed during the course of these tests.

In the case of overhead lines, bundle spacers have been known to be squeezed-out completely by the contraction of conductors during severe short-circuits. The mechanical stresses acting upon spacers can be recorded even during tests at 100 kA using sensitive strain gauges that work even in this hostile environment. In other tests, 'kissing' overhead line conductors became permanently deformed due to the impact of collisions.

The violent motion of jumpers and other tower accessories can then cause serious damage to transmission lines.

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The trouble caused by the fault arc itself is another story. In distribution switchgear panels, for example, a very common test is the 'internal arc' test, during which the effects of a fault arc inside the cubicle are observed. Cloth covered indicators serve to simulate the clothing of personnel in close proximity.

Tests here are intended to verify that the pressure rise and expulsion of hot gases and debris due to internal arcing will not ignite the indicators, assuring that workers will not suffer severe burns. Such tests are usually performed over a weekly basis and demand for them has only been accelerating in recent years.

In the case of GIS, internal fault arc tests are far more complicated due mainly to the SF6 environment. In order to prevent the exhaust of contaminated SF6 gas, such tests must be performed inside a huge collector tank where the GIS section is embedded. Another problematic factor is that the fault arc is by nature mobile and tends to wander away from the supply source. That means that the arc will travel to the far end of the section and reside there, although it must not burn through the GIS enclosure. In order to avoid hassles, laboratories sometimes perform these tests in air-filled GIS sections but, as under study by CIGRE WG A3.24, the conditions of arcs in air are completely different from those in SF6.

Overhead transmission system components must also routinely deal effectively with fault arcs. KEMA, for example, has ample experience with high-current power arcs that stress overhead line insulators. In the case of a lightning back flashover, fault arcs might not endanger the mechanical integrity of the insulator string since normally they are diverted away by arcing horns. However, it has sometimes been observed during tests that such power arcs leave the arcing horn and develop ‘footprints’ on the conductors. To ensure proper simulation of real service conditions, the electromagnetic environment in which these arcs burn has to be represented in a proper circuit layout in the laboratory.

For their part, surge arresters are prone to explode in the case of internal fault arcs. Tests have therefore been designed to verify that in such a case their pressure relief system will react sufficiently quickly to avoid expulsion of metal oxide block fragments and pose risk to nearby personnel or equipment.

We recently took this to the extreme by testing arresters that had to suffer from superimposing the complete discharge of a life-size series compensation capacitor bank upon a power frequency fault current. An astounding fault arc current of 450 kA (peak value at 3 kHz) was achieved.

The ‘emperor’ of fault currents is perhaps hiding in circuits between the generator and the step-up transformers in large power plants. Generator circuit breakers have to interrupt such currents and bus ducts have to endure them. Unleashing all four of KEMA’s generators and bringing this huge power directly to the test-bay can duplicate such a condition in the laboratory. With adequate management of all the electrodynamic forces, interruption tests have been realized well above 200 kA with corresponding bus duct withstand tests above 800 kA peak.

Not exactly what one might call ‘the path of least resistance’.

Professor Rene Smeets Rene.Smeets@kema.com

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Much Progress in Standards & Testing I have been active in the T&D unit of Wacker for nearly 25 years and, with my forthcoming retirement, now seems a good time to look back at the application of silicone within the power industry.

When I first started at our Technical Marketing Dept. in 1988, the use of silicone rubber for insulators was in its infancy and there were only a few suppliers – mainly in Germany, Switzerland and the USA.

Silicone Technology

REVIEW

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At this early stage, composite long-rod insulators were produced by injection molding a high consistency silicone rubber (HCR) filled with aluminum trihydrate (ATH). Hollow core insulators, by contrast, were being manufactured using a low pressure casting process that required a less viscous silicone. And, in Hungary, a research institute was conducting its first trials of liquid silicone rubber (LSR). However, since the viscosity of commercially available LSR was still comparatively high, the size of insulators that could be produced this way remained limited.

Today, current manufacturing processes are basically similar to what was done in the early days. One might therefore be tempted to conclude that there has been little progress in the field over the past two decades. However, the fact is that much has changed in this industry.

For one, silicone rubber formulations have been optimized in regard to processing as well as mechanical and electrical properties. Wacker, for example, was among the first to offer silicones for all T&D applications and also introduced, in 2001, a platinum cure, one component system for HCR that offered fast curing without undesired by-products. Similarly, manufacturers of mixing and dosing equipment, injection molding machines and the molds themselves all improved their products. Indeed, as a result of advancements such as these, any companies starting a new production line for composite insulators today have access to fully packaged solutions – something unthinkable only 20 years ago.

Still, I feel that perhaps the greatest progress has been in evaluating the key properties of silicone rubber as well as in defining new and reliable test standards for these. When first introduced, silicone was known mostly for its stability against UV and climatic variables and this was confirmed by the excellent long-term performance of silicone composite insulators. Then, in 1989, J. Kindersberger and M. Kuhl issued a paper dealing with the unique hydrophobic behavior of silicone rubber 1.

This started a ‘hydrophobicity boom’ of sorts as numerous papers and theses followed that investigated silicone’s unique hydrophobicity recovery and transfer properties. Still, it soon became apparent that the industry lacked reliable test methods, apart from measuring water drop contact angle – something of limited practical value.

CIGRE took up the issue and set up Working Group D1.14, where we participated by providing sample plates for Round Robin Tests. Findings were later published in 2009 in ELEKTRA and a test procedure for hydrophobicity transfer was proposed to the IEC2. Based on the modified rotating wheel test (which was developed in collaboration with the University of Applied Science in Zittau, Germany), this WG has also been investigating a dynamic hydrophobicity test, soon to be proposed to the IEC3.

According to the old standard (1984, second edition), test specimens first had to be sanded to remove any extrinsic materials that could falsify the result from the plate’s surface.

For example, an epoxy resin specimen often contains silicone mold release residues and might show a far longer lifetime than a sanded sample. This procedure, however, is not applicable to silicone rubber. Grinding the surface of an ATH filled specimen seriously affects its properties and leads to incorrect results. This point was taken into account in 2007 when revising the inclined plane test according to IEC 60587 (3).

Perhaps the greatest progress over the last 20 years has been in evaluating the key properties of silicone rubber as well as in defining new and reliable test standards for these.

There are other examples as well of how test standards have evolved over the years, including measurement of dielectric strength of elastomers. The corresponding standard, IEC 60243-1, up to now has not offered any guidelines in this respect. For the current revision, still in process, a new test procedure has been introduced that can cope with the high dielectric strength typical of silicone rubbers.

Power utilities have tended to be conservative insofar as accepting new outdoor insulation materials and it was therefore not always easy to introduce silicone rubber. But now, after 40 years of service experience, silicones are not only well-accepted but there is an expanding range of applications under review.

As a result, a variety of new products are in our R&D pipeline, including silicone rubbers for DC, conventional and modified gels, a new generation of insulator coatings as well as new HCRs with higher thermal conductivity and also UV-initiated cure materials.

1

Kindersberger, J., Kuhl, M.: Effect of Hydrophobicity on Insulator Performance, 6th International Symposium on High Voltage Engineering, New Orleans, August 1989, paper 12.01

2

Evaluation of dynamic hydrophobicity properties of polymeric materials for non-ceramic outdoor insulation – Retention and transfer of hydrophobicity on behalf of Working Group D1.14 “Material properties for non-ceramic outdoor insulation”

The world of silicones has indeed changed for the better.

The number of suitable silicone rubbers to choose from has increased. Industry knowledge of the material’s key properties and testing methods has improved. And, looking back, I am proud to say that I feel we at Wacker have contributed to this process by sharing our knowledge and expertise, developing new products and technologies and continually expanding our presence in the T&D field.

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Bärsch, R., Lambrecht, J., Winter, H.-J: On the Valuation of the Early Aging Period of Polymer Insulator Surfaces During Accelerated Aging Tests, 9th International Symposium on High Voltage Engineering, Proceedings, Volume 3, Pollution Phenomena, paper 3023, Graz, 1995, pp. 3023.1 - 3023.4

Dr. Hans-Jörg Winter Hans-Joerg.Winter@wacker.com

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Due to their excellent resistance to pollution flashover, silicone composite insulators are often the preferred choice for service in polluted areas where they become exposed to partial discharges and electrical arc stresses. These conditions are simulated in IEC 60587 that regulates the required tracking and erosion resistance of sample plates.

Editorial

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REPORTING FROM CIGRE

Investigating Polymeric Housings Using the Inclined Plane Test

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CIGRE Working Group (WG) D1.27 (chaired by Dr. Jens Seifert) is charged with investigating polymeric materials used as external housings for insulators, arresters, bushings, instrument transformers, etc. This topic is now of growing importance given greater demand for such equipment and includes analytical methods to better distinguish these materials by means of their unique ‘fingerprints’. Specifically, there is much interest to investigate possible changes in such housings after a certain period in service and one of the questions is whether an additional Inclined Plane Test under DC stress is required to better evaluate tracking and erosion performance. Evaluating relevant properties of ‘used’ insulator housings is seen as very helpful to estimate the residual life of a power line and also to better define a broad inspection and maintenance regime covering components such as insulators, towers, conductors, dampers, line hardware, etc. In this regard, Technical Brochure 481 (Guide for the Assessment of Composite Insulators in the Laboratory after their Removal from Service) was published last December by WG B2.21 and serves as a guideline for assessing insulators.

aged phase’ of the material from the test’s outset.

For example, inspection of insulators with silicone rubber housings after 10 years’ service revealed a certain ‘silification’ (i.e. formation of a surface layer of silica). Service stress in this case was characterized by moderate pollution, intense sun and an annual rainy season. The question was whether this type of change was only cosmetic or if it might negatively impact functionality. The insulators were studied closely and no reduction of hydrophobic properties or mechanical strength was found. The influence of the ‘silificated’ surface to tracking and erosion performance was then investigated. Parts of the housings were extracted to obtain test specimens having the pollution and silica layer generated during service. An Inclined Plane Test at constant voltage of 4.5 kV was used for six hours. The findings (Fig. 1) reveal very little erosion and the test was passed.

Independent tests performed at the University of Zittau in Germany offered an answer (Fig. 2). Three families of materials were investigated: Material A, a structural material typically used mostly for distribution; Material B, a castable silicone rubber with no added filler to improve erosion resistance; and Material C, an HTV silicone with ATH filler. For polymeric Material A, practically all tests under DC resulted in failure with a conductive path that exceeded the 60 mA threshold.

• leakage current and root temperature of the arc • mobility of the discharge (influenced by electrode roughness or contour) • impedance and ‘stiffness’ of voltage source • physical structure of damage path In the case of DC voltage stress, other test parameters also influence results, including: • polarity • chemical composition of the electrode material • ripple shape of voltage Given such test parameter uncertainties, an obvious question that arises is whether a ranking of materials in regard to tracking and erosion performance under AC would provide appropriate information and be fully transferable to DC.

Some specimens of Materials B and C failed, but in these cases by a different failure mode than Material A. Their failure mode was erosion and the test time required for this to occur was always much longer than for Material A.

The same test was then performed on virgin material specimens cut from the housing of an identical new insulator and again the test was passed. However, the new specimens showed a trend toward slightly greater erosion. Based on this, it was concluded that ‘silification’ of the housing surface does not reduce tracking and erosion performance in the case of the ATH-filled HTV silicone rubber material investigated.

Fig. 2: Investigation of three families of polymeric materials.

Fig. 1: Inclined Plane Test specimens cut from insulator housing.

More investigation is now ongoing to determine whether or not this ‘silification’ process might actually provide a type of ‘shield’ against its propagation, i.e. in other words, does the first layer inhibit further ‘silification’? If anything, results during accelerated UV tests seem to confirm this.

The main purpose of the Inclined Plane Test is to evaluate the tracking and erosion performance of a polymeric housing material. If the material has inherent hydrophobicity, this property is destroyed using a wetting agent, which then simulates the ‘late

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In spite of the fact that the first standard for the Inclined Plane Test was published as far back as 1977, results can vary quite a bit depending on certain test parameters:

These results seem to prove that it is not generally possible to transfer AC test results to expected DC performance under conditions such as a simulated late-age phase of the material. The WG will perform more tests with the round-robin philosophy to verify if this is true.

Nevertheless, it can still be concluded: 1. Erosion or failure rate under the same test conditions is higher for DC stress; 2. Positive polarity DC stress causes more severe damage; 3. Secondary material reactions can occur, leading to accumulated damage. This means that certain polymeric materials are just not suitable as housings for DC applications.

Dr. Frank Schmuck frank.schmuck@sefag.ch

Editorial

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Using Arresters to Meet Learning Objectives I’ve had several opportunities in recent years to bring my technical presentations on lightning protection and insulation coordination to a more practical level. The goal was to make them better suited for various training and graduate education courses I teach. During the course of modifying my materials, I’ve also managed to develop an appreciation of what it really means to define learning objectives.

Transient Thoughts

My introduction to this concept came from two directions: first of all, professional courses in the nuclear power industry had already helped me formalize a more systematic approach to training. I was now being asked to develop courses that follow this same philosophy but involve mature students who learn best when teachers share not only theoretical concepts but also practical knowledge derived from case studies and project experience.

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At the time, I also had the opportunity to visit fellow columnist Professor Stanislaw Gubanski at Chalmers University of Technology. This gave me another perspective when it comes to university-level teaching. For example, Stanislaw told me about the EU Bologna Process (1999) that includes 12 years of elementary and secondary school, a three-year baccalaureate and two years at the masters or license level. The Bologna Process apparently also calls for defining a course’s learning objectives to shift from focusing on what a lecturer teaches to what students are expected to learn. Incidentally, Stanislaw also told me how densely packed the undergraduate electrical engineering curriculum already was. Adding any new topic would therefore almost certainly require dropping some other competence.

Chalmers now routinely shares learning objectives with students at the start of each course. The specific means by which each objective is to be verified (e,g, exams, lab exercises or project reports) is also clarified. The main benefit of this learning-outcome approach to teaching is improved student results. When I faced the task of defining what a student or adult learner would be expected to know, understand and/or be able to demonstrate after completing one of my courses, I turned to my wife for help. She’s an experienced high school teacher familiar with defining terms such as intended learning outcomes and course-specific goals. She made it seem easy by providing an example from her field of teaching foreign language: recall the foreign word for ‘year’, distinguish the noun’s gender, choose appropriate articles and adjective endings and finally arrange these words correctly, e.g. “Last year I visited Korea,” … “L’année dernière j’ai visité la Corée”, or “Letztes Jahr habe ich Korea besucht.”

Like many INMR readers, I find lightning protection and insulation coordination extremely interesting. The issue at hand is how these topics can be more effectively taught within the undergraduate electrical engineering curriculum. Most programs, for example, show students how to compute the surge impedance of a coaxial cable and maybe also offer lab experiments as to why and how cables should be terminated.

This background makes it easier to understand a familiar case study, namely the direct-stroke lightning protection problem.

The choice of two adjacent spans, rather than a single arrester, to limit the voltage Vpk, is deliberate. In my teaching experience, perhaps the most difficult concept to explain to students in back flashover and shielding failure calculations has been why the current (I) divides equally in two. Expressing this problem in terms of learning objectives, the successful student should therefore:

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Simplified circuit for learning objectives when it comes to arrester lightning protection.

• Recognize that the wires over ground form distributed surge impedances; • Recall the formula to calculate the surge impedance of a wire pair directly or from its per-unit length inductance and capacitance; • Extend the surge impedance formula to the case where two impedances are in parallel, over a ground plane; • Identify the lumped circuit components (arrester, resistance, ground); • List the physical attributes that affect the voltage at the evaluation point (Vpk); • Construct a suitable electrical model; • Calculate the voltage in response to a specified current wave; • Examine the relative roles of arrester voltage, footing resistance and span length; • Compare the calculated voltage with a model of insulation breakdown strength; • Modify the circuit to reduce Vpk to a desired level.

Multi-disciplinary engineering teams could address different aspects of such an arrester protection problem assignment as a team project. For example, a student in geo-technology could develop and test the relationship between footing resistance (Rf) and local soil resistivity. A mechanical engineering student could refine the estimates of conductor sag as a function of span length, leading to realistic conductor heights at the stricken tower for constant mid-span clearance to ground. A specialist in high-speed instrumentation could build a practical scale model using a small gas-filled spark gap and low voltage surge arrester along with a low-cost, safe impulse source such as an electric fence pulse generator. The student in electrical power system engineering would be responsible for modeling the arrester’s characteristics as well as the methods for secure measurement of voltage and current without damaging test equipment. The student in the combined engineering/MBA stream could then evaluate the social benefits and business case for fitting arresters to any existing line (as was done in the article on p. 100).

Advanced treatment of the arrester lightning protection problem at the postgraduate level could cover the volt-time curve of the insulation, the statistical distributions of peak return stroke current and soil resistivity, the V-I curve of the arresters and the benefits of adding an overhead ground wire to the circuit.

This basic problem not only demonstrates some important engineering concepts but also seems to fit gracefully into my improved learning-focused curriculum, and possibly yours.

Dr. William A. Chisholm W.A.Chisholm@ieee.org

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Coating Line Insulators with RTV Silicone In his column In Praise of Coated Insulators in the past issue of INMR (Q4, 2011), Ravi Gorur raised an interesting question: since application of RTV silicone coatings to porcelain at substations has enjoyed success for 25 years now, why has this practice not yet become widespread for line insulators as well?

The study of RTV coatings to combat pollution flashover of line insulators began in China during the early 1980s. The first trial application at a local power supply company came in 1986 when Tsinghua University co-operated with the Tianjin Power Bureau and applied coatings to 15 insulator strings on a 35 kV line running near a coastal chemical factory. This area was so polluted that, even with regular cleaning, contamination flashovers still occurred.

SCENE FROM CHINA

The insulators already in use on this particular line were strings of 4 double umbrella-shed porcelain disc insulators of the type commonly used in China in areas of moderate to heavy pollution. However, it was decided to conduct the trial application of coatings on strings of only 3 regular shape disc insulators. Evaluations were then performed to monitor the relative performance of these different strings on the same towers. As part of this work, maintenance staff would visit test towers during severe weather to observe what was taking place. In their final report, they noted that during heavy fog, for example, the uncoated insulator strings had loud noise from corona as well as very serious discharge activity – sometimes looking like a string of lanterns at night. By contrast, the RTV coated insulators with only 3 discs in the string showed no evidence of discharges. Indeed, over years of monitoring the coated insulators, during which no cleaning was conducted on them, noise and discharges during severe weather were comparatively minor and always much lower than on the uncoated insulators installed on the same towers.

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A few of these coated insulator strings were subsequently removed for further analysis and it was found that they still had excellent hydrophobicity and a very high contamination flashover voltage. Based on this result, RTV coating technology was approved for widespread application in Tianjin, including on substation support insulators and bushings as well as for important overhead lines from 35 kV to 500 kV. Of the 15 strings of coated insulators in that first trial application, a number more were removed after 6, 9 and 14 years of service for detailed observation and testing. It was found that, in spite of some minor surface damage, contamination flashover voltage remained very high. In fact, there are 5 of these strings still in service after 25 years, without any cleaning or other maintenance required over that time.

Hydrophobicity of trial coating application still excellent after 6 years’ service.

Given this success with RTV coatings, there are now two effective options when it comes to selecting line insulators for areas exposed to moderate or heavy pollution: silicone rubber insulators and RTV coated porcelain or glass insulators. The main advantages of composite insulators are their comparatively low price, less weight as well as easier transport and installation. Their disadvantage is that service experience is still limited compared to ceramic insulators.

Moreover, problems such as core rod fracture or unexplained flashover have occurred periodically to the extent that, even though such failure rate is low, there is still a degree of worry.

In fact, some Chinese power companies still will not use composite insulators on tension strings, especially at the higher transmission voltages.

Coated tension strings on 500 kV line in Hubei Province.

The key advantage of RTV coatings on porcelain or glass is that they combine the reliable mechanical properties of inorganic insulators with the excellent pollution withstand of silicone rubber. Their main disadvantages are higher price and the risk that not every insulator on the tower receives a consistently good coating quality. Indeed, for newly built lines, application of RTV coated insulators is clearly more expensive than the option of using composite insulators. By contrast, in the case of existing lines, the cost to coat ceramic insulators already in service is not high when viewed against the expense of replacing all of them with composite types.

In terms of quality problems related to coatings, it has been difficult to ensure a consistent application by the initially used method of brush coating. Coating quality then improved greatly with the arrival of spray coating even though some power utilities still prefer to remove the insulators first, coat them on the ground and then re-install them afterwards. There have also been several porcelain and glass insulator manufacturers who have offered the option of carrying out the RTV coating in the factory prior to delivery. This ensures more consistent coating quality.

Thanks to these improvements, Chinese power utilities have become more willing than ever to employ RTV coatings on important transmission lines. For example, the Tianjin Power Bureau applied coatings to a key 220 kV line connecting it with In the early 1990s, Beijing. Moreover, for tension tower insulators in areas with RTV coatings also high pollution, most utilities these days opt for RTV coatings became increasingly on porcelain or glass strings. popular in Henan, Shandong, Liaoning, In summary, my response to the question raised by Prof. Gorur Jilin, Heilongjiang and Shanxi – all having such good is that, in China at least, the technology of applying RTV silicone experience that the coverage rate of coating application in coatings to combat insulator pollution flashover on lines is in these provinces has become as high as 80-90% of all insulators. fact already well established.

Apart from the benefit of significantly decreasing contamination flashovers, maintenance work such as insulator cleaning has also been reduced. Indeed, RTV coating technology is now so

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widely accepted by power companies across China (see article on p. 50) that there is already a domestic industry comprised of a number of competing local manufacturers. Many of these co-operate with local universities or research institutes to continually improve product quality and some even have their own specialized teams to carry out on site coating installation.

Prof. Guan Zhicheng Tsinghua University, Shenzhen Campus guanzc@tsinghua.edu.cn

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DC Insulator Market Poised for Explosive Growth The battle between the DC espoused by Thomas Edison and the AC of George Westinghouse and Nikola Tesla was decided at the start of the last century when AC became the dominant technology for transmitting electrical energy. It now appears, however, that DC was never really defeated at all. In fact, given its modern synergy with power electronics, DC seems to be on the rise and leading the way for much of what’s taking place in the otherwise mature field of electrical transmission.

P I G I N I Commentary

Indeed, much has changed since the first commercial exploitation of HVDC transmission in 1954 when the Swedish mainland grid was connected to the island of Gotland using static mercury arc valves. Mercury valve technology was eventually replaced by thyristor valves with gates activated by line voltage. In fact, HVDC bulk transmission utilizing basically this technology has perhaps reached its peak with the recent record-setting Xiangjiaba-Shanghai ± 800 kV link that transports 6400 MW over a distance of some 2000 km.

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Now, there’s a virtual ‘revolution’ underway that’s bringing the energy field much closer to that of power electronics.

This involves development of externally-controlled thyristors, typically insulated gate bipolar transistors (IGBT) and gate turn-off thyristors (GTO), which together have made even small HVDC systems economical to operate. Because of benefits in terms of reduced power losses, these may one day even become a substitute for the traditional ‘thyristor solution’ that still prevails for large systems mainly because of lower power losses. For example, a recent application of this latest technology was made with the Caprivi Link (350 kV, 300 MW, 950 km) that connects Namibia to Zambia.

These technical developments as well as the many advantages of DC transmission for long distance connections have led to a virtual exponential growth in total power being transmitted worldwide using DC (see Fig. 1).

Total power transmitted by DC surpassed 100 GW in 2011 and the clear trend suggests a more than tripling of all power transmitted this way by 2020. This projection corresponds well with what we know about planned short-term projects such as upcoming new 800 kV and 1000 kV lines in China, EHV and UHV lines in India and the many projects now underway in Brazil, including the 2700 km Rio-Madeira line that will soon enter operation. On top of these are new DC lines foreseen in Latin America (e.g. the HidroAysen Project in Chile as well as the Colombia-Panama and Peru-Brazil interconnections).

This optimistic long-term forecast for DC may well even prove conservative since ongoing innovation will only serve to make this technology increasingly competitive for power

Fig. 1: Growth in total power transmitted by DC.

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transmission and indispensable for interconnecting different countries. Then, there’s the growing need to transfer energy resources such as hydroelectric and other renewables to huge urban demand centers located far away. For example, DC systems that integrate and superpose on AC systems are already under advanced study both in the U.S. and Europe.

All these current and future DC projects involve long distance connections by both cable and overhead lines but overhead transmission is clearly prevailing. More than 30,000 km of overhead DC lines are installed today against only about 6500 km of cable, with an exponential trend expected in regard to power being transmitted overhead (see Fig. 2).

On the basis of the above, I’ve tried to estimate the installed population of DC insulator strings, making certain simplistic assumptions (e.g. the majority of lines being double circuit with a prevalence of I strings on V strings and considering some 2.5 towers per km). The resulting ‘market prognosis’ for such insulators is depicted in Fig. 3. Against only a few hundred thousand DC strings installed today, numbers approaching 1 million such strings seem reasonable by 2020 and an order of magnitude higher (i.e. close to 10 million) by 2050.

The same trend can be extended to station insulators as well, although of course the numbers will be much lower and depend on specific application.

This virtually explosive market development could prove particularly favorable for composite insulator manufacturers given the significantly better DC service performance of silicone compared to ceramic insulator technology under pollution.

In my view, ongoing research and product development by manufacturers of composite insulators should aim to achieve the following key goals: 1. Optimizing housing material, e.g. by nanotechnology, to assure a long-term high level of hydrophobicity will be maintained under DC; 2. Identifying the ideal insulator geometry and profile for DC, which may well prove significantly different from that for AC; 3. Finding solutions to better control voltage distribution as is particularly critical in DC, e.g. by utilizing layers of zinc oxide.

Those manufacturers who succeed to reach these goals first will probably gain a significant share of this future ‘exploding’ market.

Fig. 2: DC connection lengths by cable versus overhead line.

Alberto Pigini pigini@ieee.org

Fig. 3: Expected growth in population of DC line insulator strings.

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Increasing Transmission Capacity: The View From a New CIGRE Brochure

From the Research View

Evolution of existing as well as any new system assets to the level of Smart Grid is now the dominant scenario expected in the power industry for years to come. This will be achieved mainly by adding ‘smartness’ (i.e. intelligence and controllability) to the various components used in electrical grids, e.g. through added functionality. Other important goals ahead for network operators will include extending the operating limits of networks, increasing robustness, guaranteeing safety and security, ensuring environmentally friendly designs, developing components with reduced losses and providing a consistently high quality of energy supplied. Achieving all these targets will undoubtedly present a formidable challenge.

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For example, it’s expected that 20% of total energy consumption in Europe by the end of the decade will originate from renewables. (Germany, incidentally, has already met this goal and plans to increase its own ratio to 35% by 2020.)

Making this a reality will clearly require building new lines as well as modernizing existing networks in order to allow energy exchange between small as well as large producers. This will call for new solutions to traditional ways of designing power lines and, given the difficulty and long lead times associated with obtaining new right-of-ways, more efficient utilization of present transmission capacity.

CIGRE has begun studying these challenges and recently published a number of important documents intended to provide guidance. One of these was issued by Working Group B2/C1.19 in the form of Technical Brochure No. 425, titled “Increasing Capacity of Overhead Transmission Lines – Needs and Solutions” and reviews the various technologies that could allow this to happen.

The document is addressed mainly to system planners and among the alternative solutions discussed are: (1) technologies to increase line ampacity, (2) technologies for uprating voltage, and (3) miscellaneous other largescale actions. The last category includes re-shaping the configuration of EHV AC lines to improve surge impedance as well as more use of HVDC systems. The brochure’s authors also stress the importance of close co-operation between system planners and line designers, whose main responsibility is usually focused only on understanding and dealing with electrical and materials related phenomena.

Below are some of the new brochure’s key contents: Increasing line ampacity is considered from two points of view: as a design change and as an operational change.

The first approach is achieved either by raising the thermal limit of existing conductors or replacing them with ones that have higher ampacity. A good example of the latter solution has been adopted in Egypt and reported on in my column of INMR Q4, 2010.

Operational methods, on the other hand, rely on using reserve capacity without any basic change in line construction. This approach assumes that a line’s thermal rating will always vary depending on prevailing climatic conditions. In this regard, various real time monitoring systems are utilized, including not

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only weather monitoring but also measurements of conductor temperature and tension.

Solutions based on uprating any line’s voltage offer another effective ways to increase transmission capacity while allowing involved energy losses to be reduced. Typically, the new voltage level is at least double the former. This means there is a need to re-insulate the line while respecting various constraints such as safety clearances. There may also be the occasional need to replace existing conductors as well in order to reduce possible corona losses. The brochure references some practical case histories of this solution, including where a 66 kV line in Spain was upgraded to 220 kV (i.e. a 233% capacity increase) while still using the original conductor and foundations.

In the category of large-scale actions, the brochure concentrates on adopting high surge impedance loading (HSIL) lines and on conversion from AC to DC transmission.

As regards HSIL technology, the main issue is to reduce line voltage drop to increase a line’s natural power. This is attained by increasing line capacitance and decreasing its inductance – something which can be achieved by adding sub-conductors per phase, increasing bundle size, using asymmetric bundles or decreasing phase distances. Successful adoption of HSIL technology has already been reported in countries with long transmission distances, such as Brazil, Russia and China.

Application of HVDC technology is also becoming more popular. Indeed, one of the most advantageous – albeit also most expensive – ways to increase transmitted power is by converting existing AC lines into elements of an HVDC system.

Among the factors that allow higher efficiency to be reached are the absence of reactive power and the ‘skin effect’. The CIGRE brochure provides examples of various solutions to optimize such conversion, including monopole, bi-polar or tri-pole DC systems – each offering different possibilities to utilize the conductors of the existing line. However, the high cost of adding converters must not be overlooked.

By way of conclusion, the authors of the new brochure recommend not considering individual projects separately but rather looking first for an overall re-structuring strategy based on large-scale analysis of network development in the region under study. This should ideally be done in co-operation with all responsible authorities so as to deal effectively with environmental issues and this way allow for continuing network expansion.

Whichever technology is eventually selected to increase transmission capacity, one thing seems clear – there will be all sorts of new market opportunities ahead for insulator suppliers as well as line constructors. They should therefore look to the brochure for guidance in their own strategic planning.

Prof. Stanislaw Gubanski Chalmers University of Technology stanislaw.gubanski@chalmers.se

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10 Things to Always Remember About Arrester Applications Over the years, I’ve written on a broad variety of topics when it comes to surge arresters. So, to start this 20th Volume of INMR, it seems to me that there might be great value in condensing this knowledge into some basic arrester principles that ideally must always be understood and respected. To that end, below I humbly offer my selected ‘10 Nuggets of Arrester Knowledge’.

Woodworth on Arresters

1. The Terms that Really Count While U c and MCOV (maximum continuous operating voltage) are the most important voltage ratings of an arrester, unfortunately they have not always been appropriately promoted within IEEE or IEC application areas. Basically, these are the numbers that should be used whenever referring to an arrester’s voltage rating. In my view, the term ‘rated voltage’ in IEEE is of little value. Also, in the IEC market areas, the term Ur should not be taken as rated voltage.

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2. Closer is Always Better Whether applying arresters on a distribution transformer or at a 500 kV substation, they should always be installed as close as possible to the equipment they are intended to protect. In distribution, where lightning is the main concern, inductance of the leads into and out of the arrester can cause large voltage drops that effectively increase clamping voltage. A higher clamping voltage then means less protection for the transformer.

At substations, where both switching and lightning surges are of concern, too lengthy a distance between the arrester and the protected transformer can result in reflections of the traveling surge. This can increase the voltage at the transformer to a level significantly above the arrester’s clamping voltage.

3. Line and Ground Lead Diameters The diameters of arrester line and ground leads are too often larger than necessary. For surge reasons alone, a small wire (~5mm) is all that would really be needed. However for mechanical, fault, and corona reasons the leads have to be larger.

When sizing arrester conductors, always ensure that they can handle fault current without melting or breaking from the magnetic forces. If the arrester is on a transmission line, also ensure that the leads can withstand a million bends at common bending points. At substations where voltage is above 100 kV, lead diameters will need to be increased to eliminate risk of partial discharge or corona.

4. Extra Creepage Can Reduce Margin of Protection If an arrester is increased in length to achieve a higher creepage distance, the arrester’s inherent inductance will also increase.

This means that, in the case of fast rising surges such as lightning or insulator flashover, clamping voltage increases – in other words, the same negative effect as having too long a lead.

If extra creepage is achieved by broader diameter or more external sheds and arrester length not increased, then there is no such risk of a drop in performance. Note that this impact of length on protection capability is not always indicated in manufacturer data sheets. Also, note that if the insulators at a substation or on a line have longer specific creepage due to pollution, then the arrester’s creepage will also have to increase.

5. Use a Low Residual Voltage Arrester at the Riser Pole Underground circuits, whether at distribution or transmission voltages, are among the most costly to repair in the event of a line fault, e.g. caused by dielectric failure induced by a lightning or switching surge. Using arresters with the lowest possible clamping voltage at the transition pole ensures that the line

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has the maximum capability to deal with the inevitable voltage doubling that can occur in such cases. If a separable – also known as deadfront – arrester is used on the underground circuit, there is less need for ensuring the best possible arrester at the transition pole.

6. Disregard the Longstanding Practice of ‘Four Per Mile’ Spacing In the U.S. and other countries, one sometimes finds a policy of distribution line arresters being installed on every 4th or 5th pole. It is not known how this ‘rule of thumb’ originated but it is clearly incorrect since it has little effect on improving line performance in case of lightning strike. If a direct strike takes place near a protected pole, this simply transfers the surge to the next (unprotected) pole and the flashover occurs there. If the strike is near an unprotected pole, then the insulators on that pole will flash over. There are obviously better ways of deploying line arresters to maximum benefit.

7. Make Sure Animal Guards are Completely Effective Guards or wildlife protectors on arresters can prove ineffective if they do not keep the animal out of the critical flash area surrounding an energized line. Covering the terminal alone so that it is invisible to the eye may not be sufficient to eliminate animal induced outages. To be truly effective, a guard must also provide a barrier that does not permit the animal to stand on an earthed surface and somehow have any part of its body come into proximity with this critical live zone.

8. Overhead Ground Wires are Not Always Needed to ‘Lightning Proof’ a Line There are a variety of situations (e.g. hoar frost) where overhead ground wire is not ideal. In such circumstances, using arresters in place of shield wire has been proven to work. However, the arresters must then be installed on each phase of each tower to achieve a zero lightning outage rate. While this might seem an expensive option at first glance, it can actually prove cost effective since the towers themselves can be shorter and have lower foundation costs – and of course there is no cost for the shield conductor.

A secondary savings of such a shieldless yet fully protected line design is that costly grounding efforts (depending on soil conditions) can be eliminated.

9. Energy Ratings are Not What They Seem (At Least Not Yet) With no standardized test yet available for this arrester characteristic, published values are based on either one or two impulses. That means that equally rated arresters from two suppliers might actually be different. When the newest revisions of IEC 60099-4 and IEEE C62.11 are released later this year, arrester energy rating method will be the same, regardless of manufacturer. At present, there are no energy ratings for distribution arresters – only current ratings and this will change to a charge rating in the near future.

10. Ground Lead Disconnectors Are Not Reliable on Delta and Ungrounded Circuits The ground lead disconnector is a useful device since it not only disconnects distribution arresters from the circuit after an overload but makes their failure visible to maintenance staff.

But the disconnector is also activated by ground fault current and there is no ground fault current on ungrounded as well as delta circuits.

Jonathan Woodworth Jonathan.Woodworth@ArresterWorks.com

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Database to Make PD Measurements of MV Cable Systems More Effective As quality of service become ever more important, regulatory authorities in Europe are introducing a ‘bonus-penalty’ type system to push electricity supply utilities to improve their network reliability.

F

CUS ON CABLE ACCESSORIES

However, as is evident from Fig. 1, between 70 and 80 % of all supply interruptions to LV power consumers are caused by outages affecting the medium voltage network. This means that interruption frequency and duration affecting the MV network (i.e. cable systems and overhead distribution lines) will together play the major role in determining overall level of availability. Transmission lines play a far lower proportionate role in this regard.

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At the same time, it is important to also note that countries with a higher ratio of their MV network made up of overhead lines (e.g. Spain and Italy) typically have lower network reliability than do countries with a higher proportionate use of cable systems, such as the Netherlands and Germany (see Fig. 2).

Apart from the above is the fact that, when it comes to pure MV cable networks, there’s still great potential for network operators to improve reliability by performing partial discharge (PD) measurements.

Most cable systems today are a mix of older paper-insulated and new polymeric (e.g. XLPE or EPR) cable technologies. In the case of paper cable systems, it is fair to say that failure rates are dictated by both the accessories and the cables themselves. In the case of polymeric cables, however, outages are most often linked to problems caused by the accessories and in particular to defects resulting from mistakes made when installing them.

Whatever the cable technology, measuring PD offers one of the most effective tools to estimate system quality and (with some important limitations) to also predict the residual life of its components.

Fig. 1: Portion of supply unavailability to a low voltage customer caused by interruptions in different voltage levels of network.

Looking only at accessories for polymeric cables, the nature of defects that can be detected by PD measurement is quite specific to different designs and types. Moreover, the severity of such defects depends on the nature of the installation mistake that was made. For example, air gaps in a heat shrink joint can cause a PD level of more than 1000 pC at service voltage.

Nonetheless, the defective joint might manage to survive for anywhere from 6 to 10 years. Other defects, as in the interface of a slip-on joint, could result in a much lower PD level (e.g. several hundred pC) yet fail after only months or even weeks.

That’s why it’s always necessary to compare the results of the measurement on a certain accessory with other tests results on that very same type of accessory.

The main problem for electricity supply companies who perform such PD testing therefore lies in the database available to them for comparison purposes – simply because of the large number of different joints and terminations installed in their network and the typically low failure rates of these accessories.

To get more information in this respect, a joint working group of the VDE in Germany, representatives from the manufacturer of measuring devices, electrical network operators and universities proposed developing a web-based database.

The objective is to collect PD measurement data from many electricity supply companies across different countries.

This idea of a web database was presented in two papers at the 2011 CIRED Conference in Frankfurt and also at the 2011 Jicable in Paris. Hopefully, developing it will represent an important first step toward better evaluating PD measurement results – and from there toward further improving cable system reliability. (please visit www.vde-kabeldatenbank.de)

Fig. 2: Mean value of supply unavailability in minutes/year for countries with different cable rate of the MV network (Netherlands 100%, Germany 65%, Italy 30% and Spain 25%).

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Given these considerations, one obvious way to improve network reliability is to increase the role of cable systems within the overall medium voltage network – much like Sweden is currently doing with the goal of reducing the impact of severe weather events such as major snowfalls and windstorms.

Another opportunity, of course, could be setting up a ‘smart grid structure’ that offers new possibilities for achieving higher availability in the case of networks comprised mainly of overhead lines.

Professor Klaus-Dieter Haim University of Applied Sciences Zittau/Görlitz, Gemany KDHaim@hs-zigr.de

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UTILITY PRACTICE & EXPERIENCE

Turkish Transmission Utility Looks to Silicone Insulators to Reduce Construction & Maintenance Costs Strategically located at the crossroads of three continents, Turkey has for centuries stood as a meeting point between Asia and Europe, between east and west. With a mostly young and growing population, rapid urbanization and until recently strong economic growth, this emerging country now faces numerous challenges to meet its future energy needs. Electricity consumption, for example, is increasing by an average of 8 to 9% a year, and significant investments will be needed in generation, transmission and distribution to balance power system supply with this spiraling demand.

to as low as -43 °C in eastern Anatolia. Moreover, typical daily fluctuations can be as high as 20°C, making it seem that all four seasons can be experienced on the same day. Few other countries face such dramatic climatic variations in service conditions for the power grid.

Helping the country to satisfy future generation needs are some positive climatic factors. For one, there is high potential for wind energy along the western coast while an average of 7.2 hours of sunshine each day means an abundance of solar energy. Moreover, with plentiful rain and generous water resources, there seem ample available sources of renewable energy.

But Turkey’s geography also poses certain problems for its power system. The country is surrounded by vast bodies of water: the Black Sea to the north, the Marmara and Aegean on the west and the Mediterranean to the south – in all cases with long coastlines. Each has its own specific character in terms of salinity, with salt concentrations that can vary from relatively low in the Black Sea (e.g. 18 parts per thousand) to more than double that level in the Mediterranean.

As the natural bridge between Asia, Africa and Europe, Turkey also seems to have inherited several distinct climatic regions from each. With an average monthly relative humidity of 80% and annual precipitation of up to 1400 mm, daily temperatures can range from a sweltering +45°C in southeastern Anatolia

Turkey’s Power Sector

The Turkish electricity market has undergone significant structural change over the past 15 years.

As a first step towards an open, competitive market, the former stateowned utility (TEAŞ) was divided up into three successor organizations: the Electricity Generation Company (EUAŞ), the Turkish Electricity Transmission Company (TEIAŞ) and

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INMR Contributor and T&D specialist Raouf Znaidi visits the Turkish Electricity Transmission Company to report on future plans for expansion as well as how this challenging environment is affecting selection and maintenance of critical components such as insulators for lines and substations.

the Turkish Electricity Trading and Contracting Company (TETAŞ). In addition, an entirely new entity – the Energy Market Regulatory Authority or EMRA – was established to oversee all energy market activities. Any prospective investor in this sector must apply to EMRA to obtain the license required to develop and operate an energy facility.

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As a second step, liberalization of the electricity market through privatizing distribution and generation assets has brought several local and international companies into the Turkish energy sector. One of the most important of these, Enerjisa, was awarded the first privatization license and is also among the local companies most involved in developing hydraulic and wind power generation.

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lines in accordance with all TEIAŞ technical specifications but also with a view to optimizing costs, which we estimate at about US$ 190,000/km and US$ 250,000/km for 154 kV & 380 kV respectively.”

Ayranci adds that, apart form its role in line construction, Enerjisa is also involved in alternative power generation, particularly thermal and wind. Future plans in this regard are to expand the large natural gas-fired plant in Bandirma as well as two wind power plants in Mersin and Balikesir.

This, he notes, will represent the second phase of the company’s capital investment plan aimed at developing at least 5000 MW of new generation capacity by 2015.

Silicone insulators recently installed at Turkish substation.

According to Enerjisa’s Directorate Project Manager, Mustafa Ayranci, total installed power in Turkey as of August 2011 stood at approximately 55,000 megawatts, produced predominately from thermal plants but also increasingly from hydroelectric, geothermal and wind power. Enerjisa Generation, for example, is targeting a minimum installed power of 5000 MW by 2015, with the large majority based on renewables. Ayranci goes on to indicate that wind power offers perhaps the greatest opportunity for future development and that, with an estimated total of 48 GW of

onshore and offshore, Turkey exceeds the wind power potential of most European countries.

Nafiz Ozcan, the company’s expert in overhead transmission, points out that all this power is transferred to TEIAŞ as the main customer, using existing and newly built lines. In certain cases, based on mutual agreement, new lines have been constructed entirely by Enerjisa but under a Ayranci. Enerjisa involved in both power supervised contract for TEIAŞ. generation and distribution.

Says Ozcan “basically, we built these new high performing transmission

Fig 1: Installed power in Turkey by energy source (2010).

Photos: INMR ©

Glass strings have traditionally been the dominant type of insulator used on Turkish transmission lines.

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“Since many of our lines do not suffer from pollution, turning progressively to silicone-housed insulators for these during the past three years has been based mainly on other factors, particularly their attractive acquisition cost.”

improving existing transmission infrastructure so as to assure a more consistent high quality of service.

This goal has become ever more important since TEIAŞ expects electricity demand to grow by about 7.5% annually over the course of its 2012-2018 Plan.

Photos: INMR ©

New investment program by TEIAŞ aims to improve reliability with optimized costs.

Turkish Electricity Transmission Corp. (TEIAŞ):

Ozcan. Transmission lines built according to TEIAŞ technical specifications. with more than the equivalent of US$ 320,000,000 allocated to expanding our transmission system.

At the same time, we are aiming to optimize these investment costs as much as possible.”

In contrast to the role of private companies such as Enerjisa, the Turkish Electricity Transmission Corp.

(TEIAŞ) is the sole owner of the HV Yildir goes on to point out that and EHV network and also responsible resources will also be devoted to for planning, construction, operation and maintenance of this system.

TEIAŞ also prepares a Transmission System Investment Plan as well as a 10-Year Statement Report for development of the country’s transmission capabilities.

Among the key projects when it comes to expansion of the grid will be future dual interconnections of Turkey’s national electricity system to the European system to the North and to a group of countries to the south, including Egypt, Jordan, Libya and Lebanon among others.

Glass has traditionally been the dominant insulator technology specified on the Turkish overhead transmission network with some use of porcelain on lines but more

TEIAŞ General Manager, Kemal Yildir, explains that one of the principal goals of the investment program in 2012 will be to renew and further develop the country’s electrical infrastructure. “As during the past year,” he says, “significant new projects will be undertaken, Kemal Yildir (center) reviews TEIAŞ investment program with INMR’s Raouf Znaidi (right).

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Transmission lines near Istanbul suffer from high humidity combined with marine and industrial pollution, with additional vandalism problems that lead to shattered glass discs.

frequently at substations. Up until recent years, composite insulators have played little to no role. But now that seems about to change.

While a CIGRE study on experience with composite insulators found that superior pollution performance was the main reason utilities decide to use them, this has apparently not been the sole consideration at TEIAŞ. Says Yildir, “many of our lines where we are now installing composite insulators do not suffer from pollution. Our turning progressively to siliconehoused insulators in these cases has therefore been based on other factors, especially their increasingly attractive acquisition cost. In addition, we feel that such insulators offer us further potential for savings when it comes to installation and maintenance costs compared against conventional insulator technologies.”

Table 1: TEIAŞ Transmission System (Aug 2011) Voltage

380 kV

220 kV

154 kV

66 kV

Total

Substations

Circuit Km

16,000

85

35,000

510

51,595

588

Table 2: Number of Outages on TEIAŞ Transmission Lines (by end of 2010) Voltage (kV)

Length of System

Total Number of Outages

Period of Availability(days)

Outage Rates/100km

380

16,000

1264

345.95

8.36

154

35,000

3051

352.38

8.80

66

510

89

365

13.32

Total

51,595

4404

354.45

8.63

Yildir also observes that there seems to be a growing cost efficiency when it comes to the production and application of the latest generation of silicone composite insulators, meaning there may be even further cost savings to be realized in the future.

Turkish Transmission System

As of August 2011, the TEIAŞ network comprised some 51,600 circuit-km of overhead lines operating at 380 kV, 220 kV, 154 kV & 66 kV.

Photo: INMR ©

Since part of this network is exposed to vast open sea, insulation often has to be designed to resist continuous exposure to marine and in some cases industrial pollution as well. There are also high levels of ultraviolet radiation as well as frequent large daily temperature fluctuations. For example, with the Sea of Marmara in the country’s northwest, some HV lines near the commercial hub of Istanbul are especially vulnerable.

380 kV tower near Ankara employs 24 standard profile glass discs in tension and 22 in suspension.

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According to Transmission Maintenance Manager Hikmet Ozdemir, TEIAŞ experienced a total of some 4400 transmission outages during 2010, with an overall outage rate for the entire system of 8.63

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Trial application of RTV silicone coatings in Turkey increased initial insulator costs at substations by about 60%, but has greatly improved performance under polluted conditions.

Golbasi Substation is an example of a location where composite insulator technology is being progressively applied to reduce costs. per 100 km. Ozdemir says that new maintenance policies are now being developed with the objective of reducing this rate. Table 2, provides an overview of outages recorded at each transmission voltage and highlight the problems affecting the country’s old 66 kV network.

According to Ozdemir, major 154 kV and 380 kV lines near the capital city of Ankara and the surrounding area are generally insulated with 10 or 11 and 22 to 24 standard profile glass discs respectively. These, he says, have typically not experienced any serious operating problems. By contrast, lines in other parts of the country suffer from high humidity combined with marine and industrial pollution. Vandalism problems affecting glass discs have also been reported.

For example, the 380 kV HabiplerZekeriyaköy line west of Istanbul has had a history of extensive problems related to pollution flashover and vandalism and experienced up to 3 outages per day, increasing outage rate in the Istanbul area to 11.43 per 100 km. This was one of the lines

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where TEIAŞ finally decided to turn to composite insulators to deal with these problematic service conditions.

Ozdemir points out that the benefits of this move have extended beyond overcoming pollution and also included important economic gains.

Explains Ozdemir, “the insulators we selected to replace existing glass anti-fog disc insulators, were siliconehoused alternating shed designs that allowed us to effectively deal with the pollution problem. At the same time, the acquisition cost of these new insulators was less than half that of the equivalent glass cap & pin string – and this does not even factor in the savings related to their being mostly maintenance free.”

Indeed, reducing maintenance requirements at substations as well as on lines has been one of the key goals at TEIAŞ. In this regard, alternatives have been looked at to replace the costly frequent washing of porcelain at stations in industrial areas such as the North.

As first step in this direction, 4 years ago trials were begun with RTV silicone coatings applied to substation porcelain by an outside contractor. While this increased initial insulator cost by about 60%, these coated insulators are still

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Ozdemir. Some TEIAS lines suffer heavily from pollution flashover as well as vandalism.

performing reliably and Ozdemir reports that maintenance staff are very satisfied with how this technology has improved substation insulation performance under pollution.

The Golbasi Substation, however, provides a good example that composite line insulators are being installed in Turkey even when pollution is not the dominant concern. Located in a rocky, agricultural area just outside Ankara, there is little visual evidence of pollutants accumulating on insulator surfaces in spite of noise from surface discharge activity clearly evident, even on a sunny afternoon.

Mustafa Danaci, who has worked at the substation for more than three decades, explains that noise level here is more or less constant

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Porcelain housings at Golbasi substation.

280 mm to 430 mm respectively.

Alternatively, there is some use of porcelain long rods as well and these are usually designed with from 900 mm to 1200 mm of creepage. In the case of Ankara, cap & pin strings account for about 70 percent of all insulators now in service on regional distribution lines.

throughout the day and night and is probably linked to localized high humidity. The important point, he says, is that he cannot recall any major pollution flashover incidents.

Photos: INMR ©

Turkish Distribution System

Almost all 36 kV lines in Turkey insulated with three suspension or four tension locally produced porcelain cap & pin insulators with cement pole structure.

While the Turkish Electricity Distribution Co. (TEDAS) and its regional affiliates still dominate distribution of electricity across Turkey, this is gradually changing as new players enter the business.

In fact, the country’s distribution system has been divided into 21 regions with the goal of privatizing all but one of these. Enerjisa Baskent Distribution Co. is an example of Members of Enerjisa’s engineering team discuss the new private firms resolving high outage rate on distribution system. in this sector and operates and maintains a network of some 27,000 circuit-km of 36 kV, 15 kV & Kalkan and Aydoğan explain that in order to reduce outages a global 6.3 kV lines that supply 6.6 million maintenance scheme will be people across 7 cities, including conducted each year that includes Ankara. a program to wash insulators, particularly on de-energized coastal Traditionally, the large majority of lines. This will contribute to the 36 kV lines in Turkey are insulated with a string of three porcelain cap & goal of reduce pollution flashovers and related power interruptions and pin insulators of standard or anti-fog improve service quality.  design and with total creepage of

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As of the end of 2010, Enerjisa Distribution’s system had experienced a total of 5344 outages, corresponding to an annual rate of about 20 per 100 km. According to Engineers Emrah Kalkan and Mustafa Aydoğan, the main cause of this relatively high rate is the ageing distribution system acquired

by Enerjisa only two years ago. Other problems affecting line performance have been pollution as well as birdtriggered outages, especially during migration season.

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UTILITY PRACTICE & EXPERIENCE

RTV Coatings Play Key Role at Chinese Substations

After years of rapid urbanization and industrial development, many of China’s cities suffer from high pollution levels that also adversely impact performance of outdoor electrical insulation. In fact, reducing the risk of pollution flashover at substations located near major urban centers has become a key part of their design and maintenance strategies.

INMR visits two substations from northwest to southeast China to report on application of RTV silicone coatings as one of the remedial measures increasingly used to combat pollution flashover.

Porcelain of GIS bushing at new Guandu 500 kV Substation near Zhengzhou shows evidence of contamination build-up and related arcing activity during wetting by light rain.

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Wubei Substation is located only some 30 kilometers from Urumqi, capital of China’s Xinjiang Province.

Built in 2008, it operated for two years at 220 kV before being uprated to its full design capability of 750 kV in October 2010.

Wubei’s pollution exposure is classified as Category IV (very heavy) due to the presence of a large local chemical industry and many coalfired power plants. A nearby waste incineration facility only adds to the problem as does the region’s topography and climate.

Porcelain tension insulators to gantry at Wubei when newly installed (middle) and one year later (bottom).

Photos: INMR ©

Li YaoZhong, Chief Engineer at the Electric Power Research Institute in Urumqi, points out that most of Xinjiang is barren and treeless. As a result, the province’s power network is typically exposed to high ESDD levels due to wind-blown dust. “While we are far from the ocean,” remarks Li, “the soil here is full of salt and other conductive contaminants.

There are also large daily temperature fluctuations, especially during winter.

If you combine this with frequent fog, it’s the classic recipe for flashover problems.”

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t Waste facility (smokestack in background) adds to pollution concerns at Wubei and resulted in all 750 kV breakers being equipped with silicone-housed bushings. q

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All porcelain support insulators on the 220 kV side at Wubei have RTV silicone coatings as well as antiicing booster sheds. q

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Li explains that this service environment has resulted in most overhead lines in Xinjiang being equipped only with silicone insulators and this policy also applies to the growing 750 kV transmission network that is a unique voltage level used only by the power grid in northwest China. For example, the first such line, built in 2006 between Manas Power Station and Wubei Substation, employs silicone insulators for all suspension applications. Explains Li. “they are not only less costly to buy but more important also require much less maintenance than porcelain strings which must be inspected annually for punctures.”

There are also a growing number of 750 kV substations already in operation at this voltage – all, like Wubei, upgraded from their initial operating voltage of 220 kV. Li says that there are plans over the coming years to build additional 750 kV lines and that together these will result in a circle network covering much of China’s northwest.

Photos: INMR ©

Heavy pollution levels near Wubei have made it necessary to look for high performance insulation for most applications at the substation, from post insulators, to switches, to bushings and the preferred solution has been to coat all porcelain with RTV silicone. In most cases, booster sheds have then been placed on top of the coated porcelain to prevent ice bridging on relatively short insulators during the winter months.

Booster sheds offer anti-icing functionality and are placed on RTV-coated porcelain on the 220 kV side at Wubei.

Insulation at substations in Xinjiang is exposed to typically high ESDD levels.

With large daily temperature fluctuations and frequent fog, it’s a classic recipe for flashover problems.

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According to engineers based at Wubei, the application of RTV silicone material was performed there late in 2010 and about two weeks were required to coat the entire station. Most RTV suppliers recommend cleaning and degreasing the porcelain prior to applying the coating evenly across the entire porcelain surface, ideally with a thickness of between 0.3 and 0.5 mm.

However, some coated insulators at Wubei appear to have variable coverage and offer evidence that weather conditions during application as well as skill level of the maintenance staff involved can impact results.

For example, if coating is carried out on a windy day, application thickness and extent of coverage can vary from insulator to insulator unless special care is taken.

Indeed, to avoid the potential risk of variable quality of application, some porcelain insulator manufacturers in China and elsewhere now offer new pre-coated porcelain straight from the factory. In such a case, coating conditions are always controllable and this usually means more consistent application quality from one unit to the next.

At almost the diagonally opposite side of China from Wubei is Luochong 220 kV Substation, near the center of Guangzhou in the southeast and one of the country’s largest urban centers. This substation belongs to the Guangdong Province Power Grid and is situated directly below a busy highway overpass, meaning constant high

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Evidence that RTV layer does not fully cover surface of porcelain, especially near flange.

Note high levels of dust and contamination already deposited on coating.

levels of dust, which eventually settle onto insulators.

Luochong was built in 2000 next to an oil-fired power plant that has since been taken down and replaced by numerous high-rise residential buildings.

Liu Pingyuan, an Engineer with the Guangdong Power Grid, explains that the high local pollution affecting Luochong requires the station to be washed live at least once a year.

However, this apparently is not sufficient and certain vital types of equipment, such as breakers and transformer bushings, have recently been coated with RTV silicone.

Liu also notes that the use of such coatings by the Southern China Power Grid is still relatively new, dating back only to 2004/2005.

is whether those units with less than ideal coating coverage will perform as well as other insulators that have been properly coated.

A paper on RTV coated line insulators published last year by researchers at the Shenzhen Campus of Tsinghua University reported that electrical and physical tests on field-aged samples taken down from 500 kV, 220 kV and 110 kV lines suggested that a service life of at least 6 years

could reasonably be expected.

Of course, this applied to line insulators, which on average would probably have a much lower exposure to contamination than those at substations exposed to more or less constant heavy pollution.

Other researchers from Tsinghua University took a different approach and looked at the contamination layer itself, studying whether this might affect flashover performance

One of the key questions ahead for maintenance staff at Wubei and Luochong will be what type of service life can be expected for the existing RTV coatings. Another issue

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RTV coatings applied in controlled factory environment ensure more consistent thickness and coverage.

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of the coating. Atomic absorption spectrophotography and ion chromatography were employed to study the composition while a scanning electron microscope analyzed the configuration of this contamination on coatings aged in service.

220 kV switchgear and transformer bushings on older transformers at Luochong are RTV coated.

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Findings suggest that while there were indeed significant changes in the polluted coated insulators studied compared to their original state, flashovers still did not occur during withstand tests, even at high voltage.

In this regard, the researchers determined that the impact of different wetting behavior on the lower and upper insulator surfaces increased pollution flashover voltage more significantly than any reduction due to the presence of soluble salts

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Luochong lies directly below a busy highway overpass and extent of pollution accumulation is evident from glass discs at the end of one of the station’s insulator strings.

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Application of coating near flange of bushings in Luochong varies in thickness while some sheds show evidence of undesirable drip marks (see upper left).

on the coating surface. They therefore concluded that RTV coatings do indeed provide long-term flashover protection, even in areas with severe pollution problems.

As for the issue of whether less than ideal coating application (as clearly evident on some units at Wubei) might affect service performance, experts from the RTV silicone coating industry stress that it is always optimal to ensure coatings are applied uniformly and thoroughly to the entire porcelain surface. In those cases where the coating is either unevenly applied or does not reach the edge where the

porcelain meets the flange, coating performance will likely be reduced, although not necessarily critically. The main factor, it seems, is that a large portion of the coated surface retains its hydrophobicity, which will then play the dominant role in preventing pollution flashover. 

Photos: INMR ©

In the case of Luochong, Liu reports that actual service experience seems to back this conclusion since, in spite of the obvious presence of contaminants on the coatings, there have been no flashovers recorded on coated insulators at the substation during the past 5 years.

One of the current transformers at Luochong has oil leaking onto the porcelain (above). RTV coating on porcelain switchgear housing (left).

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UTILITY PRACTICE & EXPERIENCE

Application of Composite Interphase Spacers to Prevent Conductor Galloping

T

he use of composite insulators on transmission lines has increased steadily in recent years due to their well-known performance advantages as well as their lower acquisition and life cycle costs.

At the same time, these insulators have also offered solutions in other areas where ceramic insulators have not proven ideal due to their weight and fragility. One such application is as interphase spacers to prevent conductor galloping.

In this case, composite spacers serve to improve the aerodynamic or ‘aeroelastic’ behavior of lines exposed to difficult climatic effects such as changing temperatures and wind as well as combinations of wind with ice or sleet deposits. Interactions of these have the potential to produce harmful stresses on different parts of the line through induced motion and conductor vibration – stresses that can cause extensive damage and also endanger line service performance.

This article, extracted from an INMR World Congress contribution presented by insulator specialist Alajos Bognar from Hungary, examines the problem of conductor galloping and how application of composite interphase spacers can help control it.

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Spacers installed on lines near Shenyang, in northern China.

Because of complex physics and the highly unpredictable way conductor motions and vibrations can develop over time, it is often impossible to predict and deal with all potential future galloping problems during the design stage of an overhead line.

wind. This makes them susceptible to development of sustained, cyclic conductor motions or vibrations. The specific type of vibration depends on several factors and can take different forms:

1. Aeolian Vibrations These are caused by alternating wind forces and impart an oscillating Finding the best solution when such problems arise first requires evaluating lifting force to the conductor. This type of vibration is characterized by service experience to gain the frequencies in the range from 5 to necessary insight into all the factors 100 Hz, with amplitudes of only a that influence onset of destructive few mm up to the full diameter of stresses and vibrations on the line. the conductor. The phenomenon is This way, there is the greatest chance dangerous, because fatigue-bending of finding the most effective measures stress can occur at or near conductor to keep these within allowable limits. fixation points such as clamps and cause breakage of individual strands.

Interphase spacers are among the It is common to use special dampers control devices available to engineers (e.g. the Stockbridge damper) to to prevent the most dangerous minimize the level of such vibrations. kinds of vibrations, such as those caused by ‘sleet jump’. But in order 2. Wake-Induced Oscillations to understand how these devices Such oscillations are typical only for can help and to better assess their bundle conductors for which some effectiveness, it is necessary to first sub-conductors are in the wake review some important principles. induced by those to the windward side. The latter then provide a Development of Vibrations shielding effect on their leeward counterparts.

Along Transmission Lines Even though transmission lines have Four types of such wake-induced a highly flexible form and relatively slender profile, they are continuously motion can be distinguished: subspan mode (in a section between exposed to the forces of climate and

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two spacers of the bundle); vertical galloping; horizontal galloping; and rolling. Basically, the oscillation is produced by the wake, which induces lower drag coefficients and creates lifting forces on leeward sub-conductors. Due to this kind of aerodynamic instability, the leeward conductor starts to move and, because of conductor spacers inside the bundle, the windward conductor is forced to participate in the movement.

While this kind of vibration does not usually lead to a pronounced

Photos: INMR ©

Introduction

Transmission line in Canada during high crosswinds.

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reduction in phase-to-phase distances, a portion of the wind’s mechanical power is absorbed by the conductors and can cause fatigue, especially if repeated over long periods of time.

line design and specific mode of excitation. This form of conductor galloping can affect several neighboring spans and last from only a few minutes to much longer.

3. Galloping Conductor galloping (also referred to as dancing) is a phenomenon where transmission line conductors vibrate with very large amplitudes.

This produces violent dynamic stresses in the conductors that can lead to damaged insulator strings and tower structures. Under certain conditions, conductors having different potential can come into contact or dangerously approach one another such that a short circuit occurs. Preventing this phenomenon is therefore of fundamental importance in maintaining reliable service of the line.

Fig.1: Development of galloping as a standing wave.

Interaction of Iced Conductors & Wind

One of the most common ways galloping develops is as a result of interaction between a steady, moderately strong crosswind that acts on an asymmetrically iced conductor surface.

Through this type of ‘self-excitation’ process, periodic, high amplitude vibrations (or oscillations) can occur on either single or bundle type conductors.

The amplitudes can approach the value of the sag and have motion mainly in the vertical plane, with frequencies of between 0.15 and 1 Hz depending on

Example of snow adhesion to conductors in China.

Most of the observed galloping in this case takes the form of standing waves that occur with one, two or sometimes as many as 10 loops in a span (although galloping events with three or less waves per span are most common). In the case of more than three loops, the galloping generally has smaller amplitudes.

When this problem develops, the oscillating motions of the phase conductor and overhead ground wire can lead to contact. Alternatively, the critical air gap breakdown distance between conductors of different phases or between the phase conductor and ground wire can be reduced. This will result in short-circuit and operation of the relevant circuit breakers to switchoff the line.

In the case of automatic reclosing, switch-on will be successful.

However, contact or approach of the conductors will quickly reoccur because of the periodicity of the galloping phenomenon. The repetitive short circuit can then damage the conductors through the high energy, high temperature power arcs that develop between the conductors as they approach too close to one another or come into contact. The negative mechanical effects have to be considered as well (as mentioned above).

One of the main processes contributing to galloping is the formation of an ice coating on the

Predicting the exact dynamic behavior of a transmission line can prove highly complex and studying it can be done only under real service conditions or alternatively using test lines that duplicate all the relevant climatic conditions.

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A CIGRÉ survey reported that the majority of power supply companies use interphase spacers with polymeric insulators due to their low weight and high flexibility under compression loading. conductor. Since a conductor’s surface is not perfectly smooth, some snow adheres to it and marks the onset of ice deposit formation. From that stage on, the torsional rigidity of the conductor (or conductor bundle) becomes important. The rotating (or twisting) torque that develops as ice forms causes the conductor to turn and the snow deposit gradually increases to the point that a complete sleeve has been created around the conductor.

A secondary problem – known as ‘sleet jumping’ – is caused by the large ice deposit that has formed.

This is because when conductors become covered by ice there is a tendency that the conductor closest to the ground will shed its ice first. Upon unloading, this lower conductor can jump toward the upper conductor, possibly leading to a temporary short circuit. Once the lower conductor has recovered from its initial ‘ice jump’, it often settles into a new position with less sag than before and this can persist for a long period of time. If the upper conductor has still not shed its ice load, the reduced separation distance between the two can result in air gap breakdown between the phases.

These examples serve to illustrates the complexity of the problem and that predicting the exact dynamic behavior of a transmission line can prove challenging. Studying it can therefore best be done only under real service conditions or

Examples of application of composite interphase spacers.

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alternatively using special test lines located in areas that exactly duplicate all relevant climatic conditions.

Recently, simulation tools have become available to track development of the conductor galloping process. Some even take into account whatever control devices have been put in place to prevent or reduce the problem.

However, these simulations need to be validated by continually analyzing actual phenomena and parameters obtained from real service conditions in the field.

Controlling & Preventing Galloping

Because of the dangerous effects of galloping, several alternative approaches have been used to avoid or at least reduce the problem.

These include: • Changing Line Routing or Design Fully eliminating the potential for galloping is often impossible without high incremental costs in line design or construction. For example, the route of the line can be changed to areas where ice deposition becomes far less likely.

Similarly, conductor cross-section can be increased as can clearance between conductors.

• Application of Damper/De-tuners These devices are based on the fact that torsional (or twisting) movement of a bundle conductor arrangement and its vertical motion (because of lifting forces) interact with one another. The instability

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that causes galloping is enhanced when the frequencies of these two motions are close to each other (i.e. there is synchronization / resonance).

is to prevent the most dangerous consequence from conductor galloping, namely phase-to-phase short-circuits. Attaining this is relatively straightforward and involves The solution is therefore to separate providing in-span mechanical the frequencies through some form of couplings between phases, thereby external intervention – a solution such maintaining phase-to-phase as offered by so-called ‘pendulum clearances at some pre-determined, de-tuners’. The de-tuning pendulum acceptable limit. increases torsional frequency but is not able to dampen the torsion.

Moreover, from the oscillation One such anti-galloping device damping point of view, these devices with promising results on e.g. twin provide further benefit by limiting horizontal bundle test lines is the any torsional (twisting) motion of Torsional Damper and De-tuner (TDD), conductors or conductor bundles. which combines a de-tuning effect (as Such control (damping) is achieved above) with high torsional damping. since the flexible composite insulators act to increase the conductor’s • Interphase Spacers torsional stiffness. This type of The main concept behind the arrangement offers other advantages application of interphase spacers as well that arise from the flexibility

of the composite spacer. Should conductor vibration act to reduce phase-to-phase clearance, its energy can be absorbed by the spacer’s inherent resilience.

Indeed, in several practical applications phase spacers are even placed to connect all three-phase conductors (conductor bundles), resulting in a several times over cross-connected system of sub-spans.

This serves to greatly enhance the dynamics of the original system.

For example, it could happen that not all phases are unstable when one or two phases are galloping.

However, the connections through the spacers between the stable and unstable phase(s) provide the latter with additional aerodynamic and mechanical damping.

Power frequency electric stresses across an interphase spacer are about 73% higher than for phase-to-ground insulation because spacers are subjected to lineto-line voltage.

Interphase Spacers as Anti-Galloping Devices General Set-up of Interphase Spacers As discussed, perhaps the most serious consequence of large amplitude galloping is electrical failure caused by contact or critical approach of conductors with different

Photos: INMR ©

Two grading rings are typically required for interphase spacers.

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Example of conductor clamp after galloping test. Only minor wear on hinge part will not affect continued functionality.

Typical test set-up with 4 conductor bundle for galloping test on interphase spacers.

The power frequency electric stresses across an interphase spacer are about 73% higher than for phase-toground insulation because spacers are subjected to line-to-line voltage.

Stress release grading rings must therefore be applied at both ends for voltages of 154 kV and higher.

The electric field distribution of phase spacers is different from that of typical suspension insulators in that the spacer's field is symmetrical. The field closer to the end fittings is also higher than for suspension units.

Typical set-up of snow dropout test.

potentials. To avoid this, it is logical to mechanically fix the clearance between the conductors inside the span utilizing insulating structures. While this solution has been applied in the past using ceramic insulators, their weight has created problems because of the modification needed to the preadjusted sag and stress of conductors, especially in the case of lower voltage transmission lines. Their installation and periodic servicing have also proven to be comparatively difficult.

For these reasons, this kind of application is virtually completely covered these days by composite insulators. In fact, starting in the late 1980s and early 1990s, installation of composite interphase spacers accelerated rapidly and a CIGRÉ

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survey at the time reported that the majority of responding power supply companies used interphase spacers with polymeric insulators due to their low weight and high flexibility under compression loading.

Electrical Design of Interphase Spacers

When it comes to electrical design, the requirements and test methods for interphase spacers are similar to those of normal suspension or tension insulators. Key design parameters include:

Regarding pollution performance, because of the excellent surface hydrophobicity of silicone, no problems should be expected for composite phase spacers equipped with housings made from this material. For example, outdoor weathering trials were conducted with such spacers at a test site on Japan’s Okinawa island, where they were exposed to continuous contamination by salt wind as well as frequent typhoons. In spite of the heavy coastal pollution and wind loads, no evidence of erosion or other deterioration was found after several years of service.

Taking into account that typical applications for such spacers are in areas with cold climates, tests must also be made to evaluate spacer performance under accretion of ice • Power frequency voltage distribution and snow. Typically, such tests have along the unit and flashover under confirmed that the insulator portion contamination; of a composite interphase spacer • Switching impulse flashover voltage properly designed for heavy pollution across conductor-to-conductor gaps; will be able to withstand these high • Lightning impulse flashover voltage. service stresses as well.

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Mechanical Design of Interphase Spacers

The mechanical stresses on an interphase spacer through galloping and ‘sleet jump’ include tension, bending (buckling) and fatigue caused by oscillation. The requirements for tension strength are not special but still have to be taken into account in the design of suitable hardware such as end fittings, clamps, etc. In this regard, tensile tests must be carried out on the complete arrangement. For example, the requested mechanical strength of a 275 kV interphase spacer used for 4-bundle conductor of ACSR 410 can be about 79 kN – a level which will easily be met by any well designed unit.

Far more important when it comes to mechanical stress, however, is a spacer’s bending/buckling behavior.

In this respect, one of the principal merits of composite insulators as interphase spacers is that the onset of buckling or a certain amount of deflection at bending presents no danger so long as the stress remains below the critical limit at which deterioration starts. To provide some indication of such stress levels, relevant figures for allowable bending stress by utilities in Japan are as generally follows: 1. allowable stress: 294 MPa; 2. proposed range of maximum design cantilever load: circa 450-500 MPa. To put these limits into perspective, it should be noted that the normal breaking stress of a good quality glass fiber-reinforced epoxy (FRP) rod is usually more than 800 MPa. Frequent galloping/vibration can produce fatigue in interphase spacers, especially in their metallic end fittings and connections, with risk of wearing out movable parts.

Therefore, it is necessary to test their design and performance by simulating the galloping oscillation during a relevant large-scale test, which can be conducted with typical parameters such as:

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Example of bending of spacer after snow drop-out test.

To simulate how this process can develop, a large-scale test set-up is utilized where the effect of an ice load is provided by several weight blocks that hang on the lower conductor by means of an electro magnet. When the magnet is switched off, the weights fall and conductor jumping occurs.

In a typical such test, span length can be set at 70 m and the length of the spacer at 10 m while a twin-bundle ACSR 410 conductor is used. The interphase spacer will experience a fairly large, flexible, bending during the test but there should be no visual evidence of defect. The maximum stress in the FRP rod can reach a value of 290 MPa and once the stress is removed, the spacer must return to its original shape.

Installation of Interphase Spacers

Installation methods for interphase spacers.

Frequency of oscillation: approx. 1Hz Amplitude of oscillation: 10 to 20 cm Number of oscillations: 106 cycles Such a test is deemed passed when there are no visible changes in the structure or in parts such as conductor clamps.

A further important design feature that must be tested is the ability of the interphase spacer to withstand the stresses when heavy ice/snow loads on the conductor (bundle) in the lower position suddenly drop. In this situation, the lower conductor suddenly jumps upward even as the conductor (bundle) in the upper position remains loaded. An interphase spacer located between the two will therefore be subjected to great compression stress.

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After clarifying the electrical and mechanical loads as well as geometric data for the critical spans where interphase spacers are intended for application, it is necessary to decide where to locate them within the span.

In this regard, a variety of factors usually have to be taken into account.

However, it must be emphasized that symmetrical distribution lengthwise should be avoided since this placement could promote the development of standing waves in the span or sub-spans.

Some typical arrangements can be seen in the accompanying Figure.

Generally, the ideal number and location of the interphase spacers in each case will then depend on the line’s length of span.

Final Remarks

To collect some of the knowledge and data presented in this article, a full-scale test line was set-up in a mountainous area of Japan and this subsequently became the site of numerous different tests. The above explanations on how composite interphase spacers can combat conductor galloping and ‘sleet jump’ problems on transmission lines up to 500 kV were obtained based on the results of these tests. 

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GIS TECHNOLOGY

Supplier of GIS Equipment & Components Looks to Rising Demand

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Pinggao Group is among China’s largest firms in the power sector, producing a range of substation apparatus including SF6 breakers, isolating and earthing switches as well as enclosed and hybrid-type GIS systems. A subsidiary of state-owned China Electric Equipment & Technology Co., Ltd., Pinggao distributes this equipment primarily in the domestic market, where there have been many projects in recent years as part of the continued expansion of the country’s electrical network.

Because of its reliance on GIS technology, Pinggao has also developed expertise in the area of gas-liquid engineering, specifically relating to handling and

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purification of SF6. This expertise has resulted in a range of portable equipment for use in testing, extracting and recycling the gas.

In addition, about 10 years ago Pinggao diversified into areas related to its core business – one of which was composite hollow core insulators for applications such as bushings and switchgear housings. This operation has recently moved to a new production workshop alongside the site where the company’s huge GIS production facilities are soon to be re-located.

INMR travels to the city of Pingdingshan, in southern Henan Province, to report on recent issues in China surrounding the use of SF6 and also on the Pinggao’s new insulator production facility.

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Growth Expected in Portable SF6 Handling Equipment Since HV gas switches were first introduced in China during the early 1970s, SF6 has seen a rapid rise in its application as an insulating medium for electrical equipment. Indeed, the volume of SF6 gas now in use across the country’s vast power network is estimated to be some 18,000 tons and this amount has been expanding at a rate of at least 20 per cent in recent years, i.e. from 5000 to 6000 tons annually.

Yet, for all its benefits as an insulating and arc-extinguishing medium, SF6 comes with some serious drawbacks. Most notable among these is the fact that it is a highly potent greenhouse gas whose impact is about 24,000 times that of CO2. Given that it remains stable in the atmosphere for over 3000 years, the gas has been specifically targeted since the Kyoto Protocol as one of several whose release into the atmosphere has to be strictly controlled.

SF6 has almost 24,000 times the greenhouse effect of CO2 and remains in the atmosphere for over 3000 years.

In this regard, a series of standards and specifications have been issued in China over the years defining local regulations when it comes to the manufacture, handling and application of SF6. These include, “Guidelines for the administration and test of the gas in SF6 electrical equipment”, “Guideline for SF6 gas seal test of HV switchgear equipment” and “Detailed rules for the operation, test and maintenance staff safety of SF6 electrical equipment”.

However, in spite of all the rules and procedures outlined in these documents, local maintenance personnel do not always observe them properly. Combined with the fact that there is sometimes no recovery and handling equipment available on site, the gas is often emitted arbitrarily during routine maintenance or periodic refurbishment at existing power system installations.

For example, the State Grid Corp. of China (SGCC) estimates that, given proper management and support installations, hundreds of tons of SF6 could effectively be recycled each year in China during the course of such work. The environmental benefit would be enormous and equivalent to eliminating more than 14 million tons of annual CO2 emissions.

(from top) Examples of 1000 kV, 500 kV, 220 kV and 110 kV GIS installations in China.

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The State Grid Corporation estimates that, given the proper facilities, hundreds of tons of SF6 could effectively be recycled each year in China.

Photos: INMR ©

According to Jin Guo Hao, Manager of Pinggao’s Gas & Liquid Project Equipment Division, these types of considerations have influenced the State Grid’s policy of promoting re-cycling as the best way to limit future emissions of the gas.

Implementation of this policy by Pinggao has already resulted in a range of portable equipment that is intended to facilitate handling and re-cycling of SF6. “In a nutshell,” says Jin, “our division’s goal has been to develop relatively smallscale, easy-to-use equipment that allows for a more responsible and sustainable use of SF6.”

Pinggao’s own SF6 treatment and storage facility in Pindingshan sits alongside the sprawling GIS production operation and it is here that the gas is purified to meet Chinese specifications. However, there is a realization that such centralized storage facilities by GIS equipment manufacturers must be supplemented by smaller regional centers in order to make re-cycling more practical and cost-effective.

Over the period 2012 to 2013, the SGCC plans to start the process of setting up a network of SF6 recycling and treatment centers in areas close to the various provincial grids so as to make them more accessible to grid construction and maintenance companies.

Photos: INMR ©

Production plant at Pinggao for portable gas vacuum SF6 extraction and recovery equipment.

Table 1: Required SF6 Specifications after Purification* Quality grade

≤ 0.04

CF4 Air (N2+O2)

≤ 0.04

Water (H2O)

≤ 0.0005

Acidity (calculated in HF)

≤ 0.00002

Hydrolysable fluoride (calculated in HF) ≤ 0.00010 Mineral oil

≤ 0.0004

Purity (SF6)

≥ 99.9

Total decomposition

≤ 0.0001

With gas purification in the treatment centre through absorption, deoiling, filtering and dust filter, SF6 gas can reach these specifications that meet Chinese Standard GB/T 12022-2006: *

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“Our division’s goal has been to offer a range of portable equipment that promotes more responsible and sustainable use of SF6.”

For example, Jin notes that cooperation has already started with a Research Institute and Power Grid Company in Anhui Province aimed at establishing convenient local handling and purification of SF6.

In this regard, a number of options have been proposed for relatively small-scale purification/storage centers that would occupy from 200 to 360 m2 and which would be equipped with some of the portable testing and handling apparatus now being offered by Pinggao. The goal would be to have such a network become operational by 2014-2015.

Pinggao’s product portfolio in this field already includes several models of SF6 testing, vacuum extraction, handling and purification equipment and Jin sees a rapidly growing market for such apparatus developing both in China and abroad. Says Jin, “apart from the obvious benefit to society of reducing greenhouse gas emissions, there are also other issues involved.

These include lowering operating costs and increasing awareness among maintenance staff of the need to re-cycle as many components in the grid as possible.”

Photos: INMR ©

Pinggao’s main SF6 purification and storage facility in Pindingshan.

GIS equipment leaving plant for installation on site.

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Another factor notes Jin is that commercial exploitation of advanced technologies such as SF6 re-cycling and treatment systems offers a model for development of other power technologies that have yet to reach maturity.

As for future sales prospects, Jin is equally sanguine.

“If we succeed to eventually re-use 90% of the SF6 now being lost during routine maintenance,” he points out, “the total volume of the gas processed each year will exceed 500 tons. This will have a significant impact on our business and more than double our domestic sales, quite apart from the growth we expect to achieve in export markets.”  Alternative configurations proposed for small-scale SF6 treatment and recycling centers.

Portable apparatus developed for SF6 testing and re-cycling.

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GIS Supplier Relocates Insulator Production to New Plant GIS bushings and other housings used in current transformers and switchgear in China are still made predominantly of porcelain. However, a fast-growing local market has emerged in specialized applications such as UHV, DC as well as for service areas with seismic or pollution concerns.

The result has been a burgeoning domestic demand for composite housed apparatus with silicone sheds and this has motivated Pingao’s Insulator Division’s recent investment in a new and larger plant able to supply housings up to 1100 kV. According to Plant Manager Zhao Yunjie, the new facility already produces between 4000 and 5000 pieces for voltages of 500 kV and higher and he believes this business volume will soon increase rapidly. Says Zhao, “we have invested in some of the best injection machinery available to be in a position to meet the expected rise in demand.”

Perhaps the showpiece among the new equipment from several domestic suppliers is a huge horizontal molding machine with a clamping force of 2800 tons and shot volume of 100,000 cc of silicone material – claimed to be one of the biggest of its kind in the world. This machine, intended mainly to produce 500 kV housings in two molding cycles, is also used for molding 220 kV housings in a single shot.

Zhao explains that since molding cycle time is the key variable that influences this machine’s productivity, two ovens capable of accommodating different tube diameters are used to pre-heat the FRP tubes before they are lifted into the mold cavity.

Composite insulators produced at new factory range from 35 kV switchgear housings to huge hollows for applications up to more than 1000 kV.

Photos: INMR ©

The new Pinggao insulator plant already produces between 4000 and 5000 pieces each year for voltages of 500 kV and higher.

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“Our sales up to now have been mainly to meet Pinggao’s internal needs but we expect volume to increase rapidly as we start to look for new customers outside the Group.”

Photos: INMR ©

“Without heating,” says Zhao, “the typical molding cycle on this machine can be as long as two hours. But if we heat the tubes to between 70 and 80°C before inserting them into the mold, we can reduce this cycle time to only about 40 minutes.” The decision whether to pre-heat or not then becomes a matter of optimizing production costs and throughput volume, depending on the number of units needed to meet current demand.

One of the elements of Pinggao’s production strategy is that tubes are molded before addition of the flanges by gluing at both ends. This process differs from that used at certain other suppliers who choose to attach flanges prior to molding. Zhao claims that the time of flange attachment makes no real difference in terms of final product quality and that both methods are equally effective and accepted within the industry.

At the moment, most housings at the new plant are made using high consistency (HCR) silicone rubber, but Zhao reports that a range of production and dosing equipment for liquid silicone rubber (LSR) are also available. He and production engineer, Xu Weixing point out that

(Top) Some of range of hollow insulators now offered. New shed geometry developed for enhanced pollution performance of 220 kV housings.

LSR use by the plant is currently comparatively small but expected to grow significantly this year with the decision to also supply wall bushings.

These will be offered as an addition to the current line of housings for CTs, CVTs, breakers and GIS bushings but will be made using only LSR material for the highest voltage

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applications up to ± 800 kV DC and 1100 kV AC. Another development in the shortterm will be internal production of the FRP tubes, which at the moment are being purchased from outside suppliers. This transition is planned for 2013 with the purchase of new

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New horizontal clamping machine.

Oven at front left used to pre-heat tubes to accelerate molding cycle time.

Photos: INMR ©

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220 kV housing with new shed design ready for cantilever testing.

tube manufacturing equipment and Zhao notes that the move will allow the plant even greater selfsufficiency.

Says Zhao, “we already make all our own molds as well as meet all our needs for HCR silicone. With the addition of internal tube capability, we will have full control over all our costs and production lead times.”

“Our sales up to now have been mainly to meet Pinggao’s internal needs but we expect volume to increase rapidly as we start to look for new customers outside the Group.”

The Insulator Division has been supplying composite insulator housings since 2000, mainly to meet Pinggao’s internal needs. According to Wei Lü, Marketing & Sales Manager, the new plant and recent investments

in molding and other machinery signal volume to increase rapidly as we an interest to grow this business. “At start to look for new customers and the moment”, he says, “our sales are opportunities outside the Group.”  about RMB 100 million (approximately US$ 16 million) but we expect this

Photos: INMR ©

(from left) Sales Manager Wei Lü, alongside Xu and Zhao inspect 220 kV housings

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INSULATORS

Modern Pollution Monitoring Principles Allow Better Selection of Insulators for Polluted Service Conditions (Part 1 of 2)

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Failures of insulators due to pollution flashover can prove very costly, causing potentially long outages and requiring expensive and time-consuming maintenance.

By most accounts, one of the leading causes of such failures is improper specification of insulators in the first place – to the extent that the designs selected are unable to cope with all the stresses imposed by pollution in the environment. Therefore, it is vital for engineers at power supply companies to correctly understand the real pollution characteristics of the service environment into which insulators will be placed into service.

According to present IEC recommendations, the process of selecting and dimensioning insulators for use under polluted conditions requires an accurate site severity evaluation. Ideally, this should be performed over a minimum of one year in order to best choose the maximum stress level and corresponding Site Pollution Severity (SPS) class.

This process is defined as pollution monitoring and is normally done

by Equivalent and Non-Soluble Salt Deposit Density (ESDD/ NSDD) measurements on reference insulators (i.e. cap & pin or long rod) as well as using widely accepted Directional Dust Deposit Gauges (DDDG) data. For coastal areas, there may also be a need to estimate Site Equivalent Salinity (SES). Once the correct SPS class is obtained, the

This article, contributed by Igor Gutman of STRI in Sweden and Wallace Vosloo of Eskom in South Africa, introduces practical field and laboratory experience with different pollution monitoring techniques as well as presents principles to convert from one parameter to another. It also discusses how to use these parameters for insulator dimensioning according to the requirements of the IEC 60815. There is also discussion of the latest version of a specialized software program called the Insulator Selection Tool (IST).

It has to be kept in mind that input data for insulator selection obtained via pollution monitoring must always be reliable.

methods. These methods have been used for decades for switching overvoltages but are now also well established for insulator dimensioning under polluted conditions. For example, IEC TS 60815-1 contains two clear messages: “Both deterministic and statistical design methods are available to design and select appropriate insulator solutions based on site pollution severity (SPS) and laboratory test results” and “Software packages are available for the statistical approach”.

specific creepage distance of the selected insulator (in terms of phaseto-ground voltage) is chosen, as per Figure 1-1.

The above may appear simple and straightforward. But it has to be kept in mind that input data for insulator selection obtained via pollution monitoring must always be reliable.

However, optimal insulation dimensioning can be achieved only through application of statistical

Standard Pollution Monitoring Techniques The recently issued IEC 60815 recommends the following diagnostic techniques to define pollution severity for selecting and dimensioning outdoor insulators:

Fig. 1-1: Example of dimensioning (mm/kV) based on SPS class.

• Equivalent Salt Deposit Density (ESDD) • Non-Soluble Deposit Density (NSDD) • Directional Dust Deposit Gauges (DDDG) conductivity • Surface Conductivity (SC) • Site Equivalent Salinity (SES)

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ESDD/NSDD is already well known in the power industry since the 1960s and therefore not discussed here.

Similarly, SES is a rather unpractical parameter and also not worthy of discussion. Instead, the two other more useful parameters are explained below:

1. Directional Dust Deposit Gauges (DDDG)

Fig. 2-1: Examples of typical DDDG installations in South Africa, Sweden, and Norway (top to bottom).

A DDDG consists of four collecting tubes, each with a vertical slot on the side into which rain and any airborne pollution are blown.

The slots are arranged to face north, south, east and west while removable containers are attached to the bottom of each tube to collect the deposits. Examples of typical installations are shown in Figure 2-1.

The containers are taken out at monthly intervals and their contents mixed with given amounts of distilled water to determine resulting solution conductivity. The pollution index (PI) is then defined as the average conductivity of the four directions expressed in µS/cm and normalized over a 30-day collection period.

Moreover, in addition to the pollution index, the amount of non-soluble deposits is sometimes also of interest since it can also influence insulator performance. As such, if considered useful, the amounts of these deposits in the solution

Table 2-1: Different Precipitation & Corresponding ’Danger Factors’ Type of Precipitation

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Factor of Danger

1. Dew with temperature increase

1.0

2. Fog

1.0

3. Light fog

0.5

4. Drizzle

0.8

5. Frost while temperature > -5°C

0.8

6. Rime frost while temperature > -5°C

0.5

7. Wet snow

0.8

8. Rain

0.7

9. Rain with conductivity >1000 μS/cm

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should be measured using the same procedure applied for standard NSDD measurements.

If weather data at the site under study is available, the pollution index can be modified to also take climatic influences into account. This is done by multiplying the pollution index (PI) with the climatic factor (Cf), calculated as follows:

where Fd is the number of foggy days/ year (i.e. < 1000 m of horizontal visibility) and Dm is the number of dry months/yr (i.e. < 20 mm of precipitation). Note that this equation is based mainly on South African experience and should be used with caution in much different types of climates. In particular, other types of precipitation factors could also be taken into account in moderate climates. For example, observe in Table 2-1 (taken from a Russian Guide) where various types of precipitation are related to different ‘danger factors’ from the viewpoint of insulator pollution performance.

DDDG measurements appear to be quite sensitive to the immediate environment. Two test sites, Horred and Ringhals, are both located in western Sweden with Ringhals directly at the coast, while Horred is about 10 km inland. Average DDDG measurements at these sites are different by a factor of 2.

An important question that arises in this regard is whether knowing DDDG measurements would enable converting this into standard ESDD data. The answer can be seen from Table 2-2 where site severity classes at 12 South African locations are presented in the range from I to IV (according to the old version of IEC 60815). Results obtained utilizing ESDD and DDDG values seem to correspond very well and seem to

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Table 2-2: Various Site Severity Classes Based on ESDD and DDDG Measurements in South Africa (2001-2005)* ESDD

DDDG Monthly Monthly average maximum over one over two year years (in μS/cm) (in μS/cm)

Monthly maximum over one year

Site Severity Class

Acacia Park

0.22

III

247

573

III

Ascot sub

0.10

II

273

599

III

Atlantic sub

0.18

III

247

534

III

Belhar sub

0.09

II

143

328

II

Bluedowns

0.13

III

133

367

II

Duine sub

0.27

IV

489

1407

IV

Khaylitsha

0.18

III

419

1246

IV

Mpilo

0.19

III

373

1094

IV

Muldersvlei

0.06

I-II

111

309

II

Plattekloof

0.06

I-II

189

479

II

Stickland

0.14

III

150

366

II

Vlakte

0.10

II

210

502

II-III

KIPTS

1.78

IV

4845

14608

IV

Site

Site Severity Class

* Site severity classes based on old IEC 608156

confirm that using DDDG data for site this has reportedly only been severity evaluation is indeed feasible. achieved in Russia, where outdoor insulation is selected based on direct 2. Surface Conductivity (SC) artificial pollution tests at about Theoretically, measurements of 1.7 x maximum operating voltage as insulator resistance with further well as at 2, 5, 10 and 20 µS for site conversion using a ’form factor’ into severities from I to IV respectively surface conductivity (in µS) offers (according to the former version of many advantages. For example, it IEC 60815). allows automatically taking into account the impact of any nonuniformity in pollution layer or combining the effect of pollution and wettability parameters in the case of composite insulators.

However, in order to use this parameter as an alternative to standard ESDD/NSDD measurements, it should be applied and standardized in a laboratory environment, in service measurements and also in any guide materials. Up to now,

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Fig. 2-3: Example of insulator pollution monitoring device (IPMD) in the field.

In South Africa, by comparison, a practical monitor called the Insulator Pollution Monitoring Device (IPMD) as well as similar devices whose primary diagnostics parameter is surface conductivity have been developed (see Figure 2-3). These allow more detailed and frequent assessment of site conditions by automatically determining severity of pre-deposited pollution at selected intervals and by recording any instantaneous pollution events. Some also allow monitoring leakage current amplitudes on in-service insulators.

Typical measurements performed by such devices include: • Insulator surface conductivity under natural pollution and wetting; • Insulator surface conductivity under natural pollution but with artificial wetting; • The leakage current amplitude on in-service insulators.

Examples of findings obtained from an in-service IPMD are shown in Figure 2-4. As both pre-deposited and instantaneous pollution levels are recorded daily, natural pollution and wetting events are not averaged out over a month. This eliminates any risk of missing any singular severe pollution events, as is the case with both the surface pollution deposit and directional dust deposit gauge methods.

Implementation of Insulator Selection Tool (IST) Program Input data from pollution monitoring can serve as a direct input for the IST software program that follows IEC 60815. The latest version of this program was developed as a result of close collaboration between STRI and Eskom and includes the following new features: • Possibility for direct dimensioning in the case of DC applications (see Figure 2-5). Indeed, users can now choose between AC application or negative and positive

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DC applications. The latter are separated because the electrical strength of a line insulator is lower for negative polarity. The program therefore provides separate calculations per pole.

• Direct comparison of up to three different insulators per screen dump (see Figure 2-5 “Add to comparison” button).

Fig. 2-4: Typical field measurements obtained by IPMD. • Additional input data option in the form of DDDG measurements.

For example, if no standard ESDD data is available, Equivalent ESDD button and a special window for DDGI-S (Dust Deposit Gauge Index – Soluble) are activated.

The user can then write in DDDG data, which the program will convert into standard ESDD data (see Figure 2-5).

Fig. 2-5: Screen dump of IST program: AC/DC and DDGI-S are available. (Note: This is an example, not any specific insulator-related calculation).

• Two independent databases are also provided. The first, called “STRI proprietary” is created

by STRI who is responsible for all the data contained, i.e. the user has no right to enter and make any changes. The second database called “Customer”, by contrast, is open for the user, who can create their own insulation data (both geometry and flashover performance). STRI then bears no responsibility for this database.

• An economics-oriented section of this program allows cost comparison of different insulator options from the viewpoints of initial investment, maintenance expenses and potential outage costs (see Figure 2-6).

Editors note: Part 2 of this article will appear in INMR Qtr 2, 2012. 

Fig. 2-6: Screen dump of IST program: economics part. (Note: This is an example, not any specific insulatorrelated calculation).

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INSULATORS

Improved Insulator Design for Performance Under Icing

C

hina has the largest number of composite insulators in service on its overhead networks and also the world’s biggest and most competitive insulator industry. Indeed, by some accounts, there are already about 200 Chinese manufacturers of these types of insulators and this number seems to be growing each year.

Given this combination of factors, it is perhaps not surprising that some unique designs of composite insulators intended for specialized applications now also come from Chinese suppliers.

One such innovation is a type intended for superior performance under icing and at the same time offering enhanced protection against flashovers triggered by so-called ‘bird streamers’ – a common problem in countries with large populations of migrating birds.

INMR visits a supplier that has recently optimized its ‘ice design’ to make it less vulnerable to problems of shed deformation under snow and wind loading.

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Among the service situations when insulators become most vulnerable to pollution flashover is when ice that forms on tower cross-arms and conductors during winter melts and drops onto the insulators suspended below. A further potential threat is when the icy water re-freezes and forms conductive bridges over the sheds.

Indeed, one of Japan’s most serious outages in recent years took place in the country’s coastal region of Nigata, where ice laden with sea salt had formed on towers and insulator strings from a severe winter storm.

As melting progressed, widespread flashovers occurred.

Central and northern regions of China experience similar climatic conditions to those affecting northern Japan. But the main source of pollution, especially during wintertime, is not the sea but conductive dust from coal-fired power plants that become trapped in u Special anti-icing design of insulators on this tower in Xinjiang, also protected by many bird discouragers. t Example of how melting ice can re-freeze on structure.

Photos: INMR ©

One of the potential problems of a large diameter single silicone shed is that there is a risk it can deform under the weight of snow and ice or through long exposure to constant wind.

Wide aerodynamic porcelain discs are alternative solution to protect composite suspension insulators below from impact of ice shedding and re-icing.

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Tenet anti-ice insulators now feature large shed in third position as well as reinforced structure for improved mechanical performance.

According to General Manager, Qi DongBao, one of the potential problems of a very large diameter single shed is that there is a risk it can deform under the weight of snow and ice or through constant exposure to wind and rain. Such deformation risks diminishing its strength and also reduces the unit’s effectiveness in preventing snow or icy water from dripping directly onto the sheds below.

Another possible complication is tower clearance distance, especially when this design is being applied to existing towers with cross-arms not dimensioned for an extra-wide insulator shed at the top.

Photos: INMR ©

With these types of considerations in mind, Tenet has re-designed its antiice insulator by incorporating underribs that provide greater mechanical strength to the wide silicone rubber shed. Another innovation has involved moving this large shed to the third position from the insulator’s tower end, to eliminate any possible concerns relating to clearance.

ice that forms on overhead lines. To deal with this type of problem, a number of Chinese insulator suppliers have begun offering a special composite design equipped with an extra wide shed at the tower-end of the unit.

Tianning Electrical Engineering (Tenet) is among the largest insulator suppliers in China and, being located in the country’s cold northwestern province of Xinjiang, has traditionally been one of the major suppliers of this ‘anti-icing’ design. Recently, their design has been re-engineered with a view to achieving superior long-term service performance.

Moving wide shed to third position assures better clearance from tower.

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u Insulators with earlier anti-ice design removed from service and awaiting electrical testing.

Production of anti-icing designs normally requires a second molding operation.

Photos: INMR ©

p Qi holds newly optimized anti-ice insulator design.

Because of the unique new wide shed, production of these types of anti-icing designs typically requires a second molding operation. This means there is higher labor as well as machine and energy costs. However, Qi explains that, notwithstanding these higher costs, the price of these units is similar to or only slightly higher than conventional design geometries.

Second molding operation needed to add large diameter shed.

Photo: INMR ©

Qi reports that field experience with the anti-icing composite insulator design has, in general, been positive and adds that units removed from service are routinely tested to verify that their electrical characteristics have not diminished.

He also notes that this optimized new design is currently being offered for applications up to and including 220 kV. 

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INSULATORS

Porcelain Insulator Manufacturer Undergoes Factory Expansion

The area surrounding the city of Liling in China’s Hunan Province has traditionally been home to manufacturers of porcelain. Liling Huaxin Electrical Porcelain Technology (also known as PK Insulators after the town of Pu Kou, where it is based) was founded in 1985 and moved to its present location in 1992. Recently, it has embarked on a major program of capital spending aimed at significantly expanding production operations.

These investments – expected to total about US$ 8 million once complete – will feature some especially notable additional equipment, including the largest ball mills in the industry and one of the world’s highest kilns, capable of firing insulators of more than 12 meters in height.

INMR travels to Liling to visit PK’s facilities and report on the main factors behind the investment program.

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The past two decades have witnessed a dramatic expansion of the Chinese power grid, with many large projects completed or underway across the country. All this sustained infrastructure investment has helped fuel double digit growth within the domestic insulator industry, which has supplied the large majority of the country’s insulation needs, both on lines and at substations. In fact, such has been the demand in recent years that new insulator factories have had to be built while many existing ones have undergone significant expansion.

Within the sector that manufactures hollow porcelains for all types of substation applications, PK Insulators (PK) ranks among the suppliers that have benefitted most from this strong market growth. According to cofounder and President, Shi Jungshen, sales have increased significantly during the past five years alone, reaching some RMB 300 million (circa US $ 50 million) in 2011. In the process he claims that the firm has also now developed into China’s single largest supplier of hollow core

porcelain insulators and also among the world’s leaders in this sector.

This system will replace the current reliance on mostly manual dosing.

Expecting the upward domestic business trend to continue and with a growing volume of exports as well, PK has recently embarked on an ambitious investment program. Over the next two years, two new production lines will be added to complement the four already in operation. A large plot of land adjoining the existing plant has been purchased and work is already underway on a new building that will house one of these additional lines. The second new line is scheduled for completion by 2014.

Says Shi, “we are spending to expand our capabilities since customer technical requirements are becoming higher. At the same time, sales volume continues to grow each year.”

A related investment in upstream porcelain production involves two huge 60-tonne capacity ball mills that are nearing completion and will soon be housed in a new building being constructed around them. “These computer-controlled ball mills will feature variable frequency control,” says Shi, “and will operate with high efficiency to allow more accurate batching as well as reduce energy costs.” Shi also notes that the new ball mills will serve the needs of the three newest production lines, including Line 5 that will be complete by the spring of 2012.

Apart from adding entirely new facilities, Shi explains that PK is also investing to update its current production facilities, including a planned new system for automatic dosing of the various ingredients used to produce the porcelain mass.

Further downstream is perhaps the centerpiece of the new investment program – a 12.5-meter kiln that is reportedly one of the highest of its kind in the world. Housed in the same building is a new 12 m cutting machine for insulators of these huge dimensions as well as a hot water temperature testing facility able to accommodate such pieces.

Photos: INMR ©

HV porcelain housings to be supplied to customer in Africa.

Shi. Tripling of sales over the past 5 years has been behind new investment program.

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Shi explains that this towering new gas-fired kiln will enable second firing of jointed porcelain sections to yield uniformly glazed housings intended mainly for specialized UHV applications. Sensors are placed at strategic locations throughout the kiln’s vast surface of refractory brick to allow consistent temperature to be maintained throughout and ensure that there is even firing of the massive pieces of porcelain placed inside.

Like most manufacturers in the insulator industry, up to now PK has relied on using a special epoxy to glue

pre-fired porcelain sections together in order to produce housings whose dimensions exceed the heights of normal shuttle kilns. While acceptable to customers for most oil applications, this technology is not considered ideal, especially when it comes to apparatus such as pressurized UHV GIS bushings. Says Shi, “the Chinese government has supported more GIS development in the grid and, when it comes to 800 kV or 1000 kV, engineers prefer to have a second glazing and re-firing for all porcelain used in such SF6 applications.”

Perhaps the centerpiece of PK’s new investment program is the towering 12.5-meter kiln that is one of the highest in the world.

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Two new ball mills each feature a 60-ton capacity.

Epoxy glue joint is common in the industry for connecting sections to make very long hollow insulators.

Shi reports that since the kiln is still relatively new there have not yet been any orders for such large pieces. Rather, the new kiln has been used several times only for test firing initial production samples. These will be then tested and allow staff to gain experience into the know-how required to optimize handling and firing of such long pieces.

markets outside China include the U.S., Japan, Germany, Canada and Sweden.

Given the large investment being made in adding more porcelain capacity, Shi and Huang are not concerned about any large-scale replacement of ceramics by composite technology, which they regard as serving mainly a niche market. Says Huang, “porcelain and composites both have their relative strengths and weaknesses. But if you look at the big picture, the key advantage offered by porcelain is its proven long-term strength with no risk

Photos: INMR ©

Huang Gang (Banks) is PK’s manager for foreign business and states that exports in recent years have climbed even faster than the growth in domestic business to the point where they now account for about half of all sales. Major

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“We have come to see ourselves not just as another insulator supplier but rather as one that has attained a world-class level.”

of ageing. This makes it the preferred choice for most installations and especially so in areas where there are potential weather extremes.”

For his part, Shi sees that the new investment is serving two key objectives – improving quality through better processing methods and increasing overall production capabilities, especially when it comes to extra long pieces that will serve the growing UHV market. He also notes that while some 90 percent of current business is in hollow core, there are plans to expand the product range to include more types of solid core insulators as well.

Says Shi, “we have come to see ourselves not just as another insulator supplier but rather as one that has attained a world-class level. To maintain this position, we have to invest to keep our prices competitive and to ensure continuous quality improvement.” 

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INSULATORS

Insulator Manufacturer Implements New Production Scheme The past several years have seen large investments made at many insulator factories across China as more and more suppliers aim to optimize production to combat rising costs as well as bolster their quality image among international customers.

manufacturers in other countries. However, it is worth remarking that steps such as these are largely eliminating any past perceived quality differences between Chinese-made insulators and those produced in other parts of the globe.

The changes being made are certainly not revolutionary in the insulator industry and in fact have already been put into force by some

INMR visits one manufacturer based in central Henan Province to report on their relatively new production management scheme.

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For example, production of a composite insulator involves several discrete steps, from crimping on the end fittings to molding of the housing. There is also inspection of incoming components such as rods and fittings as well as in-house manufacturing of the silicone rubber material.

A unique bar code system now identifies each insulator under production.

“With more and more competition, quality has fast become the lifeblood of our business and the key factor in continued growth,” says Zhang Feng as he strolls through the production area at Xianghe Electric’s plant in Zhengzhou. Zhang is the company’s General Manager and he and his team have devoted the past year to developing an improved system for managing the flow of insulators as they move through the different stages of manufacture.

“The new PPM system represents a large step forward for us in maintaining quality and also in better tracking the status of any order.”

According to Zhang, the new system called PPM (Production Program Management) represents a major step forward for Xianghe not only in maintaining quality but also in better tracking orders. A unique bar code is assigned to every single insulator and allows anyone in the plant to monitor every stage of its production. Says Zhang, “with this system, production staff can know the exact rod and fittings that are in each insulator, the batch of silicone rubber used to mold it and even who was involved in every step, from crimping to molding.”

To make the system even more transparent and accessible to plant workers, a large board in the main production area displays the unique code of each product and links it to its current production status. “This”, remarks, Zhang, “allows everyone to quickly track how an order is progressing. More importantly, it also promotes higher productivity since we can see at a glance how many units are being produced at any time.”

Photos: INMR ©

Apart from the new bar code system, Xianghe has also changed its method of linking its name to each unit manufactured. In the past, this was done using a metal nameplate attached to the end fitting.

However, such a system was seen as presenting a risk of damaging the galvanized zinc coating of the fitting and has now been entirely replaced by a system of laser marking on the top shed of each unit.

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Computer system accessible to workers provides complete details on any unit by scanning bar code.

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Electronic board in plant allows managers and workers to keep track of production status of insulators.

All these changes in production control and monitoring come at a time when Xianghe’s business outside China is expanding rapidly. According to Li Xia, Director of International Sales, orders from new customers in the Middle East and South Asia have created a need to further expand plant capacity. She and Zhang mention a new, latest generation molding machine that has just recently been put into operation and which is expected to help to meet the burgeoning demand. Says Zhang, “our renewed focus on maximizing quality and plant productivity has required a large investment but we are already starting to see the returns.”  Laser system imprints brand and company information on each unit.

Shed design for areas of high pollution and new molding machine expected to produce these and other types more efficiently.

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ARRESTERS

Utility Reduces Lightning Outages With Line Arrester Investment Program The urban area of Shenzhen is China’s first and perhaps still most visible economic success story after the country opened itself to the outside world during the early 1980s. Once little more than a sleepy fishing village across the strait from bustling Hong Kong, Shenzhen has been transformed in less than three decades into a financial megalopolis of graceful skyscrapers, luxury condominiums, shopping malls, broad tree-lined avenues and world famous golf courses.

Its location in the south of Guangdong Province facing the South China Sea puts Shenzhen squarely in a sub-tropical region with an average of some 70 thunderstorm days each year. Lightning activity – particularly focused between the months of April and October – can be especially severe, lasting for prolonged periods with a high ground flash density as well as high peak currents.

INMR travels to Shenzhen to report on a program begun by local power authorities in 2000 to mitigate lightning outages on 110 kV and 220 kV lines by selectively installing externally gapped line arresters (EGLAs).

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Electricity supply in Shenzhen is the responsibility of the Shenzhen Power Bureau (SPB), a part of the Southern Power Grid – one of China’s two main grid operators. SPB operates a fast-growing transmission system comprising numerous 110 kV and 220 kV lines that traverse the spines of the region’s steep terrain. High local isokeraunic levels combined with poor ground resistance of the coarse, rocky soil on which towers are sited means that many of these lines are particularly vulnerable to the impact of periodic lightning strikes.

Indeed, the impact of lightning on SPB transmission lines has

The impact of lightning on transmission lines in Shenzhen has traditionally been high and once accounted for as much as 60 percent of all outages recorded on the HV network.

Photos: INMR ©

traditionally been high and in the past accounted for as high as 60 percent of all outages recorded on the HV network. For example, according to published reports, between 2004 and 2007 the transmission line outage rate in Shenzhen varied from 2.77 to 3.88 per 100 km – a level generally regarded as unacceptably high.

Remains of punctured porcelain disc insulators litter ground below mountaintop 220 kV tower. Local regulations require regular inspection of porcelain strings and, whenever punctured insulators are found and replaced, they are shattered to recover metal fittings.

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Typical EGLA arrangements on transmission tower. Note presence of counter allowing SPB to track extent of arrester activity.

It was with a view to reducing the extent of this problem that engineers at SPB embarked on a program to equip some of the most problematic line sections as well as most exposed towers with line arresters having an external gap design (EGLA). Arresters became the preferred choice since, in most cases, alternative solutions for lightning mitigation were not considered practical given the difficulty and expense of lowering the high grounding resistance in mountainous areas.

In order to make the program most cost-effective, the installation process began with identifying which specific lines were most at risk, based on past lightning strike data and related outage statistics. Moreover, other factors were also considered, such as the extent to which towers were prone to shielding failure or whether there was any possibility to avoid problems Photos: INMR ©

by increasing insulation levels while still respecting tower clearances.

For example, in order to optimize the arrester investment, the policy was that in those cases where grounding resistance was low, EGLAs would be installed on the upper phase or on both side phases. However, when the grounding resistance was high and difficult to lower (e.g. as in the case of towers whose foundations sit on rock) or where lightning activity was severe with high risk of shielding failure, EGLAs would be installed on all 3 phases of the conductor.

By all accounts, the installation of arresters on the most vulnerable line sections and towers of Shenzhen’s transmission network has already resulted in significant improvements in service performance. For example, local engineers point to one 220 kV line where there were at least

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Examples of one type of EGLA where external gap (either by arcing horns or corona rings) is on composite insulator in series.

Connection between arrester and insulator allows for broad range of installation options and configurations.

2 lightning related outages each year before the arrester program began. Recently, however, no lightning outages were recorded even though special counters installed on the arresters indicated ongoing lightning activity on the towers. Similarly, one formerly troublesome 110 kV line that had several outages attributed to lightning in 2004 and 2005 also saw improved performance even as counters indicated numerous cases of arrester activity.

Indeed, a decade after the program first began, there have been only a handful of lightning outages reported on those towers equipped with the EGLAs and together these represented less than 3 percent of all transmission line outages. In one such case, for example, the SPB’s lightning detection and locating system showed that the lightning peak current was 150 kA while the grounding resistance of the affected and neighbouring towers exceeded 25 ohms. The resulting outage was then found to have been due to the arrester external gap being to large which was the result of improper installation. Other defects affecting line arresters were found to include dropped leads on arrester counters or unexpected repositioning of the discharge ring.

Shenzhen’s power network is still expanding rapidly with some 100 km of new transmission lines being added each year. With this type of growth, it is not seen as practical or financially feasible to equip all of these with arresters.

Instead, the plan is to continue to selectively install EGLAs based on a review of yearly lightning outage statistics.

EGLA installed on 220 kV tower of line from Daya Bay nuclear power plant in Shenzhen.

Looking back over the past 10 years, the program to equip more than 80 multi-circuit 110 kV and 220 kV lines in Shenzhen with line arresters has succeeded in reducing lightning outages and increasing overall network reliability. 

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ARRESTERS

Supplier Adjusts Strategy and Design For Arresters & Cutouts The past several years have seen higher labor and raw material costs take hold throughout China. Manufacturers across diverse sectors have felt the impact, including those in the large industry that supplies components for electrical transmission and distribution. Combined with the rising exchange value of the Chinese currency against the U.S. dollar, many of these suppliers have felt it necessary to re-position themselves in their international business with more focus on product features or quality and correspondingly less emphasis on purely low price.

INMR visits one such firm, Yikun Electric – a supplier of surge arresters, MOV blocks, fuse cutouts and polymeric insulators – based in the southeastern city of Wenzhou, to learn how management there have adapted to this changing business environment.

“Rising labor and material costs have required us not only to re-orient production lines for greater efficiency but also to modify our corporate image.”

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Insulators for applications in traction are a special strength.

Yikun’s General Manager, Dick Yu, is quick to point out that the current economic climate in China has imposed a new challenge, especially on companies such as his with a substantial proportion of their business in exports. Says Yu, “rising labor and material costs have required firms such as us not only to re-orient production lines for greater efficiency but also to modify their corporate image. For example, we used to see ourselves primarily as a supplier of electrical components offered at a very competitive price. Today, our focus has shifted to providing good quality at the right price.”

Yu explains that part of this shift has been greater emphasis on products where Yikun has traditionally enjoyed a strong market position in terms of design or production capabilities. For example, he mentions traction arresters for high speed railways as one product where the company has long excelled and claims it has recently become one of the leading brands in China, taking over from foreign-based suppliers. “This,” he says, “is just one example of what we are hoping to achieve, namely trying to substitute imports with locally-made products that we think are better suited for the Chinese operating environment.”

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In fact, Yu points out that the past two years have seen a large investment made in arrester and MOV block production in general. Capacity has increased by about 40 percent to some 100 tonnes/month with the addition of two new tunnel ovens where pressed blocks undergo heating cycles to burn off binders and moisture followed by sintering under very high temperatures.

Moreover, a range of new high performance blocks up to 115 mm diameter have been developed for Class IV and Class V arrester applications.

On top of the recent investment in metal oxide block production, an equally large expenditure has been made to automate testing downstream with the goal of reducing labor input and increasing capacity. Says Yu, “this is another example of re-orienting our production to focus more on what we

Steps in production of MOV blocks include pressing, sintering, ultrasonic cleaning and glazing.

Photos: INMR ©

More automated testing has aimed for greater efficiency.

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do best. We have aimed to make our whole testing process for blocks more efficient by automating as much as possible. For instance, one new machine can simultaneously test five blocks for reference voltage and store the results, which then correspond with the product data imprinted on the blocks themselves. All this used to be done manually.”

Another example of Yikun’s growing emphasis on products with improved design or functionality can be found in their line of fuse cutouts, where close to all sales are exported.

According to Sales Manager, Jim Wang, while porcelain still dominates as the main housing material, Yikun has actively participated in the trend toward a new generation of cutouts made from silicone.

This structure in Florida, features older porcelain cutout (right) and also the newer preferred silicone units.

“Five years ago,” reports Wang, “polymeric types accounted for less than 10 percent of our total cutout sales. But this proportion has been rising steadily each year. Today, almost half of our sales of cutouts are designs with silicone housings and, for some markets such as the U.S. and Brazil, this proportion is now more than 50 percent.”

Wang and Yikun’s other Sales Manager, Thomas Fang, point out that while polymeric cutouts can be up to 15 percent higher in price than porcelain, they offer users important cost reduction advantages, including less risk of breakage in transport and during installation. Another benefit lies in superior service performance since silicone cutouts offer higher creepage yet with less than half the weight of porcelain.

Porcelain cutout production at Yikun being increasingly replaced by units made with silicone.

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For example, at 15 kV a porcelain cutout typically has 220 mm of creepage and weighs about 6 kg.

By comparison, Yikun’s siliconehoused equivalent features 380 mm creepage at a weight of only 4 kg. “It is basically a completely interchangeable design as far as the user is concerned,” adds Wang, “with

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Photos: INMR ©

Interchangeable 15 kV cutouts with porcelain and silicone housings offer different features and sealing systems. Wang holds new cutout.

the only difference being in housing material.”

Another advantage of the silicone cutout design, claims Wang, lies in its better sealing system. With the porcelain-housed design, a special cement needs to be applied to fix the porcelain to the fuse barrier assembly and fittings. By contrast, the silicone design is molded directly over the internal assembly and includes a superior seal, meaning less risk of service problems due to moisture ingress. Wang goes on to point out that Yikun’s silicone cutout design has already passed the 5000 h accelerated ageing test and has also been submitted to thermal cycle testing to ensure the seal remains in good condition over a wide range of operating temperatures.

Wang explains that these types of advantages have already started a technology shift in many countries where utilities now either prefer or accept only silicone-housed cutouts. Apart from its own line of cutouts, Yikun also serves as sub-

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Assembly of fuse tube holder (right) is followed by heating and drying, prior to circa 15 minute molding cycle for housing.

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Jiang and Wang examine composite insulators.

Final assembly of arresters.

supplier to an OEM market, where it manufactures fully assembled units or even cutout components such as fuse tubes based on designs provided by outside suppliers.

Another example of improved designs aimed at avoiding competition based solely on a low price are Yikun’s line of MV surge arresters, which now feature a new wrapped module

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that replaces the former tube type.

According to Lili Jiang, Yikun’s Sales Manager, the new method of winding the epoxy impregnated fiberglass over the MOV blocks results in higher mechanical strength and superior short circuit performance.

Looking back over all the changes in the factory and product line over the past three years, Yu remarks that

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New wrapped design of arrester module.

Yikun has gone through a substantial transformation. “Today,” he says, “the focus here has shifted to reliability and better quality achieved at lower costs through better production methods and testing. We have also invested more in our internal R&D center to provide a higher level of technical support to customers and to be able to continually improve our products.” 

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BUSHINGS

Experience in Mexico With Non-Ceramic Bushings in Contaminated & Seismic Areas The first application of composite insulators in Mexico began in the mid 1990s with initial use limited to transmission lines in areas with pollution problems. A growing body of service experience eventually resulted in a national standard for such insulators on Mexican HV lines. However, because of lack of sufficient local operating experience, the application of polymeric-housed substation apparatus such as bushings, arresters and station posts has yet to be standardized in that country.

breaker operating in an environment of severe pollution.

Their performance – as well as that of 230 kV polymeric bushings installed four years earlier in a known seismic area with high industrial and coastal pollution – was then evaluated using inspection methodologies such as corona, electric field distribution, leakage current and hydrophobicity measurements.

Introduction

in the western part of the country. These insulators suffered from defects and failed even before the line’s commissioning due to poor mechanical strength and low UV resistance of their housings.

Because of that experience, non-ceramic insulation was for years essentially banned from all outdoor applications in Mexico.

This article, contributed by Ruben Sáldivar-Guerrero of the country’s Electric Research Institute (IIE) as well as staff To gain experience with composite insulator technology in from the Substation Department of the Comisión Federal the Mexican substation environment, polymeric bushings Electricidad (CFE), reviews the research and findings. were installed several years ago on a 230 kV circuit

The specification of composite-housed components such as bushings has grown worldwide due to the advantages offered compared to traditional porcelain housings. Much lower weight yet high mechanical strength and hydrophobic surfaces combine to make these types of components ideal for areas with severe pollution as well as high seismic risk.

Unfortunately, initial Mexican operating experience with composite insulator technology was not encouraging. It began during the 1980s when the first overhead line equipped with these insulators was constructed

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Then, in October 1995 and January 2003, two powerful earthquakes, measuring 7.9 and 7.6 respectively, shook the coastal area of Colima – a region of high seismic activity and also vulnerable to climatic problems such as tropical storms and hurricanes.

Figure 1: Seismic regions of Mexico. In Zone A, no major seismic activity took place over past 80 years while Zone D has seen the most powerful earthquakes. Zones B and C are regions falling between these extremes.

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Damage to Mexican substations from recent earthquakes was extensive.

Colima coastline. Between them are six generators that supply four 400 kV transmission lines, a 230 kV line and five substations with a total capacity of 3,747 MVA. The intensity of the recent quakes caused severe damage to switches, transformer bushings and other equipment, with much apparatus destroyed due to the poor seismic withstand of porcelain.

Apart from the constant threat of earthquake, Manzanillo I and II are also affected by pollution from the sea as well as from local combustion of fossil fuels that together impact performance of external insulation on equipment such as bushings.

In order to prevent pollution flashovers and interruption of service, Mexican national standards were revised to permit the use of nonceramic bushings on transformers operating in this environment.

Subsequently, this was extended as well to all substation equipment located in areas of high seismic activity and pollution and also for mobile substations. Silicone rubber housings were considered a better option than porcelain in such applications due to their superior pollution performance as well as lower inertia and higher flexibility, with a circa 10 to 1 reduction in weight compared to ceramic material.

A research project was therefore undertaken at the Manzanillo substation where the previous porcelain bushings on a total of 10

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Live washing of substations in Colima was required regularly due to build up of pollution.

In order to prevent failures and interruption of service, local standards were revised to permit non-ceramic bushings on transformers and other substation equipment located in areas of high seismic activity and pollution. power transformers were replaced by units with silicone composite housings. The surfaces of these new bushings were then closely monitored using diagnostic tools such as measurement of contact angle and surface resistivity.

Since installation in 2003, these new bushings have demonstrated excellent service performance.

Moreover, their silicone housings

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400/230 kV transformers in Manzanillo equipped with silicone bushings.

have remained hydrophobic in spite of the build up of a pollution layer.

This meant that live washing, which in the case of the previous ceramic bushings had to be performed regularly during 7 months, could be reduced to less than once a year.

The 400 kV air-insulated substation at Manzanillo was subsequently replaced by a GIS with a total of 21 silicone bushings interconnecting to transmission lines. This made it the countryâ&#x20AC;&#x2122;s largest installation employing silicone bushings.

The next step in the application of non-ceramic bushing technology in Mexico was to investigate how long these silicone housings could maintain good performance under the demanding local service conditions â&#x20AC;&#x201C; in other words what life expectancy could be expected for them. With this in mind, the CFE initiated a project to install similar non-ceramic bushings at another substation with the same type of service conditions as Manzanillo.

Test Methodology

Two non-ceramic bushings were installed on a 230 kV dead tank breaker at the Cerro Prieto generation plant in Mexicali, located in the same seismic region as Manzanillo.

This geothermal facility consists of four generation units: Cerro Prieto I with an installed capacity of 180 MW; Cerro Prieto II and III, each with 220 MW capacity; and

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Non-ceramic bushings installed on 230 kV dead tank breaker at Cerro Prieto.

Cerro Prieto IV with a capacity of 100 MW. Because equipment at the substation is continually exposed to a combination of industrial contamination and condensation, all insulation there has historically had to be washed at least several times a year.

Non-ceramic bushings were installed at the substation in March of 2007 and monitored closely since then using leakage current measurements where the frequency and range of current peaks were recorded using a custom-built data acquisition system. In addition, their electric field distribution was measured both before and over the course of the test period using an instrument originally developed at Hydro-QuĂŠbec. Finally,

Figure 2: Electric field distribution along non-ceramic bushing connected to incoming line.

A key question at CFE was how long silicone-housed bushings could maintain their good performance under the demanding local service conditions.

observations were also made with a UV camera to identify risk of corona damage to housings.

In addition, a series of specialized tests were also performed on each insulator including static contact angle measurement, chemical characterization by infrared spectroscopy as well as analysis of surface microstructure.

For example, digital imaging analysis involved a water drop being placed on the non-ceramic surface and recorded with a highresolution camera. The file was then analyzed using software to measure static contact angle. Similarly, microstructure analysis was done by replication â&#x20AC;&#x201C; a non-destructive

Figure 3: a) Visualization of corona effect on the line connectors of non ceramic bushing connected to line. b) Same image in darkness.

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Housings of non-ceramic bushings maintained their hydrophobic property, even after deposition of contamination.

technique that records topography of surface microstructure as a negative on a thin cellulose acetate film.

Although low molecular weight silicone chains from the surface are consumed initially due to moisture on the sheds, with time these molecules migrate from the bulk polymer and accumulate on the contaminated surface, restoring surface hydrophobicity

Findings

To benchmark their condition when new, the non-ceramic bushings were evaluated at the start of the test using the characterization techniques discussed above. Figure 2, for example, shows the electric field distribution for the bushing connected to the line, both in 2007 when first installed and then again in March 2011.

The principle of measurement in this case involves tangential electric field along the bushing. As shown, electric field is concentrated toward the bottom of the bushing and also at the top where it connects to the line. The shape of the curve suggests that there are no obvious problems with the breaker. A similar pattern of electric field distribution was also noted for the non-ceramic bushing connected to the outgoing line.

Nor were any corona effects detected along the lengths of the bushings being monitored, in spite of some isolated ionization of the air near the line connectors. This absence of corona provides evidence that there has been no external degradation of the insulation housing even though the bushings have not been washed since installation. Because silicone rubber has the property of transferring its hydrophobicity into the contamination layer, this allows them to remain hydrophobic even under the heavy service pollution.

Fig 4: Schematic of dead tank breaker shows points selected for evaluating hydrophobicity along non-ceramic bushings.

A leakage current system was installed near the breaker and downloaded data indicated current

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peaks of only around 8 mA â&#x20AC;&#x201C; much less than the 50 mA needed to trigger onset of dry band activity on the surface. Even during sporadic rains, maximum peaks recorded were in the area of 24 mA, not enough to cause any deterioration of the silicone insulation.

Moreover, to further evaluate hydrophobic behavior of the housings during the test period, static contact angle was measured on sheds located near the top, middle and bottom of the bushings.

Although low molecular weight silicone chains from the surface are consumed initially due to moisture on the sheds, with time these molecules migrate from the bulk polymer and accumulate on the contaminated surface, restoring surface hydrophobicity Table 1 provides the static contact angle values recorded. As can be seen, contact angles on almost all sheds of both test bushings decreased during the first months (June 2007) but then increased up to the last evaluation. Similar behavior has been observed during accelerated ageing tests in the laboratory and can be explained by the fact that, initially, low molecular weight chains from the surface are consumed due to rain or condensation on the sheds. However, with time, these types of molecules migrate from the bulk polymer and accumulate on the contaminated surface, thereby restoring surface hydrophobicity and increasing contact angles.

It is noteworthy that while the non-ceramic bushings were never washed during the entire course of

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Table 1: Contact Angle Values for Non-Ceramic Bushings at Different Test Times*

*Last row shows images of water drops on top shed for each evaluation.

the test, they continued to maintain hydrophobic surfaces. This reduced the risk of leakage current under rain and condensation and helped preserve the integrity of the housings, even in the challenging service environment.

In each evaluation of the two test bushings, their chemical composition was also verified using Fourier Transform Infrared Spectroscopy (FTIR). Figure 5 shows typical FTIR spectra of the connected bushing housings at different evaluation times, measured on the bottom shed.

Peaks a, b and d are absorptions attributed respectively to the C-H, Si-CH3 and Si-O-Si bonds, which are components of silicone rubber and serve to identify silicone as the main component of the housing material.

Mexican experience with non-ceramic bushings has not only suggested excellent service performance can be expected but equally important that maintenance costs can be significantly reduced.

Fig 5: FTIR spectra of bushing housings at different evaluation times.

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No changes were detected in these absorption peaks over the course of the different evaluations, meaning that the silicone rubber material did not degrade during the test period.

Moreover, the housings showed no evidence of chemical changes along the full length of the bushings, as shown in Figure 6 which presents FTIR spectra measured at different shed positions of the bushing four years after installation. Again, there are no changes in absorptions peaks, from which it can be concluded that the chemical composition along the housing has been maintained during the test time, with no degradation.

Finally, the microstructure of the housing surface was analyzed using

Fig 6: FTIR spectra on different shed position of bushing housings four years after installation.

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Porcelain transformer bushings were damaged near the flange due to earthquake of April 2010. Note extensive pollution accumulation on surfaces.

a non-destructive technique that looks for changes on polymeric surfaces. Figure 7 shows that some microstructural changes were evident after four years of service, possibly due to the contamination layer not having been removed by washing.

However, by both chemical and physical characterization methods, it is still clear that the non-ceramic bushing housings have not suffered any significant deterioration.

A strong earthquake, measuring 7.2 on the Richter scale, occurred in Mexicali on April 4 of 2010. The intensity of the tremors severely damaged electrical equipment such as circuit breakers and transformer bushings with porcelain housings, which shifted out of their normal flange position. By contrast, the two non-ceramic bushings did not suffer any damage. This event only confirmed the wisdom of applying non-ceramic insulation on substation equipment, especially for such seismic regions with high contamination.

Summary

Fig 7: Images of microstructure of bushing housing during different evaluation times as analyzed by scanning electronic microscope.

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The application of non-ceramic insulation on various HV equipment started in Mexico only during the past decade. Initial use was focused on 400/230 kV transformer bushings at the Manzanillo II Substation with the goal of preventing failures due to earthquake.

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Up to now, these bushings have demonstrated good service performance, even here in one of most severe seismic regions of the country.

To gain additional experience, two non-ceramic bushings were installed on a 230 kV dead tank breaker in an equally seismic region in order to study possible degradation of their polymeric housings as well as possible loss of electrical performance.

These bushings were then subject to extensive electrical and physicochemical diagnostic tests.

After four years of service, there have been no signs of deterioration. Corona, leakage current and electric field distribution testing have all confirmed that the new bushings were able to withstand both seismic movement and heavy industrial contamination, even without washing. Contact angle evaluation and chemical analysis of their surfaces showed no longterm loss of hydrophobicity, nor of any noticeable degradation to their silicone housings.

This experience with non-ceramic bushings has not only suggested excellent service performance can be expected but equally important that maintenance costs can be significantly reduced. ď ¸

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CABLE ACCESSORIES

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Overview of Testing Requirements for Cable Accessories With the introduction of XLPE technology, the reliability of cables – especially those from manufacturers with strict quality control – has become very high. As a result, a large proportion of cable failures these days seem to involve problems with accessories, such as terminations, rather than with the cables themselves.

subject to on-site commissioning testing – something that becomes increasingly important at higher voltage levels.

In order to maximize customer acceptance of associated test reports and documents, design and type tests of this sort are often performed at independent facilities, such as laboratories accredited according to ISO/IEC 17025.

Tests of accessories according to published standards These laboratories offer the advantage of meeting all are carried out with samples taken at random from the required standards in quality management and use of a production line in order to ensure that findings represent common interpretation of the standards. an evaluation of a ‘typical’ product. This way, they also help confirm that both the design and production process The following article, contributed by Heiko Jahn, are satisfactory. Moreover, since cable accessories are Technical Manager at one such independent facility – subject to possible installation errors or other problems FGH Engineering & Test in Germany – reviews some of in the field, the whole cable system is sometimes later the present day test requirements for cable accessories.

Figure 1 depicts some of the IEC and CENELEC standards for testing medium and high voltage cable accessories. While these standards define the minimum requirements to satisfy the quality demands of customers, certain large power utilities sometimes issue their own additional specifications, based on

these standards but placing higher stress on the accessories to be tested.

As shown in Figure 2, some tests (e.g. the thermal cycling test) are similar Tests on Cable Accessories across all applicable voltage levels. By contrast, short-time current and peak Humidity & Salt Fog Testing of withstand current tests are requested Terminations mostly for MV cable accessories.

Given the importance of reliable service performance, even under demanding service conditions, non-ceramic insulation has become the state-ofthe-art for terminations in most MV cable systems. Moreover, such insulation is now increasingly found in HV cable terminations as well.

Figure 1: Selected test standards for medium and high voltage accessories.

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Pre-qualification tests are generally required only in the case of HV accessories.

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Nonetheless, there is always some concern that the surfaces of such polymeric accessories might be adversely affected by tracking and erosion. In the case of MV terminations, for example, these are usually installed with a significant mechanical strain on the cable. In the event that tracking affects a termination’s edge or that any surface erosion becomes too deep, the material could conceivably rupture.

This, in turn, could lead to moisture

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ingress and the eventual need to replace the termination.

Given this, a humidity test for indoor terminations as well as a salt fog test for outdoor terminations have been introduced into the standards with the goal of assuring that the required performance could be met in both types of applications. The main differences between the two are in test duration as well as in the conductivity of the salt water being used.

A corrosion-proof test chamber with inclined roof is essential to avoid any influence from water that might drip down onto the test samples and affect test results. Moreover, the feeding circuit has to provide low impedance so that there is only a small voltage drop under high test currents. Testing three-phase cable accessories with spreader caps may require significant additional preparation to build up the appropriate test circuit.

Figure 2: Tests for medium and high voltage accessories.

Since humidity and salt fog testing lasts for up to 1000 hours, most customers seek additional information on the accessory, apart from simply monitoring its surface condition.

Therefore, while only detection of over current (flashover) is requested in the standard, todayâ&#x20AC;&#x2122;s test systems also provide for long-term measurement of leakage current. From such data, any loss of hydrophobicity can be monitored and the stress on the cable termination estimated.

Photos: INMR Š

With the latest test systems, it is also possible to save all relevant test data in the event of flashover, thereby providing the opportunity to carry out additional investigation into its cause (see example in Figure 3). Moreover, using currently available technology, test bays can even be observed by remote access to the measuring system, allowing the test engineer to be kept fully informed by phone or e-mail.

Non-ceramic cable terminations are already standard for distribution and rapidly gaining in use at transmission voltages as well.

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While the pollution behaviour of MV terminations is tested as described above, no similar test procedure is as yet requested in the case of HV terminations equipped with non-

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The fact that non-ceramic housings are now increasingly common in HV terminations means there will clearly be a need to test their surface behaviour.

Example of typical salt fog test chamber for terminations.

Fig. 3: Long-term current (left) and current oscilloscope graph of flashover (right).

ceramic housings. This stands in contrast to cable terminations using porcelain, which are required to be tested according to either IEC 60507 for AC or IEC 61245 for DC. Still, given the fact that nonceramic housings are now increasingly common in HV terminations, there is clearly the need to test their surface behaviour as well.

At the moment, the only applicable IEC test for the hollow core composite insulator is the tracking and erosion test according to IEC 62217/IEC 61462.

Unfortunately, this is not a full-scale test and therefore does not account for the influence of length and field grading of the termination (as is the case of bushings). As such, the only information arising from this test is the quality of the cover material and design. Given this, some power utilities have started to issue their own test specifications to verify the

performance of HV cable accessories under pollution.

Electrical Type Tests A thermal cycling test under applied voltage is requested for MV and HV accessories with load current up to the maximum conductor temperature.

Since conductor temperature must then be measured, a so-called dummy loop has to be used. The heating current of the test loop is regulated such that sheath temperature of the test loop is the same as for the dummy loop. As such, it can be concluded that conductor temperature has this same value. Figure 4 shows a typical such test circuit for the electrical type test of HV cable systems as well as their accessories.

Proper set-up requires the possibility to adjust and measure voltage and current in the test loop as well as in the dummy loop. A typical thermal cycle then consists of:

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1. A heating interval where the test loop conductor is heated to the maximum temperature; 2. A stabilization interval during which temperature is held close to this maximum; and 3. A cooling part with the heating current switched off.

A specific number of such cycles have to be carried out followed by whatever high voltage withstand tests and measurements are specified.

Since the heating current is mostly inductive, the feeding current can be reduced by the use of capacitive compensation circuits. This way, the requested power is influenced mostly by losses in the cable and in the heating transformers.

Pre-qualification Tests Pre-qualification testing is similar to the electrical type test described above except that its duration is 8760 hours (equivalent to one

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year) and that it is performed under realistic installation conditions for the complete cable system, e.g. in natural soil, a concrete tunnel or a PE duct.

Photos: INMR Š

The relevant standard for the prequalification test requires a test loop length of at least 100 m. Because of this large size and the installation conditions to be simulated, these tests must be carried out in an outdoor test field. As defined in IEC 62067 for cable systems with rated voltage of 170 kV and higher, a total of 180 thermal load cycles at 1.7 times the rated conductor-to-earth voltage have to be applied over the course of the one year test period.

Moreover, with the recent upcoming edition 2.0 of IEC 60840, this test is now also recommended for systems from 36 kV to 170 kV in cases where the dielectric field strength is similar to that for cables with higher rated voltages, i.e. 8 kV/mm at the inner conductive layer. This means that manufacturers of such products will now also be required to provide evidence of performance. By contrast, this test is not required for complete systems (including accessories) in the MV range and only a long duration test of the cable itself is needed.

Figure 4: Typical layout of test circuit for electrical type test on cable systems.

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Example of installation work for pre-qualification testing.

The application of heating cycles to a cable system installed in an outdoor test field is far more difficult than performing a type test in an indoor HV test laboratory. For example, the ambient temperatures of systems can differ by more than 40°K over the course of the one year test period and this has to be taken into account in terms of feeding power of the heating circuits.

SCR-controlled heating circuits are therefore necessary to ensure the heating cycle will follow the defined temperature curve exactly. This type of system takes into account the maximum heating current allowed as well as the conductor and sheath temperatures of the test and dummy loops and applies precisely the

The performance of any cable system depends greatly on the quality of installation of its various accessories since these are assembled on site, often employing a range of different materials.

current needed to follow a predefined temperature curve. Even higher dielectric losses of the test loop on which voltage is applied have to be taken into consideration using an offset and any deviation between target and real conductor temperature must be kept to less than 0.2°K. The resulting curves are then shaped perfectly and the number of invalid cycles significantly reduced. Figure 5 shows an example of such a heating cycle.

As shown, there is an offset between the heating currents of the test loop and of the dummy loop and this offset is necessary to get the sheath temperatures of both loops as close together as possible. Even though the dummy loop conductor temperature

may appear artificial, it is in fact a real measured temperature curve.

This curve consists of several basic points connected by straight lines. The position of these points are stored as constants within the program for the SCR controller but can also be changed manually. The controller regulates the heating currents so as to follow the straight lines precisely. This, of course, is not possible manually if an operator is not constantly observing measured values during the heating period. Manual adjustment of the curve parameters can be used to minimize energy consumption of the heating system.

Introduction of pre-qualification tests to lower voltage levels has required additional testing capacity and today large laboratories, such as CESI Group, offer test bays for several test loops, in some cases up to a test voltage of as high as 800 kV. A growing request these days when it comes to testing is the possibility to observe test loop behaviour on a real time basis as the test progresses.

Upon request, some facilities offer remote viewing access to the

Figure 5: Typical curves measured during prequalification test on 220 kV cable system.

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On Site Tests The performance of any cable system depends greatly on the quality of installation of its various accessories.

Since the cable itself is routinetested at the factory, the probability of it failing is very low. Accessories, by contrast, are assembled on site, often employing a range of different materials. Final system quality, therefore, can be assured only by a clean installation with no damage to any of its components.

Example of lightning impulse test on cable loop using mobile impulse generator.

measuring system via the internet.

This allows not only the client but also test engineers to access data from the systems, in the latter case with writing capabilities to the equipment as well so as to permit process parameters to be adjusted if necessary without the need to be physically present.

Further evidence of progress in this field is the possibility to test the whole test loop with lightning impulse upon conclusion of the cycles. In the past, the standard

was met simply by choosing the most stressed part of the cable and cutting out a section of 30 meters to test. With mobile impulse generators, however, it is now possible to place the generator close to the test loop and carry out the lightning impulse test on the whole loop, including accessories, without any need to cut or move the cable.

While this type of test procedure is not required by the standards, it increases confidence in test results.

To ensure that no serious defect has occurred during installation, on site commissioning tests can identify any possible weaknesses.

For example, modern equipment now allows measuring partial discharges in the field and this means that the likelihood of detecting installation problems has increased significantly.

Since the capacity of an installed cable system is relatively high, resonant test transformers are the preferred equipment for AC tests.

Test frequencies vary between 20 and 300 Hz while test current is in the range from 80 to 240 A. The photo below illustrates an example of such on-site measurement which involves mobile test platforms that guarantee high availability and reduced transport costs.

Summary

Most tests of cable accessories are electrical tests. Moreover, since the accessory cannot be used without a cable (or at least a cable simulation), these tests are typically carried out with the accessory installed in such a system.

Photos: INMR Š

While well defined in the case of MV terminations, there is currently no specification for pollution performance of HV cable terminations, especially not for terminations with composite insulators. With the new edition of IEC 60840, prequalification tests as well as tests after installation of the cable system are also recommended for the range between 36 kV and 170 kV. This will result in increased demand for test capabilities. ď ¸

On-site test of cable system before start-up.

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A City Built on Insulators

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P

orcelain insulators are something one usually expects to find only as part of an electrical installation such as a substation or overhead line.

However, to the people of the Chinese city of Liling and its outlying communities, electrical porcelain has become very much a part of everyday life – not only providing

them jobs and income but also fulfilling a broad variety of distinctly ‘non-electrical’ needs.

INMR travels the countryside outside Liling to capture some of these diverse and often imaginative new applications for porcelain insulators.

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Photos: INMR ©

In Liling, insulators serve not only on lines and at substations but also as guardrails, pillars, terraces and conduits.

The production of porcelain insulators has historically been scattered widely throughout the world. Generally, the development of this industry in any one region has depended on two key ingredients – an abundant supply of clay and the entrepreneurs prepared to invest in transforming this material into a variety of electrical insulation products.

And nowhere on earth are these two factors in more abundant supply than in the small city of Liling. Located just south of Hunan’s capital, Changsha.

Liling has been bestowed with an abundant supply of high quality clay that has made it an ideal place for manufacturing porcelain. In fact, porcelain production there dates back some 1700 years to the era of the Eastern Han Dynasty.

Stacked porcelain plates such as these inspired the basic design of most porcelain insulators.

By all accounts, there are at least 100 different porcelain insulator manufacturers officially registered with the local government and probably an equal number who operate ‘below the radar’. Together, these hundreds of plants supply porcelain for virtually every possible low, medium and high voltage application.

With such a large population of suppliers, one might expect that landfills in the area would be overflowing with the tens of thousands of pieces

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Even in death, one finds that insulators again play a prominent role, this time adorning monuments to the departed.

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In the countryside of Liling, one can find electrical porcelain almost everywhere.

rejected each year because they do not meet basic quality requirements. But, fortunately, a far more environmentally friendly solution has emerged over time. Factory rejects are often left by the sides of roads and then quickly recycled by local residents into building applications of every imaginable sort, from structural supports for terraces, roads and balustrades to pillars that hold up sheds.

They have become so fundamental to local culture that one even finds them intertwined with the city’s religious life and observances. For example, a temple high in the mountains overlooking Liling has traditionally had a day

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where local farmers come to a ceremonial well to pray for abundant rain for their crops. The well, known as ‘dragon well’ because of the dragon’s head in the small shrine, is adorned by a display of – what else? – insulators. (Perhaps local entrepreneurs come here as well to pray for an abundance of orders.)

INMR salutes the good citizens of Liling who not only help ensure the transmission and distribution of electrical power worldwide but also remind us that everything produced by the ‘hand of man’ can also be used creatively by the ‘brain of man’. 

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VOLUME 20 NUMBER 1 • QUARTER ONE - 2012

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