South African Utility Maintains Key Role Testing Insulator Performance

Sep 30 to Oct 6, 2013

UTILITY PRACTICE & EXPERIENCE

20

YEARS

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South African Utility Maintains Key Role Testing Insulator Performance

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f one looks back some 30 years or so, large national power companies such as EDF in France, CEGB in the UK or ENEL in Italy were important participants in the development process for electrical insulators. In co-operation with local manufacturers, these utilities often set the applicable standards and testing requirements and sometimes even provided test sites (such as Martigues or Dunkirk in France or Dungeness in the UK) to assess field performance.

with insulator suppliers has diminished and even disappeared ‌ except in a handful of countries.

But that was the model of the past.

One of these exceptions is South Africa, where the national utility, Eskom, continues to play a high profile role in the field of insulator development. Indeed, a fixed proportion of turnover is allocated each year to R&D activities that, among other things include monitoring the service environment and assessing performance of different insulator types and designs.

Eskom has also helped pioneer new insulator test methods and related monitoring equipment.

In more recent years, with de-regulation and the refocusing of the utility industry on its core business of electricity supply, such close co-operation

INMR travels to South Africa to meet Eskom engineers and also visit substations and lines scattered across the vast Western and Northern Cape.

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Photos: INMR ©

With a sprawling network comprising some 300,000 km of transmission and distribution lines, Eskom has had a long history of problems due to insulator failures. When the country’s 400 kV ‘backbone’ transmission grid was established between 1966 and 1969, the first such lines employed glass cap & pin strings with the relatively short creepage of 15 mm per kV. In coastal areas, however, industrial and marine pollution combined with local weather patterns caused a high incidence of flashovers, especially during late winter and early spring. In addition, vandalism at various locations resulted in numerous broken glass discs. Locally produced porcelain cap & pin insulators were tried as an alternative but soon abandoned because of frequent punctures in areas with lightning.

Given these types of problems, by the late 1970s some of the glass cap & pin strings on Eskom’s transmission system began to be replaced by porcelain long rods having a specific creepage distance of 25 mm/kV. Although superior performance was expected due to their single unit construction and aerodynamic profile, the number of flashovers actually increased. Various mitigation techniques, such as hand washing, were then considered but such maintenance proved costly and mostly ineffective. By 1987, a program of washing from helicopters was introduced in its place, partly because of the speed with which the work could be done and also because it was often difficult for trucks to reach affected lines.

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Blackened corona rings on polymeric tension insulators, surge arresters and composite transformer bushings attest to localized pollution affecting 400 kV Stikland Substation near Cape Town.

According to Eskom maintenance personnel, porcelain long rods required frequent washing in some areas and it was difficult to predict when was the correct time. To help, a device was built based on evaluating the radio interference generated when pollution reached a critical level. Nevertheless, the cost of helicopter washing also proved high and, during extreme pollution events, even periodic washing did little to prevent flashovers. Nor was increasing specific creepage distance of strings an option because of limitations associated with minimum clearances.

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During the late 1980s, mass pollution flashovers occurred in the eastern province of KwaZulu Natal and it was at this stage that Eskom became among the first utilities in the world to consider widespread application of composite insulators.

Actually, several hundred silicone insulators had already been installed on lines located in heavily-polluted areas and, except for a handful of mechanical failures, these performed well electrically – even with a specific creepage distance of only 23 mm/kV. A decision was therefore made in 1990 to specify silicone insulators with 30 mm/kV to progressively reinsulate all 275 kV and 400 kV

Mass pollution flashovers during the late 1980s made Eskom among the first utilities in the world to consider widespread application of composite insulators.

Photos: INMR Š

lines located in areas exposed to severe pollution. As a result, by 1995 Eskom had over 15,000 composite insulators in service on its transmission network and overall performance was judged very good.

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Examples of pollution deposition on glass insulators in South Africa.

Shattered glass stub shows onset of corrosion.

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Photo: INMR ©

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This Eskom line near coast still equipped with porcelain long rods but apparently operates at lower voltage than design limit.

“Many design engineers worldwide thought that IEC 61109 Annex C was sufficient to guide insulator specification at voltages of 132 kV and below, as at higher transmission voltages.

But we quickly found that it was simply not applicable to our service environment.”

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This selective re-insulation reportedly resulted in customers experiencing from 100 to 200 fewer voltage depressions each year. In addition, there were cost savings as well from no longer having to wash lines. Excluding customer benefits, the US $ 7 million reportedly invested in the project had a payback of less than 10 years while also solving persistent past problems such as vandalism and pin corrosion on glass strings located near the seacoast.

Over the following years, successive generations of composite insulators found their way onto Eskom’s network and the utility began developing expertise in assessing which designs were good and which were not. The basic strategy was to become ‘an informed buyer’ rather than simply depending on external sources. Part of this philosophy also involved fully understanding the actual service environment for insulators by monitoring pollution levels on a regular basis (see article on p. 76).

Eskom insulator expert, Wallace Vosloo, has helped implement this

These days, even with a fastgrowing population of composite insulators selectively installed throughout the country, widespread flashovers still occur in parts of South Africa due to different underlying causes. Sanjay Narain, one of Eskom’s Chief Engineers in transmission line engineering and insulation design, explains that in KwaZulu Natal, for example, these problems are usually linked to high local humidity combined with the As proof of this statement, Vosloo frequent burning of sugar cane. notes that in 2003, when Eskom The Western Cape, by contrast, instituted its program of qualifying tends to be dry and widespread potential insulator suppliers at bush fires occur in cycles of about the Koeberg Insulator Pollution every 7 years, due to the time Test Station (KIPTS), only 60% needed for re-growth of vegetation. of the products tested over the These fires create high loading of subsequent 3-year period passed. airborne particulate matter that Moreover, many of those that failed drifts out to sea and mixes with the tests also failed in service, salt, before being blown back to even though these often fulfilled land and settling onto lines. Clean all the requirements set down insulators can become critically in IEC 61109 Annex C. This, he polluted in a very short time. The concludes, demonstrated that last major flashover problem in the meeting this standard alone – at area occurred in 2007 and the one least as far as South Africa was before that, in 2000, apparently concerned – was not sufficient proof blacked out almost the entire Cape of expected good performance in Town region. service.

(left) Greased porcelain breaker housing at Ascot 66/11 kV Substation in Cape Town shows high pollution loading.

Pollution deposition on polymeric arrester at 132/66 kV Belhar Substation.

Photos: INMR ©

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policy for years and published an influential text covering the field.

He remarks, “Many utility people at distribution voltage levels of 132 kV and below thought that IEC 61109 Annex C was all that was needed to guide their specification of composite insulators, as at transmission voltages. But, in our case, we soon found out that it was simply not applicable to our environment.”

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Photos: INMR ©

Test line at Gochang in Korea where 765 kV insulators are being continuously monitored and compared in a coastal environment.

According to Narain, at the time many local lines were still insulated with glass and it was only these that experienced flashovers. Moreover, some substation porcelain with as much as 31 mm/kV specific

creepage also flashed over.

“Because this problem was absent on lines insulated with polymers,” he reports, “we decided to carry out a massive project where nearly a dozen of our 400 kV lines were

re-insulated with silicone rubber in place of glass. Hopefully, by 2014/2015, we will see if this has solved cyclical pollution flashover problems due to bush fires.”

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At the moment, Eskom’s transmission network is about to undergo a fundamental shift, with a planned new backbone of 765 kV. Says Nishal Mahatho, an insulator specialist at the utility’s Research & Innovation Department, “much of our system is still based on 400 kV and 275 kV, although we have two 765 kV lines insulated with glass that operate in the middle of the country. But now we have changed philosophy and are moving in a phased approach toward a 765 kV grid and by 2020 to a ±600 kV DC system as well. This means that there is even more work than normal going on when it comes to insulator research.”

One example is a project already underway at KEPCO’s Gochang Test Station in Korea where five different designs of composite insulators, already approved by Eskom, are being compared to one another and also to the standard glass strings used on existing 765 kV lines in South Africa. The environment is coastal and every effort has been

made so that the comparison is as realistic as possible, even using the same planned conductor bundle configuration.

Mahatho explains that leakage current is being measured on each insulator design and correlated against weather conditions. The insulators are installed in double I-strings and are apparently among the first to be monitored using sophisticated new leakage current sensors from EPRI in the U.S. Periodic UV and IR testing with special cameras is also being performed on each.

Says Mahatho, “the idea was to compare how the different materials and designs from Eskom approved manufacturers perform in a full scale energized environment. For example, because of the high voltage, electric field will be one of the issues that might contribute most to premature ageing.” He goes on to state that the test is expected to last 2 years with at least 90% of this time under voltage and that parameters

“With the new 765 kV composite insulators planned for our network, corona ring design and placement will be even more vital to performance than the silicone rubber housing material.”

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400 kV lines re-insulated with silicone rubber.

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“If you go from guessing ESDD to actual measurement, your band of uncertainty reduces and you can fine tune selection of suitable insulator dimensions and geometry.”

Photos courtesy Eskom

I examined for each insulator will include hydrophobicity and electric field distribution, in addition to leakage current. It is also planned to remove the insulators after a year to allow close-up visual examination before putting them back up.

Narain refers to the upcoming generation of insulators planned for Eskom’s future 765 kV network and notes, “we are dealing in this case with composite insulators where corona ring design and placement may prove even more vital to performance than the silicone rubber housing. Narrain also points out that Eskom philosophy has always been that insulator corona rings must take care only of the insulator and not necessarily aim to also protect associated hardware.

“For this reason,” he says, “you will find separate grading rings for hardware on a lot of our 400 kV and 765 kV lines.”

In many ways, South Africa’s topography can be compared to an

upside down saucer in that there is a sharp rise from most coastal areas to plateaus that typically reach 1500 m or more. This means that a high proportion of Eskom lines have their insulation affected by altitude and therefore local test laboratories, such as the National Electric Test Facility have been intentionally sited to operate under reduced air density (in the case of NETFA at 1625 m).

The entire process of insulator selection at Eskom has been systematically refined in recent years so as to remove decisions based mostly on past experience.

Instead, a large body of reliable information has been compiled on pollution across the country to record and track environmental stresses at every line and substation.

Says Narain, “in the past, selecting insulation was driven mainly by experience and perceptions. Now, we are moving to eliminate such subjectivity from decision-making

Example of testing at NETFA for corona on different insulator and grading ring assemblies.

and rather base our line insulation specifications not on guessing ESDD levels but rather on actual measurements that provide us with a statistical representation of the real pollution situation. This reduces the band of uncertainty and means we can fine-tune our selection of ideal insulator dimensions and geometry. For example, instead of specifying from 30-40 mm/kV, we can be precise and state we want e.g. 35-36 mm/kV.” Narain goes on to explain that pinpointing pollution levels across the country and correlating these against the flashover behavior of actual insulator profiles from a test laboratory makes it possible to plot stress versus strength. “We put all this type of information together to arrive at the optimal insulator choice,” he says. “In other words, once we know the flashover characteristics of any particular insulator’s profile (i.e. the flashover voltages under different pollution severities) we can be certain of

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how it will respond in our service environment. It also means that different insulator creepage values can be specified in the case of a single very long line.”

to go with the higher level along the whole route. Environments are dynamic, with possible increases in pollution levels, and we could therefore gain extra security.”

One example of this type of thinking put into practice is the planned extension to Cape Town of the 765 kV Gamma-Kappa Line. The first 200 km of this 350 km line were insulated with glass strings having 20 mm/kV. However, once the line passes the mountains near the heavily agricultural Ceres Valley, it enters a zone of high pollution and insulation will therefore be changed to composite types with 31 mm/kV. “Of course”, notes Narain, “if there is not much difference in acquisition cost between low creepage and high creepage insulation, we might decide

At the same time, he and colleague Vosloo warn that line design engineers must be cautious in specifying high creepage in every situation. Says Vosloo, “there is an optimized creepage which balances the risks of flashover and ageing.

Too low a creepage increases risk of flashover but too high may increase risk of premature ageing.”

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Bottom photos: Extensive network of dust deposit gauges and insulator reference strings allow pollution environment to be regularly monitored.

“There is an optimized insulator creepage in the case of every line which balances the risks of flashover and ageing. Too low a creepage increases risk of flashover but too high may increase risk of premature ageing.”

Photos: INMR ©

With the planned re-structuring of the transmission grid to one based on 765 kV AC and ±600 kV DC, there is now a big drive to establish a new KIPTS with greatly expanded

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Incoming line at newly-built 765 kV Sterrekus Substation includes separate corona protection for insulators and hardware.

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facilities to handle testing at much higher voltages and also under DC. According to Vosloo, who was based there from 1993 until recently and who oversaw much of its expansion

from the initial 22 kV and 66 kV rigs to 11 kV, 33 kV and 132 kV, work is scheduled to begin in coming months and expected to last about five years. The enlarged site will also

include a high voltage hall as well as pollution chamber where flashover performance will be measured of test objects taken down from the station.

The largest hurdle still ahead is seen

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as installing sophisticated power source equipment.

Given the amount of time needed for the expanded Koeberg site to be finished and commissioned, insulator testing at Eskom will continue using a modularized scheme based on facilities enclosed in large shipping containers placed inside a substation. Among these

Photos: INMR ©

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will be a salt fog chamber, with office, test bay and associated power sources and support equipment. There will also be an inclined plane test set-up for AC and DC as well as a tracking wheel.

Says Vosloo, “we feel confident in saying that our formalized testing requirements at KIPTS, imposed on every insulator supplier to Eskom

since 2002, have helped advance insulator technology in general. The proof is that today some 90 percent of products tested pass – a 50% increase over the pass level at the start. More important to Eskom, in the process we have also eliminated a lot of the problems we once had on transmission lines.” 

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Opposite page and top photo: Soon-to-be-completed Sterrekus Substation will become key facility within new 765 kV Eskom transmission grid.

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Paved driveway at 400 kV Stikland Substation expected to be location of Eskom’s planned new modularized testing facilities.

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