MEASUREMENT OF PARTIAL DISCHARGES IN AN INDUSTRIAL HV TEST LABORATORY

摘要

Partial Discharge (PD) level has to be measured duringrnacceptance-test of power transformers, HV bushings,rninstrument transformers, cables and other HV apparatus.rnAn acceptable PD level is specified for each kind ofrnapparatus, and ranges from a few hundreds ofrnpicocoulombs (pC) for power transformers, down to arnsingle pC for cross-linked polyethylene (XLPE) cables.rnUnfortunately, the PD measuring circuit acts as a largernantenna and collects disturbances induced by radiornbroadcasting stations or by industrial sources ofrnelectromagnetic radiation. The measured PD signalsrnwill be masked by these disturbances if they are not bernsuppressed by an electromagnetic shield. The shield hasrnto reduce disturbances below the PD acceptance level,rnspecified for a given type of HV equipment under tests.rnIn the case of HV power transformers the backgroundrndisturbance level should not exceed a few tens of pC.rnHowever, to test XLPE HV cables, the backgroundrndisturbance should be reduced to approximately one pC.rnObviously, more stringent requirements are imposed onrnthe electromagnetic shield for testing XLPE cables thanrnfor HV power transformers.rnThe shielding efficiency (SE) of a large HV laboratoryrnshield depends mainly on the continuity of conductingrnenclosure. This calls for very low contact impedancernbetween the metal panels that form the enclosure, andrnbetween the door and its frame.rnUsually, the door is large enough to allow an entry ofrntall HV apparatus. The closed door has to provide arnlow-impedance path for the minute current induced inrnthe enclosure by the disturbance electromagnetic field.rnA well-designed electromagnetic shield can effectivelyrnreduce such field. However, the shield does not suppressrndisturbances conducted by the supply and controlrncables, or by the laboratory grounding system. Radiofrequencyrn(RF) filters have to be inserted in the supplyrnwires to stop the conducted disturbances. A specialrndeep-driven grounding rod is required to isolate the testrnlaboratory from ground currents that circulate in the soilrnupper layers.rnA thyristor controlled drive generates higher harmonicsrnof current and voltage in the supply system. Suchrndistorted supply voltage affects the test voltagernwaveform, and increases the PD level measured on thernexamined test object. To reduce the test-voltagernwaveform distortion, the HV laboratory should suppliedrnby a separate power transformer.rnDesign features of the shielded HV laboratories andrntheir operational experience are discussed in this paper.rnExamples of HV test laboratories designed and built torntest XLPE HV cables and transformers are presented asrnan illustration of the successful protection against bothrninduced and conducted disturbances.