Inmr Issue 97

VOLUME 20 NUMBER 3 • QUARTER THREE - 2012

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Issue 97 • Quarter 3 - 2012 • Volume 20 - Number 3

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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 97.indd 3

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Coming in the next issue of

INMR

When people think of mechanical failure of a composite insulator, there is a tendency to think only of brittle fracture. However, the incidence of this mode of failure has been significantly reduced in the latest generation of these insulators through better materials and product designs. Nevertheless, other factors such as the inability of the specified insulator to cope with dynamic line stresses or improper molding of the housing over the core rod can also result in fracture-type failures that are as yet not well categorized and differ substantially from what is normally observed during brittle fracture.

The next issue of INMR will feature an article that deals with the specific case of a 500 kV AC composite insulator in a V string configuration that fractured yet showed none of the classic characteristics associated with brittle fracture. One of the objectives behind the detailed forensic analysis was to provide test data to allow comparison with any similarly fractured insulators appearing on other networks. Another goal was to encourage both manufacturers and power supply companies to retain their focus on the quality of the critical sheath-core interface in order to avoid premature ageing and failure.

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Coming in the next issue of

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Porcelain as a dielectric used across electrical insulation applications for over a hundred years is familiar to everyone in the power industry. What is not so well known, however, is that the main materials that make up the porcelain mass are the result of unique geologic conditions that have been at work for as long as 45 million years. Indeed, porcelain’s excellent electrical and mechanical properties derive from exploiting rare natural deposits of ball clay whose chemical and physical properties are ideal for optimized production and long-term field performance.

The next issue of INMR will contain a special article from the site of one of the world’s oldest and still most important ball clay quarries in southern England. This article, involving experts from French-based ceramics giant, Imerys, will allow readers to better understand that the origins of a reliable, high quality porcelain insulator are not simply in the factory but also in nature.

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Weather Extremes Highlight Need to Monitor for Changes in Pollution Severity Just as nobody on the way to the beach wants to listen to a forecast of bad weather, most people today prefer not to focus on warnings from climatologists that our lives may soon change in a big way. Still, anyone who’s been following events over the past year has to be asking themselves: ‘what’s going on?’ Blistering heat and drought across the central U.S.. Floods of epic proportions in Russia, Thailand and, most recently Japan. Downpours in Beijing. Torrential rains unleashing landslides in Brazil. Freak windstorms and lightning storms in the U.S. northeast. Tornados, typhoons, hurricanes, wildfires – all arriving either sooner, more frequently or with greater impact.

In July, the U.S. National Oceanic & Atmospheric Administration (NOAA) announced that mainland USA has just experienced its hottest 12-month period since record keeping began some 115 years ago. The hottest year on record had been 1998; but now 2012 seems well on track to beat it. Especially ominous, according to the NOAA, 9 of the 10 warmest years globally have occurred since 2000.

We can debate the reasons behind these statistics but what is beyond dispute is that earth is rapidly becoming a place of weather extremes. And few industries should be more concerned about this than operators of power networks that are often the first and most severely affected infrastructure.

Perhaps little can ever be done to avoid loss of overhead lines under cataclysmic events of wind, snow or ice loading.

But there are other aspects of earth’s changing climate – far subtler – that could also impact system reliability, even in the absence of toppled power structures.

Most insulation on existing lines and at substations has been dimensioned according to standards based on historical experience with service parameters such as precipitation, wind, humidity, temperature, etc. These are the factors that most influence global dust patterns as well as deposition rate and type of pollution settling on insulator surfaces. Yet with

projected rising sea levels, more desertification and higher wind activity, it is precisely these variables that are now changing at an unprecedented rate.

Earth has rapidly become a place of weather extremes. And few industries should be more concerned than operators of power networks.

There may be a tendency to believe that adding creepage is all that’s necessary to provide an added margin of security to power network insulation in the face of changing pollution conditions. But, as discussed on p. 30, this is not necessarily true. Shed shape and geometry affect how an insulator ‘self cleans’ while distance between sheds impacts icing behaviour as well as potential bridging during heavy rain.

Moreover, at DC there is limited margin to add creepage without affecting insulation distances.

At the same time, the relative proportions of ESDD and NSDD levels on surfaces impact how insulation performs and also the onset of problems, such as corrosion, from excessive leakage current. Indeed, an article in this issue by William Chisholm discusses the importance of non-soluble pollutants in terms of their impact on flashover performance. As Chisholm points out, “NSDD is important in electrical performance of insulators because it affects the nature of surface wetting. A surface with a heavy but inert dust deposit will stabilize whatever conductive pollution there is and promote repeated development of partial discharges and dry bands.

The settled layer of dust can also absorb sulphur dioxide and water vapor directly from air. This adds to the risk that hygroscopic salts will scavenge enough water from humid air to ‘self-wet’, thereby forming a continuous conductive surface that reduces insulator flashover performance, even if there is no rain or fog.”

One of the keys to avoiding reliability problems due to pollution flashover problems in service environments marked by weather extremes will be maintaining reliable data through more frequent site pollution severity assessments as well as greater on-line monitoring of insulation.

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INMR® Q3 2012

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Editorial

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PREVIEW

ofofthis thisissue issue

Issue 97 0 Quarter Quarter03− - 2000 2012

40

10 Perspective

Utility Practice & Experience

Weather Extremes Highlight Need to Monitor for Changes in Pollution Severity

16 Editorial

Does Your Grid Look ‘Smart’?

20 Inside Track on Smart Grid Everything Becoming ‘Smarter’

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Volume Volume 20 − 00 Number − Number 3 0

22 From the World of Testing Synthetic Testing Leaps Over Earlier Limits

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Canadian Utility Confronts Challenges to Network Components

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World’s Highest HVDC Line Delivers Power to Tibet

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Norwegian Utility Re-Dimensions Insulation in Voltage Upgrade Project

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Algerian Grid Operator Focuses on Harsh Service Environment (Part 2 of 2)

24 Silicone Technology Review

HCR Offers Range of Manufacturing Options

26 Reporting from Cigre

Guide for Handling Composite Insulators: Update

28 Transient Thoughts

94

Insulators

94

Non-Soluble Surface Deposits on Insulators

100

Glass Insulator Manufacturer Invests in Production Facility

Selecting Insulators Using Satellites

30 Scene from China

Shed Profiles for Composite Insulators

32 Pigini Commentary

Insulator End of Life: Recycling vs. Disposal

34 From the Research View

100

Arresters/Cable Accessories 108

Sheath Voltage Limiters Protect HV Power Cables

Surface Charge & DC Flashover Performance of Polymeric Insulators

36 Woodworth on Arresters

Developments in Epoxy Materials

38 Focus On Cable Accessories

114

Lessons Beyond University

Trends in Conductor Cross-Section & Connector Design

Compaction Driving Design of New Epoxy Insulation Components

Advertisers AdvertisersininThis ThisIssue Issue ABB Components & Insulation Materials 57 ABB Distribution Automation & Components 49 Alstom Grid 33 Balestro 43 CSL Silicones - SiCoat Outside Back Cover DTR Corp.

43 Dalian Composite Insulator 4, 35 Dalian Insulator Group 14-15 Dekuma Rubber & Plastic 51 Desma 71 Dextra Power 13 Dongguan Gaoneng Electric 1

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EGU HV Laboratory 8 Glasforms 13 Huayi Machinery Group 11 Hubbell Power Systems Inside Back Cover Hübers Verfahrenstechnik 77 Huntsman Advanced Materials 89 Jinan Meide Casting 77 KEMA 23 Motic Electric 35 Omni LPS 43 PK Insulators 9 QingZhou Shi Liwang Electric Technology 69

Reinhausen Power Composites 63 Rugao Dasheng Line Material 5 SGD La Granja 45 STRI 83 Shaanxi Taporel Electrical Insulation 21 Shandong Peiport Electric 75 Shanxi Century Metal Industries 71 Shenma Electric Power 6-7 Sichuan YiBin Global Group SYGG 81 Siemens, Arresters Div. Inside Front Cover TE Connectivity 39 Tianning Electrical Isolating Materials 2-3

Tridelta Überspannungsableiter 37 Trench GmbH 59 Vogel moulds and machines 65 Wacker Chemie 25 Wellwin Precision Moulds 11 Wenzhou Yikun Electric 29, 71 W.S. Industries 107 Xi’an Gaoqiang Insulation 27 Yizumi Rubber Machinery 18-19 Zhejiang Fuerte 35 Zhengzhou Xianghe Group Electric Equipment 17 Zibo Taiguang Electrical Equipment 31

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Does Your Grid Look ‘Smart’?

EDITORIAL EDITORIAL EDITORIAL

It’s fair to say that moving high voltage electricity reliably across vast distances and with minimal losses is already challenge enough. So why force the clever engineers who accomplish this to also make the resulting systems look ‘stylish’. Does anyone really care? And even if they did, how can you ever hope to make a huge substation or a 50 m high power structure look ‘trendy’?

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This GIS substation near Montreal, Canada benefits from interesting concrete fascia, circular features and tall elegant entrance gantry.

Richard Schomberg’s column in this issue talks about how everything in our world is becoming smarter. Not just power grids, but also buildings – even entire cities.

But let’s stop a moment to consider what the word ‘smart’ really means. Being from the English language, the word ‘smart’ is obviously British in origin. And, if I’m correct, in England, if you say to someone: “Don’t you look smart!” they’re not remarking on your intelligence. Rather, in Great Britain, ‘smart’ means something like: ‘stylish, trendy, fashionable’. So, do power grids merit being labeled as ‘smart’ – at least according to the common British use of the word.

220 kV towers in Dalian, China are minimalist yet grand.

Is the power delivery industry missing the opportunity to make its structures and installations look ‘smart’ as well as increasingly being smart.

Of course, style is a matter of taste. What looks appealing to one person might make another wince. Still, any decent architect can tell you While over 40 m high, this that there are fundamentals Chinese design DC tower is when it comes to aesthetics bold, balanced and quite simply, that dictate whether any beautiful – in spite of competing structure will be appealing against majestic surroundings.

– or not. For example, having graceful lines, being well proportioned, symmetrical, uncluttered all will make any power structure – including an entire substation – more friendly to the eye. Here are some examples of power structures I regard as truly ‘smart’, in the sense that they manage to appear both trendy and elegant in spite of their relatively massive scale.

Hopefully, this is the direction all electrical system operators will follow when it comes to designing the overhead lines and substations of tomorrow. Then, perhaps if someone from the U.K. were to visit your power installation, they might well remark: “My word! Doesn’t that look smart.”

Marvin L. Zimmerman mzimmerman@inmr.com

INMR Issue 97 • www.inmr.com

ISSN 1198-7332, E-mail: info@inmr.com • Editor & Advertising Sales: Marvin L. Zimmerman: mzimmerman@inmr.com, 1-514-939-9540 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, 1-514-509-0888, 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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220 kV towers in Seville, Spain are the epitome of style and grace.

Yet the fact that something is enormous and made of concrete or steel doesn’t mean it has to be ugly. Take, for example, the Eiffel Tower – an enduring symbol that is known and beloved worldwide. I can’t imagine someone renting an apartment in Paris and complaining “why the heck must I have a view of the Eiffel Tower from my bedroom? Yet most people bemoan structures along power lines as something they prefer never to be forced to look out upon.

Editorial

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Inside Track on Smart Grid

Everything Becoming ‘Smarter’

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After Smart Grid and Smart Buildings, the latest buzzword in our industry now seems to be Smart Cities.

So, what exactly are the commonalities and differences that require yet another terminology? Let’s explore.

Adding intelligence to cities represents quite a challenge but also offers wonderful business potential. Key roadblocks will be determining which technologies to use as well as how to finance and make them operational through suitable policies.

Smart Buildings: Self-Contained Energy Systems Globally, the commercial sector presently uses over 40% of energy produced (much of it as electric power) and buildings such as retail outlets, data centers, office buildings and hospitals consume the lion’s share.

A Systems Approach Many technologies to solve urban problems are now readily available but being applied independently. Standalone solutions may provide short-term benefits but little in the way of long-term progress. To achieve a truly ‘smart city’, infrastructure and data need to be shared in order to reduce cost and increases benefits. High-level roadmaps are needed to connect technologies and allow multiple companies to participate in technology solutions. Standards are a key to this process because they facilitate interconnectivity. They also reassure the financial community and accelerate ‘go’ decisions since they allow more compelling business cases to be made. Investment is shared but investors enjoy benefits across a variety of infrastructure.

Buildings can be complex structures integrating a variety of different internal power users, e.g. air conditioning, refrigeration, lighting, elevators, escalators, computers, communication and surveillance systems, among other machines, sensors and activators. Heating is still achieved largely through fossil fuels; however cooling, ventilation and refrigeration account for some 20% of electricity consumed, depending on climate and location. Lighting usually consumes around 40% of the electric energy used by these sites, including within the offices, warehouse, factory and any external lighting.

Looking ahead, designing ‘zero-energy buildings’ that produce as much or more energy than they use will become essential. More important still will be retrofitting existing buildings, especially since the renewal rate of commercial and public buildings in many countries is low and their lifespan can vary up to 50 years. These buildings will need to be updated for added intelligence, automation and more energy efficient systems including sensors, timers, programmable thermostats, lighting, heat pumps, energy storage and renewable energy sources.

Fortunately, the IEC has already developed numerous International Standards that apply to all these technologies and which will help manufacturers to build reliable, efficient and safe products. Moreover, given existing technologies, a reduction of 30% (and perhaps even as much as 50%) of energy consumption by buildings is certainly achievable.

Research suggests that, at current energy prices, such investments can be quickly recovered.

More Sustainable & Healthier Smart Cities There are now 21 ‘megacities’ having over 10 million inhabitants. Moreover, over half the world’s population lives in cities and, by 2050, projections suggest that this proportion will be 70% (i.e. some 6.4 billion in all). Yet, many cities are already near their limit in terms of congestion, pollution and overstrained infrastructure.

Cities are hugely complex and bring together a multitude of buildings, infrastructure and control/information systems.

The problem is that many city services (including power generation, water, gas, transportation and emergency services) function independently from one another. That’s why motorists and storeowners alike curse as roads are repeatedly torn open, e.g. first to fix the gas, then the water and finally the electrical lines. In order to increase the ‘livability’ index and sustainability of cities, digital infrastructure will be needed to improve sharing real-time information across departments and different technologies.

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Smart Grids – The Key Enabler Smart Grids are the essential first step toward making cities smarter. They will permit balancing the huge electrical power consumed in order to maintain a steady supply and also providing security to residents. Temporary interruptions of power are not only an inconvenience but also carry a potentially serious economic impact on the business sector and may even cost lives if key systems fail.

Storage to Balance Peak Demand Moving forward, cities will need to add energy storage to respond to peak consumption and this can potentially include the batteries of plug-in hybrid and electric vehicles. Another form of energy storage can be achieved when clusters of buildings are able to exchange excess heat or cooling (or share large scale HVAC plants that have higher efficiency).

Storage resources will also permit cities to integrate increasing numbers of solar panels and other intermittent power that customers supply back into the grid.

‘Microgrids’ will allow communities to plan how much energy they take from the grid and when it’s preferable to use distributed and stored energy. Together with smart meters, they will encourage more efficient energy use. In the future, direct current microgrids may even help reduce power losses as well as consumption.

Smarter Energy Consumption for Better Life Quality Smart Grids integrate and enable Smart Buildings as well as Smart Cities. But addressing future energy challenges and reaping the benefits will depend on how fast we can add intelligence, automation and a systems perspective to buildings as well as to cities - making everything ‘smarter’ in the process. That will be the real challenge!

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

Editorial

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from the world of TESTING

Synthetic Testing Leaps Over Earlier Limits

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Short-circuit power, and current as well, have only been increasing on electrical networks throughout the world.

The development of circuit breakers to interrupt such a growing current level has of course had to keep pace.

voltage, including transients, has to be imposed between the breaker terminals to verify that dielectric withstand remains unaffected by the hot, ionized gases exhausted into the space between live internal parts and the grounded enclosure.

The interrupting power that a breaker can handle is measured in GVA per break, with one break involving a single interrupting chamber. Up to about 1980, when air-blast breaker technology was dominant, there was a slow but steady increase of up to a few GVA per break. Since then, the workhorse of the breaker industry – SF6 technology – has evolved rapidly and managed to increase this level to about 20 GVA/break by 1990.

A single voltage source to cover UHV synthetic tests requirements is therefore clearly no longer adequate. As a result, a new development has started in which more than one synthetic voltage sources is involved. One line of development in this regard has been in Asia where a solution was adopted in which the complete breaker, in all its grandeur, is installed on a platform with isolation to ground of many hundreds of kV.

With the development by Japanese manufacturers of breakers able to interrupt 63 kA of fault current at a rated system voltage of 550 kV, a maximum breaking capacity in a single gap seemed to have been reached and indeed has not since been surpassed. The driver here was a planned 1000 kV transmission grid for Japan. Combining two breaks of this new ‘super breaker’ in series, could allow faults at this ultra high voltage to be managed. However, economic stagnation hit the country and the super-grid never became reality, meaning there was never a domestic need for these UHV breakers.

Higher interruption capabilities naturally create the need for larger test facilities. The upper limit to three-phase ‘direct’ test solutions (i.e. fault current and voltage supplied from a single source) was attained 40 years ago. Although this test condition comes closest to reality by allowing the three phases to interact naturally, only the largest laboratories were equipped to test HV breakers under these conditions. In KEMA’s case, for example, this limit was a rated voltage of 145 kV. The solution to overcome limitations in testing power was to combine two sources: a current source, i.e. one or more generators supplying the current necessary to maintain the fault arc inside the breaker; and a voltage source consisting of a pre-charged capacitor bank that creates the required voltage (transients) across the breaker immediately following interruption. Such a solution, first explored as far back as the 1960s, is referred to as ‘synthetic testing’ and over the years was adopted by all major high-power test laboratories.

The challenge, of course, in applying synthetic testing is the seamless transition from the arcing stresses before the exact moment of current interruption to the voltage stress that immediately follows. In addition, synthetic testing, being traditionally single phase, has to be cleverly enhanced to better simulate three-phase conditions.

The limits of classical synthetic testing, however, quickly became apparent once China commissioned its own 1100 kV AC grid in early 2009. For this extreme voltage, circuit breakers have to endure multi-frequency voltage transients up to 2000 kV, appearing right after short-circuit interruption. Since UHV breakers consist of at least two interrupting chambers in series, one might think that one feasible solution might be to test only one chamber with half or its proportionate share of the voltage.

Unfortunately, while dramatically reducing necessary test power, such a solution is not acceptable technically when metal-enclosed dead tank or GIS switchgear are involved. Full

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Keeping the breaker literally on ground in all tests implies shorter test duration, higher mechanical stability and no problems with breaker secondary control.

For our part, we at KEMA have worked toward what we regard as a more ‘down-to-earth’ philosophy by literally keeping the breaker on ground in all tests. This implies shorter test duration, higher mechanical stability and no problems with breaker secondary control. For this solution to work, however, a supply of ultra high voltage is required from a two-stage synthetic voltage injection installation. By properly timing the triggering of various voltage sources, the necessary transient recovery voltage can be synthesized to its standard wave shape in a step-by-step manner – all the way to its maximum.

This concept was demonstrated initially using a temporary installation, erected in the test bay. In 2006, for example, fullpole 800 kV circuit breaker testing was demonstrated and then, two years later, the first UHV breaker was tested for 1100/1200 kV rated voltage.

Over the past months, this temporary synthetic installation was replaced by a permanent facility located in its own building and dedicated to testing breakers rated 800 kV and higher. A major spin-off of this latest laboratory expansion is the possibility to perform full three-phase tests on threephase metal enclosed switchgear, e.g. with rated voltage of 145 kV. Up to now, there had been no way to perform such tests adequately under effectively earthed system conditions.

Looking at these developments, synthetic testing certainly seems to have at last freed itself from its earlier single-phase constraints and can now climb to unprecedented heights.

Professor Rene Smeets Rene.Smeets@dnvkema.com

Editorial

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HCR Offers Range of Manufacturing Options Although ceramic insulators made from porcelain and glass have a long-established position in the T&D market, a variety of composite, elastomeric and plastic materials are now increasingly being used to replace them. Still, in spite of the relatively large variety of nonceramic materials, utilities that select such insulators mainly request silicone as the material best able to meet their service needs.

Silicone Technology

REVIEW

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Silicone polymer is highly non-conductive with demonstrated volume resistivity of at least 1x1014 ohm cm and the electrical breakdown strength of silicone fluid even makes it suitable for transformer applications. Those formulations of silicone used to make insulators all start with the base silicone polymer whose consistency can vary from the low viscosity of water to a barely flowable material, as in silicone gum.

These silicone gum based elastomers are highly viscous because they are reinforced with pyrogenic silica, which results in a solid material prior to curing. Such formulations are better known as high consistency rubber (HCR) and tend to be stronger mechanically than elastomers that start off as pourable liquids. Even though HCR is solid when uncured, it can still be easily processed into insulators using a range of manufacturing techniques. These include simple compression molding, automated injection molding and different types of extrusion processes.

HV insulators must be able to withstand electrical discharges and this is achieved using additives or fillers that help the cured elastomer resist damage due to the heat and burning of arcing. Such additives increase viscosity to varying degrees depending on surface area of the filler and complete dispersion of the additive is critical to good discharge performance.

Forming rubber parts is as simple as filling a mold to create a shape and then applying heat, as in the case of compression molding of insulator sheds. Due to HCR’s high viscosity, presses are needed to push the two mold sections together.

The high pressure forces the uncured rubber to flow and completely fill the form. Pre-form cutting and weighing may be manual, but compression molding is still capable of producing many identical parts both quickly and efficiently.

Injection molding is the logical extension of compression molding. A press is also needed, but the molds are engineered to allow the uncured rubber to be pushed into the closed mold cavities through openings. To facilitate this process, the press must have additional moving parts to convey the uncured rubber into the injection area and this is done in two basic ways: one is to use a cylindrical stuffer box with a ram to press the HCR material into a screw which then fills the injection ram in a controlled fashion; the other is to feed the screw with a strip of uncured rubber. In either case, the injection ram is set up to push the same precise amount of rubber into the mold during every ‘shot’. The speed and efficiency of this process is more a function of mold engineering than material parameters since the HCR can be compounded to meet any processing requirements.

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Injection and compression molding can also be ‘blended’ allowing manufacturers to mold directly onto an insulator’s structural elements. For example, producers commonly mold directly onto the fiberglass rod or even onto end fittings. In these cases, silicone primer is needed to enhance bonding.

HCR silicone can be formulated to bond without primer, but then sticking to the mold may become a concern and formulations for insulators are typically designed rather to release easily from the steel or metal alloy tools to reduce cycle time.

The options for curing (i.e. cross-linking) the silicone are diverse. For example, since there is usually high pressure and exclusion of air, addition of dialkyl peroxide is a common and efficient curing methodology. Platinum catalyzed addition curing is another and more elegant choice. If temperatures are carefully controlled, it’s even possible to injection mold with diacyl peroxides.

Extrusion is a continuous manufacturing process in which silicone rubber is squeezed through a die to create an extruded shape and then vulcanized. A tube shape extruded onto an FRP rod is the basis for continuous production of rubber sheathed rods, usually cured in seconds at around 500°C. For this type of fast curing without pressure, the cure agent is usually a form of diacyl peroxide (e.g. benzoyl peroxide). Since such cure systems result in unwanted by-products that must be burned off by post curing, a more elegant solution is platinum catalyzed addition cure, which can be compounded either as a milled and mixed two part product or as a ready-to-use product with no mixing required.

One application for addition cure extruded solid silicone is production of spiral wrapped hollow core insulators. Hollow tubes tend to deflect or change shape under the pressures used to injection mold HCR and this is to be avoided since it can negatively impact symmetry of design and reduce performance. An alternative solution is therefore required.

In the spiral wrap process, a unique extruded silicone profile is wound onto a fiberglass tube resulting in a hollow core insulator with sheds.

Platinum catalyzed addition and diacyl peroxide curing systems can both be cross-linked using an oven without pressure or in an autoclave with pressure and heat. As stated above, platinum catalyzed addition cure systems do not result in unwanted peroxide by-products, which can have a distinct odor and appear powdery on the surface of the cured part.

Another advantage of platinum catalyzed addition curing is that there is less risk of oxygen inhibition. This can manifest itself as a soft and tacky outer surface on the cured elastomer when curing takes place without pressure.

Polymeric insulators are typical of a variety of electrical components that are enhanced by using silicone elastomers such as HCR. User benefits lie in the high service performance of the finished silicone component, while, for manufacturers, the main advantages are the range of available processing options.

Toby Vick Toby.Vick@wacker.com

Editorial

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Guide for Handling Composite Insulators: Update In 2001, growing evidence of service failures of composite insulators due to mishandling caused CIGRE Working Group 22.03 to issue Technical Brochure 184 titled Composite Insulator Handling Guide. Pioneering utilities as well as manufacturers contributed to the document, which aimed to educate users about the risks of inappropriate handling during transport, storage and installation.

REPORTING FROM CIGRE

Although composite insulators are often still thought of as ‘indestructible’ (at least compared to ceramic equivalents), experience has shown that this is only partly true. In spite of their flexibility and non-brittle nature, improper handling can lead to modes of failure that are quite different from those of other insulator technologies. For example, cases of damage to an insulator’s sealing system or to its FRP core rod have been reported in tension applications due to torsional overloading while stringing conductor (e.g. see Fig. 1).

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While the existing CIGRE handling guide focuses mainly on mechanical damage and corresponding premature failures, growing experience has made many utilities customize and extend the content of this document. Indeed, some of the areas added over time should now be considered a necessary update to the original document.

Fig. 1: Composite insulator electrically failed by combined flashover and flashunder caused by torsional rod crack.

1. CORONA/POWER ARC PROTECTION Composite insulators require more attention when it comes to designing adequate protection against corona, i.e. ionization of the air adjacent to high voltage or earth side due to excessive electric field gradients. This is due to their relatively small diameters which, while advantageous in terms of pollution performance, makes their housings and sealing systems more vulnerable to ageing and damage under continuous corona discharge (e.g. see article on p. 40). In the event that corona or combined corona/ power arc protection devices are deemed necessary for an insulator string, detailed instructions should also be available from the supplier to guide those who carry out installation on the line.

Fig. 2: Example of damage to seal and degraded shank surface of 245 kV tension insulator after 1 year service without appropriate corona protection.

2. MOULD GROWTH In a service environment characterized by elevated humidity and temperature as well as the presence of various species of mould, surfaces of both ceramic and composite insulators can become colonized. The effect on inert glass or porcelain surfaces is relatively minor because these materials are already hydrophilic. Mould growth can therefore only serve to increase thickness of the pollution layer. In the case of composite insulators with hydrophobic surfaces, however, this valued performance-enhancing characteristic can temporarily be diminished or even lost.

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Fortunately, experience has shown that in only relatively few cases is the mould growth such that removal is required. The decision whether or not to remove is then usually taken only as a preventive measure once heavy colonization has been identified. Positive field experience in this regard is confirmed by laboratory investigations that have shown that only white rot fungus Phanerochaete chrysosporium can actually degrade the main components of the silicone rubber polymer.

It has recently been reported that mould growth occurred during long-term storage of composite insulators in crates exposed to rain and warm temperatures. These crates are typically made of wood and import into many countries requires pre-treatment (i.e. ISPM 15). Research, however, has demonstrated that such pre-treatment has no influence on potential for subsequent mould growth and practical experience suggests that the design of wood crates calls for a compromise: on the one hand, holes or slots are needed to ventilate the internal space; on the other hand, their size should be small enough to prevent ingress of pollution or entry by animals.

Depending on climate, wooden crates can quickly deteriorate when left fully exposed to the environment. If the insulators inside the crates are unprotected or if the protection was removed from earlier use, water with organic content from the wood can come into contact with the insulator surface and lead to colonization by fungus. A wide variety of species are available worldwide to create such ‘biogenic’ pollution.

From the user perspective, there are two possible scenarios: 1. insulators can be covered by fungal species such as Aspergillus niger and Cladiosporium which are superficial and can easily be removed, with the previously colonized areas again becoming fully hydrophobic, or; 2. combined colonization by mould species Chaetomium and Cladosporium, which have a symbiotic interaction with Methylobacteria, may create a pink appearance on insulator surfaces. While the mould is easily removed, the colour diffuses into and remains in the silicone rubber bulk material. Surfaces with such pinkish residue return to being hydrophobic after cleaning and therefore regain their original performance, but lose their aesthetic appeal.

Fig. 3: Colonization by Aspergillus niger and Cladiosporium is easily removed.

Fig. 4: Colonization by Chaetomium, Cladosporium and Methylobacteria leaves pinkish residue.

After more than 11 additional years of service experience with composite insulators in different applications and under a range of environmental conditions, updating the original Insulator Handling Guide is now under consideration. This may therefore become the responsibility of a new CIGRE Working Group, which will be the successor to B2.21 (Insulators). Readers are of course always invited to contact me to propose further aspects that should be included in an updated version of this handling guide.

Dr. Frank Schmuck frank.schmuck@sefag.ch

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Selecting Insulators Using Satellites This past April marked the sad end of a fascinating program that had monitored the earth’s environment. The European Space Agency’s Envisat satellite, which had operated for double its planned 5-year life, suddenly stopped transmitting. After trying to reboot it for about a month, the operators declared the mission over. It was a bittersweet moment since, with 10 sophisticated sensors, Envisat had already delivered over a thousand terabytes of data that found their way into many different scientific publications.

Transient Thoughts

Fortunately, as one satellite image faded from view, another was picked up. On May 8th, the first NASA Ozone Mapping and Profiler Suite (OMPS) posted on-line images of sulphur dioxide levels near the Nyiragongo volcano in the Congo. It was interesting to discover that NASA had in fact been maintaining such an on-line archive of SO2 observations going back to 1979.

Indeed, this July 23 will mark the 40th anniversary of the first US Geological Survey with satellite images from Landsat-1.

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Among numerous other benefits, satellites provide an independent surveillance capability that monitors long-term trends in pollution exposure of insulators. This allows making better use of IEC guidelines when it comes to specifying leakage distance, whether for polymeric, ceramic disc or long-rod type insulators. In this regard, INMR readers might be interested to follow recent progress in validating satellitebased pollution measurements.

The IEC 60815 Guide calls for establishing the non-soluble deposit density (NSDD) as well as the equivalent salt deposit density (ESDD) in the relevant service area. Traditionally, this has been accomplished by collecting accumulated pollution, wiping it from the test insulators, analyzing its chemistry and filtering and weighing inert deposits. CIGRE Technical Brochure 333 (Oct 2007) has promoted the additional benefit of energizing these insulators so as to also monitor leakage current.

However, fixed-site monitoring such as this comes with some fundamental limitations. For example, while it can establish a representative assessment of site severity at a small substation, it cannot capture rapid changes in pollution that may occur across a large station or a line running close to a road or other pollution source.

Monitoring leakage current on different energized insulators (either within a substation or along a line) seems appealing, especially since Smart Grid initiatives reduce the cost of collecting and processing such data. However, in the case of new designs, remote monitoring of pollution levels will probably provide data of sufficient quality and with lower cost.

From the viewpoint of remote monitoring, I was intrigued by a study that appeared in the journal Science in 2005 and whose goal was to develop a global dust accumulation map.

The authors compared three studies that correlated satellite optical depth estimates for dust deposition with locally recorded concentrations of iron. They then constructed a map of average annual dust deposit density (in g/m2/year).

This map, resembling the inverse of a satellite map of global lightning flash density, shows areas of 20 g/m2/year deposit rate in northern Africa, the Middle East and northern China.

The satellite estimates of dust deposit rate show the Persian Gulf region with a deposit rate of 10-20 g/m2/year.

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Annual Dust Deposit Rate (in g/m2/year) from Satellite Optical Depth.

Jickells et al., Science, 308, 67-71 (2005).

Annual Lightning Flash Rate, (CC+CG)/km2/year, from Satellite Optical Transient Density. Use CC=2 CG to relate Cloud-to-Cloud and Cloud-toGround activity. NASA OTD/LIS

Some INMR readers may recall an article by Claude de Tourreil that described state-of-the-art site severity assessment in the Persian Gulf. (This article can be conveniently accessed from www.inmr.com with the article archive search term < NSDD >).

The minimum NSDD values recorded by Iran’s Niroo Research Institute (NRI) exceeded 0.2 mg/cm2 and were usually high enough to increase the IEC 60815 site pollution severity assessment by one category – from ‘Heavy’ to ‘Very Heavy’.

Recently, I had the chance to correspond with Dr. Evgeni Volpov of the Israel Electric Company. His assessment of the Jickells et al. dust deposition map was that, although constructed using relatively large grids and only a few coarse levels, it still offers useful estimates and trends. He went on to note that an NSDD/ESDD ratio of 10 seems quite reasonable for the Middle East. This trend was also evident in the results of the NRI work in Hormozgan State as shown in Claude’s article.

There has been significant progress over the years in improving the resolution of satellite measurements of pollution, particularly in regard to mapping NOx, SO2 and particulate matter. Users as well as manufacturers of insulators for outdoor applications should definitely look into the possibilities that accompany such sources of additional satellite-generated data. These sources can help developing better designs and, more important, also ensure insulator specifications are always optimized for actual pollution conditions. As a step toward this, please refer to the article on p. 94.

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

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Shed Profiles for Composite Insulators Finding the ideal profiles for porcelain and glass suspension insulators has proven to be a relatively complex task, with a succession of different designs introduced over the past 100 years – all aimed at achieving optimal pollution flashover as well as self-cleaning performance. Nor has this process stopped with research still ongoing into new improved designs.

Insulator number As a result, today there is a relatively broad range of different profiles of ceramic insulators in service on transmission Figure 2: Comparison of flashover voltage of 36 types of networks worldwide. Whatever preferences have emerged composite insulators with same insulation height. from one region to the next have typically been based on climatic variables as well as local topography and even level of economic development. In the case of China, for example, As part of this same research, a total of 19 test specimens of double or triple-shed aerodynamic profiles are preferred by alternating shed design with different shed spacing and radii were compared in terms of relative pollution flashover most domestic power companies. performance. As can be seen in Fig. 3, all 5 designs with By contrast, the main factors affecting shed profiles in the different radii have superior flashover performance when the case of composite insulators have traditionally been related shed spacing is 100 mm. Moreover, it’s also obvious that it is to production constraints. Depending on manufacturing not necessarily true that the bigger the diameter the higher will technology, most sheds are smooth and cannot be made too be the flashover voltage. Rather, an optimal value exists and complicated in order that they can be easily removed from the flashover voltage was found to be highest with large shed/small mold cavity. What variations exist tend to be mostly in shed size, shed radii of 90 mm and 66 mm. spacing and angle of inclination.

SCENE FROM CHINA

As of now, there has still not been that much systematic research on how shed profile impacts pollution flashover performance and the level of knowledge on the subject can perhaps best be described as ‘fuzzy’. Some believe that specific creepage distance plays the decisive role while others disagree.

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In the case of China, close co-operation between Tsinghua University and the China Southern Power Grid, along with active support from local manufacturers, has resulted in 36 different shed profiles for composite insulators. Basically, all of these have been classified into one of four possible categories, depending on the number of different shed diameter sizes within one repeating shed unit, as follows: one large, one small; one large, two small; one large, one medium and two small; and one large, one medium and four small. Figure 1 depicts the first of these categories, namely alternating large and small sheds.

Figure 3: Impact of spacing of large sheds on flashover voltage.

Figure 4 shows the impact of average radius of large and small shed diameters on pollution flashover behavior under 3 different types of spacing of the large sheds. It can be seen that flashover voltage is highest at a spacing of 80 mm.

Shed Unit

Insulation Distance (a) One big one small

Figure 1: Schematic of alternating one large, one small shed composite insulator.

Figure 2 then shows the DC pollution flashover voltage obtained for these 36 different possible shed design categories using the solid pollution layer method and with test voltage applied by the constant ‘up and down’ method. The values shown represent a proportionate per unit comparison of flashover voltage of each different design versus that of the best-performing geometry, namely alternating one large and one small shed (#22). These findings clearly indicate that shed geometry does indeed have a profound impact on pollution flashover performance of composite insulators, i.e. by as much as 22% in the case of units with an identical insulation distance.

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Shed spacing S1 (mm)

Average shed radius Pav (mm)

Figure 4: Impact of average diameter of big and small sheds on flashover voltage.

Of course, other factors besides pollution performance also have to taken into account in deciding on the best geometry for composite insulators and these include protection against flashover due to bird streamers or ice bridging. In this regard, there is clearly a need for yet additional research.

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

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Insulator End of Life: Recycling vs. Disposal The focus when it comes to electrical products and components has traditionally been on quality and performance in service. Increasing public awareness of environmental issues, however, has now led both manufacturers and users to pay much greater attention to the overall ecological impact of these types of products – from extraction of their raw materials to what happens to them at their end-of-life. Methodologies such as Life Cycle Assessment (LCA), which evaluate the full environmental impact of any product from ‘cradle to grave’, are therefore becoming ever more important to this industry.

Indeed, a clause similar to the one below appears more and more these days in technical specifications for insulators and other apparatus, adding an environmental dimension to past requirements that talked only about manufacturing, testing and delivery:

P I G I N I Commentary

“Suppliers are required to comment on the environmental soundness of the design and on the materials used in the manufacture of the items offered. In particular, comments should address such issues as recyclability and disposability at the end of service life.”

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Support toward the development of this area will come from IEC TC 111, which is dealing with Environmental Standardization for Electrical and Electronic Products and Systems and working to produce relevant Standards, including: • IEC Standard 62430:2009, Environmentally Conscious Design for Electrical and Electronic Products, specifying requirements and procedures to integrate environmental aspects into the design and development processes for electrical products; • IEC Standard 62474, Material Declaration for Products for the Electro Technical Industry, moving toward a standardized declaration about the environmental compliance of all materials that make up the final product;

From the viewpoint of material utilization, it is usually preferable to achieve the highest level of recycling possible, e.g. secondary better than quaternary recycling. However, from an economic and resource utilization perspective, this need not always be the optimal scenario. Secondary recycling might require excessive amounts of energy and other resources (e.g. facilities, manpower, additives, additional energy, etc.) whereas quaternary recycling may be simple and not require energy or any additional resources.

Primary, secondary and tertiary recycling processes all involve collecting materials, identifying them, reclaiming the remnants and then marketing them. For this process to be justifiable economically, the value of the reclaimed materials must of course be higher than the cost needed to recycle them.

Let’s examine the typical current recycling scenario when it comes to insulators: Secondary recycling of porcelain insulators (e.g. as fillers for concrete or roads after being treated in hammer-type grinders) is possible, but not especially advantageous. As a result, most ceramic material is still disposed of in landfills with only their metal parts recycled. In the case of glass insulators, secondary recycling is more viable.

• IEC Technical Report 62635, Guidelines for End of Life Information Provision from Manufacturers and Recyclers, and for Recyclability Rate Calculation of Electrical and Electronic Equipment, intending to provide information to recyclers so as to enable appropriate and optimized end-of-life treatment. This report also evaluates recyclability and recoverability rates based on product attributes that reflect real end-of-life practices.

One of the aspects that most affects the environmental impact of products such as insulators and related apparatus is the strategy usually adopted at their end-of-life, i.e. disposal versus recycling. Each year, power transmission and distribution utilities generate significant quantities of waste such as epoxy, silicone and porcelain materials, especially during any projects to renew ageing infrastructure. At the moment, this kind of waste frequently ends up only in landfills.

But now, waste management directives in places such as Europe have encouraged recovery and recycling through more restrictive use of landfills. The European Parliament, for example, has issued directives on waste electrical and electronic equipment (WEEE) stating that all such equipment must be recycled and effectively banning the disposal of such waste in landfills. Greater restrictions on electrical waste, now and in the future, mean that producers as well as users of high voltage apparatus will increasingly have to identify alternative waste-management methodologies. Similar tendencies are evident in other areas of the world as well.

Basically, there are four general classes of recycling techniques: • Primary recycling, i.e. conversion of waste into materials having properties equivalent to those of the original materials; • Secondary recycling, i.e. conversion of waste into materials having properties inferior to those of the original materials; • Tertiary recycling, i.e. conversion of waste into chemicals or fuels, and • Quaternary recycling, i.e. conversion of waste into energy

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Most techniques when it comes to primary or secondary recycling involve mixing some waste with virgin raw material, which is then processed as if it were all virgin. Designation as primary or secondary recycling is then really a matter of how successful this process is. Tertiary recycling refers to chemical de-composition of the material (e.g. de-polymerization into chemicals and fuel) while quaternary recycling is synonymous with incineration and utilization of the energy released.

Composite insulators cannot easily be dismantled and re-processed for use in new products. Cost and environmentally efficient approaches for primary, secondary and tertiary recycling are therefore not yet feasible and only the metal fittings can effectively be recycled. As a result, apart from metal recovery, quaternary recycling (i.e. incineration with recovery of energy) is presently the most effective way to treat composite insulators at their end-of-life.

At the same time, it should be noted that using polymeric waste as fuel for generating power or heat benefits from the fact that these materials have a high energy content to release.

Due to the massive quantities of insulators, apparatus and other line components that will need to be replaced in the near term, there will clearly be a need to find clever recycling or re-utilization options.

Achieving this goal will also contribute to greater sustainability of power transmission and distribution systems as a whole.

Alberto Pigini pigini@ieee.org

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Surface Charge & DC Flashover Performance of Polymeric Insulators

From the Research View

My last column explained that testing DC withstand voltage of polymeric composite insulators must be performed with great care due to the possibility of long lasting electric charges accumulating on their surfaces. Indeed, the impact of surface charges has now become a primary concern since investigations of full-scale polymeric insulators, prototypes and material samples revealed that flashover characteristics can be affected by charges residing at the gas-solid interfaces.

Clearly, this phenomenon will need to be better understood and considered when designing and testing HVDC insulation systems, especially for UHV.

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Surface charging of polymeric materials has already been studied for decades with regard to various industrial applications. In the case of HV outdoor insulation, it had not been considered a serious issue in the past because of the relatively high conductivities of the non-organic, ceramic insulating materials most often used. For these materials, deposited charges decay quickly and do not alter electric field applied to the insulators. However, with the progressive introduction of polymeric materials to HVDC insulation systems, phenomena associated with charging such insulators have begun to receive growing attention as the times such charges reside on surfaces are much longer.

Experiments have been performed to better understand the phenomenon and these showed that electric charges can be deposited on polymeric surfaces in different ways, e.g. activated by corona discharges in the surrounding air. Those experiments involving corona discharges were therefore able to simulate real operating conditions and, at the same time, allow the charge deposition process to be well controlled.

The problem was that most of these investigations on coronacharged polymeric surfaces were conducted on flat samples with thicknesses ranging from tens to several hundred of micrometers and placed on a grounded metallic electrode.

Few studies involved thick samples (i.e. several mm), which are more relevant to the case of HV insulation.

Given this, students at Chalmers University of Technology recently attempted to analyze surface charge behavior on materials used for outdoor insulation with the goal of studying the influence of charging on DC and impulse flashover voltage. Experiments were performed on cylindrical models of polymeric insulators, consisting of a glass fiber reinforced

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epoxy core (108 mm length x 30 mm diameter) covered by a 4 mm thick layer of silicone rubber and held between two spherical metallic electrodes. Since 3 mm long portions at both ends of the insulator were inserted into grooves in the electrodes, effective surface length was 102 mm. When charging such models using a concentric corona source (i.e. a needle belt placed around it), the resulting charge distribution profiles tended to be bell or saddle-shaped, depending on charging voltage level.

Deposition rate and dynamics of electric charges on insulator surfaces are conditioned essentially by their electric field, which, in the case of a HV composite insulator, typically contains a dominant tangential field component at the surface.

Therefore, if an external voltage is applied to the insulator, the resulting electric field (DC or impulse) is either strengthened or weakened by the charge field.

Experiments were performed to measure the negative DC flashover voltage under different charging conditions, achieved by varying the intensity of the corona source as well as its polarity. Moreover, a model based on streamer discharge criteria was developed and employed to calculate flashover voltage while accounting for surface charge density profiles registered for the specific mode of charging being used.

Both measured and calculated negative DC flashover voltages are shown in the accompanying chart, where results correspond to six corona charging voltage levels with indicated experimental error bars. The dashed and dotted lines represent the calculated flashover voltages for positive and negative corona charging respectively. The amount of charges on the surface (X-axis) refers to the quantity of homogeneous charges deposited on the insulator surface.

As can be observed, statistical variations in flashover voltage are high in the case of uncharged surfaces, with values falling between 80 kV and 100 kV (87.5 kV being the average).

Charging of the surfaces, by contrast, resulted in a much narrower statistical spread.

It should be noted that charging with positive corona reduced flashover voltage levels while negative charging led to an increase. For each charging condition, calculated flashover voltage levels fell within the corresponding statistical variation of the measured value. As such, predictions from the model seem to agree with experimental findings, especially in the case of negative charging. One might also note that both experimental and theoretical flashover voltage levels vary linearly with the amount of charges deposited on the insulator surface. Now seems the time to confirm these findings on a larger scale.

Readers interested to learn more about this research should refer to publications by Sarath Kumara, Yuriy Serdyuk and Stanislaw Gubanski in the recent issue of IEEE Transactions on Dielectrics and Electrical Insulation (Vol. 10, No. 3, 2012).

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

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Lessons Beyond University This summer saw the retirement of Michael Comber, one of the leading figures in the surge protection industry for almost four decades. Michael managed the arrester development team at Hubbell Power Systems over the past 20+ years and before that he led the R&D team at GE for more than 15 years. For the last several years, he has headed the IEEE SPD Working Group 3.3.11 toward the improved, soon-to-be-published C62.11 Test Standard for MOV Arresters. He has also been Secretary of IEC TC37 for the past 10 years. Fortunately, Michael indicates that he plans to remain active in the IEC standards world.

Woodworth on Arresters

Congratulations to Michael! I hope the next chapters in his life will be as fruitful as all that’s gone before.

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The departure of industry ‘icons’ such as Michael brings to mind a lingering and not yet well addressed issue, namely how to fill the voids created when people of this caliber leave the scene. Indeed, depending on country, one finds a looming shortage of highly experienced professionals in the field of power engineering. There’s therefore a pressing need for young engineers, recently out of university, to develop their skills to levels that can allow them to fill the shoes of industry giants who’ve gone before.

In this regard, I’d like to offer some suggestions to those of you who hope one day to rise and become the ‘Michaels’ of tomorrow: Get an Assignment in a Laboratory To my mind, there’s no more beneficial learning experience than testing components such as arresters in the laboratory. Here is where the true meaning of Ohm’s Law and the real effect of inductance can be directly observed and where it quickly becomes evident that no test always goes right the first time.

Develop Presentation Skills Being an expert will have little impact in the industry without the skills needed to effectively transmit knowledge to others.

Power engineering conferences such as CIGRE, IEEE, etc. provide the perfect forum for experts to disseminate knowledge and I find it fascinating to observe that some do a wonderful job at such events while others miss the opportunity entirely due to poor communication skills. Today, there’s a growing confluence of engineering and business. Technical specialists must therefore not only be expert in their field but also understand and participate in how this expertise will translate into commercial success in the marketplace.

Don’t Kill an Idea Until It Kills Itself Thomas Edison once famously said, “I haven’t failed – I just discovered 10,000 ways that won’t work.” Nothing blocks innovation faster than those who can’t see beyond the possible and think only of why it may not work. Just because something hasn’t been done before doesn’t mean it shouldn’t be considered.

Finally, I say to all those already in the industry – mentor any bright young talent whenever you see it. Perhaps, before it’s too late, together we can succeed to identify and nurture the next generation of skilled engineers to fill the growing voids in power engineering expertise. If not, just Google the word ‘consultant’.

Jonathan Woodworth Jonathan.Woodworth@ArresterWorks.com

Find a Mentor There’s a Chinese proverb: A single conversation with a wise man is better than ten years of study. How true – and especially in our industry.

Only so much can be learned from books.

Young engineers should search out and form relationships with experienced ones, even if not in the same discipline. Every successful engineer I’ve met had a mentor they could bring their problems to, whenever needed.

Learn to Use Simulation Software This is where a sample can be blown up – and it doesn’t matter. Where the circuit can be changed a thousand times over without taking a month each time. Where organization of data can be learned quickly, simply because so much is being created.

Generalize Before Specializing Learn the basics first and only then find a niche that few others understand and where it’s possible to specialize. At the same time, be careful to ensure that the area selected shows promise and won’t soon become obsolete. In the process, try to become the ‘go-to-person’ that others

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respect. Take the time to study the niche chosen deeply and with passion.

Who will design the surge protection solutions of tomorrow?

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Trends in Conductor Cross-Section & Connector Design Achieving ever-higher transmission capacities for HV cables systems has made the steady increase in AC and DC cable cross-sections an ongoing element in the industry’s R&D agenda. In 1998, during installation of perhaps the world’s first 400 kV AC polymeric cables under Berlin, the then ‘impressive’ conductor crosssection of 1600 mm² copper was deemed sufficient.

F

CUS ON CABLE ACCESSORIES

Today, by contrast, cross-sections of as much as 2500 mm² for 110 kV up to 500 kV cables are not regarded as anything exceptional. Indeed, maximum cable cross-sections these days typically fall in the range of 3200 mm². That represents quite a change, and in less than 15 years!

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To reduce the influence of the conductor ‘skin effect’ in AC applications, so-called Milliken conductor designs are available and consist of strands that are individually insulated (e.g. see Fig. 1c). Another possibility to counteract this phenomenon is by using an enameled wire conductor, i.e. with an enamel coating on all copper strands. In this case, a special connector or special conditioning of the conductor is necessary.

a)

appears that connector technology will increasingly move in this direction.

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

c)

Fig. 1: Conductor design for different cable types. a) 400 kV DC submarine cable (paper insulated) 1 x 1850 mm² Cu; b) 400 kV AC XLPE cable (Berlin cable) 1 x 1600 mm² Cu; c) 1 x 3200 mm² Cu Milliken conductor with compression connecting sleeve.

Actually, the connector is one of the most important components of a cable joint and can often be the main reason behind any malfunction. For example, in the event of high rate of current and insufficient connector resistance, temperature rise and resulting deterioration of a joint’s insulation becomes possible. As such, stable connector technology is important not only for the reliability of the joint itself but also for that of the entire cable system.

a)

Fig. 2: Connector solutions for HV and MV cables a) HV compression connector for 3200 mm² Cu conductor; b) Design variants of four different MV shear bolt connectors.

At the medium voltage level, the use of mechanical or shear bolt connectors (shown in Fig. 2b) is now becoming more common. Considering the many different conductor sizes, materials and shapes available on the market, the advantages of a bolt type connector compared to a compression connector quickly become evident.

For higher voltages and conductor sizes of more than 1000 mm², the use of classical compression connectors (see Fig. 2 a) remains common. Still, even for such applications and cross-sections up to as high as 2500 mm², new shear bolt connectors are now becoming available. It certainly

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b)

The connector is one of the most important components of a cable joint and can often be the main reason behind any malfunction.

b)

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UTILITY PRACTICE & EXPERIENCE

Canadian Utility Confronts Challenges to Network Components

Photo: INMR ©

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Pollution is one of the most common and often among the most serious problems that affect overhead transmission and distribution networks (e.g. see article on p. 94). Yet challenges to the reliability of overhead lines also come from a range of other sources, not related to pollution. Unsuitable design, improper handling, fluctuating weather, abundant wildlife and vandalism are among a multitude of such risk factors that also impact performance of critical line components.

FortisBC, a gas and electric utility in the western Canadian province of British Columbia, operates in a relatively pristine environment with only isolated pockets of industrial activity and almost no natural contamination.

Yet, like many other electricity suppliers, it must cope with a variety of issues that can reduce network reliability or availability. This article discusses some of these issues as well as how they are being addressed.

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“Since many of our lines are strategic and access to them depends on time of year, we jump at every opportunity to do pre-emptive studies ahead of any problem rather than simply responding to an outage.”

(from left) Leclair, Khalil-Pour and Williams examine samples of degraded insulators and cutouts.

F

ortisBC serves the electrical power needs of some 160,000 domestic and industrial customers in the southern interior of British Columbia. With a history going back to 1897, the utility operates an overhead distribution system of more than 4500 km – mostly at

13 kV – as well as nearly 1400 km of transmission lines, mainly at 63 kV and to a lesser extent 132/138 kV, 161 kV and 230 kV. Gary Williams, Manager of Network Services for the picturesque surroundings of Okanagan Lake – a

hilly, forested region comprising several small cities and towns – explains that because FortisBC is vertically integrated into generation, distribution and transmission, the organization maintains a blend of competences among its Power Line Technicians (PLTs). This, he

230/138/13 kV Lee Substation is starting point of 74 Line where possible corona-induced degradation of insulators has been identified.

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Evidence of corona activity (white spot) on end fitting and degradation to area near seal. notes, allows them to handle a wide range of different emergencies.

“Still,” says Williams, “many of our lines are strategic and access to them depends on time of year. We therefore jump at every opportunity to do pre-emptive studies ahead of any possible problem rather than simply responding to an outage.”

our lines.” Says Leclair, “we assess condition in cycles of every 8 years and then rehabilitate any issues identified from this the following year. If the matter is urgent on our scale from low to very high, we may address it the same year.”

One example of such a developing problem involves degradation of 15-year old composite insulators on Engineering Manager, Mike Leclair, heads the department responsible for a 230 kV single circuit line (74 Line) condition assessment of the network running some 35 km from Lee and this involves periodic inspection Substation in Kelowna northward to the city of Vernon. According to T&D of lines and substation assets.

Lines Engineering Supervisor, Aram “Like many power utilities these Khalil-Pour, climbing assessment days”, he remarks, “our business and removal of samples from this is primarily operations focused and line revealed areas of what appear has little internal R&D resources. minor damage near the end fittings We rely instead on staying active in on some insulators not equipped utility best practice groups in order to be more proactive in identifying with corona rings. problems that might develop on

Corona on end fittings as well as adjoining sheds generally results from localized high electric fields and has the potential to degrade both the sheath and seals of insulators, particularly those made from polymeric materials. Once a seal has been compromised in this way, moisture can penetrate into internal interfaces and trigger failure mechanisms such as brittle fracture.

“We have taken down samples of affected insulators from lines at altitudes below and above 1000 m since corona activity will depend on altitude,” reports Khalil-Pour. “Now, we will send these to a laboratory for testing. While there have not been any outages or operational problems so far, we still feel that the line inspection in this case identified an issue that needs to be addressed.”

Khalil-Pour goes on to mention that the importance attached to resolving this matter is only heightened by reports of a recent failure in the U.S. due to exactly such corona-induced degradation that ultimately resulted in brittle fracture and line drop.

To gather more information, KhalilPour contacted several large insulator manufacturers to canvas industry opinion on the need for corona rings across a range of applications, from 63 kV up to as high as 500 kV. Replies received allowed him to develop a Table that he says will now help guide future insulation design practices on FortisBC lines.

Examples of alternative insulator designs and resulting distribution of electric fields in the absence of grading (corona) rings.

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“But,” says Khalil-Pour, “the immediate questions to resolve

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will be what is the present risk and whether we will need to retrofit corona rings on these polymeric insulators in both vertical and horizontal configurations. Knowing this is important since such work will require scheduling an outage on a strategic line.”

Requirement for Corona Rings on Various Insulator Applications* System Voltage

63 kV

115 kV

160 kV

230 kV

500 kV

Polymeric horizontal post

No

No

No (< 1000 m) Yes (> 1000 m)

Yes

Yes

Polymeric vertical post

No

No

No (< 1000 m) Yes (> 1000 m)

Yes

Yes

Polymeric suspension/dead-end

No

Yes

Yes

Yes

Yes (at both ends)

Glass

No

No

No (< 1000 m) Yes (> 1000 m)

Yes

Yes

Porcelain

No

No

No (< 1000 m) Yes (> 1000 m)

Yes

Yes

*Based on survey of industry opinion.

Williams observes that decisions about what to do with the insulators on 74 Line are typical of other network concerns as well. “We typically develop a capital project whenever we see a possible need such as this,” he points out. “But first there has to be a risk analysis and justification to get approval for the capital spending. Here is where we rely on Engineering to provide a thorough investigation. This way, the failures can be catalogued internally and used whenever needed to provide input for future decision-making.”

Williams also notes that if it is eventually decided to install corona rings, the live ends of insulators to be retrofitted – especially those made from silicone – will have to be examined for evidence of degradation since adding rings will not reverse such problems.

A second problematic transmission line according to Leclair has been 43 Line, where flashovers have been reported to the wooden structure and where condition assessment revealed potentially damaged insulators. As in the case of the composite insulators without corona rings, samples have been removed and will be sent for laboratory testing. Leclair predicts that there will probably be a need to rehabilitate that line later this year.

Yet another example of a network concern at FortisBC, this time affecting distribution lines, relates to failures of fuse cutouts made with porcelain housings. While pollution is rarely a source of problems in the

All photos: INMR ©

72 Line and 74 Line (on steel towers to left in upper photo) feature 230 kV and 138 kV insulators, some of which have no corona rings.

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Photos: Courtesy of FortisBC

Porcelain cutouts in the Okanagan have experienced cracking due to cycles of moisture penetration combined with cold temperatures.

Photos: Courtesy of FortisBC

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Photo: Courtesy of FortisBC

interior of BC, wide temperature swings over the seasons impact how certain line components age.

Summers here are usually hot and dry while winters can be cold, often with substantial precipitation. This type of weather pattern has caused some fuse cutouts to encounter problems, most often due to moisture penetration and subsequent cracking of the porcelain.

Schematic of porcelain cutout design. Example of broken cutout due to moisture ingress and repeated freeze-thaw cycles.

Early in 2011, Khalil-Pour conducted an investigation into the problem and discovered that of approximately 32,700 cutouts installed on the FortisBC network, there were 132 documented cases of failure since 2004, representing a cumulative failure rate of 0.4%. He then correlated this rate against date of manufacture of the failed cutouts and found that the relative age of the affected units did not significantly impact incidence of failure. Units that had been in service only a few years were failing at about the same rate as much older cutouts. His conclusion was that these failures were as much related to poor design and quality control problems during manufacture as to ageing under the local service conditions.

Correlation of failures of cutouts with their date of manufacture revealed that units that had been in service only a few years were failing at about the same rate as much older ones.

According to Khalil-Pour, if the metallic fittings cemented into the cutout housing come into direct contact with the porcelain through improper manufacturing, an inherent weakness is created.

When the cutout is exposed to high temperature, the metal expands and can crack the porcelain. He also notes that another more common mode of failure involves the paint or epoxy sealant used to prevent

Photos: INMR Š

Polymeric-housed cutouts are now preferred for all new installations at FortisBC.

Old cutout design shows impact of UV in degrading fuse tube.

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ABB silicone cutouts. Offering enhanced material properties for reliable outdoor performance.

ABBâ&#x20AC;&#x2122;s silicone cutouts offer superior performance, durability, and insulation properties. Single-piece, injection molded insulators eliminate moisture ingression and thus prevent breakage from freeze/thaw cycles. Hydrophobic properties allow silicone to outperform other materials by increasing tracking resistance. ABBâ&#x20AC;&#x2122;s silicone cutouts also offer the highest creepage distance in the industry, making them a better alternative for high contamination environments. The fuse tubeâ&#x20AC;&#x2122;s protective coating offers additional protection from UV exposure. Utilized for more than 30 years as an outdoor insulation material, silicone is the most used, fastest growing polymeric material for high voltage insulation. www.abb.com/mediumvoltage

ABB Inc. MV Distribution Components Phone: +1 252 827-3212 Fax: +1 252 827-4286

All photos: INMR ©

Bird protectors now standard on pole-top transformer bushings and arresters.

moisture from entering the unit. If this sealant deteriorates, there can be moisture ingress. Subsequent freeze-thaw cycles will then lead to cracks and eventual failure. A further cause of failures, notes Khalil-Pour, is damage occurring due to improper handling or installation, causing chips or creating stresses on the porcelain body.

Yet another problem affecting cutouts on FortisBC distribution lines involves not the housing but rather the fuse tube assembly. “Longterm exposure to UV can cause the fuse barrel to degrade noticeably,” explains Construction Foreman, Tom Harrison. “So now we require a special gel coating containing a UV inhibitor to protect the tube. Unlike cracking of the porcelain, which is a safety concern for our PLTs (linemen), this is more a nuisance but still something we are trying to resolve in our specifications and purchasing.”

To deal with the problems affecting porcelain cutouts, Williams reports that FortisBC began using polymeric cutouts in 2010, shortly after the relevant CSA standard (C310-09 Distribution Class Polymer Cutouts) was issued. “Although other utilities in Canada began to use them sooner”, he says, “we decided to wait for the standards. Now, we have

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approved both EPDM and silicone rubber designs.”

A related issue has involved nesting on structures by local species such as osprey – a raptor type bird that Bird and other wildlife-induced preferentially builds its nest on outages are a concern for most distribution poles because their operators of distribution lines and height is optimal for hunting. Harrison FortisBC is no exception. Indeed, acknowledges that removing nests to the utility has recently adopted another location in such instances a standard requiring insulated is often not successful since birds protectors on all new equipment such typically return to the original site and as transformer bushings and surge re-establish another nest. arresters for pole-top application.

Even the insulated hangers on However, Harrison reports that arresters are covered by polymeric FortisBC has still had notable material to minimize possible shortsuccess in a few high-profile cases circuit by birds. where osprey nests have been moved to special nearby ‘non-line’ poles while the tops of the previously affected structures were covered by an inverted ‘V’ shaped plastic material to prevent re-nesting. A The decision to closed circuit camera was then increasingly specify mounted above the relocated nest to observe life inside with the images composite line transmitted to a web site for public insulators was viewing: (http://www.fortisbc.com/Electricity/ not related to Environment/EnvironmentalInitiatives/ acts of vandalism Pages/Osprey-nest-camera.aspx).

but rather driven mainly by their comparative ease of installation and transport.

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According to Williams, vandalism is regarded as yet another network concern but not especially severe compared to most other issues. Still, a visit to the Hollywood Substation in Kelowna, located along a popular riverside walk outside the city core, reveals several examples of broken

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Special structures for osprey nests (in one case incorporating CCTV camera and modem).

All photos: INMR ©

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Examples of shattered sheds due to vandalism.

Photo: Courtesy of FortisBC

used to arrive packed only in cardboard boxes that collapsed easily if wet. This, he says caused sheds to become distorted, especially on those units at the bottom of the box.

The response to this experience, he says, has been to change purchasing specifications to request shipment in wooden crates, as is more common for transmission class insulators.

New packing requirement for composite distribution insulators.

Based on this experience and to eliminate future risks of damage due to mishandling, Khalil-Pour has prepared and disseminated an internal training manual for use by PLTs (linemen) as well as outside contractors who return insulators from the field.

porcelain sheds on a CT and on switch supports. Harrison observes that even the high fence surrounding the substation was apparently not sufficient to deter rock throwing.

Williams notes that the decision

by FortisBC to increasingly specify composite line insulators in place of the porcelain and glass used in the past was not related to such sporadic acts of vandalism but rather driven mainly by their comparative ease of installation and transport.

As at many other utilities dealing with such a changeover in technology, the process of adopting composite insulators has involved a learning curve when it comes to shipping and handling practices. For example, Khalil-Pour mentions that composite distribution insulators

Sheds on porcelain switch support at Hollywood substation in Kelowna show evidence of leeching of rust from fittings.

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Reflecting on how their utility has responded to this range of challenges to network reliability, Williams and Leclair feel that a program of periodic inspection followed by investigation and shortterm response to any concerns identified is the best strategy. Says Williams, “we see that the key to our staying ahead of any developing problems will be to identify them at as early a stage as possible, followed by investigating the cause and deciding on the correct remedial action.” 

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UTILITY PRACTICE & EXPERIENCE

Tibet

World’s Highest HVDC Line Delivers Power to

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T

he recently completed 750 kV/±400 kV Qinghai-Tibet Intertie certainly deserves to be included among China’s most ambitious network projects. Commissioned late in 2011, it connects with 750 kV AC lines running a distance of about 1500 km between the cities of Xining and Geermu, in central Qinghai Province. From a newly built converter substation in Geermu, it then continues as a ±400 kV DC line to Tibet’s capital, Lhasa, more than 1000 km away.

Apart from these lines, the project also includes two 750 kV substations, a 750 kV switching station and the two ±400 kV converter stations. A 220 kV AC circle grid in Tibet is part of the same scheme which, in total, is said to have involved an investment equivalent to some US$ 2.5 billion.

and also the longest HVDC line ever built at such altitudes – quite literally along the ‘rooftop of the world’. For example, it traverses terrain that averages 4500 meters (nearly 15,000 ft) with the highest point reached at the dramatic 5300 m mountain pass at Tanggula – China’s gateway to Tibet (known in Chinese as Xizang).

Apart from the obvious construction challenges across the vast, rugged Qinghai-Tibet plateau under extreme cold, permafrost, high UV and fragile local ecosystems, altitude impacted factors such as corona as well as electromagnetic field. Designing the most suitable line insulators for this environment and meeting performance requirements of external insulation on HV equipment also proved demanding.

What makes the new ±400 kV line portion of the INMR visits Geermu to report on aspects of this project particularly noteworthy is that it is among unique network project. the world’s highest overhead transmission lines

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Investments made in expanding China’s power grid over the past 10 years have been nothing short of extraordinary. Indeed, they are perhaps best put into context if one considers that, between 2005 and 2010 for example, the country added, on average, the equivalent capacity of the entire power system of the United Kingdom every single year.

Overview of recent UHV projects in China and location of Geermu–Lhasa ±400 kV line.

Prominent among these investments have been ground-breaking UHV projects – both AC and DC – such as the 640 km 1000 kV AC line between the provinces of Shanxi and Hubei (see INMR Q1, 2010) or the rapidly expanding 750 kV AC supergrid in Western China (see INMR Q4, 2010). When it comes to DC, China has also seen an increasing array of ambitious

Courtesy of State Grid Corp.

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All photos: INMR ©

750 kV AC lines from Xining arrive at Geermu Substation.

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Some 300,000 km of new lines with nominal voltage of 110 kV and higher are expected to have been added by the time China’s 12th 5 year plan ends in 2014.

projects such as the 1400 km long ±800 kV line between Chuxiong and Suidong Converter Stations in south China (see INMR Q4, 2009).

And, in spite of a recent economic slowdown, the pace of growth in this sector seems only to be accelerating.

Driving these investments is a commitment by the State Grid Corp. of China (SGCC) to increase electricity transmission as a proportion of total energy delivered.

At present, energy sources in China are still transmitted mainly by road and rail, with as much as half of all railway capacity used just to move coal. Now, the SGCC intends to increase the share for electrical

transmission such that by 2020 the ratio of coal to energy transmission will drop significantly.

Achieving this will require construction of numerous new lines and substations. Indeed, some 300,000 km of lines with nominal voltage of 110 kV and higher are expected to have been added by the time China’s 12th 5 year plan ends in 2014, transmitting a total of nearly 250 TW. Yu Jiajun is Sales Manager at Shenma Electric Power’s office in Beijing and his work focuses on new electrical infrastructure in China. He reports that the State Grid Corp. and the China Southern Power Grid will together invest more than RMB 2 trillion as part of the latest 5 Year Plan to strengthen and optimize the country’s power grid and that much of this will involve EHV and UHV projects. For example, he says that a dozen UHVDC projects (most at ±800 kV but with one at ±1100 kV) are on the drawing boards over the coming years. Among

Suspension and tension towers (bottom and left) on 750 kV lines leaving Xining.

All photos: INMR ©

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Typical 40 m high suspension tower along ±400 kV line to Tibet. Height of towers determined by maximum permitted electro-magnetic field in populated areas.

these are the Hami–Zhengzhou and Xiluodu–Zhejiang ±800 kV DC projects due for completion this year. Shenma is among China’s largest suppliers of hollow and solid core composite insulators for substation equipment applications.

More recently, the company, based in Jiangsu Province near Shanghai, has expanded to also offer overhead line insulators and has already supplied several key EHV and UHV projects.

Shenma was also awarded the contract to supply about 60% of the

composite insulators installed on the new ±400 kV Geermu–Lhasa line.

Professor Liang Xidong of Tsinghua University’s Dept. of Electrical Engineering and now serving as President of Qinghai University points out that the project to connect Tibet with the rest of China is vital to the future economic growth of the region. Power shortages in winter, when river levels are low, are common.

Another advantage of the new Intertie will be to increase the reliability of the Tibetan power grid. For example, a major outage occurred several years ago when Yanghu Hydropower Plant, which supplies a high proportion of

All photos: INMR ©

Double string on highway crossing. Each suspension insulator 8 meters long and offers some 28 m total creepage.

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Tension tower shows comparative arcing distance of porcelain cap & pin strings and silicone long rods.

In China, silicone insulators are typically dimensioned with 75% of the creepage distance normally required for ceramic equivalents.

the province’s total power, had to interrupt operation because of an accident. The new ±400 kV line, notes Liang, will also permit power flow between Qinghai and Tibet to be adjusted to meet the needs of better exploiting the region’s rich natural resources.

Apparently, the original intent was for the line to be designed for ±500 kV. But constraints in terms of external insulation at these altitudes combined with the realization that

power demand in Tibet probably did not require such a system voltage resulted in the decision to scale the project down to ±400 kV. Wu Guangya, an insulator specialist from the High Voltage Research

Institute in the central Chinese city of Wuhan, served as UHV expert to the State Grid Corporation on this project. Wu explains that one of the key considerations when it came to specification of insulators and hardware such as fittings and corona

Key considerations in selection of insulators and related hardware involved the high altitude, perpetually high UV, frequent cold and remoteness of the area.

All photos: INMR ©

Corona rings on single and double ±400 kV strings have to cope with increased potential for corona activity at high altitude.

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Cap and pin porcelain insulators on gantry tower to Geermu are among substation’s major sources of corona noise.

have been able to satisfy the line’s pollution performance criteria.

rings involved the extreme altitude.

Other factors included perpetually high UV and frequent cold in the area through which the line runs.

Wu says that the decision to select silicone composite insulators for all suspension applications on the new line was based on

an assessment of their relative advantages and disadvantages in relation to porcelain or glass cap & pin strings. He remarks that both ceramic technologies have a history of successful application on Chinese power systems and that a variety of possible shed designs might

However, he also notes that regulations in China require that porcelain discs be checked regularly for loss of dielectric. Conducting such inspections annually on every tower across remote regions of Qinghai would represent a real maintenance challenge. “Moreover,” notes Wu, “should the number of zero value insulators in any string become critical due to insufficient inspection, this could quickly lead to line flashover.

All photos: INMR ©

Wall and converter transformer bushings at Geermu Substation all equipped with silicone housings formulated to resist high UV exposure.

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As for glass, Wu observes that higher thunderstorm activity south of the Yangtze River (the demarcation line between north and south China) has made toughened glass a more popular choice than porcelain.

Experience with glass in the north, by contrast, has been less and he says that replacing any discs that self-shatter across the rugged terrain in Qinghai and Tibet would also present a maintenance headache.

Wu also notes that dust and pollution problems in north China mean that double shed discs on insulator strings are preferred, but there is still not sufficient operating experience with such designs in the case of glass.

Some silicone insulators used on 750 kV AC lines in northwest desert regions of China have experienced tearing of large diameter sheds under continuous high wind shear.

Given the decision to specify composite insulators for suspension applications on the new ±400 kV line, Wu points out that there were still two key issues to resolve given the service environment: possible premature ageing under the almost constant high UV and risk of reduced performance under frequent cold temperatures. According to Shenma’s Yu, the issue of UV resistance of silicone is one

Operating experience in China suggests that hydrophobicity of silicone is reduced at ambient temperatures of between -2 and -10°C.

that has been addressed by the Chinese insulator industry through incorporation of fillers that prevent breakdown of the chemical bonds in the silicone polymer by high-energy UV rays. Moreover, he claims that the silicone rubber used for this line is a unique formulation of high quality in respect to tear strength of the shed material, which is greater than 20 kN/m versus the more typical 7 kN/m. This apparently was another issue to resolve since recent experience with high winds along 750 kV AC lines in north-western desert regions has demonstrated that wide sheds can be susceptible to tearing under the impact of constant wind shear.

All photos: INMR ©

Wu addresses the impact of sustained cold on performance of composite insulators by noting that operating experience in China suggests that hydrophobicity of silicone is reduced at ambient temperatures of between -2 and -10°C. “For example,” he says, “we have identified flashover behaviour of such insulators at 220 kV, typically

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±400 kV insulator geometry has three different shed diameters spaced 40 mm apart, with small size sheds on both sides of mid size sheds and also on both sides of large size sheds.

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consecutive large sheds amounts to 160 mm (less than the 180 mm spacing Wu says is normally required in China for UHV applications).

Table: Design Parameters of Composite Insulators for ±400 kV & 750 kV QinZang Intertie Lines* Voltage Level

±400 kV

750 kV

SML

Height mm

kN

Insulation Length

Creepage Distance

mm

mm

160

8000

7490

28030

210

8000

7490

28030

300

8000

7490

28030

210

7150

6840

23530

420

7300

6890

23530

* Source: China EPRI

on rainy mornings when temperature falls into this critical band. Research in Wuhan then confirmed that this was due to a relative loss of hydrophobicity.” Wu goes on to emphasize that a related finding from the same investigation was that water on composite insulators turns to ice not a 0°C but rather at -3°C. Overcoming this potential risk of reduction in hydrophobicity of silicone rubber under persistent cold temperatures, according to Wu, involved developing the most suitable shed geometry for the line’s pollution environment. In the case of the ±400 kV Qinzang Project insulators, this turned out to be three different shed diameters: small, midsize and large, each separated by a distance of 40 mm along the shank

“There has been a lot of controversy whether tension strings on UHV lines should be composite and, if so, what proportion to use for the purpose of testing.”

and in a configuration where there is a small shed on either side of both the mid-size and the large sheds. As such, the distance between any two

Wu explains that there are five main pollution classes in China – ‘A’ being the lowest to ‘E’, the highest. Unified specific creepage distance (USCD) then varies from a typical low of 22.0 mm/kV to as much as 53.7 mm/kV for ‘E’ type service environments. This line, he states, has been classified as passing through a type ‘C’ service environment with normally prescribed USCD requirement of 34.7 mm/kV. Allowing for the fact that the line is DC and that about 88 percent of its total length traverses areas with an altitude exceeding 4000 m, a correction factor of 1.28 was applied, resulting in a final USCD of somewhat more than 44 mm/kV. While suspension insulators on the new ±400 kV Geermu–Lhasa Line are silicone composite type, porcelain cap & pin strings dominate most tension applications. A small number of porcelain long rods were also selected for use on a handful of towers in order to obtain operating experience with how well these perform in terms of self-cleaning in such a service environment.

All photos: INMR ©

Single phase 750/330 kV AC transformers, 330 kV GIS side and spare transformer for converter station (right).

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All photos: INMR ©

Disc insulators on discontinued line near salt lake demonstrate impact of local dust and pollution. Paving stones at Geermu Substation show presence of salt deposits from constant truck activity outside.

According to Wu, the use of ceramic disc strings for tension is typical of what is found on most UHV AC and DC lines in China where composite insulator technology has been dominant – but only in suspension.

This is due mainly to concern about

possible damage to insulators from linemen walking across strings for conductor maintenance or repair.

Says Wu, “there is still a lot of controversy here as to whether tension strings should ever be composite and, if so, what proportion

to use for the purpose of evaluating how well this works. While some experts recommend as much as half of all strings, others suggest only 5%.

My own view is that 20% would be a better target for testing purposes.”

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All exterior porcelain at Geermu coated with RTV silicone. Booster sheds added along 750 kV GIS bushings.

All photos: INMR ©

As part of the final process to confirm design parameters of the new ±400 kV line in terms of tower clearances, on site testing was carried out in the summer of 2011. The findings allowed researchers to compare lightning and impulse switching data obtained from simulations performed before construction at the UHV test laboratory in Beijing (at 54 m) as well as at the new 4300 m high test station in Tibet. This work was carried out by the China Electric Power Research Institute (CEPRI) on a simulated tower structure located near Tanggula Mountain, the highest point along the line (some 5000 m). According to CEPRI, minimum length here would have to be 12 m (with 14 m the longest) versus the more typical 9.75 m for an insulator used in UHVAC. These findings applied to silicone insulators and would be longer in the case of porcelain.

Main applications for composite hollow core insulators at Geermu are converter transformer and wall bushings.

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One of the problems during the construction phase of several UHV projects in China has involved bird attack on the sheds of silicone insulators before the line was energized. This same problem has also been reported in places such as Australia, where local species with powerful beaks have, in extreme cases, been known to chew away many if not most of the sheds. CEPRI researchers report, for example, that about 2000 composite insulators on the 640 km 1000 kV line from Shanxi to Hubei were damaged by birds during the period after installation but before energization. While in most cases the damage was

minor and affected only a few sheds, in some cases, the sheath itself became punctured and the insulators had to be replaced.

Equipment such as arresters and live tank breakers outfitted with leakage current monitoring devices and surge counters.

Apparently, this same problem of bird pecking also occurred during completion of the ±400 kV line to Lhasa, although by most accounts, it was never a major problem. A more serious incident involving birds rather was the line’s first flashover, recorded in January of this year and attributed to short circuit from a bird streamer.

All photos: INMR ©

The origin of the ±400 kV line to Tibet is the Geermu 750/330/±400 kV Converter Station, a huge facility surrounded by vast stretches of open desertlike landscape. The area has comparatively little industrial activity but the site has nevertheless been classified as type ‘C’, meaning moderate to high levels of pollution.

The contamination comes mainly from a nearby salt lake with China’s largest extraction facility for alkalis.

Solid core composite reactor supports as well as silicone housed arresters and CT used for indoor DC field.

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Indeed, the impact of this salt lake is so widespread that much of the windswept ground surrounding Geermu is tinged by a whitish coating of salt. The salt mixes easily with dust and is then kicked up by trucks driving along dirt roads running past the substation. Only months after commissioning, the cut stone pavement at Geermu already shows clear evidence of its presence.

Indeed, the pollution environment at Geermu is so challenging that all porcelain equipment housings and support insulators were coated with RTV silicone before the station was energized. Moreover, because of the high specific creepage distance and relatively narrow shed spacing required for porcelain, booster sheds were installed for sensitive applications, e.g. 750 kV GIS bushings, to protect against bridging during heavy rain or from ice formations during winter.

Another remedial measure against pollution at Geermu Converter Station was the construction of buildings to house the DC fields, making this one of only three such fully enclosed facilities in all of China. According to CEPRI researcher, Wei Jie, the added cost of the enclosures was justified because power arcs in DC are more difficult to extinguish than for AC and, considering the substation’s altitude of 2800 m, the problem would be even greater.

All photos: INMR ©

Given the risks and the high level of pollution, solid core reactor supports as well as arresters and CTs specified for the indoor DC fields all have silicone housings for added security.

This applies as well to the converter transformer bushings outside the valve hall as well as to the huge wall bushings.

High metal walls intended keep noise levels outside valve hall to a minimum (positive pole at background). DC field enclosed in large building to limit pollution exposure. Interior section of silicone wall bushing.

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world’s highEst hVdC linE dEliVErs powEr

Another safety feature at Geermu are high metal walls around the converter transformers, intended to shield substation staff from high noise levels when equipment is operating at or near capacity. This, apparently, is still

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Substation engineer Liu reviews images of discharge activity along arresters during first heavy rainfall.

Photos: INMR ©

not the case, as engineers report that only some 1200 MW of the project’s 2000 MW capacity is currently being delivered to Tibet.

In spite of all the pollution countermeasures, night time video inspection at Geermu recently revealed surface discharge activity near the flange of 330 kV arresters.

This occurred in mid-May, during a period of heavy precipitation, reportedly the first sustained rainfall since the substation was put into operation. Substation staff indicated that they would respond to this by verifying ESDD measurements to assess the current level of pollution affecting the substation. Depending on results, a decision would be made whether any remedial measures, such as washing, might be necessary.

Looking to expectations for the performance of the new line to Tibet, CEPRI’s Wei notes that China’s expanding HVDC and UHVDC networks have provided engineers with a growing body of service experience on which to base future insulation decisions. The country’s first HVDC line was built in the late 1980s using only porcelain string insulators but, although designed

for ±500 kV, operated in areas of extreme climatic conditions at a lower voltage until 1997. The flashover rate on the line to that time was 0.07/100km/yr but then increased to 0.27/100km/year once the line was brought to its full design voltage.

According to Wei, research was carried out into why flashover rate increased and the insulation was subsequently changed to silicone rubber composite type, at which point the rate dropped back to 0.07/100km/yr. Wei also notes that post insulators at the line’s converter stations had originally been designed with 40 mm/kV specific creepage but that this was eventually considered too low for a ±500 kV application.

Over the coming months, RTV silicone and booster sheds were applied while some porcelain posts were gradually exchanged for composite ones made with silicone rubber housings.

Recent experience on UHVDC lines has shown problems of corrosion affecting the metal fittings of porcelain double ‘V’ strings – a situation which so far has occurred predominantly on the 1400 km line running from Yunnan to Guangdong and affecting some 20,000 discs. No ‘I’ strings were involved since the phenomenon seems to be initiated by rain coming from the side of a string in the ‘V’ configuration. Apparently, the problem involved the line’s negative pole and affected only insulator caps that, unlike pins, were not outfitted with zinc anti-corrosion rings.

Ferrous oxide and zinc oxide residues migrated down onto the porcelain surface turning it yellowish in color.

Problem of electrolytic corrosion of caps on UHVDC lines has affected mainly porcelain V-strings but only a few tension strings.

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Fittings on Geermu-Lhasa insulators equipped with fixed zinc ring.

Photos: INMR ©

Wei, who participated in the research into this phenomenon, says “the conductor is at negative polarity, and the cap has a relatively positive polarity. During periods of heavy rain, some electrolytic activity can take place, which results in

corrosion. While it is not necessarily a serious operating problem, Chinese standards still require that affected porcelain discs must be replaced.”

to Shanghai ±800 kV line but apparently involved a much smaller number of discs, only about 2100.

Wei goes on to report that much of this problem of corrosion of caps was geographically focused due to local weather conditions and that no program of replacing affected discs has as yet been undertaken.

He also remarks that other ±800 kV DC lines that traverse the same type of service conditions, such as the Jinping to Jiangsu line, could be expected to encounter a similar

A similar problem, notes Wei, was reported on the Xiangjiaba

Photo courtesy of CEPRI

Porcelain disc insulator outfitted with zinc ring on cap to prevent corrosion from electrolysis.

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world’s highEst hVdC linE dEliVErs powEr

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First tower on Geermu-Lhasa line.

problem so remedial measures were undertaken. “In the case of this line,” says Wei, “zinc rings have been fixed to the bottom of the caps of porcelain discs. Our research has shown that such a solution will prevent possible corrosion.”

Wei explains that this type of problem affecting other DC lines in China is never likely to be a concern in the case of the Geermu-Lhasa line. “To begin with,” he notes, “leakage current on the silicone rubber insulators here is much lower than on the porcelain V-strings used elsewhere. Also, there is much less likelihood of heavy rain along this line’s route. And, in any case, a zinc ring has already been fixed on all the fittings of these silicone insulators.”

“Our focus from the viewpoint of future research in this case,” adds

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Wei, “will be mainly on the risk of premature ageing of the silicone material under continuous high UV.” Part of this research will apparently take place at the high altitude test base in Tibet, where leakage current will be precisely monitored on energized composite suspension insulators in co-operation with a large international engineering company.

The Geermu Substation, while in a difficult service environment of dust and salt, relies mainly on RTV silicone coatings to ensure pollution performance. However, according to Shenma’s Yu, future substations planned in China’s expanding northwest grid are expected to make much greater use of composite insulators. For example, he claims that a 750 kV AC substation in Gansu Province

world’s highEst hVdC linE dEliVErs powEr

will be commissioned before the end of 2013 and that a decision has already been made that the entire station will rely on composite insulation – from busbar supports to bushings to disconnectors to live tank breakers to CTs, and so.

Says Yu, “composite insulation is very suitable for UHV projects, especially under DC, since there is no need for the expense of having to put on a new RTV coating every few years. So, even if composite insulators are more expensive to buy than porcelain, over the lifetime of the equipment they will turn out to be much cheaper.” Referring to the planned complete dominance of composite insulation at the 750 kV Xiazhou Substation, Yu feels it will represent a milestone for the industry, not only in China but also all over the world. 

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UTILITY PRACTICE & EXPERIENCE

Norwegian Utility Re-Dimensions Insulation in Voltage Upgrade Project

V-string on 300 kV tower extended by 4 additional glass discs using live working methods.

All photos courtesy of Statnett

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norwEgian Utility rE-dimEnsions insUlation

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The Norwegian power utility, Statnett, operates a network of over 10,000 km of overhead lines from 132 to 420 kV. Recent changes in power flow have made it necessary to increase capacity.

So, like many transmission system operators worldwide, Statnett is now working towards building the next generation grid. Given public opposition to new lines and the difficulty in obtaining additional right of ways, this will be accomplished in large part through a program of upgrading existing lines – apparently one of the largest projects of its kind anywhere.

At present, there are 2,200 km of 420 kV lines in Norway. But by the time the project has been completed in 2030, the length of this voltage system will have grown more than three-fold to 7,500 km. Similarly, there are some 5,500 km of 300 kV lines of which circa 1,500 km have twin conductor bundles.

The plan is to upgrade these to 420 kV, in the process boosting line capacity by about 40% while keeping existing towers and conductors.

Red lines show 420 kV system currently and by the time upgrade is completed in 2030.

Lightning and switching performance of both the insulators and air gaps was also undertaken, both in the laboratory and in the field. The goal was to minimize the required extension In order to optimize insulation of existing insulators and to keep air coordination for the upgraded lines, a clearances to a minimum in order to systematic program of research was ensure the lowest incremental visual carried out into icing and pollution behaviour of different insulators options. impact from the voltage upgrade.

This article, contributed by Sonja Berlijn of Statnett (Norway) as well as Igor Gutman and Jan Lundquist of STRI (Sweden), reviews the tests and programs required for re-dimensioning the voltage upgraded lines, which in many ways proved a greater technical challenge than designing entirely new lines.

Most voltage upgrading projects in recent years have involved maintaining existing towers and conductors with the main change being rather in line insulation.

Either existing insulator strings are made longer, e.g. by adding discs (see Fig, 1) or one insulator configuration is replaced with one of a different type (e.g. as in a ‘V’

string or braced line post being substituted for an existing ‘I’ string).

such an upgrade would not be easily implemented.

For example, in order to comply with the insulation level required for a 420 kV line, 300 kV ‘I’ strings would typically need to be extended from 14 to 18 standard (21 ton) glass discs, having a total string length of between 3.5 and 3.6 m. At the same time, the required basic clearance for this insulation level would have to be 2.8 m, according to the EN 50341 European standard. Considering that clearances to cross-arms and guy wires for typical 300 kV towers in Norway are only 2.4 m, it was obvious from the start that

With this limitation in mind, Statnett engineers began to ask some basic questions: Could insulator strings be made shorter and, if so, by how much? Moreover, if insulator strings could be shortened from the expected circa 3.5 m, how much could internal clearances also be reduced without negatively affecting line performance?

Fig. 1: Portion of typical 300 kV ‘V’ string extended by 4 discs.

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in VoltagE

Relying on ‘smart’ engineering methods based on a probabilistic approach and supported by precise data about insulators and air gaps, it turned out that most towers on Statnett’s 300 kV network did have just enough space to make the upgrade feasible. In the case of other towers, however, depending on wind, icing, line temperature,

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Photo courtesy of Statnett

Fig. 2: Typical 300 kV tower at Statnett.

geography and line angles, the constraints proved too tight and different solutions had to be identified.

Performance of Insulators A. Pollution & Icing Tests

The two main environmental stresses considered when selecting line insulation in Norway are icing and pollution and the resulting traditional insulation solution on Statnett 300 kV lines has involved glass insulator strings. These were preferred because they were deemed the best choice in regard to performance under icing and also in terms of low maintenance requirements combined with long expected service life. Moreover, since it was desired to conduct as much as possible of the upgrade to 420 kV under live working conditions, the most cost-effective strategy was, wherever possible, to simply increase the length of glass strings.

Given the rationale discussed above for seeking to minimize this required extension of existing insulators,

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a series of ice and pollution tests were carried out at STRI in Sweden.

Laboratory test procedures were specially developed to simulate local environmental conditions and these methods were deemed to adequately represent Statnett’s service

Since it was desired to conduct as much as possible of the upgrade to 420 kV under live working conditions, the most cost-effective strategy was to simply increase the length of installed glass strings.

norwEgian Utility rE-dimEnsions insUlation

environment. This ensured that test results would indeed be applicable for use in any statistical-based method for insulator dimensioning (according to IEC 60815-1) – now available in a proprietary, easy-to-use software called Line Performance Estimator (LPE).

Icing tests were performed according to the widely accepted Ice Progressive Stress (IPS) methodology (developed by STRI in collaboration with Statnett and the Swedish grid operator, Svenska Kraftnät) and which has now been included in the IEEE Standard 1783-2009. These tests were performed over a broad range of melting water conductivities, from 100 to 300 µS/cm to establish 50% flashover voltage levels.

Solid Layer Method tests were then performed according to IEC 60507 and modified to comply with the low levels of NSDD measured on insulators removed from service in Norway. Again, 50% flashover voltages were obtained across a broad range of salt deposit densities, from 0.03 to 0.16 mg/cm2.

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Photos courtesy of Statnett

air gaps as well as lightning and switching performance. The key parameters to resolve in this regard were the minimum clearances required not to jeopardize line performance as well as what distance was needed to ensure no flashovers would occur to tower parts such as guy wires.

Fig. 3: Insulator set-up for icing tests (top).

Fig. 4: Close up of insulator string with ice accretion (left).

The laboratory tests were performed on the specific type of glass cap & pin insulator shell currently used on most of Statnett’s 300 kV network.

Pollution and icing tests were also performed on other types of glass shells as well as on composite insulators. This allowed a database to be created of comparative results, which could find use one day in future projects. Recently, similar test results were also obtained for RTV-coated insulators and also simulating wet snow conditions.

As a basis for applying the statistical dimensioning method, results from the pollution and icing test were then expressed in the form of flashover performance curves where the key parameters in the calculation included: U50 voltage level with a 50% flashover probability; l - axial length of the insulator; g - pollution or ice severity; A and a - experimental constants derived from the tests.

Results from this program of testing confirmed that the number of insulator discs in strings on uprated lines could be reduced from 18 to 17 if applying deterministic

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insulation dimensioning. Moreover, if relying on statistical insulation dimensioning, it was found that it could even prove acceptable to use only 16 discs on a limited number of 420 kV towers.

B. Lightning & Switching Performance of Insulators & Air Gaps With the expected pollution and icing performance of insulators on upgraded lines established, Statnett engineers turned their attention to

One of the keys during any voltage upgrade is to find the optimal insulator length and matching air clearance so as to ensure the least number of failures.

norwEgian Utility rE-dimEnsions insUlation

To answer these questions, lightning and switching impulse tests were carried out on a model of a fullscale tower set up at Graz University of Technology in Austria. Actual gap factors – relating flashover voltage of the actual gap to that of a standard rod-plane gap – were established for the different air gaps of suspension towers, e.g. across the string, as well as between phase conductor and tower or guy wire given different insulator swing angles. It turned out that actual gap factors for Statnett suspension towers were greater than typical values presented in the standards and this provided a valuable margin utilized in the upgrade. For a standard ‘I’ string configuration, this meant that clearances required for ‘no wind’ conditions could be reduced by about 10%.

In the case of tension towers, it was important to determine the optimal position of the conductor bundle in relation to cross-arms and guy wires. For safety reasons, it was deemed preferable that the majority of flashovers should take place between conductor and crossarm. Test results confirmed that this was the case for about 80% of all recorded flashovers if the conductor bundle was placed so that the distance from it to the guy wire equaled that from the corona ring to the cross-arm.

Statistical Insulation Coordination

With expected insulator performance under pollution, icing, lightning and switching known and gap factors confirmed, Statnett engineers looked to statistical coordination (in place of standard ‘black or white’ deterministic methods) and used

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All photos courtesy of Statnett

Fig. 5a: Tower window in upright position for testing flashover voltage across insulator string (top right).

Fig. 5b: Tower window tilted for testing flashover voltage between conductor and tower leg or guy wire (top left).

Fig. 5c: Full scale model of tension tower for flashover testing (bottom right).

the LPE program to calculate future • gap factors for lightning and performance of the upgraded 420 kV switching impulse in outer lines. To predict line performance, and center phases and across this program relies on data such as: insulator strings.

• system voltage • annual number of pollution and icing events • pollution and ice severity • ground flash density • soil resistivity • tower footing resistance • geometric data for insulator strings (e.g. position of arcing horn, length of hardware) • tower geometry

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In general, during the course of any voltage upgrade project it becomes necessary to optimize use of available tower top clearances and certain trade-offs may have to be made. In the case of the Statnett upgrade, these can best be explained with the help of Figure 6, which represents a ‘screen dump’ of the LPE program outcome when applied to this project.

norwEgian Utility rE-dimEnsions insUlation

For example, assuming that insulator string length would be shortened by 10 cm versus the optimal selected value (corresponding to -0.1 in the X-axis versus 0.0), the number (or %) of failures due to pollution or icing would increase. At the same time, lightning and switching performance would decrease because of larger available air gaps.

As such, one of the keys during any voltage upgrade is to find the insulator length and matching air clearances that ensure the least

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number of failures and best overall line performance. Moreover, since tower designs in Norway often vary due to changes in local topography, this optimal solution for Statnett had to be tailor-made for every specific type of tower while at the same time considering overall line performance, which of course was the main goal.

In fact, based on experience from projects such as the Statnett upgrade, the functionality of the LPE program was extended to also make use of data imported from the design software for each individual tower type. Such data includes conductor position, insulator length and calculated tower top clearance under different wind loading conditions. This would then allow lightning, switching, power frequency, pollution and icing performance all to be evaluated for every different tower. In this manner, the statistical insulation performance for the complete upgraded line could be optimized.

Indeed, using results from the LPE, Statnett line designers were able to verify calculated lightning, switching, pollution and icing performance results against targets set for the whole line in terms of e.g. acceptable number of failures per 100 km/year. If these targets could not be satisfied, line designers could then identify which specific tower types were responsible for inferior performance and propose appropriate remedial measures.

Figure 7 shows how the result calculated for a particular section of line is presented as a pie chart as well as how performance for five selected towers is depicted.

For enhanced lightning performance in the case of the Statnett upgrade, the primary action would be to replace ‘I’ strings with ‘V’ strings.

If that were not possible, improved tower grounding or installation of line surge arresters would be considered. Line arresters could

Fig. 7: Presentation of calculated performance of uprated line using Line Performance Estimator.

also be an option should switching performance need to be improved.

Similarly, inferior pollution performance could be overcome in a variety of ways, such as by installing ‘V’ strings, composite insulators or by using longer glass strings. Icing performance could be improved by e.g. specifying insulator designs with large shed spacing.

Statnett has recently become involved in one of the world’s largest voltage upgrading projects.

Since the intent was to utilize existing towers and conductors to the maximum, there was only limited margin in terms of extending insulator strings and allowing greater air clearances. Insulation coordination also had to be performed precisely since available clearance margins were very small.

Using statistics-based engineering methods combined with findings from extensive testing on insulators and air gaps, it was deemed feasible to uprate existing 300 kV lines to 420 kV in a cost-effective manner and with relatively small added visual impact – all accomplished using live line working methods.

Insulator string length and internal clearances were kept as small as possible while not sacrificing overall line performance.

Fig. 6: Screen dump of Line Performance Estimator (LPE) program. Fault contribution (in %) for different causes of outages.

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Summary

in VoltagE

To be able to make proper use of such precise engineering methods, several R&D projects had to be performed but, in retrospect, this proved money well invested. 

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UTILITY PRACTICE & EXPERIENCE

Part 2 of 2

Algerian HV Grid Operator Combats Harsh Pollution Part 1 of this article by INMR Contributor and T&D Specialist, Raouf Znaidi of Tunisia, appeared in the Q2, 2012 issue of INMR. It reported on how Algeria’s transmission grid operator, GRTE, deals with a combination of pollution challenges that negatively affect reliability of the overhead network. This second part focuses on one particular region of the country where pollution is especially severe and where findings on comparative insulator performance at a special test station are being applied across the entire network.

To make matters worse, this part of Algeria is home to large populations of migrating storks that usually nest on towers and further contribute to the widespread problem of pollution flashover. Regional Transmission Director, Mokhtar Said, explains that, in spite of frequent live washing, several 60 kV and 220 kV lines – insulated with strings of 7 and 18 fog-type glass cap & pin insulators respectively – routinely experience as many as 20 outages/100 km each year. He goes on to say that the local combination

of pollution sources is so severe that Oran has been chosen as a pilot site for GRTE maintenance staff to assess the relative performance of alternative insulator designs as well as remedial countermeasures.

For example, in 2007, a research study was launched on pollution phenomenon with the aim of identifying which insulators and profiles perform best under the harsh local service conditions.

Another goal was to optimize the costly washing program in terms of

Photos: Courtesy of R. Znaidi and GRTE

When it comes to a potent combination of industrial and marine pollution, few places rival the northwestern city of Oran – one of five maintenance regions managed

by Algerian HV grid operator, GRTE. A vast industrial zone near the city is dominated by a panorama of flaming petrochemical plant chimneys that release unending clouds of black smoke. This smoke combines with salt from the nearby Mediterranean and quickly settles on insulators of nearby 220 kV and 60 kV lines that run parallel to the coast.

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identifying the most appropriate washing cycle.

Said notes that rapid industrial growth in the region around Oran has created a sharp increase in electricity demand and resulted in expansion of Algeria’s Western Transmission Network. Indeed, between 2007 and 2011, the length of regional transmission lines increased by nearly 80% to the point where they now make up nearly a third of the country’s grid.

Tewfik Bentabet, Head of GRTE’s Maintenance Department for Oran region, reports that persistent flashovers in the area are typically triggered by layers of solid pollution that quickly form on insulator surfaces and then become wetted by salt spray or nighttime humidity. These problems, he notes, have led to a progressive shift away from the fog type and instead toward aerodynamic glass disc profiles. There has been an increasing application of composite insulators as well. Says Bentabet, “we are looking to benefit from the self cleaning properties of aerodynamic strings or the superior flashover performance of composite insulators, even with little to no maintenance.”

Pollution Test Station

In order to evaluate how best to deal with high levels of industrial pollution mixed with salt spray and dry salt, GRTE recently embarked on a south-south and south-north study of polluted insulators. This work, undertaken in co-operation with STEG International Services from Tunisia as well as consultants from a French utility, aimed, among other things, to establish a pollution map of Algeria. “The main goal of the study,” explains Bentabet, “was to better understand the pollution environments through which our transmission lines are routed with a view to better selecting and dimensioning insulators for our service conditions.”

Photos: Courtesy of R. Znaidi and GRTE

Growth of Oran’s Transmission Network

Polluted 220 kV lines occupy a narrow corridor.

Table 1: Growth of Oran Regional Transmission Network (2007 to 2011) Km of Lines (2007)

Km of Lines (2011) 60 kV (includes.

Total

2108

1853

4941

848

2398

4346

7592

16,765

3236

9806

10,245

23,288

25.5

26.2

24.5

42.4

32.6

System Voltage

400 kV

Algiers

-

1961

1529

3498

981

Oran

-

2049

2219

4267

Total GRTE

263

9195

6651

Oran % Total

0

22.2

33.4

220 kV

60 kV

Total

400 kV

90 kV, 150 kV)

Table 2: Oran Regional Lines Most Impacted by Pollution* Problematic Lines In Oran Region

System Voltage (kV)

Total Outages 2011

Saida-Mechria /Bougtob

60

59

Ain Skhouna-El Bayadh

60

32

Zahana-Senia

60

23

BHE-ZAH

60

13

Tlemcen-Zahana

220

35

Marsat Hadjadj Station-Relizane

220

25

Ghazaout-Oujda

220

26

Oued Sly-Relizane

220

10

*In spite of frequent live washing and other maintenance, certain 60 kV & 220 kV lines (insulated respectively with 7 & 18 special fog type glass insulators) still experienced over 20 outages/100 km per year.

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pin insulators were third in terms of performance (see Table 3).

Photos: Courtesy of R. Znaidi and GRTE

According to Beloufa, among the insulators tested at Marsat El Hadjaj (i.e. standard profile glass, aerodynamic profile glass, silicone insulators with alternating sheds and EPDM insulators of the same geometry), accumulation of pollution varied greatly by insulator.

Moreover the distribution of pollution deposition was not uniform such that the non-soluble pollution accumulating on lower surfaces of sheds was usually higher than on upper surfaces (e.g. see Fig. 1).

Parallel 220 kV lines insulated with composite and glass string insulators.

Table 3: Ranking of Alternative Insulators at Test Station

Reference Surface

ESDD Rank

NSDD Rank

A: Upper

2

3

nd

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Global Ranking

3rd

3rd

1st

1st

2nd

2nd

rd

A: Lower

rd

3

3rd

B: Upper

1st

2nd

B : Lower

1st

1st

C: Upper

2nd

1st

C: Lower

2nd

2nd

As part of this work, the Marsat El Hadjaj Substation near Oran was selected as site of a pollution test facility with a system for monitoring leakage current on five different types of insulators energized at 17.5 kV phase-to-ground through a single-phase transformer and fuse cutout. The site was also equipped with a full-fledged meteorological station allowing climatic variables such as humidity, temperature, rainfall and wind speed to be measured and then correlated against leakage current data.

Assessment of site pollution severity was based on 3 to 4 ESDD and NSDD measurements conducted per

Self Cleaning Properties Rank K

Chemical Analysis

Analysis of pollutants accumulating on the insulators installed at the Marsat test station as well as on the standard profile glass disc used as reference revealed that the composition of artificial pollution within IEC Standards is not representative of actual service conditions in countries such as Algeria (see Table 4).

year on standard profile glass cap & pin insulators.

Mohamed Belouf of GRTE’s Maintenance Dept. for Oran reports that the first 18 months of findings from the test facility provided valuable information on the comparative performance of different insulators under local conditions. For example, based on ESDD and NSDD measurements, a preliminary ranking of the various insulator designs tested confirmed that aerodynamic glass cap & pin discs had the best ‘order of merit’, followed by silicone composite insulators with alternating shed profile. Standard glass cap &

Photos: Courtesy of R. Znaidi and GRTE

Insulator Type & Profile

In regard to developing a pollution map covering all of Algeria’s service conditions, as many as 36 non-energized test stations were eventually set-up across the five different regions of the GRTE network. Both ESDD and NSDD measurements are performed there each year by a joint research group.

Pilot test facility at Marsat El Hadjaj Substation.

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Bentabet points out that such findings on pollution composition and extent of accumulation have made GRTE question the applicability of guidelines within IEC as regards optimal insulator selection for locations such as Oran. In particular, these findings have led to a process of gradually replacing unsuitable fog as well as special fog type glass discs with aerodynamic designs or composite insulators on all transmission lines that run parallel to the coast as well as pass near industrial areas up to 30 km inland.

Figure 1: Pollution distribution on upper and lower surfaces of standard profile insulators.

Impact of Birds on Transmission Outages Table 4: Analysis of Natural Pollution in Algeria Compared to Artificial Pollution Required in IEC Standards Chemical Element

Natural Pollution in Algeria (%)

Tonoko in %

Kaolin in %

(according to IEC)

(according to IEC)

Si

8.3

61

73

Fe

-

17

1.5

Al

-

14

23

Ca

100.6

-

-

K

13.28

6.5

_

Mg

6.47

0.6

_

Na

32.5

_

_

Ti

-

0.6

1.6

Photos: Courtesy of R. Znaidi and GRTE

Storks regularly nest on towers during their seasonal migration and their conductive streamers (conductivity > 25 mS/cm) accumulate on insulator strings, reducing their effectiveness.

Umbrella type aerodynamic discs used to protect strings from such problems.

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As reported in past issues of INMR, experience in countries such as China has shown that up to 43% of flashovers on lines (even those equipped with composite insulators) can be attributed to bird streamers (often included within the category ‘unexplained causes’). This is the case in Algeria as well since seasonal migration of large stork populations that nest on towers is among the major problems affecting performance of local transmission lines.

“Contact of stork streamers with conductors is a serious concern for maintenance staff in our region,” notes Mrs. Goumari, the country’s first ‘lineswoman’ and who also analyzes outages in Oran. “As a result, we have had to rely on remedial countermeasures, the most effective of which we call the ‘umbrella technique’, i.e. installing a wide aerodynamic glass disc at the top of the suspension string (or mounted on the dead end side). This protects fog type discs from possible short circuit not only from bird droppings but also due to local acid rain.” According to Goumari, the technique also has the benefit of reducing the accumulation of pollutants on lower surfaces of discs in the string.

Another valuable tool has been the ‘stork road map’ developed in cooperation with the R&D center of SONELGAZ and which shows stork migration routes across Algeria in relation to overhead transmission lines.

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Beloufa explains that one measure adopted to reduce the impact of storks on flashovers of suspension strings was to build secondary nest sites, typically on less vulnerable nearby tension towers. However, in a pattern generally noted by those who study the impact of birdlife on overhead networks, storks will often set up new nests very close to the ones that have been removed and relocated.

Silicone Grease versus RTV Coatings

Figure 2: Stork populations and migration patterns in relation to GRTE overhead lines.

Photos: Courtesy of R. Znaidi and GRTE

“We coat insulators and porcelain equipment housings here quite generously with more than a 0.5 mm total thickness of RTV material but applied to no more than a half or at most two-thirds of the total length of insulator from the dead end.”

Overview of Marsat El Hadjaj substation.

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Prior to 2009, the main solution used by GRTE for protecting substation porcelain from the effects of petrochemical, industrial, marine and desert pollution was based on application of silicone grease. While the life expectancy of the greasing technique generally would not be expected to exceed 3 to 5 years in a heavily polluted environment such as that near Oran, the first grease applied to insulators at the Marsat el Hadjaj substation in 1992 has proven effective for almost 20 years in preventing high leakage currents and discharge activity.

Given the difficult service environment as well as experience with diminishing efficacy of grease under heavy pollution, GRTE began applying RTV silicone coatings in 2009. Since then, more than 1000 kg of such material has already been applied on critical substation equipment such as breaker housings, surge arresters, current transformers and bushings. At the time of the visit by INMR, coating of insulators at the Marsat El Hadjaj Station was almost complete and one of the points of interest involved the unique application technique selected by local maintenance staff.

Says Beloufa, “we coat insulators and porcelain equipment housings here quite generously with over a 0.5 mm total thickness of RTV material. But we apply this coating to only half or at most two-thirds of the total length of insulator from its dead end.”

Cleaning the porcelain in preparation for coating is done using a range of different manual techniques

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Photos: Courtesy of R. Znaidi and GRTE

Applications of silicone grease in Oran region. how various remedial measures are reducing pollution problems in the Oran region. Nevertheless, they agree that more appropriate insulator design combined with live line washing and growing application of composite insulators has already improved performance of those lines that were most problematic in the past.

These are now insulated mainly with combinations of aerodynamic glass and composite insulators, made from both silicone and EPDM housings.

RTV silicone coating applied to no more than a half to two-thirds of porcelain from dead end side. and involves much time devoted to removing all solid pollutants as well as dried grease still adhering to surfaces. Later, once surfaces have been wiped clean, application of the coating is relatively quick.

At present, substation operating staff for Marsat El Hadjaj and surroundings report that, with the exception of audible noise from corona activity during wetting under high humidity, the RTV coatings seem to be performing remarkably well.

Summary

Bentabet and Beloufa look back over recent service experience and report that it is probably not yet long enough to make a definitive assessment of

Looking to the future, they see that similar efforts will also be undertaken within other regions of GRTE with the goal of improving overall service quality by systematically reducing pollution flashovers. Back at Head Office in Algiers, GRTE Maintenance

Director, Fathallah Souker sees a steadily growing but controlled application of composite insulators in Algeria. “To maximize performance of insulation on our transmission lines,” he says, “we will still rely on live line washing even as we use progressively greater quantities of composite insulators. But in spite of our efforts to ensure proper selection criteria for these insulators and even considering their economic and performance advantages in overcoming pollution flashovers, GRTE will continue to follow a cautious approach. What we wait for is the time when their actual life expectancy in our environment is clearer and there are no longer any perceived operating risks in terms of inspection and live-line work.” 

Table 5: Evolution & Assessment of Composite Insulator Use in Algeria Total Length

2002

2006

2008

2010

12,234

17,403

21,683

circa 23,000

306

475

913

1184

Number of 220 kV lines

9

9

10

26

Number of 60 kV lines

3

-

2

3

GRTE network (km) Total km of lines with composite insulators

Main causes of outages & operating problems

• Industrial, marine, bird & agricultural pollution • Low insulation level & insulation coordination • Vandalism

Main selection criteria & advantages of composite insulators

• Acquisition costs: 36 to 44% less, • Transport & installation costs: 37% less • Longer creepage distance for same axial distance in comparison with glass string, i.e. 31 to 66% higher • Maintenance free (no washing) & fewer flashovers (up to 90% less)

Disadvantages & risks of composite insulators

• Uncertain service life • Not ideal for live line work • Relatively difficult inspection, fault detection & replacement of any defective units

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INSULATORS

Non-Soluble Surface Deposits on Insulators H

igh voltage engineers well understand that insulators usually remain quiescent when dry – even if highly polluted. However, they can quickly light up with partial discharge activity once dirty surfaces are wetted by rain or condensation. The extent of such discharge activity will depend on whether the surfaces are hydrophilic, allowing continuous water films, or whether they bead water through inherent hydrophobicity.

Since excessive arcing activity can lead to flashover, this potential reliability problem is best managed by selecting profiles with sufficient leakage distance for the system voltage and pollution environment. This article, contributed by INMR Columnist William Chisholm, looks at dust and pollution accumulation on insulators, with a view to more correctly identifying applicable service conditions and better specifying insulators.

All photos: INMR ©

In places such as China and North Africa, NSDD levels far exceed ESDD.

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Photo: INMR ©

When in contact with metal and combined with moisture, non-soluble dust can lead to problems such as accelerating metal corrosion.

Site Pollution Severity According to IEC 60815 IEC 60815 basically recommends adjusting leakage distance stress of an insulator over a 5:2 range, from 22 mm/kV of line-to-ground voltage in the case of very light site pollution severity (Class A) up to 55 mm/kV for very heavy pollution (Class E). Of course, the key to applying this standard properly is establishing the correct site pollution severity before insulators are specified. If site pollution is underestimated, the standard provides little value in avoiding discharge and flashover problems downstream.

Fortunately, there are many guides to help the power engineer assess the site pollution severity (or SPS) of any service environment. Perhaps the best such practice is described in INMR Issue 69 (Qtr. 3, 2005) and involves sampling the pollution from exposed insulators as part of a systematic longterm program to develop a country-wide pollution map. This, for example, has been carried out in countries such as Iran (by the Niroo Research Institute) and China (by the State Grid and the China Southern Power Grid).

Assessing electrically conductive deposits forms a fundamental part of these studies and selection of leakage distance based on measuring equivalent salt deposit density (ESDD) is well established. However, IEC 60815 also calls for an estimate of non-soluble deposit density (NSDD) in the accumulated pollution.

The key to applying IEC 60815 properly is establishing the correct site pollution severity before insulators are specified.

Effect of NSDD on Electrical Strength of Insulators NSDD deposits often far outweigh ESDD levels that accumulate on exposed insulator and metal surfaces (in China by some 5 to 1). When in contact with metal and combined with moisture, this non-soluble dust can lead to different problems, such as accelerating metal corrosion.

For example, one way to put NSDD into proper perspective is to compare its weight (in mg/cm2) with the amount of water on a surface under conditions that lead to corrosion. At the ‘critical relative humidity’, where water is absorbed directly onto a surface containing salt pollution, the resulting water layer thickness weighs roughly 0.001 mg/cm2. This increases to 0.1 mg/cm2 at 100% relative humidity, then to 1 mg/cm2 when covered with dew and finally to 10 mg/cm2 when wetted by rain.

NSDD is important as well in electrical performance of insulators because it affects the nature of surface wetting.

A surface with a heavy but inert dust deposit will stabilize whatever conductive pollution is there and promote repeated development of partial discharges and dry bands.

The settled layer of dust can also absorb sulphur dioxide and water vapour directly from air. This adds to the risk that hygroscopic salts (including chlorides and sulphates) will scavenge enough water from humid air to ‘self-wet’, thereby forming a continuous conductive surface that reduces insulator flashover performance, even if there is no rain or fog.

Indeed, the role of NSDD has been recognized for years in contamination testing using the clean-fog method. Test standards require that, whatever electrical conductivity is used in the pre-contamination slurry, it must always have the same 40 g/l concentration of kaolin clay. This yields a repeatable clean fog test result and, according to work in 1996 by Prof. R. Matsuoka, the resulting low NSDD level of 0.05-0.07 mg/cm2 has only minimal influence on insulator selection according to IEC 60815.

At higher levels (e.g. up to 1 mg/cm2), however, the effect of NSDD on electrical strength becomes progressive.

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For example, test results plotted in Figs. 1 and 2 show that a 5:1 increase in ESDD (from 0.04 to 0.2 mg/cm2) causes flashover performance to drop by 30 to 40%. A 7:1 increase in NSDD (from 0.14 to 0.95 mg/cm2), by contrast, results in a reduction of performance from 20 to 25%.

The important performance advantages of silicone rubber surfaces over glass and porcelain – whether as polymeric insulators or RTV silicone coatings – relate mainly to the ability of these high surface energy materials to break up continuous water films. This ensures that there is no direct electrical path from the end fittings across the insulator surface.

The according-to-some ‘unresolved’ or open question in application of silicone materials is whether this performance advantage can be maintained under all circumstances. There may be some critical loading level above which inert dust accumulation overwhelms the ability of the silicone material to produce a waterbeading film of low molecular weight (LMW) entities that are responsible for hydrophobicity transfer.

Photos: INMR ©

Estimating Global Dust Deposit Rates

Fig. 1: Critical flashover stress of ceramic disc insulator with bottom-ribbed profile.

The NSDD can of course be estimated using some generalized multiplier based on experience, such as 5 times the ESDD as suggested for China or 10 times ESDD in the Middle East. However, it is usually far better to obtain independent estimates of both nonsoluble and soluble pollution. In this regard, satellite observations seem to offer a new and economical way to gather useful NSDD data covering large areas of the globe and also to study how the NSDD/ESDD ratio changes across the different regions.

NSDD is also important in electrical performance of insulators because it affects the nature of surface wetting.

Fig. 2: Critical flashover stress on ceramic disc insulator with external-ribbed profile.

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Nineteen authors of a 2005 article in Science used three studies that matched satellite optical depth estimates for dust deposition with locally recorded concentrations of iron. They then proposed a map of average annual dust deposit density, which highlights areas with a rate of 20 g/m2/year deposit rate in north Africa, the Middle East and north-west China.

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Fig. 3: Annual dust deposit rate (g/m2/year) from satellite optical depth (detail from Jickells et al., Science, 308, 67-71/2005).

Dust Accumulation Model

Dust Accumulation Observations

Dust is removed from the atmosphere by dry or wet deposition. A simple model for accumulation of NSDD on insulator surfaces would therefore be as follows:

At this stage, there are relatively few points of confirmation to guide application of dust deposit rate to estimate NSDD levels on insulators. For example, along the route of a proposed 1000-km transmission line in Russia, NSDD levels from 40 test stations were reported to be in the range from 0.02 to 0.14 mg/cm2, with a median value of 0.04 mg/cm2. The global map shows annual dust deposit rates on the order of 1-2 g/m2/year, which when converted gives 0.1-0.2 mg/cm2 per year.

• The dust flux is uniform throughout the year • Days without precipitation allow the dust to accumulate • Days with precipitation wash all dust away This model would clearly be better for upper surfaces than those facing the ground. The rate of increase of NSDD, with a heavy dust flux of 10 g/m2/year in Fig. 3, would be 0.0027 mg/cm2 per day. To reach the level of 1 mg/cm2, at which the influence of NSDD becomes very strong, would therefore require (1/0.0027) days – roughly a full year without rain. Unfortunately, there is no well-defined computer model yet to establish how air flow past insulator surfaces influences rate of increase of NSDD on bottom surfaces, or how it reaches an equilibrium after long-term service.

Photos courtesy of George and del Bello, CIGRE

Top Surface

In Iran, NSDD levels on test insulators were found to be in the range 0.2 to 0.8 mg/cm2 in the Bushehr region, compared to values of 0.5 to 3 mg/cm2 in the Hormozgan region. These were then plotted on the IEC 60815 classification chart for SPS, along with the measured values of ESDD. Close inspection of the Jickells map for this region reveals that annual dust deposit rates also differ (i.e. 10-20 g/m2/year for Bushehr and 20-50 g/m2/year for Hormozgan). In both

Bottom Surface

Accumulation of dust on upper and lower surfaces after 30 years service in Iran.

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Bushehr State

Courtesy of NRI

Hormozgan State

Fig. 4: Pollution levels in Iran classified using IEC 60815 site pollution severity chart.

cases, proximity to the sea ensures that ESDD levels exceed the high level of 0.1 mg/cm2/year.

The use of logarithmic scales on both axes in the IEC chart is deliberate since such distributions are more helpful when selecting insulators. They faithfully capture the wide dispersion in the test results, whereas simple averages and standard deviation values for normal distributions do not. Scatter in NSDD values proved the same or less than variation in corresponding ESDD measurements.

dust map and also suggests that an NSDD/ESDD ratio of 10:1 is indeed appropriate for the Middle East. The Hormozgan observations above support this refinement.

After long-term service on a Âą600 kV DC line near the Itaipu dam in Brazil, measured NSDD levels exceeded 2 mg/cm2. Given local ESDD of 0.4 mg/cm2, a 5:1 ratio seems to apply here. As discussed, this ratio has also been proposed for many insulator applications in China.

Differing NSDD/ESDD ratios may also be appropriate for other regions and exposure conditions. For In Israel, Dr. Evgeni Volpov of the Israel Electric example, Ontario, Canada is an area with low Company confirms his confidence in data from the global annual levels according to the global dust map

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(i.e. 0.2 to 0.5 g/m2/year). In a season of winter measurements with frequent intervals of natural rain, the observed NSDD/ESDD ratio was less than 3:1.

Statistically, the standard deviation of the natural logarithm of the NSDD values (sln NSDD =1.5) was three times higher than the corresponding ESDD value (sln ESDD = 0.5). This contrasts with pollution levels in Iran, where there was less variation in NSDD than in ESDD. On the other hand, local sources of non-soluble pollution deposits, such as cement plants, can lead to extreme levels of NSDD on all surfaces. For example, heavy deposits accumulated over a period of 27 years on insulators near a cement plant in Indonesia, which has about the same dust deposit density as Canada and should otherwise have low pollution levels due to frequent rain. The ESDD level of 0.7 mg/cm2 would otherwise be considered ‘medium’ without the NSDD, but ‘very heavy’ with NSDD (i.e. 26-29 mg/cm2, higher than the maximum level shown in the IEC 60815 classification chart). In this unusual case, the overall NSDD/ESDD ratio was about 400.

Conclusions This article has discussed using global satellite observations to establish a suitable local insulator design given NSDD levels. Additional validation, especially in those countries having a strong gradient in annual dust deposit rate, together with improvements in map detail will help towards the goal of refining the model of dust accumulation on insulators.

All photos courtesy of George and del Bello, CIGRE

All photos courtesy of George and del Bello, CIGRE

At the same time, a computer model of airflow may be needed to project the flux density of surface deposits onto an insulator body of specified geometry in order to estimate NSDD accumulation rates on bottom surfaces. 

Pollution after 27 years near cement plant in Indonesia.

Top/bottom surface ESDD: 0.077/0.06 mg/cm2.

Top/bottom surface NSDD: 24/29 mg/cm2.

Pollution observed in Brazil after 25 years service at ±600 kV DC. ESDD = 0.4 mg/cm2; NSDD = 2 mg/cm2

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INSULATORS

While there are hundreds of different porcelain insulator manufacturers scattered across the globe, those who produce toughened glass cap & pin discs number barely over a dozen.

The reason for this discrepancy is certainly not a function of customer preference since glass and porcelain strings have traditionally had a roughly similar market share on overhead lines worldwide. Indeed, many countries, including France, Spain and Italy, have grids dominated by glass.

China – with the world’s largest power network – also sees an important role for glass strings on transmission lines and has the largest number of glass insulator plants of any country. INMR visits Sichuan YiBin Global Group (SYGG) – a glass insulator manufacturer based in central Sichuan Province.

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Photos: INMR ©

Glass Insulator Manufacturer Invests in Production Facility

“The quality of an insulator is one of the keys to guaranteeing the reliability of a power system,” observes SYGG’s Deputy General Manager. “And achieving a consistently high quality depends on several factors including the right production technology, the latest equipment and skilled workers. It was with these targets in mind,” he adds, “that we decided to invest some RMB 280 million (circa USD 40 million) with the goal of establishing a world-class production line for glass insulator discs here in Chengdu. This investment was then supported by our Group’s more than 20 years of relevant glass manufacturing experience.” According to the Deputy GM, this process is ongoing as an additional RMB 40 million was invested in R&D just over the past year to introduce new, very high strength insulator designs.

Indeed, the company’s Technical Manager reports that SYGG has several R&D projects underway, including a triple shed design as well as glass shells pre-coated in the factory with RTV silicone. “Our target,” he states, “is to research all insulator designs up to 300 kN and 500 kV. With this range, we know we can cover the large majority of needs worldwide.”

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The annual domestic market for glass insulators in China is estimated at about 20 million discs and reflects a fairly stable market in recent years.

New double ribbed aerodynamic glass disc.

The annual domestic market for glass insulators in China is estimated at about 20 million discs and reflects a fairly stable market situation over the past several years. This volume is predominantly for AC, with about 7-8 percent for DC – a higher level than in the past. Moreover, because of the number of major DC projects now being planned by China’s State Grid, the local market for DC insulators is expected to continue to grow rapidly.

All photos: INMR ©

Production of glass insulators begins with the various raw materials, mainly quartz and sand, but which also include soda ash, calcitite, feldspar and dolomite. The Director of the Raw Materials Workshop, explains that these materials are combined using a sophisticated computer-controlled batching system that guarantees a consistently precise final composition of the glass. These materials are mixed

Computer-controlled batching and mixing of raw materials that feed furnace.

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Quality control staff use counters installed on each production line to record and track the rate of every type of defect identified during visual inspection.

All photos: INMR ©

The importance of consistent thickness is especially evident whenever there is a need to change molds in order to produce an insulator of different design. Such changeovers may require a different volume of molten glass to be drawn from the furnace for each gob and breakage rate can rise slightly until the adjusted process stabilizes.

Molds are pre-heated just before gobs of molten glass drop into them. Spinning, after removal from mold, allows internal screw-shape to form inside glass shell. and fed into the furnace using two separate channels, each equipped with a powerful magnet to remove any metallic contaminants.

About 1/3 of the mass fed into the furnace is cullet, i.e. previouslyformed glass discs that shattered subsequent to forming or were rejected downstream by quality control inspection and ground up.

This cullet, which also includes any unused gobs of glass during mold changeovers, is a common feature of the industry since it contributes to the consistency of the glass manufacturing process.

One of the key production parameters for insulators is ensuring homogeneity of the molten glass and this requires continuous stability of the furnace, which is designed to run at temperatures of up to

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After molding and spinning, the glass shells undergo the important toughening process. Here, surfaces are exposed to repeated cycles of rapid cooling from jets of compressed air while their inside continues to remain very hot. As shells move through the machine, their insides 1000°C for years without need for cool and shrink – a process that a maintenance shutdown. A control room therefore monitors every aspect creates the desired compressive of the process until the molten glass stresses at the surface. To achieve this gob – free of any bubbles, inclusions effect, the glass must be very clean since the presence of even a tiny or impurities – is delivered by inclusion can lead to shattering. This special feeders directly into pretoughening process is an important heated molds. determinant of performance by These individual molds, placed on a ensuring the absence or propagation of cracks in the dielectric over the rotating machine, receive the glass gob in a precise quantity and dictate insulator’s service life. the outer shape and dimensions of After toughening, the glass shells are the shell. Internal geometry is then determined by devices inserted into transferred through another thermal shock by immersion in cold water, the molten mass within the mold as required in the standards. This cavity. Pressing is critical because it not only imparts the final internal entire process of repeated cycles of alternate heating and cooling shape but also assures a uniform thickness throughout – a factor that helps eliminate the large majority of units containing inclusions or will minimize any risk of shattering other defects since they will usually during the toughening process that shatter under the rapid temperature follows. change. Finally, there is a close

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Glass shells are toughened by process of forced cooling, which imparts desired compressive stresses at surfaces.

visual inspection of each shell and any defective units are placed onto a conveyor for re-cycling as cullet.

Those units that pass inspection are basically complete and ready for assembly of fittings.

All photos: INMR ©

SYGG’s Director of Glass Shell Production reports that acceptance rates for glass shells in the Chengdu factory typically average around 90 percent – a level that he claims is probably higher than in the industry at large. He also explains that quality control staff use a series of counters installed on each production line to record and track the rate of every category of defect identified during visual inspection. “Production staff look at the relative incidence of every type of defect on a daily basis,” he says “and discuss whether certain process parameters need to be adjusted, such as angle of burners, furnace temperature or gob feeding.”

Glass shells then move on to the next step in manufacturing, which consists of attachment of the metallic cap and pin to every unit.

All hardware is first dipped in pitch to reduce risk of corrosion at key interfaces and also to provide a

After final thermal shock in cold water bath, glass insulators go to quality control inspection. Defective or damaged units are returned for use as cullet (left conveyor in photo at right).

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protective buffer between the glass and metallic surfaces that have different coefficients of expansion with temperature change.

The fittings are then transported using an elaborate system of overhead conveyors to work stations, where production workers attach them to the glass shells using precise amounts of pre-mixed special cement. After assembly, there is curing in a hot water bath, whose temperature depends on the mechanical rating of the insulator.

The last stage in production consists of testing, most of which is performed on production samples at SYGG’s internal laboratory facilities.

All photos: INMR ©

Counters keep track of every type of defect, such as the strips in middle photo, for remedial action that may require adjusting process parameters.

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All fittings dipped in pitch before being transported to final assembly stations.

One test that is performed on every unit just after curing is for tensile mechanical strength, which, as per the standards, is done at 50% of the SML rating.

According to the Testing Manager, SYGG has built facilities to conduct the majority of any testing either as per the standards or according to occasional special demands from customers. He notes that several power supply companies in India, Brazil and the United Kingdom have their own test requirements that are often different from or more stringent than those mandated by IEC. One example is RIV testing, which is usually done at 10 kV, 1MHz with a maximum permissible 34 dB. But, he notes, some customers in Brazil and the U.K. request testing at 30 kV, 1 MHz and 52 dB. Similarly, some Indian customers require insulation resistance measurements in the HV lab. Yet another example is the impact test, which he claims is almost no longer required today within IEC but which is nevertheless demanded and sometimes at an even higher level than specified in the standards.

Yet another example of a special test requirement is asked for by the grid operator in the U.K. whereby a glass string is hung at angles of up to 30° inside a special tunnel and exposed to wind speeds of 20 m/second. If any noise is emitted, the string does not pass.

While a relatively new player in an industry with a history going back more than 100 years, SYGG management claim they have already established their presence not only in China but in several overseas markets as well. In fact, Frances Wen, who heads the Export Division, reports that a substantial contract for a few million pieces was recently signed with a large Asian customer while new orders are coming from South America, Europe and most recently, the U.S. She also indicates that while about a third of sales went to export markets in 2011, the

All photos: INMR Š

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Tensile strength testing carried out on every unit (left).

In-house test capabilities include fullfledged HV laboratory as well as thermomechanical, RIV and other test facilities (bottom).

proportion this year will likely be closer to 50 percent.

The Chinese market for transmission insulators has undergone a gradual shift in recent years. Whereas 5 years ago, it is estimated that glass accounted for some 40 percent

of all insulator strings in service, with composite and porcelain types at about 35 and 25 percent respectively, composites have today basically exchanged the share with glass and, if anything, are only growing in importance. Recognizing this trend, the company’s Deputy

General Manager indicates that SYGG is now actively exploring composite insulators as an addition to their mainstay glass technology. “We are now developing our own line of composite insulators,” he indicates, “and we hope to offer these in the short term.” 

All photos: INMR ©

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ARRESTERS/CABLE ACCESSORIES

Sheath Voltage Limiters Protect HV Power Cables Over the past decade, demand for longer lines and higher current capacities for HV power cables has required new methods of loss prevention. At the same time, ensuring high reliability of these lines is progressively more important. Together, these developments have sharply accelerated application of surge protection on underground cable networks.

This article, contributed by arrester expert and INMR Columnist, Jonathan Woodworth, explains the surge protection scheme offered by sheath voltage limiters (SVLs) – devices intended to protect the cable jacket from electrical stresses during transient events. Since high voltage cables these days are available in an array of different types and designs, for the sake of simplicity the focus here is on a single core HV cable with metallic sheath and polymeric outer jacket (as shown in Fig. 1).

Introduction

Growth in installation of underground cables has focused more attention on some of their potentially negative environmental effects. Because cable is often installed with metallic sheaths, current is induced onto the sheath from the primary conductor and flows directly to earth, representing a 100% loss of energy. In the process it can also raise the temperature of the cable, which then becomes a limiting factor in the system’s overload capability.

A common means to reduce such losses is to segment the cable sheath (as in Fig. 2). However, if segmentation is used to interrupt induced sheath current, measures must also be taken to limit voltage induced on the sheath during transient events. Otherwise, the voltage differential between sheath and earth can exceed the cable jacket’s

If segmentation is used to interrupt induced sheath current, measures must also be taken to limit voltage induced on the sheath during transient events.

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Fig. 1: Simple HV cable showing polymeric jacket that may require surge protection.

withstand, leading to puncture. This can become a point of moisture ingress, which can lead to long term dielectric and failure issues.

While a range of configurations is used to reduce losses in cable systems, (including cross-bonding of the sheaths and transposing phase conductors) segmentation along with surge protection of the cable jacket is considered the most effective. The link box in this case is a universally used sealed junction box placed either in manholes or cabinets and that accommodates surge protectors as well as a point to cross-bond the sheaths. Figure 4 shows a typical such link box setup that provides a location for the sheath voltage limiters as well as for cross bonding of the sheath.

The phase conductors do not enter the link box, rather only the sheath or sheath extension.

Fig. 2: Loss reduction in cable systems using segmentation and sheath voltage limiters.

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The SVL

A sheath voltage limiter (SVL) is basically a surge arrester under a different terminology. It functions as an arrester and, in most cases, is in fact a re-labeled distribution arrester.

Selecting an SVL

As stated earlier, the main purpose of the sheath voltage limiter is to clamp or limit the voltage stress across the cable jacket. If the cable sheath is grounded at both ends, the voltage stress across the jacket is quite low during steady state and also relatively low during transients. However, if the cable is segmented to reduce losses or if there are link boxes along the cable at locations of transposition or cross-bonding, it is important to install the SVLs here to eliminate any risk of insulation breakdown of the cable jacket or link box.

Photo courtesy of contributor

Two examples of sheath voltage limiters are shown in Figures 5 and 6. In Figure 5, the arrester has no sheds because this particular design is intended only for use in the dry environment of a link box. By contrast, the SVL model shown in Figure 6 has sheds similar to an arrester because it is intended for outdoor application.

Typical installation of an SVL.

There is no standard method prescribed by IEC or IEEE for selecting the optimum rating for cable sheath/jacket protection. The following method is therefore proposed based on discussion with cable suppliers, arrester suppliers and with the aid of transient modeling of the system to determine the effects of a surge during transients.

This analysis assumes sheath segmentation is a single point bond (earthed at one end of the sheath) and an open point at the other end.

Sheath Voltage from Power Frequency Sources

Figure 8 depicts an example where a 27 kA fault results in 3800 Vrms on the sheath. The most common rationale in selecting an arrester for protecting the sheath is to select an SVL with a turn-on level above the worst case induced power frequency voltage. This means the SVL does not need to dissipate any energy during a temporary overvoltage (TOV) caused by faults.

For overhead arresters, this is generally not the rule and, in those cases, the arresters are sized to conduct current during the TOV but not enough to cause it to fail. The overhead sizing rationale utilizing an arresterâ&#x20AC;&#x2122;s TOV capability is not used for SVL selection unless it is necessary to achieve a better margin of protection.

Fig. 3: Typical configuration of cable, SVL and phase arrester on transition pole with SVL mounted near bottom of termination.

Photo courtesy of contributor

Because the sheath of a cable is in such close proximity to the conductor, the voltage appearing on an open sheath can be substantial and is directly related to the current flowing through the phase conductor. This relationship applies during steady state as well as during faults.

Fig. 4: Link box with 3 SVLs and cross-bonded sheaths.

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Sheath Voltage Calculations

Fig. 5: Sheath voltage limiter with typical ratings 0.8 to 4.8 kV Uc (MCOV) used inside link boxes.

Photo courtesy Tridelta

Fig. 6: Sheath voltage limiter with typical ratings 4-14 kV Uc (MCOV) for use outdoors.

Photo courtesy Tridelta

Steps to Select the Optimum Sheath Voltage Limiter Step 1: Determine the voltages that will appear on the sheath during transient events Step 2: Select AC Rating and TOV Rating Step 3: Check Margin of Protection of the Selected Rating Step 4: Check that the Energy Rating of the SVL is adequate Step 5: Check mounting and failure mode for fit and function Figure 7: Recommended step to be used to determine the rating of an SVL

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The steady state voltage gradient is the voltage that will appear along a 1 km length of sheath with 1000 amps flowing continuously and is a function of the configuration of the cable in the trench as well as its dimensions. There are two basic trench configurations: the trefoil, comprised of three cables that are positioned equidistant from one another such that their cross-section forms an equilateral triangle; and the flat configuration whereby all cables are laid such that they are in the same plane and the same distance from one another.

If the voltage gradient is not supplied by the cable manufacturer for the configuration used, it can be calculated using relevant equations and methods derived from IEEE 575 “Guide for Bonding Sheaths and Shields of Single-Conductor Power Cables Rated 5 to 500 kV”:

Once the voltage gradient is known for 1 km at 1000 A, the voltage that will appear at the open end of a segment during a fault event can also be calculated.

It is important to determine this voltage level because the SVL voltage rating (Uc) needs to be set just higher so that the arrester does not conduct during a fault event.

Should the arrester conduct in this instance, it would need a much higher energy handling capability than generally available for distribution type arresters. If it is found later in the sizing process that a lower level Uc is needed, a transient analysis will determine the SVL’s proper Uc and energy rating.

Assuming that the margin of protection will be adequate, then the Uc rating of the SVL will be greater than or equal to the voltage at the open point (Eopen), as follows: Uc ≥ Eopen= voltage gradient x segment length x max. expected fault current where voltage gradient is V/km/1000A, length is in km and fault current is expressed in kA. For example, if a voltage gradient on a particular system is 200V/km/kA and the line is 2 km long with a potential of 17.5 kA, then the minimum acceptable Uc rating for the SVL would be 7000 V. Note that if the line were only 1 km long, the SVL’s minimum Uc would be half that of the 2 km long line and could be a minimum of 3500 V.

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Fig. 8: Example of sheath voltage during 27 kA fault on trefoil configured cable.

Fig. 9: Current conduction through properly sized SVL.

Fig. 10: Current through improperly sized SVL with peak levels in 600 A range per half cycle.

Fig. 11: Temperature rise of improperly sized SVL showing imminent failure if breaker does not immediately interrupt fault.

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Figure 9 shows the current flow through an appropriately rated SVL on a 1 km line with the above-mentioned voltage gradient and fault current. It can be seen that only some microamps flow through the SVL, which is exactly what is desired. However if the same SVL is applied to a similar line of 2 km length, the current through the SVL would be significant (as in Figure 10) and the immediate temperature rise to failure is shown in Figure 11.

When there is a switching surge event on the phase conductor of a cable, the current through it will induce a voltage on the sheath in the same way it does at steady state or during fault events, even though the wave shape is significantly different. Since the voltage and current on the conductor during a switching surge are not sinusoidal or even a simple impulse (see Figure 13), it is not possible to accurately predict the resulting voltage and current on the sheath.

Therefore, when determining the proper Uc ratings for SVLs, one cannot select one rating for all link boxes unless the lengths of all segments are equal. Moreover, if the SVL is chosen correctly, it will not be required to absorb any significant level of energy during a system fault.

The only ways to accurately determine the actual voltage and current on the sheath are through transient simulations or real field tests. Since tests are not practical, transient simulations are really the only option and some useful rules of thumb have emerged from running such simulations:

Protecting the Jacket from Switching Surge

1. If the SVL is selected to ride through a fault event with minimal to no serious conduction, then the switching surge energy withstand capability of a 10 kA rated distribution type arrester is adequate. If the SVL is not dimensioned to ride through the fault, then station class arresters may be needed.

The jacket and sheath interrupts are generally the weakest insulation in a HV power cable system. Figure 12 shows their withstand levels as per IEEE 575.

Typical BIL Withstand of Sheath Interrupt and Jacket kV Peak (1.2x50 Âľs Wave) System kV

Across Halves

Each Half to Ground

Jacket

69-138

60

30

30

161-240

80

40

40

345-500

120

60

60

Fig. 12: Lightning impulse withstand of sheath interrupts and cable jacket.

Switching surge impulse withstand of the sheath interrupt and jacket are assumed to be similar to other insulator types and are 83% of the lightning impulse withstand rating (BIL).

2. If the 1000 A switching surge residual voltage is not available, then the 1.5 kA 8/20 lightning impulse residual voltage can be used for the margin of protection calculation.

In the case study used to create Figure 14, the switching surge voltage on the sheath without SVL protection would rise to greater than 100 kV. According to Figure 12, this is more than 40 kV above what the jacket or interrupt insulation can withstand, representing certain failure of the cable jacket. In this case, with an SVL of 9.6 kV Uc, the voltage on the sheath is limited to a maximum of 33 kV. To calculate margin of protection during a switching surge, it is recommended that the 1000 A switching

Fig. 13: Switching surge on phase conductor of 345 kV cable with (green) and without (red) arrester protection on that phase.

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Fig. 14: Switching surge voltage inducted onto sheath of 345 kV cable with and without SVL protection.

3 pu switching surge on phase conductor without SVL (green) and with SVL (red).

Fig. 15: Voltage and current on phase conductor of 345 kV cable with 100 kA surge to phase several spans from transition pole.

surge residual voltage be used. Since switching surge residual voltage is not a mandated test for distribution type arresters, the 1000 A residual voltage may not be available. In this case, a reasonable substitute for the switching surge voltage is the 8x20 residual voltage at 1.5 kA. For the 9.6 kV SVL used in the above study, the V1000=1000 A 30/75µs residual voltage is 28.4 kV.

the cable with a moderate level of current as well.

Figure 15, for example, depicts the voltage and current entering a 345 kV cable given a 100 kA lightning strike a few spans away. As with switching surges, there is no accurate means of predicting the induced voltage onto the sheath during a lightning surge, therefore a transient study is required.

From Figure 12, it can be seen that the BIL withstand level of the jacket for a 345 kV line is 60 kV. This means that the switching surge margin of protection (MP2) for this case is: MP2=([( BIL x .83 )/V1000 ]-1) x 100 = 111%

Calculating margin of protection (MP1) for lightning is very similar to what is done in the case of switching surges. Here, 10 kA is used for the coordinating current and the full BIL is used for the withstand of the jacket and interrupt insulation. Using the same type of SVL as above for the switching surge calculation, the residual voltage at 10 kA is 35 kV and cable BIL is 60 kV. Therefore, MP1=([BIL/V1000 ]-1)x100 = 71%.

Again, a 9.6 kV Uc SVL will provide adequate insulation protection for the cable jacket. 

Protecting the Jacket from Lightning Surge

When lightning strikes an overhead line before the transition pole, the surge is clamped by the arrester that is universally mounted at this location and most of the surge current is diverted to earth. However, a surge voltage of significant magnitude can travel into

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Photo courtesy of Georg Jordan

DEVELOPMENTS IN EPOXY MATERIALS

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Compaction Driving Design of New Epoxy Insulation Components

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Just as demands for lower costs and less maintenance are impacting design of overhead transmission lines, one of the key drivers these days for applications at the medium voltages is the requirement for ever smaller and lighter insulation components. INMR visits a German-based supplier of epoxy insulators to look at how the latest generation of epoxy materials are helping to achieve this goal.

Epoxy resins have long been the mainstay material for a broad range of monolithic MV insulation components “Developments such as used in apparatus such as switchgear and transformers as well as in catenaries and power convertors for traction. new hydrophobic and semiOver the years, these resins have undergone periodic reformulations intended to improve their processability flexible epoxies have widely and overcome any problems encountered in service. For increased the scope of example, bisphenol A (BPA) epoxy, which is still the dominant material for most indoor MV applications, was possible designs to meet found to lose stability outdoors, particularly in tropical regions, as its double-bond aromatic structure breaks changing customer needs.” down under persistent sunshine. To overcome this deficiency, the BPA resin was chemically modified to a structure resistant to high-energy UV rays. The resulting cycloaliphatic epoxy material (CEP) has since become According to Hubert Wilbers, a European-based the norm across a variety of outdoor line and substation Technical Support Specialist at Huntsman Advanced applications.

Materials, these types of developments allow manufacturers of insulation components greater In recent years, to make this material even more possibilities to optimize design to meet changing needs versatile, it too has undergone product development, in the marketplace. Says Wilbers, “the new hydrophobic resulting in new formulations that maintain long-term and semi-flexible epoxies have widely increased the hydrophobicity and even exhibit semi-flexibility at higher scope of possible designs. Today, these materials operating temperatures. are ideal not only for classical monolithic insulators containing a single cast polymeric material but also for a range of composite insulators based on an inner rod or tube but with epoxy in place of silicone outer insulation.”

One insulator manufacturer that is using these new materials to expand their product development process is Georg Jordan, a medium-sized supplier of insulation component based in Siegburg, Germany. With a history going back some 60 years, Jordan has worked with most of the successive generations of epoxy materials.

Its current Managing Director, Ulrich Jaegers, explains that the business of producing customized insulation components made from epoxy is now changing as rapidly as the materials themselves.

“In the past,” he remarks, “customers typically came to us with a specific design and our basic role was to make

Photos: INMR ©

Jaegers (left) and Wilbers with epoxy GIS bushings.

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trains traveling up to 300 km/h. The problem is that the sheds flutter too much and because of their large diameter, lap up onto those above, effectively reducing insulation performance. Our new epoxy solution overcomes this drawback while still meeting the need for reduced weight.”

the tooling and cast the product. But this paradigm has changed. These days, customers come looking not only for a product but also for technical advice in regard to its expected mechanical and electrical behavior. They even want expertise in analysis of failure modes intended to establish what could go wrong so as to anticipate potential problems before they occur.”

Wilbers explains that the new semi-flexible version of CEP was deemed ideal for the housing in this case since the material must be flexible enough to move with the core under bending stress, without risk of de-lamination at the critical interface with the core. “A standard epoxy material is too brittle for a solution involving a central FRP rod or tube,” he says. “What’s needed instead is a material that is flexible enough to move in each direction, together with the core. This same inherent flexibility will also ensure adequate elongation and crack resistance in the case of mechanical shock.” Wilbers notes that yet another advantage of epoxy versus silicone in a composite design such as this is that there is excellent adhesion between the outer sheds and the FRP core, without the need for applying a primer during production.

Apart from this expanding role, Jaegers adds that the components themselves are changing as well, with a key factor being greater demand for more compact designs to save energy or space, or both. One of Jordan’s principal market sectors over the past 15 years has been traction and he says that manufacturers of high-speed railway systems are responding to their own customer requests by asking for smaller or lighter sizes of electrical hardware such as pantograph insulators. This, in turn, is driving suppliers of insulation components to reduce their product sizes and weights as well.

A perfect example of this trend is a new pantograph insulator in the last stage of product development at Jordan. Unlike the unit it is designed to replace, it is not monolithic but rather composite – made from a central FRP tube over which semi-flexible hydrophobic CEP is cast. This product, now in final testing, will soon be ready for the market and allow the weight of the pantograph insulator support to be reduced by as much as 40 percent.

The use of a hydrophobic version of CEP in this case was seen as beneficial since an additional requirement of the end customer was less maintenance. Pantograph insulators are exposed to contaminants dislodged from the overhead wire during motion of the train.

Hydrophobicity in this case ensures good electrical performance in such a service environment and Wilbers indicates that accelerated ageing tests have demonstrated that the epoxy shed material will maintain

“A silicone composite insulator would of course also meet the weight reduction goal in this application,” admits Jaegers, “but that solution is not practical for

Photos: INMR ©

Current state-of-the-art monolithic pantograph insulator made with standard CEP (left) and much lighter newly developed composite replacement relying on semi-flexible hydrophobic CEP.

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Vacuum casting sees resin and hardener stored in separate tanks (left photo) for delivery to static mixer, just before being transferred to vacuum vessel.

hydophobicity transfer as well as recovery over the component’s full projected life of 30 years.

“New tougher epoxy materials such as the hydrophobic CEP allow us the chance to develop quality low weight products with reduced wall thickness.”

Like most manufacturers of epoxy insulators, Jordan uses two distinct production technologies – vacuum casting and automatic pressure gelation (APG). While both employ similar epoxy resins, hardeners and accelerators, one of the distinctions between the two processes lies in the volume and relative size of the components being made. For example, vacuum casting, which sees curing temperatures of only about 80°C over a cycle of as long as two to three hours, is typically used for larger items such as generator bushings that can be up to 1.8 m or more in diameter. Because of the smaller production volumes, it is not unusual for staff to be required to

manage a number of tools for different products during the course of a single shift.

APG, which accounts for some 70 percent of Jordan’s total production, sees curing take place at up to 150°C and with much shorter cycles, sometimes only 15 to 30 minutes. Jaegers points out that because the casting takes place over a relatively short time frame, the risk of cracks is essentially higher than in the vacuum casting process. “With APG,” he emphasizes, “you have to always think carefully about the material because shrinkage is higher than during vacuum casting.”

In both technologies, one of the key production parameters is maintaining perfect vacuum to eliminate risk of air bubbles, whose presence effectively requires the product to be scrapped. Indeed, components

State-of-the-art mold for production of 15 kV epoxy railway insulators by APG.

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routinely undergo x-ray examination to search for and measure the scale of any possible inclusions that would require workers to adjust production parameters.

The development process for the new composite pantograph insulator is typical of what Jaegers says is the changed business environment in this industry.

“Today,” he says, “we increasingly search for product ideas at places such as trade exhibitions and consider how any new item can be made in a different way or by using a different material.” For example, Jordan’s design department has recently been looking at developing a compact epoxy railway insulator for tunnel applications, where both less maintenance and reduced weight compared to the existing porcelain are desirable.

X-ray examination for presence and size of bubbles allows APG process parameters to be modified if necessary.

The development process in this instance includes simulations of vibration intended to establish how the proposed design would behave under normal operating conditions. Stefan Jutz, Head of the Department, explains that the insulator for this application would be attached to a 2.5 meter long rod and therefore must be capable of withstanding high mechanical force. As with the new pantograph insulator, the material being considered here is the same hydrophobic semi-flexible epoxy. Jaegers mentions an exciting more compact transformer design for power converters using conventional CEP that is now near the end of this type of development process and which Jordan hopes to bring to the market by the end of the year.

Jaegers sees that Jordan’s ability to satisfy present customer demands for specialized insulating components is supported by both the new materials and modern manufacturing methods. “The customer tells us what they want from a product and then we have to realize this with a very high production quality.

Fortunately, technology helps us to achieve this with equipment such as static mixers which, unlike the previous batch mixers, ensure better process control and consistent production conditions.”

Design department conducts simulations on new epoxy tunnel insulator.

Insofar as the epoxy materials themselves, Jaegers feels that there has been much progress in recent years, apart from past efforts to improve processability with the goal of increasing productivity. Says Jaegers, “new tougher epoxy materials such as the hydrophobic CEP give us the opportunity to develop lower weight products with reduced wall thickness to better meet changing market needs.” At the same time, he notes that a higher level of expertise is now needed since more compact designs with reduced insulation distances require greater know-how when it comes to both designing and manufacturing the product.

“All of these changes in our industry have tended to make project times longer than in the past,” concludes Jaegers. “But with greater analysis and testing, customers now tend to trust the end products more. And that customers trust our quality is the most important issue for us.” 

Jaegers. Most important issue for us is that customers trust the quality of our products.

All photos: INMR ©

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VOLUME 20 NUMBER 3 • QUARTER THREE - 2012

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Issue 97 • Quarter 3 - 2012 • Volume 20 - Number 3

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