Pollution can have significant impact on performance of insulators. For example, Spanish TSO Red Eléctrica de España (REE), which traditionally has used primarily glass insulation on its overhead transmission network, experienced high maintenance costs due to the cost of annual washing, especially along its Mediterranean coastline. In spite of this, there was still a high frequency of line faults.
This edited past INMR article, contributed by REE engineers, reported on the series of measures intended to resolve problems due to pollution as well as to increase system reliability and service life of affected insulators.
In 1994, REE first began to test application of composite insulators on the 400 kV Escombreras-Rocamora Line, one of its worst in terms of frequency of faults. The result was that frequency of faults reduced dramatically.
Due to these positive results, starting 1995 these same composite insulators were installed 50 km inland from the Mediterranean and South Atlantic coastline. Then, in 2010, REE included recently acquired circuits in the Canary and Balearic Islands in its composite insulator installation strategy.
Along 50-100 km inland South Atlantic and Mediterranean coast, REE subsequently proceeded to install either glass or composite insulators, based on related maintenance experience over the years. However, among the issues discovered were:
1. Degradation of insulator housings due to erosion in heavily polluted areas;
2. High cost of routine inspection on these insulators to detect such degradation prior to complete failure.
To resolve the problem of limited service life of composite insulators operating in special critical pollution zones, in 2009 REE began to install silicone-coated glass insulators on 11 circuits within these areas. Fig. 4 shows the representative installation of these silicone-coated glass insulators.
For example, composite insulators on the 400 kV Arcos-Puerto and Pinar-Puerto Lines, where their lifespan was only between 1 and 5 years, were replaced with silicone-coated glass insulators.
A decade after the silicone-coated glass insulators were installed, these remained in good condition, with no recorded faults and no maintenance action required.
In 2013, three different types of insulators were installed on each phase of the 400 kV Nueva Escombreras-Rocamora Line with the goal of comparing performance of different technologies in the same pollution zone. The three different insulators all had the same creepage distance and total length:
• Experimental composite insulation (Composite Class 1).
• Conventional composite insulation (Composite Class 2).
• Silicone-coated glass insulators
No faults were recorded on the line during the three years following installation of these insulators. In the case of the Class 1 composite insulators, some erosion on the housing was detected. More significant and deeper erosion was detected for Class 2 composite insulators. The silicone-coated glass insulators showed evidence of minor pollution accumulation on the underside of some insulators and some degradation of the silicone coating on the upper side. Nevertheless, there was no significant risk of mechanical failure. Based on this field test, it was concluded that the silicone-coated glass insulation offered superior performance to composite insulators in zones of high pollution.
Given this experience, between 2016 and 2017, REE installed silicone-coated glass on its 66 kV Gran Tarajal-Matas Blancas Line, located in Fuerteventura on the Canary Islands. In 2016, there had been a failure of a composite insulator failure on this line that caused a temporary blackout affecting several areas. The failed insulator, first installed in 2013 on Tower #84, was found to have suffered serious degradation of the housing with resulting exposure of the core rod and some fiber carbonization (see Fig. 8).
In Sept. 2016, based on this event, REE developed the following Action Plan:
• Analyzing faults and other incidents on the island’s lines as well as their locations and causes;
• Visually inspecting composite insulators installed on the 66 kV GTR-MTB Line and replacement for coated glass insulators as well as visually inspecting composite insulators installed on the 66 kV LSL-GTR line.
• Laboratory testing on some of the composite insulators installed on 66 kV lines, including GTR-MTB, LSL-GTR, CRJ-PLB and CRJ-LSL.
• Surveying manufacturers to establish recommended specific creepage distances & corona ring requirements;
• Analyzing degradation areas and creepage distances used in those areas;
• Determining optimal inspection frequency for composite insulators;
• Revising internal standards and regulations;
• Elaborating minimum spare requirements for glass insulator discs;
• Revising technical qualifications of maintenance personnel conducting line inspections.
Based on this Action Plan, REE concluded the following:
1. Cause of failures was degradation of the core by partial discharges in the zone with high electric gradient with subsequent carbonization of the glass fibers. This is typically associated with very high pollution service environment;
2. Visual inspection of 66 kV LSL-GTR (31mm/kV) showed that insulation installed in 2014-2016 remained in good condition;
3. Visual inspection and laboratory testing of 66 kV GTR-MTB (31mm/kV) revealed that insulation installed in 2014 on 18 towers was now in poor condition with slight degrees of erosion in the silicone housing of 5 insulators and unsatisfactory results during laboratory tests, e.g.
• Level of pollution in Fuerteventura was higher than in other Canary Islands.
• Insulators of 35 mm/kV specific creepage with 2 to 3 years of service were already in poor mechanical and electrical condition.
• Insulators having 62 mm/kV specific creepage and with 9 years in service were still in good condition
4. Survey of manufacturers indicated that
• Corona rings need to be installed in areas of high pollution and voltage < 220 kV.
• Specific creepage distances between 40 and 55 mm/kV were being supplied to countries such as Dubai, Iran, Saudi Arabia, Tunisia and Egypt for both composite and coated glass insulators.
5. There is a need to inspect live-line using a workbench so as to have a better view of insulators.
6. The terminology ‘Special Degradation Zone’ is to be assigned to Fuerteventura Island, which implies installing only silicone-coated glass.
7. Install silicone-coated glass insulators with specific creepage of 61 mm/kV along the entire GTR-MTB 66 kV Line.
Results obtained during in service performance evaluations indicated that silicone-coated glass insulators are an excellent technical and economic solution to improve the behavior of insulation in areas of high pollution and also to avoid need for periodic cleaning of insulators. Superior performance is achieved in areas of extreme pollution, combining the hydrophobic properties of silicone coatings with the mechanical reliability of glass insulators. After years of experience, it was deemed necessary to update the insulation, installation and maintenance policy at REE.
In addition, to achieve optimal insulation in each service area, a pollution map of Spain was developed according to IEC 60815. This allowed proper selection and design of the best insulation for each zone – something that is especially important in areas of high pollution based on DDDG and ESDD/NSDD measurements.