THE RELATION BETWEEN THERMAL AND ELECTRICAL STRESS AND THE PD BEHAVIOR OF EPOXY-RESIN TRANSFORMERS

This paper deals with the PD behavior of epoxy resin transformer coils in a temperature range between -30 and 180 degrees C. In particular the PD inception voltage as a very important parameter for the judgement of the PD characteristic of a transformer over the temperature is considered. The PD inception voltage is determined at different static temperatures in step test. The tempering of the coils occur from the outside by submerging the coil in tempered fluid, as well as from inside, with the help of current losses. Models of HV coils of distribution transformers are used as specimen. The windings of the coils consist of wires which have different insulation. Some windings consist of wires with a thin layer of varnish. The others have wires with a glass-silk wrapped insulation. 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. Furthermore the results of long term tests are introduced in which five coils are stressed with a voltage of 1.75 kV at a temperature of 175 degrees C for a duration of 1 yr. During this time PD measurements with a sensitivity of 2 pC were carried out.