Voltage stress on induction motors in medium-voltage (2300-6900-V) PWM GTO CSI drives

In order to reduce the cost and improve the efficiency of medium-voltage induction motor drives, it is desirable that no power transformers be used. The GTO current source inverter (CSI) drive can achieve this requirement. However, the transformerless design of the drive may introduce a high-voltage stress on motor windings, which may deteriorate the motor insulation life. In this paper, the line-to-ground and neutral-to-ground voltages of the motor fed by the GTO CSI are investigated. It is demonstrated that the maximum line-to-ground voltage applied to the motor could be twice as high as the motor-rated phase voltage. Computer simulation and experimental results from a 4000 V, 1250 HP drive are given to verify the theoretical analysis. The effects of DC link choke arrangement on the motor voltage stress are also discussed. This paper provides a valuable design guidance for the insulation of medium-voltage motors used in the GTO CSI drives.