Comprehensive Classification of Motor Winding Insulation Aging Scenarios Based on Transient Current Oscillations

Abstract

The electrical insulation of inverter-fed rotating AC machines fed by long supply cables is currently experiencing increased stress caused by both increasing switching frequencies and voltage slew rates as semiconductor technology progresses. The pulsed voltage distributes unevenly along the winding due to reflected wave phenomena in the cable-motor system creating some differential-mode and common-mode voltage harmonics at frequencies in the range of tenths of kHz up to a few MHz. This results in non-uniform insulation aging, with particular spots of the winding exhibiting higher degradation rates. The broad consensus in literature is that the turn-turn, ground-wall and phase-phase insulations near the line-end are disproportionally stressed. This paper proposes a method to, for the first time, classify among these three types of degradations. The method is based on the extraction and analysis of dedicated transient phase current oscillations. As will be shown, the transient phase current caused by a differential-mode excitation contains abundant oscillations that carry distinct insulation-related information, which can be used to differentiate among turn-turn, ground-wall and phase-phase insulation aging. The intensity of the nascent insulation degradation is also tracked. Experimental results obtained from a medium-voltage induction motor used in railway propulsion verify the applicability of the proposed method.