The Use of Partial Discharge and Impulse Voltage Testing in the Evaluation of Interturn Insulation Failure of Large Motors

Several 13.2 kV, 21,000 HP synchronous motors failed as a result of interturn insulation breakdown. Investigation of motor failures indicated that the cause was related to high magnitude, steep fronted surge waves (References 1 and 2). Partial discharge (corona) related failures were discounted without any actual measurements. To confirm the validity of the previous investigation, partial discharge testing was conducted on four new 21,000 HP motors and on one failed 21,000 HP motor. Additionally, surge impulse testing was performed on the failed motor to assess interturn insulation endurance. This paper presents the results of above tests. It is concluded that failure of motors could not be caused by corona, Confirmation of previous investigation results indicated that partial discharge testing can be used as a diagnostic tool in the evaluation of interturn insulation of large motor.