Partial discharge behavior of epoxy resin-impregnated transformer coils

Partial discharge (PD) investigations of industrially made epoxy-resin-impregnated transformer coils and coils produced under laboratory conditions in a temperature range between -30 and 180°C are described. The PD characteristics of the industrial coils can be reproduced with the simplified lab coils, and these results can be transferred to the industrial coils. The PD measurements show that the PD inception voltage decreases with rising temperature, which is explained with the superposition of two temperature-dependent phenomena. First, the temperature-dependent rising values of the relative permittivity of the epoxy resin strengthen the electrical field inside the gas cavity. Second, the ignition condition, i.e., the relative gas density in the cavity, changes due to diffusion. From the observed voltage dependence of the repetition rate and phase position of the PD pulses and with the help of a capacitive electrical circuit, it is shown that the measured PDs are discharges in gas-filled cavities