Since 2006, some utilities have been, experiencing an increased number of polymer insulator failures on their 115-kV and 138-kV transmission lines. Investigations have shown these failures can be attributed to high electric fields (E-fields) occurring close to, or on the high-voltage end fittings of, these insulators. The findings of the investigations suggest that, contrary to conventional wisdom, it might be necessary to consider the application of corona protection on polymer insulators applied below 161 kV.
A 2008 report by the Electric Power Research Institute (EPRI) was conclusive in its findings that there is an issue with polymer insulator degradation on 115-kV and 138-kV lines on certain configurations and specific insulator designs. The polymer insulator failures have raised concerns about the health of the remaining insulators in service, and the EPRI report highlighted the need for utilities to determine appropriate actions they can take to extend the life of their remaining units. Transmission line reliability can be affected if utilities do not have measures in place to minimize the effect of corona discharges on the rubber material and end-fitting seals.
Having seen increased evidence of polymer insulator failures attributed to electrical discharge, a team at Public Service Electric and Gas Co. (PSE&G; Newark, New Jersey, U.S.), a subsidiary of Public Service Enterprise Group, thought it prudent to investigate electrical discharge activity on its recently re-conductored and re-insulated 138-kV lines. The results of the investigation, using a DayCor camera, confirmed there was corona activity. Over at Public Service Company of New Mexico (PNM; Albuquerque, New Mexico, U.S.), the electric utility experienced some of its first failures caused by corona discharges at 115 kV.
High E-fields cause increased corona activity, which in turn causes polymer insulator failure. In dry conditions, high E-fields cause continuous corona activity on the metallic end fittings and nearby insulator surface. This erodes the insulator housing and degrades the end-fitting seal. Over time, this may expose the fiberglass rod to the environment, initiating a failure mode such as a brittle fracture, destruction of the rod by partial-discharge activity or an internal flashover (flashunder).
It is also possible for non-uniform wetting of the polymer rubber material to enhance any high E-fields and thus corona activity on the insulator surface. The wetting may be in the form of discrete droplets or water patches, depending on the surface properties of the rubber and whether the wetting is fog, mist or rain. This wetting enhances the local E-field, resulting in increased corona discharge activity, which occurs on most transmission insulators and is acceptable in limited
amounts. It is the unanticipated high levels of surface E-field magnitudes and the resultant corona activity that may result in accelerated aging and reduced life expectancy of insulators.
A research team examined five insulator failures at three different utilities that were recorded between June 2006 and August 2007 on 115-kV and 138-kV lines. All insulators failed mechanically
because of stress corrosion cracking (brittle fracture). All failures were on the same insulator design and on units manufactured between 1993 and 1999.
The failure investigations showed all failures could directly be attributed to continuous discharge activity from the end fitting under dry conditions. This corona cracked the rubber sheath and degraded the end-fitting seal, allowing moisture to come into contact with the rod, leading to a brittle fracture of the rod.
As a consequence of these polymer insulator failures, utilities have been forced to reexamine the use, or lack of use, of corona rings on 115-kV and 138-kV polymer insulators. For instance, PSE&G has more than 5000 polymer insulators in its network, with a majority of them at the 138-kV voltage level. These insulators represent 20% of its entire insulator population. PNM operates 1100 miles (1770 km) of 115-kV line, representing 7000 structures, of which 12% are equipped with polymer insulators. Utilities, in cooperation with EPRI, have initiated a number of specific activities to assess the risk of 115-kV and 138-kV polymer insulators aging prematurely due to high electric fields and determine what actions to take. These activities included the following:
•Conducting daylight discharge inspections. These are used to determine how many units in service are being aged prema�turely by continuous dry corona activity.