Analysis of Transition Cause of Partial Discharge Patterns in High Voltage Motor Stator Windings

A weather protected(WPII) high voltage(HV) motor stator winding has a dry band at the slot end, end-winding, and circuit ring and etc. due to environmental effects caused by the inflow of foreign substance. As a result, arcing or flashover causes insulation breakdown. This paper presents the result of an insulation diagnostic test on a WPII HV motor(2,850 kW, 6.6 kV) stator winding operated for about 4 years, where the severe contamination occurred inside of it. So, the test was performed after cleaning and drying it. The partial discharge(PD) was measured at 6,710 pC, 9,700 pC, and 12,000 pC at phase to ground voltage(3.81 kV), 1.25 times the phase to ground voltage(4.76 kV), and rated voltage(6.6 kV). The PD decreased from 12,000 pC to 10,000 pC while staying at 6.6 kV for 4∼5 minutes. The pattern of PD was converted to a discharge occurring in internal delamination of groundwall insulation from a discharge of rabbit ear-like. As a result of measuring the PD while reducing the voltage again, it decreased to 7,950 pC and 3,920 pC at 4.76 kV and 3.81 kV, respectively. Based on the results of the test, we found that when fine conductive foreign substance remains on the surface of the end-winding of a HV motor, and then energizing high voltage, the foreign substance burns, showing a rabbit ear-like discharge pattern, and then disappears altogether, converting to a discharge generated from internal delamination of groundwall insulation.