Service Experience with Hollow Composite Insulators for HV Apparatus
20
YEARS
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INSULATORS
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Service Experience with Hollow Composite Insulators for HV Apparatus Part 1 If one studies the evolution of composite insulator technology since its development starting in the 1970s, one fact quickly becomes clear: the market experience for apparatus insulators has been much different than for line insulators. For example, composite insulators for overhead lines were initially received with great interest by power supply companies due to their promised advantages of superior pollution performance as well as lightweight combined with high strength. However, this initial enthusiasm soon 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 being 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, 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, have never suffered from any single documented mode of failure. On the contrary. Service experience has, by and large, confirmed that this technology offers the high performance expected across most applications,
assuming of course that the insulators were properly specified for their service environment. Nonetheless, in spite of positive field experience, the rate of application of composite hollow core insulators until now has proven much slower 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, transformers, switches, 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 now, the application of hollow core composite insulators finally seems on the verge of growing rapidly – some predict even exponentially. While in 1993 only tens of thousands of 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 last several years. This article, edited by INMR and based on a contribution from Igor Gutman and Christian Ahlholm of STRI as well as Anders Holmberg, Lars Jonsson, Ulf Akesson and Dong Wu of ABB, is the first in an upcoming series of articles that will explore the service performance of composite apparatus insulators. Future articles will also look into some of the improvements that are being made in the equipment used to manufacture them. 83
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Composite-housed bushings for 750 kV breaker.
Photos: INMR Š
A
fter 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. 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.
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
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
Performance of silicone rubber housings under pollution is considered impressive.
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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.
Figure 2: Difference in the location of maximum E-field for line versus for apparatus insulators.
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). 2. The specific creepage distance of composite insulators could be reduced, in comparison with porcelain insulators, without sacrificing service performance.
Figure 3: Geographical location of sites included in ABB survey (red circles).
Given the above, ABB recently undertook and completed a close-up, detailed investigation into real field
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experience with composite-insulated apparatus. The specific design in this case involved the company’s helical profile sheds made from HTV silicone rubber while the locations of greatest interest included severely polluted service conditions. Site Selection for Close-Up Investigation A total of 11 sites of specific interest were selected based on local service conditions and the following criteria: • Relatively long time in service (ideally at least 5 years); • Diverse heavily-polluted environments (e.g. deserts, industrial, coastal) preferably combined with high UV radiation levels; • Less polluted environments (e.g. inland) but with low temperatures as well as frequent rain; • AC and DC voltage applications. This resulted in selection of substations to be investigated in 7 countries: Argentina, Australia, China, Iceland, Denmark, Oman and Sweden. Priority was given to AC installations, however some DC installations were also included. The inspections eventually covered sites with all of the five different environments defined in IEC 60815-1, i.e. desert, coastal, industrial, agricultural and inland. More than 50% of the climates eventually studied were humid according to the definitions in the Koeppens climate classification – one of the most widely used. About half of the sites were in areas with heavy to very heavy Site Pollution Severity (SPS), according to IEC 60815-1. Moreover, 4 sites were located in areas with high levels of UV radiation (i.e. Argentina, Australia and Oman) but below an altitude of 1000 m. The apparatus insulators inspected at these sites represented a range of voltage classes from 145 kV to 420 kV AC and from 400 kV to 500 kV DC. The total operating time of these insulators varied from 2 to 17 years. Note: Part 2 of this article will appear in the Q4, 2013 issue of INMR.
Figure 4: Summary of pollution measurements at different sites (each marked with corresponding number from Tables 1 & 2) presented in relation between Site Pollution Severity and ESDD/NSDD for a reference porcelain long rod insulator. (Numbering corresponds to numbering of sites in Tables).
Table 1: List of Sites Where Inspections Performed Site
Station Name
Voltage (kV)
Country
Station Owner
1
Echeng Converter Station
500 DC
China
SGCC
2
Wurdong Substation
300
Australia
Powerlink
3
Playford B Substation
300
Australia
Electranet
4
Bjaeverskov Converter Station
400 DC
Denmark
Energinet
5
Skagerrak Substation
420
Denmark
Energinet
6
Biskopsgården Substation
145
Sweden
Göteborgs Energi
7
SAAB Switchyard
145
Sweden
Vattenfall
8
Rangárvellir Substation
145
Iceland
Landsnet
9
Geiradalur Substation
145
Iceland
Landsnet
10
Guaymallen Substation, PIP Substation
145
Argentina
EDEMSA
11
Nimr Central Substation
145
Oman
PDO
Table 2: Detailed Site Conditions Site Pollution Severity Years in Service (IEC 60815-1)
Site
Climate (Koeppens Climate Classification)
Type of Pollution (IEC 60815-1)
1
Humid subtropical
Coastal, industrial
d – Heavy
8
2
Tropical savannah
Coastal
d – Heavy
9
3
Hot desert
Industrial, coastal
e – Very heavy
10
4
Humid continental
Coastal
c – Medium
17
5
Humid continental
Inland, agriculture
b – Light
3
6
Humid continental
Coastal
c – Medium
6
7
Humid continental
Inland
b – Light
8
8
Sub-polar oceanic
Coastal
c – Medium
4
9
Sub-polar oceanic
Coastal
c – Medium
15
10
Cold desert
Industrial, inland
d – Heavy
5
11
Hot desert
Desert
e – Very heavy
2 87
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