Evaluation of medium voltage groundwall insulation exposed to high frequency pulse voltages

Class-F 4.0 kV coils designed for medium voltage induction motors are aged under combined thermal and electrical stresses of 500h at a constant temperature of 120 degrees C and 200h at a constant temperature of 150 degrees C and stressed electrically at 60 Hz and with steep-front unipolar pulses of 40 ns rise time. The changes in the partial discharge activity and the dissipation factor as a function of time are reported. During ageing, the temperature profiles are recorded with an infrared camera as a means for determining the highest stressed regions. The experimental results show that pulse ageing is very severe when compared to 60 Hz ageing, despite using a lower pulse voltage, although the location where damage occurs on the groundwall insulation is different.