TRANSFORMER PROTECTIONWhenselectingtransformer protective relaying schemes, it isimportanttorecognizethe potentialimpactofa transformer failureTransformer Through-Fault ProtectionLimitation of Fault energy inecte! into a !istri"ution transformer limite! "y current limiting fuses# larger transformers limite! "y the im$e!ance Transformer !i%erential relays an! transformer su!!en $ressure relays are use! to &uic'ly !etect an! isolate the faile! transformer# (ac'u$ time o)ercurrent relays are use! to $rotect against faults that are not cleare! $ro$erly on circuits that are connecte! to the transformer secon!ary#Selection CriteriaThe suggeste! criteria for the selection of $rotection relays or schemes are*Pro)en e+$erience for a minimum $erio! of three yearsRelays "uilt to IEEE s,itchgear stan!ar!s or e&ui)alent IEC stan!ar!sRelays teste! to IEEE C-.#/01 C-.#/0#21 C-.#/0#3 or to IEC 403556here $ossi"le1 micro$rocessor-"ase! relays ,ith features such as self chec'ing1 self-!iagnostics1 RMS sensing1 metering1 e)ent logging1 ,a)e ca$ture1 !istur"ance recor!ing1 an! ,ith a communications $ort for !ata reco)ery an! settingRelays that $erform satisfactorily in the $resence of harmonics an! electromagnetic 7EMI8 an! ra!io fre&uency 7RFI8 interferencesRe!un!ancy re&uirements for generators1 large motors1 an! transformersA)aila"ility of a$$lication literature an! technical su$$ortNecessary Ste$s to Carry Out Relay-Setting an! Time9Current Coor!inationPre$are one 7single8-line !iagram# Recor! the !ata relate! to the relays1 circuit "rea'ers1 fuses1 current an! )oltage transformers1 an! o$erating e&ui$ment#A!! ty$e an! rating of all $rotecti)e !e)ices 7instrument transformers1 relays1 fuses1 etc#8#Recor! the rating of maor e&ui$ment1 inclu!ing transformers1 rotating machines1 fee!ers1 etc#1 that in:uence short-circuit :o,#Perform a series of short-circuit stu!ies to !etermine the ma+imum an! minimum fault-current le)els#Ascertain the ma+imum loa! currents that ,ill e+ist un!er normal an! emergency o$erating con!itions# The minimum an! ma+imum short-circuit currents !etermine the lo,er an! u$$er "oun!aries of current selecti)ity ,ithin ,hich the circuit $rotecti)e !e)ices must o$erate1 ,hile the ma+imum loa! current !e;nes the con!itions un!er ,hich the current-o$erate! $rotections must not o$erate#Re)ie, the a$$lica"le electrical co!e an! e&ui$ment stan!ar!s#O"tain $rotecti)e-!e)ice time9current characteristics cur)es an! e&ui$ment- ca$a"ility or thermal-limit cur)es## Transformer turns ratio5# Presence of no-loa! ta$ changers4# Presence of loa! ta$ changers.# Full loa! current of each ,in!ing at each ta$ )alue?# @roun! "an' action/# MagnetiAing inrush current20# CT ratios an! connections22# CT saturation for e+ternal faults23# 6in!ing im$e!ance2-# O)erloa! ca$a"ility2># Short-circuit ,ithstan! ca$a"ility25# Nee! for re!un!ant $rotecti)e relayingDiferential relays are the usual form of fault protection for transformers rated 25 MVA or more !" #V and greater$%& #V'Fusesareusede(tensivelytoprotect transformersratedat%Vand)elo**hen longfaultclearingtimes$seconds'are accepta)le+ Fusesthatarenotcurrentlimitingarenot recommended for transformer protection,udden pressure relays are used to complement diferentialrelays for internal failures in oil-.lled transformers+ Modern sudden pressure relays provide efective transformer protection /ransformer diferential protectionDifferential protection of a Delta-Wye transformera#Consi!er1 for e+am$le1 a .5-M=A1 3-0-4/-C= transformer ,ith a 23-C= tertiary ,in!ing that is use! for station ser)ice 7200 C=A8# "#a t,o-in$ut !i%erential relay is a goo! choice#c+2!0-%&-#V transformers 12 53 to 203+ inrush current 5 to %+5 times full load current+ d+%&-2-#V transformers 12 43 to 23+ inrush current could )e " to 5 times full load current+