介绍电力配电设备及系统6

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1、9576_C04.fmPage216Monday,October17,20059:12AM216ElectricPowerDistributionEquipmentandSystemsitance.TestsbytheSouthernCaliforniaEdisonCompanyonthree-phasetransformerswithungroundedprimaryconnectionsfoundthatferroreso-nanceoccurredwhenthecapacitivepowerperphaseexceededthetrans-

2、former’sno-loadlossesperphasebythefollowingrelationship(Jufer,1994):BP≥127.CNLThephase-to-groundcapacitanceofoverheadtransformersisprimarilyduetothecapacitancebetweentheprimaryandsecondarywindings(thesecondarywindingsarealmostatzeropotential).Atypical25-kVAtrans-formerhasapha

3、se-to-groundcapacitanceofabout2nF(Wallingetal.,1995).Fora7.2-kVline-to-groundvoltage,0.002µFis39vars.So,iftheno-loadlossesarelessthan39vars/1.27=30.7Wperphase,thetransformermayferroresonateundersingle-poleswitching.Normally,ferroresonanceoccursonthree-phasetransformers,butfer

4、rores-onancecanoccuronsingle-phasetransformersiftheyareconnectedphasetophase,andoneofthephasesisopenedeitherremotelyoratthetransformer.Jufer(1994)foundthatsmallsingle-phasepadmountedtransformerscon-nectedphasetophaseferroresonatewhenremotelyswitchedwithrelativelyshortcablelen

5、gths.Theirtestsofsilicon-steelcoretransformersfoundthata25-kVAtransformerresonatedwith50ft(15m)of1/0XLPEcableat12kV.A50-kVAtransformerresonatedwith100ftofcable,anda75-kVAunitresonatedwith150ftofthecable.Peakprimaryvoltagesreached3to4perunit.Secondary-sidepeakswereallunder2per

6、unit.Longercablesproducedslightlyhighervoltagesduringferroresonance.Juferfoundthatferroresonancedidn’toccuriftheresistiveloadinwattsperphase(includingthetransformer’sno-loadlossesandtheresistiveloadonthesecondary)exceeded1.15timesthecapacitivevarsperphase(PNL+PL>1.15BC).Bohma

7、nnetal.(1991)describesafeederwheresingle-phaseloadswereswitchedtoaphase-to-phaseconfiguration,andthereconfigurationcausedahigher-than-normalarresterfailureratethatwasattributedtoferroresonantconditionsonthecircuit.Itiswidelybelievedthatagrounded-wyeprimaryconnectioneliminatesfe

8、rroresonance.Thisisnottrueifthethree-phasetransformerhaswindingsonac