Service Experience with Hollow Composite Insulators for Bushings & Other Substation Apparatus
If one studies the evolution of composite insulator technology since its initial development during the 1970s, one fact quickly becomes clear: market experience for composite apparatus insulators has been much different than for composite line insulators. For example, composite insulators for overhead lines were received with great interest by power supply companies due to their promised advantages of superior pollution performance as well as light weight combined with high strength. However, much of the initial enthusiasm ‘evaporated’ once users became aware of the accompanying risk of mechanical failure and dropped conductor – a failure mode labeled brittle fracture that reached its peak during the 1990s. To some extent, mechanical fracture of the core rod is still encountered today, especially with older generations of line insulators or with units that have been damaged during handling or produced without proper quality control.
Nevertheless, after intensive research in the U.S. and China, most experts now believe that the cause of this problem is well understood and that modern composite line insulators are designed and manufactured to virtually eliminate such a risk. Composite hollow core insulators, on the other hand, never suffered from any documented mode of failure. In fact, service experience has, by and large, confirmed that this technology offers the high performance expected across most applications, assuming that the insulators were properly specified for their service environment.
For example, a survey was undertaken in recent years and involved close-up inspection of several dozen silicone rubber apparatus insulators from 145 to 420 kV AC and from 400 to 500 kV DC. These units had reportedly been in service for years across a range of different environments characterized by high humidity and/or UV as well as varying levels of pollution severity. According to the final report, no significant deterioration was observed nor had any lost hydrophobic properties, even to an intermediate degree. All still possessed a wettability class (WC) of at least 4 and typically much better, meaning good long-term hydrophobicity.
Still, in spite of positive field experience, the rate of application of composite hollow core insulators has until recent years proven much lower than for their ‘cousins’ operating on overhead lines. One of the reasons for this is that, unlike line insulators that are typically specified and purchased directly by the power companies or turnkey line contractors, equipment insulators are usually ordered by the OEMs of breakers and transformers, etc. For these types of customers, the main purchase considerations have always been cost as well as a guaranteed long service life. Here, competition from porcelain has proven challenging, especially at the lower transmission voltages where the greatest volumes are required.
Yet in spite of having lagged behind expectations for years, application of hollow core composite insulators is now growing rapidly – even exponentially. Driving this is a growing requirement for safety against risk of catastrophic failure by power utilities that, in many cases, now only accept equipment insulated with hollow core composite type insulators. While in 1993 less than 100,000 units were estimated to be in service, that population has soared and now numbers well over a million, with most installations taking place in just the past ten years.
After more than 30 years of field experience, composite insulators have become a mature technology and are widely regarded as a promising alternative to traditional ceramic insulators. In the case of overhead AC transmission lines, it is estimated that the population of composite insulators already in service numbers circa 20 million. When it comes to HV substation apparatus, it is estimated that more than a million composite housings are now in service and the trend seems to be toward exponential growth.
Composite insulators have been widely adopted for DC applications as well, however service experience here is much less due mainly to the relatively low number of DC lines and substations compared to AC. Nevertheless, exponential growth is foreseen in DC too due to the recognized advantages of superior performance under pollution, lightweight and safety in the event of explosive failure.
Composite-housed bushings for 750 kV breaker. composite insulator Service Experience with Hollow Composite Insulators for Bushings & Other Substation Apparatus Composite housed bushings for 750 kV breaker
Composite-housed bushings for 750 kV breaker.
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composite insulator Service Experience with Hollow Composite Insulators for Bushings & Other Substation Apparatus Screen Shot 2017 03 10 at 16
Performance of silicone rubber housings under pollution is considered impressive.
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There are two broad yet complementary approaches used when collecting and analyzing service experience for any new type of electrical equipment or network component. The first is based on general experience and the output here includes numbers of units installed, average or maximum service life and, if possible, some estimate of reliability as well. The second approach, by contrast, involves a close-up investigation. Here, selected in-service components operating for a long period in different specific environments or installed for many years at test stations are carefully inspected.
According to the first approach, it can be said that the overall service experience with the latest generation of composite line and apparatus insulators is satisfactory both from the pollution performance and ageing points of view. For example, a recent CIGRE Brochure indicates that the reliability of modern composite line insulators is about the same as for glass insulators.
One of the first efforts toward a close-up investigation was conducted by STRI between 1994 and 1998. In-service inspections were carried out by 36 power utilities on 279 composite line insulators and these findings were complemented by results from test stations. To allow proper comparison of the insulators removed and inspected from different sites, a Guide (similar to guides presently offered by EPRI and CIGRE) was developed showing typical examples of deterioration and damage reported on composite line insulators. One such analysis is presented in Fig. 1.
Figure 1: Example of analysis performed on line composite insulators. composite insulator Service Experience with Hollow Composite Insulators for Bushings & Other Substation Apparatus Example of analysis performed on line composite insulators
Figure 1: Example of analysis performed on line composite insulators.
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Based on this visual inspection of composite insulators installed at long-term test stations as well as in-service, the main conclusion at the time was that whatever deterioration was observed was more likely associated with design and/or manufacturing problems rather than ageing of the material. This turned out to be an important finding and supported continued development and application of composite insulator technology.
More recently, ABB undertook a general review of service experience with 110,000 of its silicone rubber apparatus insulators and the findings, summarized in 2011, were uniformly positive. The first attempt at a close-up investigation for these insulators, however, was based only on results from certain test stations. These included two coastal test sites (Kelso in South Africa and Dungeness in the U.K.) and one clean inland site at STRI in Sweden. The results supported continued application of composite insulators and could be summarized as follows:
1. No deterioration of silicone rubber insulated apparatus was observed, either in clean or in polluted environments. Moreover, the superior performance (i.e. less ageing) of these units in comparison with silicone line insulators also installed at these same sites was attributed to differences in levels and distribution of E-field (see Fig. 2).
Figure 2: Difference in the location of maximum E-field for line versus for apparatus insulators. composite insulator Service Experience with Hollow Composite Insulators for Bushings & Other Substation Apparatus Screen Shot 2017 03 10 at 16
Figure 2: Difference in the location of maximum E-field for line versus for apparatus insulators.
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2. The specific creepage distance of composite insulators could be reduced, in comparison with porcelain insulators, without sacrificing service performance.
Fig. 3: Geographical location of sites included in survey (red circles). composite insulator Service Experience with Hollow Composite Insulators for Bushings & Other Substation Apparatus Screen Shot 2016 04 21 at 3
Fig. 3: Geographical location of sites included in survey (red circles).
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