Impulse testing of distribution transformers under load

The paper describes a new functional test procedure for evaluating the impulse strength of distribution transformers under service-related conditions, including rated voltage, different current levels, and the effect of precipitation. The test procedure was applied to a representative sample of 54 commercial distribution transformers having a 15kV class insulation system. Specimen transformers used in this project were taken from utility stockpiles. The results reported here showed a significant decrease of impulse strength both with increasing load current and with aging under laboratory conditions. The paper thus presents new information pertaining to transformer failures and may contribute to improved distribution transformer design and surge protection practices.