Experience with continuous Partial Discharge monitoring of stator windings

Partial Discharge (PD) testing has been used for decades to evaluate the insulation condition in motor and generator stator windings rated 4kV and above. Most users prefer an on-line test. The predominant test performed today involves permanently installing PD sensors on the stator winding and using portable instrumentation to measure the PD activity periodically. Although being an effective approach for higher voltage machines, continuous monitoring of the stator winding is required for medium voltage machines. In addition, continuous monitoring of high voltage machines does have some advantages. This paper presents results selected from many dozens of motors and generators where the PD has been continuously monitored for up to 7 years. The comparison with portable measuring systems is discussed. The analysis shows, that it is more effective to diagnose the winding condition with continuous monitoring since the trend in PD is much easier to discern when the data is automatically collected under the same machine operating conditions.