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development of a practical continuous on-line partial - Iris Power ...

development of a practical continuous on-line partial - Iris Power ...

development of a practical continuous on-line partial - Iris Power

DEVELOPMENT OF A PRACTICAL CONTINUOUS ON-LINE PARTIAL DISCHARGE MONITOR FOR GENERATORS AND MOTORS G.C. Stone, J. Bragoszewski, S.R. Campbell, B.A. Lloyd Iris Power Engineering Inc 6380 Tomken Road Mississauga, Canada L5T1Y4 Abstract: An automatic partial discharge monitoring system has been developed to ong>continuousong>ly measure the PD activity in operating motor and generator stator windings. The system is intended for long term use by utility and industrial plant personnel, rather than researchers. The system is resistant to false indications ong>ofong> high PD activity caused by noise, since noise is eliminated on pulse-by-pulse basis, using specialized sensors permanently installed on the stator winding. The control song>ofong>tware enables recording ong>ofong> PD data under machine operating conditions and at a frequency that the user specifies, thus reducing the amount ong>ofong> data stored to realistic quantities. In the past vear, the system has been installed on 19 generators, in 4 plants and 2 utilities. The operating principles ong>ofong> the system are described, and some preliminary results are presented. INTRODUCTION Partial discharge (PD) testing has been used for over 40 years as a means to determine the condition ong>ofong> the stator winding insulation in high voltage stator windings in motors and generators [1-3]. PD is a symptom ong>ofong> loose windings, overheated insulation and contaminated windings. Various methods have been developed to measure the PD activity in a winding. The most popular methods with motor and generator users are those which can be done during normal operation ong>ofong> the machine, since outages can be avoided. Most types ong>ofong> on-line PD tests require that an expert be present to perform the test, and interpret the patterns [4,5]. The expert distinguishes stator winding PD from other (harmless) types ong>ofong> corona and PD in addition to electrical sparking noise from slip rings, power tool operation, poor electrical contacts, etc. Since noise is mixed in with the PD signals from the stator winding, the expert must usually be present for each measurement, to ensure that a trendable quantity such as peak PD magnitude can be reliably determined. The requirement for an expert at site for each measurement increases test costs, and fundamentally implies that automated, ong>continuousong> measurement ong>ofong> the PD activity can not be implemented. If the expert is not present for each measurement, then the electrical noise will occasionally result in false indications ong>ofong> high level PD, and thus imminent stator winding failure. Such false alarms reduce the confidence ong>ofong> plant personnel in the system, and inevitably results in the measurements being ignored. In research conducted over the past 15 years. PD measurement methods have been developed which make an on-line PD test easier to perform and interpret, allowing nonspecialized plant personnel to reliably perform the test. The methods depend on specialized sensors which are inherently resistant to electrical interference (noise) [2.3]. Once the PD is separated from the noise, standard electronic pulse counting techniques can be used to accurately determine the magnitude, number and phase position ong>ofong> the PD activity. The specialized sensors have been permanently installed in as many as 3000 hydrogenerators, turbine generators and high voltage motors. In the past, virtually all measurements on the specialized sensors have been made with a portable electronic instrument called a PDA (Partial Discharge Analyzer, for use with PDA couplers on hydrogenerators) or more recently a TGA (Turbine Generator Analyzer for use with “Bus” couplers and “Stator Slot Couplers” or SSCs) [2,3]. The tests are done about twice per year, usually with the motor or generator operating at full power. The instrument separates the PD from the noise on a pulse-by-pulse basis and stores the number, magnitude and phase position ong>ofong> the PD activity. The testing is usually done by local (non-specialized) plant staff. In addition, since the noise had been eliminated, simple quantities such as the peak discharge magnitude and the integrated discharge activity could be easily trended over time with repeat readings. Forty years experience has shown that it is the trend in PD activity over time that is the most powerful indicator ong>ofong> the condition ong>ofong> the stator winding insulation [2-4] The elimination ong>ofong> the need for an expert to perform a PD test, together with recent advances in computer technology, has made it feasible to develop a commercial grade system which automatically and ong>continuousong>ly monitors the PD activity. This paper describes such a system, known generically as the Partial Discharge Monitor (PDM). ADVANTAGES OF CONTINUOUS PD MONITORING In most applications, stator winding insulation deterioration is a relatively slow process, where the time between when PD can be detected and when winding failure may occur, is usually 2 or more years. Thus measurements which are done once every six months or so are ong>ofong>ten adequate for detecting stator winding problems. However, there are several situations where ong>continuousong> monitoring may be advantageous: 1. Several expert systems are being developed which ong>continuousong>ly monitor all the sensors in a generator (for

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