Load tap changers: investigations of contacts, contact wear and contact coking

The Electrical Power Research Institute (EPRI) and BC Hydro (British Columbia, Canada) sponsored a project on improving the maintenance of "on-load tap changers" (LTC). The investigation of contacts focused on stationary reversing switch assemblies (Part 1) and diverter switch contacts (Part 2). For the study of reversing switch assemblies BC Hydro provided an LTC for an in-house "case study" and the actual study lasted about nine months. The objective of this study was the evaluation of stationary reversing switch assemblies and their performance under different current loadings. Resistance, load and operating temperatures were investigated. This work compared the performance of three assemblies, two were new and one was old. The new reversing switch assemblies were made of either copper or of silver alloy (and silver-plated components), while the third, used brass assembly came with the load tap-changer (Part 1). Powertech Labs Inc. conducted in cooperation with BC Hydro, among other investigations, the study of diverter switch contacts of an LTC ("Kent" case study) at one of the utility's substations. This study lasted for nearly one year and addressed transformer oil chemistry, gas evolution and contact performance under conditions of accelerated switch operations (reported elsewhere). In addition various LTC contacts from several substations were assessed. These investigations emphasize the importance of quality control and quality assurance, especially for arcing contacts of the copper infused tungsten type (Part 2).