Voltage stress on 8000 HP induction motor due to GTO current source inverter — Case study

Voltage surges with fast rise times of the order of fractions of a microsecond can severely stress the motor winding insulation. If this insulation material does not have the appropriate dielectric strength and thickness, this voltage surge will result in the electrical breakdown in the first few turns in the stator winding of the motor. This insulation failure will result in a high turn-to-turn current eventually leading to a ground fault. The objective of this paper is to investigate the magnitude and rise time of the voltage surges on the motor winding while in service. Measurements were conducted on a medium voltage (4.16 kVac) variable frequency drive (VFD). The drive system consisted of a six-pulse, symmetrical GTO current source inverter (CSI), connected to a 4-pole, 1800 rpm, 8,000-hp 3-phase induction motor. Experimental results and computer simulation for the 4.16 kVac, 8,000-hp CSI drive are given to substantiate the results. A high frequency induction motor model is discussed and used in the simulation. This paper provides useful design information on peak voltages and dV/dt impressed by CSI on the first few turns of the motor winding.