Comparison of off-line and on-line partial discharge for large motors

Partial discharge(PD) tests were used to evaluate the insulation condition of seven large motors. These tests were conducted using a conventional partial discharge detector (PDD), turbine generator analyzer (TGA) and partial discharge analyzer (PDA). Off-line PD measurements were performed using the PDD. Offline PD pulse from one 80 pF (or 500 pF) epoxy-mica coupler was measured with the PDA instrument at 3.81 kV, 4.76 kV and 6.6 kV. On-line PD pulses from the sensors were measured with the TGA and PDA instruments during normal operation. The PDD, TGA and PDA test results showed that internal discharge was detected from voids in the groundwall insulation. It was confirmed that the PDD, TGA and PDA measurements can be directly correlated with the pattern and magnitude of PD. The results of offline and online PD measurements were judged to be indicative of stator insulator in good condition for the seven large motors.