The relation between thermal and electrical stress and the PD behavior of epoxy-resin transformers

The partial discharge (PD) behavior of epoxy resin transformer coils in a temperature range between -30 and 180°C is discussed. In particular, the PD inception voltage, which is a very important parameter for the judgement of the PD characteristic of a transformer over the temperature range, is considered. The PD inception voltage is determined at different static temperatures in step tests. The tempering of the coils occurs from the outside by submerging the oil in tempered fluid,as well as from the inside, with the help of current losses. Models of HV coils of distribution transformers are used as specimens. The dependencies between the PD inception voltage of the coils and parameters such as wire insulation and inner or outer tempering are determined and discussed. The results of long-term tests in which five coils are stressed with a voltage of 1.75 kV at a temperature of 175°C for a duration of one year are presented. During this time PD measurements with a sensitivity of 2 pC were carried out