Oil-Immersed PowerTransformerOverload Calculations by Computer

MIr. Montmeatpoints-out themerits ofutilizing station capacity backup through spare transformers ormobile substations. We agree. Theimpact ofmobile transformers onlong-range expansion plans canbetakeninto account byrevising theforced outage rates downward.This, ineffect, reduces theduration ofanextended outage to thelength oftimerequired toinstall themobile, thusreducing the expected lossoflife ontheremaining transformers. Oneofthe features oftheprogram isanoption whichutilizes thestored temperature andlife precalculations tooutput therelationship between expected loss oflife andrisk versus loadlevel foreachtransformer foreachperiod. Thispermits rapid evaluation ofchanges toanyof theinput variables. Thisoutput option canbeusedtodetermine expected loss oflife fora3-or4-day replacement period. Mr.Montmeataskswhether auniform risk pattern wouldresult ifstation loadswereheldtosomemaximumvaluewithin anexpansion plan. Thiswouldnotbetrueinmanycases. Trends inload shapes, unbalance factors, monthlypeakfactors, load-weather correlation, etc., canbeprojected intofuture years ofanexpansion plan. Suchtrends will produce different risk levels andloss oflife in different years foragiven maximumloadlimit onasubstation. Mr.Sverakcommentsthattransformer outagerates arenot constants. Theauthors areinagreement withhisthoughts. As loading policies areallowed toincrease, itwouldseemlogical that outage rates mightalsoincrease. Also,there islogic behindthe thought thattransformer failure rates areafunction ofin-service life, withtheprobability offailure being highbothatthebeginning andendofatransformer life span. Outage rates arealso afunlction of emergency switching schemes, loadtransfer schemes, supply circuit design andreliability, equipment maintenance policies, operating