Studies of Impulse Strength and Impulse Testing Problems on High-Voltage Generators [includes discussion]

Abstract

the impulsestrength of rotating machine windings isofconsiderable interest because itisnotfeasible toinsulate themfortheworst surges thatmay occur inservice. Unfortunately, there are design limitations inrotating machines whichpreclude someinternal design solutions thathavebeenapplied tostatic apparatus successfully. Theimportance ofspace andtheneed forphysical support ofcoils inmachines necessitate theuseofsolid barrier insulation andclosecapacitive coupling between thewinding andcore.The windingand itsinsulation structure present averycomplex system ofinductanceandcapacitance through whichthe energy ofasurge penetrates thenetwork ofanarmature winding somewhat asa cloud vaporentering a labyrinth. The surge enters thewinding along thecopper conductor, butfinds innumerable other paths along whichitbillows. Since this concept presents anunsolvable problem, ithasbeensimplified andistreated as acircuit problem based ontransmissionlinetheory, withsatisfactory solutions. Methodsandequipment forthesurge protection ofrotating machine windings havebeendescribed inliterature andwill notberepeated here.Thebasic principle ofsuchschemes istoemploycapacitance andinductance incombination with lightning arresters tolimit therateof voltage riseandthemaximumvoltage applied tothemachine terminals. The limitation ofvoltage magnitude isprimarily forprotecting thebasic ground insulation. Thesloping ofthewavefront provides protection forturninsulation by distributing thesurgevoltage across manyturns andpreventing itfrompiling upacross thefirst fewturnsattheterminal.