Introduction
Partial discharges (PD) are a symptom and sometimes a cause of many types of motor and generator stator winding insulation system deterioration mechanisms in utility generators. Since 1951, the on-line measurement of PD has been used to assess the need for motor and generator maintenance in stator windings [1]. More particularly, on-line PD measurement has been able to determine whether the electrical insulation is deteriorating because of loose coils in the slots, resulting in insulation abrasion; thermal deterioration or load cycling, which leads to insulation delamination; and electrical tracking caused by partly conductive contamination of the endwindings [1]-[7]. On-line PD testing is also able to determine whether manufacturing or installation problems, such as poor impregnation with epoxy or coils being too close together in the endwinding, are severe enough to shorten the winding life.
There are many methods available to measure the PD activity in operating machines. The electrical techniques rely on monitoring the current or voltage pulse that is created whenever a partial discharge occurs. The earliest methods measured the PD pulse currents by means of a high frequency current transformer at the neutral point [I], [2]. Shortly thereafter, high voltage capacitors connected to the phase terminals were used to detect the PD pulses [3]-[5]. Capacitor sizes ranged from 80 to >1000 pF. An overview of the different means for detecting PD in machines is presented in a recent IEEE guide [7].
A particular challenge with PD measurements is encountered when a machine is operating and electrical interference (noise) is often present [7]. Noise sources include corona from the power system, slip ring/commutator sparking, sparking from poor electrical connections, and/or power tool operation. This noise obscures the PD pulses and may cause the technician to conclude that a stator winding has high levels of PD, when it is actually the noise. The consequence is that a good winding is incorrectly assessed as being defective, meaning that a false alarm is given, suggesting that the winding is bad when it is not. Such false alarms reduce the credibility of on-line PD tests, and even today, many feel that on-line PD testing is a ‘black art’ best left to specialists.
Twenty-five years ago, the Canadian utility industry sponsored research to develop an objective on-line PD test for machines that could be performed and interpreted by plant staff with average training. The PD test that was developed emphasized separating PD pulses from electrical noise pulses. The noise separation methods depend on comparing the time of pulse arrival between a pair of 80-pF capacitive couplers and/or analyzing the shape of individual pulses from 80-pF capacitors or special antennae called the SSC [4], [8], [9]. To maximize the signal-to-noise ratio and thus reduce the risk of false indications, the sensors detect the PD at frequencies of 40 MHz [8]. These test methods have enabled utilities to assess the winding condition with their own staff. As a result, it is estimated that >50% of all utility generators rated 20 MW in the US and Canada have now been equipped with the required PD sensors. Globally, well over 4000 machines have the required PD sensors.
A large number of test results have been accumulated in a single database with the widespread application of the same on-line test methods. At the end of 2003, over 60,000 test results had been accumulated over 10 yr, and simple statistical analysis has been applied to the database to extract information that can help test users to interpret PD results better. In addition, experience has accrued on what operating and environmental parameters may affect the PD readings as well as the deterioration in the stator insulation condition. This article presents some of these important findings.