Statistical analysis of surface discharges in rotating machine stator insulation system

Summary

Partial Discharge (PD) phenomenon in rotating machine is one of the most degrading mode that affects the insulation system. Even though the main insulation of rotating machine is Mica, which is highly PD resistant, the prolonged discharges that take place at the surface of stator bars are highly detrimental to insulation system and can eventually lead to machine insulation failure. In this paper, PD measurements are conducted on Mica-Epoxy-Glass laminates using a cylindrical-plane electrode system. A distinct Phase Resolved Partial Discharge (PRPD) pattern was observed for surface discharges in the sample-electrode arrangement. The discharge pattern in the sample is similar to the surface discharges (end-winding discharge) pattern in actual rotating machine insulation system. Statistical analysis of the PD parameters like charge magnitude (q), phase angle of occurrence (?) and charge frequency (n) is used to classify the surface discharges. In-order to classify the surface discharges, a distribution is made using the PRPD pattern. The mean discharge magnitude distribution which represents average charge magnitude as a function of corresponding phase angle is found useful to correlate the surface discharge with PRPD patterns. This distribution has distinct shape in the positive and negative half cycle of voltage waveform. The shape of distribution is described using statistical operators: skewness and kurtosis. Finally to confirm the patterns, known surface defect was simulated in a 6.6 kV stator coils to simulate surface discharges (end-winding discharge). PRPD patterns obtained on mica-epoxy-glass laminate and 6.6 kV stator bars for similar type discharges were compared and found to have similar characteristics.

Additional informations

Keywords

Partial Discharge, Stator Insulation, PRPD, Surface Discharge, Skewness, Kurtosis