EPRI - East Kentucky Power Cooperative Polymer Failure Investigation

Paul Dolloff, PE

"When Andrew told me that one of our units that we removed from service failed in EPRI's new 230kV aging chamber test after only one month's time, I knew EKPC had avoided an outage and our decision to remove all of our high risk polymer insulators from critical lines was correct" said Dr. Paul Dolloff, research engineer at EKPC. "With our newly purchased DayCor corona camera and the inspection techniques learned from EPRI” (Guide to Visual Inspection of NCI, and Guide to Corona and Arcing Inspections of Overhead transmission Lines), Dr. Dolloff added, "EKPC will begin an in-house DayCor II™ inspection program designed to identify and remove in-service, high-risk NCI

Early DayCor II™ inspection and testing helped EKPC to identify and remove high-risk polymer insulators from service, preventing catastrophic failures and thereby avoiding unscheduled outages.

Challenge

Between 1996 and 1998 East Kentucky Power Cooperative (EKPC) experienced six failures of polymer insulators over a range of voltage levels shown in the table below. Late in 1999, with a large number of similar units still in service over a wide range of voltage levels, some of them on key transmission lines while others serving critical loads, EKPC contacted EPRI researchers to determine whether the remaining units were at a high risk of failure and to develop a plan of action.

Approach

EKPC decided to replace all polymer insulators of similar design and manufacture from key transmission lines and radial lines servicing critical customers. One of such a key transmission line was a 138 kV line that fed the Inland Container Paperboard and Packing Inc., a large industrial customer with a 50MW load.

EKPC sent to EPRI a number of units that failed, for analysis. EPRI researchers of the research center in Lenox MA, led by Dr. Andrew Philips, identified the failure modes and their causes, and claimed that the principal cause of the failures was moisture in the internal fiberglass rod that penetrated through the end fitting. When this happens, either acid builds up and threatens the integrity of the fiberglass rod (i.e. mechanical failure), or the moisture tracks up the rod electrically shortening the NCI until “flash under” occurs (i.e. electrical failure). Realizing there was a potential problem with similar units that remained in service, EKPC contracted EPRI to perform a three-day on-site inspection with the DayCor camera. After the inspection, EKPC removed from service 30 units that EPRI had identified as high-risk and sent them to the EPRI Lenox Center for evaluation. EPRI staff dissected and analyzed the units, revealing that in over 90% of the cases moisture penetrated the end fittings. These findings led EPRI to conclude that similar units remaining in service should be considered high-risk.

EKPC decided to replace all polymer insulators of similar design and manufacture from key transmission lines and radial lines servicing critical customers. One of such a key transmission line was a 138 kV line that fed the Inland Container Paperboard and Packing Inc., a large industrial customer with a 50MW load.
All polymer insulators removed from this line were sent to EPRI for further evaluation and use in other EPRI-sponsored research projects. Seven of these units were installed in the 230kV Accelerated Aging Test that was initiated in January 2001. After only one month in the aging chamber, one of the units failed electrically in a manner similar to that observed previously in the field. It is certain that this unit would have failed in-service; hence, EKPC avoided an unscheduled outage. --up>