Partial Discharges inGaseous Cavities inSolid Dielectrics UnderImpulse Voltage Conditions

An investigation isdescribed intopartial discharge phenomenainartificial air-filled cavities ofknown dimensions inpolyethylene. Duringinitial tests, the50 percent impulse- inception stress ismuchhigher thanthedischarge stress calculated fromPaschen's curve.Itisindependent oftheimpulse repetition rate,andislarger for1/50-us impulses thanfor500/3000-uAs surges;forbothwaveshapes theextinction stress islowerthanthe inception stress. Afterrepeated discharges, theinception and extinction stresses are bothreducedtothePaschencurve value. Initially, the50 percent impulse-inception stressincreases as thecavity diameter isdecreased atconstant depth andasthedepth isdecreased atconstant diameter. Afteraging, thestress becomes independent ofthecavity diameter. Details are givenofthedis- charge magnitudes andtimelags. Ithasbeenshownthata "main discharge" occursusually on ornear thecrestofthesurge,during whichall ora substantial part ofthecavity isdischarged, followed by a numberofsmaller "reversedischarges" on thewavetail. When a discharge occurs on polarity reversal itsmagnitude tendstobe appreciably greater but itcan be reducedby suitablevoltage conditioning. Experiments havealsobeen made withthecavity adjacent toan electrode insteadoftotally enclosedinthedielectric.