Effect of surface degradation on slot partial discharge activity

Abstract

To improve our understanding of slot partial discharge (PD) mechanisms, an accelerated aging test was initiated two years ago. This long-term experiment is being performed on six stator bars subjected to slot PDs under electrical, thermal and mechanical stresses. It is well known that slot PD activity in air-cooled generators is harmful to the stator winding ground insulation. The degradation induced by slot PDs will modify the physical properties of the surfaces of the cavity where PDs take place and this in turn will influence the slot PD process. To understand the evolution of slot PD activity and the changes in its PRPD (Phased Resolved Partial Discharge) pattern, it is therefore essential to recognize and understand the interdependence between surface modifications and PD signals. This paper presents results of visual observations of stator bar surface degradation, changes in surface conductivity and the effect on the evolution of PD signals in the presence of slot PD activity under conditions of electrical, thermal and mechanical stresses.

Analysis of partial discharge sources in stator insulation system using variable excitation frequency

“…The reduction in peak magnitude and a number of pulses is drastic in case negative discharge pulse. The reason for this drastic reduction in negative integrated charge was the combined effect of charge spread on anode insulation surface and oxidation of the cathode steel surface [23,24]. The effect of charge spread is discussed in detail in previous studies [20,22].…”

“…To replicate the core portion, a flat steel plate was used and it was tightly clamped over the bare insulation surface with a gap of 0.3 mm between insulation surface and steel plate. A conditioning time of 12 h at 6.6 kV, 50 Hz was maintained to obtain the typical asymmetric PRPD pattern of slot discharge [22,23]. PD measurements were carried out after immediately removing the tape, after conditioning time of 1 and 12 h at 4, 5 and 6 kV with different excitation frequencies.…”

Thermal and thermo-mechanical aging of epoxy-mica insulated stator bars

“…This behaviour can be explained by the higher relative humidity of the air in the lab ( [3,4]), probably affecting external discharges around the grading junction. In dry air, external PD activity tends to be higher because all free electrons can participate in the avalanche process, whereas at higher humidity the electro negativity of water will capture some of the electrons and reduce the overall PD activity.…”

Partial discharge detection for stator winding insulation of motors using artificial neural network

“…At this moment, when the defect is close to the high-voltage conductor but has no contact with it, electric discharge is most likely to occur. 5 The accumulation of dust and oil forming a layer of black dirt will reduce the surface resistance of the winding and will help in the formation of electric discharge. Finally, the surface insulation will erode.…”