Study on stator winding degradation under bar vibration

The stator winding insulation system on service is exposed to a combination of thermal, electrical, environmental and vibrational stresses. With the introduction of the epoxy mica insulation system, most failures that gradually degrade the stator winding are the result of bar (or coil) vibration (The term "bar" will be used generically to include coils in this article), especially in high voltage (HV) stator windings. Stator bars in slot are subjected to a certain electromagnetic force resulted by interaction of the stator current and rotary magnetic field, what would make the bars into vibration. The vibration makes the bar loosening, abrades first the slot conductive coating and then the ground wall insulation. After that, slot discharge may occur across the air gap between the core iron and the bar surface, which is a common failure mechanism of stator winding insulation. Besides that, vibration sparking may occur on the surface of the bar in slot because bar vibration is involved, which can result in rapid failure. In this paper, an actual new stator bar of 6kV/1250kW motor was used for accelerated vibration aging test. The bar was placed in a slot model and fixed tightly with slot wedges at the beginning. Then it was driven by an exciter into vibration continuously for 90 days. Partial discharge (PD) between the bar copper strand and the core iron under several different applied voltages, which was used to express the stator winding degradation, was measured before aging and after every 30 days. The results showed the stator winding degrading process and PD characteristics under stator bar vibration.