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pas trans 84 - application of partial discharge to hydrogen mtce.pdf

pas trans 84 - application of partial discharge to hydrogen mtce.pdf

pas trans 84 - application of partial discharge to hydrogen

APPLICATION OF PARTIAL DISCHARGE TESTING TO HYDRO GENERATOR MAINTENANCE M. Kurtz, J.F. Lyies, G.C. Stone, Ontario Hydro, Toronto, Canada Greg C. Stone, Fellow, IEEE, and Vicki Warren, Member, IEEE From EEE Transaction on Power Apparatus and Systems, Vol. PAS-103, No. 8, August 1984 Pages (195–200) ABSTRACT An improved partial discharge test to ascertain hydraulic generator winding condition, has been in use for the last 7 years. The test, which can be done by generating station staff without disrupting operation, has been applied to about 140 machines in eight North American utilities. Partial discharge testing is one of the main reasons Ontario Hydro has an exceptionally low winding replacement/major repair rate. Examples of results culled from thousands of tests are presented. The use of the partial discharge test for planning winding maintenance and gauging the effectiveness of repairs is discussed. INTRODUCTION The loss of integrity of any high voltage insulation system resulting from the action of thermal and mechanical service stresses is frequently manifested by changes in partial discharge (pd) characteristics. In generator stator insulation this could be symptomatic of delamination, loss of function of slot or stress-grading varnish or some other undesirable condition. If these conditions are not recognized and treated at an early stage, the cost of remedial action could skyrocket. Stator winding problems have been identified in 432 of all hydraulic generators with thermostat windings [I. Most of these problems are characterized by partial discharge activity. In this paper, we describe a practical low cost pd test now in routine use. The test data are repeatable and consistent and give clear indications not only of the need for remedial action on some windings, but also of the satisfactory condition of the vast majority of stators tested. The test also provides immediate feedback of the effectiveness of preventive maintenance procedures. PARTIAL DISCHARGE TEST METHODS Two main methods have evolved for detecting partial discharges in normally operating generators. These include neutral-coupled detection devices [2,3] specifically designed to monitor general slot discharge conditions, and line-coupling techniques [4] for regular periodic tests on individual machine phases. Most such teats, while capable of detecting insulation problems at an early stage, require elaborate equipment and/or considerable operator skill and Judgement. In a Canadian Electrical Association project initiated in 1976, these techniques were advanced, permitting objective data to be obtained at considerably less cost per machine tested, with no interruption to service and with substantial immunity from system noise [5]. 84 WM 116-0 A paper recommended and approved by the iEEE Rotating Machinery Committee of the iEEE Power Engineering Society for presentation at the iEEE/PES 1984 Winter Meeting, Dallas, Texas, January 29 - February 3, 1984. Manuscript submitted August 30, 1983; made available for printing November 11, 1983. The partial discharge test now used by Ontario Hydro and seven other American and Canadian utilities requires the installation of high voltage capacitors on the stator winding. The design and placement of these capacitors are the key to a partial discharge test which requires no service interruption. References [6] and [7] describe the test in detail. Briefly, individual partial discharge pulses originating in a generator insulation system are picked up via permanently installed capacitive couplers (about 80 pF). In hydraulic generators, these couplers are connected in pairs/phase to the circuit ring bus at the connections to parallels, or to pole jumpers or connection points near the line end. Coaxial cables connect the low voltage side of the couplers to a convenient terminal box outside the machine. A differential amplifier (Figure 1) serves to reduce common mode or system noise (including static exciter pulses) while retaining sensitivity to partial discharges which are occurring at localized sites within the winding. Figures 2 and 3 show two commonly used versions of the coupler. The installation cost per machine of the couplers in Ontario Hydro’s generators is equivalent to about 150 MH of generation. Once the couplers are installed, the generator can be tested by station staff (without a service interruption), as many times as desired, for essentially no charge. The partial discharge information is recorded by the Partial Discharge Analyzer (PDA) (Figure 4). The differential amplifier shown in Figure 1 is the input to the PDA. The PDA counts the number of partial discharge pulses per second, sorts them by magnitude and polarity, and prints the results on a paper Cape. The partial discharge magnitudes can be calibrated in terms of picocouloabs, if desired. These data can then be plotted as shown in Figures 5 Co 12. A complete test takes only five minutes per generator loading condition. TEST RESULTS The first “permanent” capacitive couplers were installed in 1976. Since then more than 140 stators have been so

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