Abstract – Several utilities have found partial discharge testing to be very useful for identifying motors or generators with deteriorated stator winding insulation. Such partial discharge tests are sensitive to problems associated with overheated insulation, loose stator bars. and pollution, all of which can cause in-service failures with long associated outages and high repair/rewind costs. Unfortunately, the partial discharge test is not widely applied since present test methods require considerable skill to perform, and/or require machine outages. These problems are due to the stator winding partial discharges being masked by high levels of internal and external electrical noise. Partial discharge tests have now been developed which eliminate the effect of noise, thus allowing plant personnel to perform the test without any machine outage.
Two broad types of noise exist: arcing from slip rings, shaft grounding brushes, etc. within the machine (internal noise); and arcing and partial discharge from the bus connecting the machine to the power system (external noise). Installations on several machines have shown that external noise can be eliminated by a pair of capacitive couplers (bus couplers) mounted on each phase of the output bus. Sensors which are mounted within the slots of the stator winding (SSCs) have been shown to eliminate both external and internal noise. An instrument called the TGA has been developed to measure the signals from either type of sensor, distinguish noise from stator winding partial discharge, and perform a pulse height, pulse phase analysis on the partial discharge signals. Using the TGA, deteriorated windings have been found to have as much as 20 times higher partial discharge activity than good windings. The SSC type of sensor is best for use on large turbine generators or other critical machines. due to its ability to effectively eliminate false indications of deteriorated windings.