Using Less Creepage for Silicone Insulators

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Composite insulators are today quite widely used and well-accepted throughout the world. In China, for example, there several million such insulators already in service on HV transmission lines up to 1000 kV AC and ± 800 kV DC. In addition, huge numbers of silicone-housed zinc oxide surge arresters as well as silicone-housed bushings have already been applied in this country for years.

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Once a decision is made to employ a composite insulator for service on a particular transmission line, the main question becomes whether or not to specify the same creepage distance as for porcelain/glass insulators or whether a lower creepage will be sufficient. In the case of China, composite insulators have been used with the primary goal of improving pollution performance of the country’s power system. Beginning at the end of 1970s, several different polymeric housing materials were tried on insulators for applications on railways or overhead lines operating in areas of heavy pollution. These included epoxy resin, PTFE and EPDM. Unfortunately, the general experience was that all of these failed relatively quickly. Only silicone rubber (SR) insulators performed successfully and, for this reason, virtually all of the millions of composite insulators used in China are today being made only from this material. Indeed, whenever Chinese utilities or manufacturers talk about composite insulators, they are referring only to SR, without any need for further elaboration.

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Because of the pace of industrialization in China starting in the 1980s, the incidence of insulator flashovers due to pollution sharply increased in frequency. This, in spite of intensive maintenance efforts, including manual cleaning using towels performed every year. Due to the generally low success rate in re-closing, in many cases these flashovers led directly to outages affecting large areas of the country. The extent of these power interruptions was made worse by the need to find and replace electrically-punctured porcelain cap & pin insulators, whose failure in many cases resulted in line drops. Because of this experience, over the years many standard profile porcelain cap & pin type insulators were replaced by double-shed open profile designs or deep under-rib (anti-fog type) units having greater creepage distance in order to maintain the same installation length. In most areas with medium or heavy pollution, however, porcelain cap & pin insulators already had increased creepage distance and therefore only a few additional units could be added to strings because of limitations in tower dimensions.

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It was typically in such cases that porcelain cap & pin porcelain units began to be replaced with SR composite insulators for trial operation between the end of the 1980s and the beginning of the 1990s. The original idea of employing a lower creepage for these insulators versus the typical target creepage distance specified for porcelain was based mainly on excellent results obtained from artificial pollution tests as well as from natural pollution testing conducted at the Dungeness Insulator Station in Brighton, England.

Because of the success of the initial trial installations, the first official document on the selection of specific creepage distance for SR insulators was approved by China’s Ministry of Energy in early 1993. In this document, it was suggested that the leakage distance of a SR insulator could be only 3/4 that normally required for porcelain insulators for application in areas having medium to heavy pollution. Another Chinese Mechanical and Manufacturing Industrial Standard – JB/T 8460-1996 – “Composite long rod insulators for use on high voltage overhead lines: dimensions and characteristics”, which mainly serves as guidance for manufacturers of SR insulators, also covered the topic of creepage distance. Basically, it suggested that the specific creepage distance of SR insulators could be simplified into only two levels, i.e. 20 mm/kV and 25 mm/kV for applications in areas with pollution levels of light to medium and medium to heavy, respectively. For heavily-polluted areas, this suggestion was actually very close to the 3/4 rule mentioned earlier.

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Because of this experience and policy, millions of SR composite insulators now in service in parts of China exposed to pollution are designed with less creepage distance than equivalent porcelain/glass insulators for these same areas. Although the typical creepage of these SR insulators is only about 75 to 80 percent that of porcelain units, they still perform well.

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There has naturally been intense discussion about this concept of shorter creepage distances. Some experts, for example, have worried about the impact on premature ageing and on the possibility of loss of hydrophobicity of such insulators. As a result, they have recommended specifying a similar creepage distance to porcelain/glass insulators – a suggestion which was in fact adopted in the case of some HV transmission line projects. Actual operational experience in China, however, has indicated that no failures or reports of damage have been related directly to the policy of a lower creepage distance. Any decrease observed in hydrophobicity of such SR insulators was typically related mainly with the type of pollution involved or the quality of the insulator as opposed to duration in service.

SR composite insulators are now employed throughout China as the major countermeasure to pollution. Most operate in areas of medium to heavy pollution and where porcelain or glass insulators had experienced severe pollution flashovers on an annual basis. Since no special cleaning is required for these insulators, maintenance work is significantly reduced compared with porcelain/glass insulators. Indeed, the overall experience so far is that after replacing porcelain/glass with SR insulators, outages trigerred by pollution flashover have basically been eliminated. It was for these reasons that for 1000 kV UHV AC and ± 800 kV UHV DC projects, where these lines pass through polluted areas, large numbers of SR insulators having comparatively short creepage distances have been applied.

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Professor LIANG Xidong

Tsinghua University