Off-line and On-line Detection of PD in Inverter-Fed Random-Wound Motor Considering IEC Technical Specifications

In the inverter-fed motor, the surge voltages with fast rise time and high voltage peak may cause partial discharge (PD) which potentially damages the electrical insulation system. Off-line tests with an impulse generator are executed under the IEC technical specifications of TS60034-18-41 (2006) and TS61934 (2011) for insulation systems of low-voltage motors fed from inverters, which are required of PD free in their expected service lives. On-line tests are executed during operation of an actual motor fed from a PWM inverter, since the PD properties may differ from the off-line's ones. This paper mainly discusses the detection sensitivity of PD for off-line and on-line tests using the PD detector at motor terminal. Firstly, effects on PD signal at the motor terminal in the stator windings under surge voltage are examined, when PD signal propagates through some coils after generating PD in the twisted pair that is connected to the motor winding purposely. In our test conditions, it is confirmed that PD signal propagating through the motor winding to the motor terminal can be detected correctly. Secondly, effects of noises on detection sensitivity are examined. It is concluded that the residual surge noise signals from an inverter or an impulse generator can be excluded using the high-pass filter of the appropriate selected frequency, then PD signals can be easily recognized, even for on-line tests. Finally, the transient noises hardly have effect on the PD detection of off-line tests under an IEC-TS61934, since the time range required for the PD distinction of off-line tests is extremely shorter than of power frequency tests. These testing results and considerations may be useful for the standardizations of the IEC TS60034-18-41 and TS 61934 in the near future.