Distortion of phase resolved partial discharge pattern due to harmonics and saturation

Field measurements on high voltage apparatus, such as voltage transformers (VT) as well as tests on gas insulated switchgear (GIS) powered on site by VTs showed up power frequency harmonics and/or distortions of the AC shape due to core saturation. The phase resolved partial discharge patterns acquired with a pattern recorder in such applications are bearing correlated distortions. Depending on the gas physics of the discharging site, for instance, some of the patterns are correlated to the AC voltage directly, while others are correlated to the voltage shape to the power of three (like with an ideal flat delamination). Further deviation of 'real-life' patterns vs. those obtained from controlled lab measurements is gained in case the measurement has to be taken far beyond the inception voltage.