Partial discharge (PD) testing has been used for many decades as a quality control test for the electrical insulation in high voltage electrical apparatus such as cables, switchgear, transformers and rotating machine stator windings. PD testing is also used on-site (i.e., at the plant where the HV equipment has been installed) as an off-line test to determine if insulation problems are developing in this equipment during service. Now it is becoming more common for on-line PD monitoring to detect insulation deterioration due to ageing in service [I].
On-line PD monitoring was perhaps first applied to motor and generator stator windings when Johnson used high frequency current transformers (HFCTs) and capacitive couplers to detect the PD during normal generator operation in the late 1940s . Shortly after, some large OEMs and a few utilities introduced their own version of the on-line PD test for rotating machines [3, 4]. These tests seemed to be particularly good at detecting problems, such as loose coils in the stator slots (which lead to groundwall insulation abrasion) and delamination of the insulation caused by long duration operation at high temperatures. The early versions of the on-line PD test for stator windings required significant skill on the part of the test technician to distinguish stator winding PD from other pulse-like signals due to interference. Such interference may be caused by power line corona, electrostatic precipitators, poor electrical connections elsewhere in the plant, power tool operation, variable speed drives, etc. Of course, if this interference is misdiagnosed as stator PD by the technician, a false indication of the condition of the stator insulation may result.
In 1976, the Canadian Electrical Association (CEA) initiated a research contract to develop an on-line PD test for stator windings that was less subjective than the tests existing at the time. This led to what are now called the PDA and TGA tests, which have been deployed on more than 12,000 motors and generators around the world since the commercial introduction of the PDA in 1986. This paper reviews the basic principles of these on-line tests and discusses its advantages and limitations. Recent progress in on-line PD test result interpretation is also presented.