Assessment of High-Frequency Common-Mode Voltage for Winding Insulation Monitoring based on Novel Voltage Step Excitation

Due to the fast slew rates of modern power switching devices, inverter-fed motors experience intensified insulation stress. In addition, inverter switching with fast slew rates excites several high frequencies including multiple antiresonance frequencies of the winding and supply cable, further suffocating winding insulation at localized winding spots. As a result of the wide range of excited frequencies, high frequency currents and voltages can be used to track insulation state. In particular, the voltage at the star point of inverter-fed motors can show the degree of motor ground-wall insulation aging. However, accessing star point might not be feasible in some applications. In this paper, a special switching scheme is proposed, turning the star neutral voltage visible at the terminal voltage without the need for additional equipment or disassembling. Via solely sensing the terminal voltage, ground-wall insulation will be monitored. In addition, the proposed switching scheme elicits additional motor resonance modes, which extends the variety of current and voltage analysis for insulation condition monitoring. Experimental validation using an inverter-fed 2MW medium-voltage traction induction motor is provided.