Surge characteristics and protection of distribution transformers. Final report

This report describes the results of an experimental study to determine the causes and prevention of anomalous failures of distribution transformers due to lightning. Utility surveys, literature searches, and the contractor's prior research determined the experiments performed to test the validity of probable causes. Scaled impulse tests on winding insulation and full magnitude tests on representative 95 kV BIL rated transformers were performed. The impulse tests simulated non-standard impulse wave shapes, methods of application, and simultaneous transient loading on new and aged transformers. Similar voltage and current surge tests were performed on typical 10 kV rated silicon carbide element distribution arresters and metal oxide varistor arrester elements. A new transient voltage computer program was utilized to predict the performance of 125 kV BIL transformers under the same type tests. The primary causes of anomalous failures were attributed to lightning surges entering shell type transformers with non-interlaced 240/120 volt coils, via the low voltage terminals; high rate of rise lightning surges impinging on transformers protected by separately mounted arresters; and shell type transformers using solid paper pad high-to-low insulation when tested to demonstrate their effectiveness.