Partial Discharge Inception Voltage of Pigtail Samples with Type II Insulation and Sinusoidal vs Switched Voltage

Partial discharge inception voltage (PDIV) under sinusoidal voltage and switched voltage with dV/dt of 20 V/ns have been investigated for pigtail samples with type II insulation consisting of Litz wire as conductor and mica-Kapton tape as groundwall insulation. Voltage was increased in steps of 100 V
peak
with duration of 60 s for both sinusoidal and switched voltage with switching frequency of 45 Hz until PDs were observed. No significant difference in the PDIV for sinusoidal and switched voltage was found. Further, sinusoidal voltage was applied until breakdown. No correlation between breakdown voltage and PDIV was found, but very large PDs (∼ 100 nC) consequently initiated 4-5 kV below breakdown voltage. Dissection of the tested pigtails showed multiple voids in the wedge between Litz wire strands and the inner insulation layer, suspected to cause the main portion of the PDs. Such voids with only a thin varnish covering the electrode surfaces would get less significant space charge build-up. Therefore, a low field enhancement from space charges is expected during the fast voltage change of switched voltage.