Preparation of Transformer Spec

8/12/2019 Preparation of Transformer Spec

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PREPARATION OF TRANSFORMER SPECIFICATIONS

By Presented at IEEE SouthernVallamkonda Sankar Alberta (SAS)-Power EngineeringE-mail:powertrans ormer!hotmail"#om So#iety (PES)-Industry Appli#ation So#iety (IAS) seminar on $o%ember &' *& "
mailto:[email protected]:[email protected]


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OBJECTIVES

+o prepare spe#i i#ations or pro#uring reliable trans ormers at market pri#es" +o prepare #lear and #on#ise spe#i i#ations with no ambiguities"

+o guide users in determining the trans ormer parameters" +o ensure that the trans ormer parameters meet system needs"

+o use e,isting standards to the ull e,tent in the spe#i i#ations"

+o amiliari e users as to how spe#i i#ations a e#t the design and the #ost"

+o en#ourage the manu a#turers to submit #ost e e#ti%e alternates with low lossesmeeting all the system needs"

+o understand the dangers when the trans ormers are used di erent to what is stated in the spe#i i#ations"

+o dis#uss the ad%antages and disad%antages o in%ited tendering %erses opentendering"

+o know and understand the #lari i#ations.#on irmations to ask in the +ender /e%iew and 0esign /e%iew meetings"

+o a%oid delays in deli%ery by ollowing up with the manu a#turer at #riti#al inter%als"

+o #he#k the 1uality o the trans ormer by inspe#ting at #orre#t produ#tion stages"

+o ensure that the a#tory tests meet system #onditions and the standards"

+o identi y the importan#e o transportation installation and maintenan#e whene%aluating tenders"

+o obtain the best bene its by knowing the #hara#teristi#s and the #osts o the

materials and the #omponents used in the trans ormers" +o intera#t with trans ormer manu a#turers or mutual bene its"

+o initiate new or e%ol%ing inno%ations in trans ormer design and manu a#tureor the redu#tion o #ost without #ompromising the per orman#e"


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FUNCTIONS OF TECHNICAL SPECIFICATIONS&" 34rom an 5ser perspe#ti%e: +o ormally and airly #ommuni#ate e,a#tly what the 6ontra#tor has to deli%er"7[&]" 34rom a 6ontra#tor perspe#ti%e: +o be able to a##urately o er ser%i#es and produ#ts whi#h pro%ide a satis a#tory solution (te#hni#al and #ommer#ial) to

user8 while remaining a long-term pro itable business"7[&]2" 34or both 5ser and 6ontra#tor: +o a%oid relationship mishaps asso#iated with

#ostly %ariation work"7[&]9" Enable the manu a#turer s design engineers to understand the #hara#teristi#s o the user s system and how the trans ormers are operated in ser%i#e";" 6reate an opportunity or manu a#turers to apply impro%ed design and manu a#turing methods bene i#ial to users" Also an opportunity to use ad%an#ed materials and a##essories"


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TOPICS

&" /atings"

" Standards"

2" Single phase %erses three phase"

9" inding #onne#tions"

;" Ve#tor group"

" +erminals"

" A##essories"

'" +ypes o #ooling"

&*" Sound le%els"

&&" +aps (range lo#ations et#")"

& " +ypes o taps"

&2" Impedan#e"

&9" 6apitali ation o losses"

&;" Short-#ir#uit withstand"

&


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RATINGS+he trans ormer s =V"A rating is usually determined based on the system e,pe#ted load

growth a%ailable generation and e,pe#ted rate o return on the in%estment" +hebusiness plan is based on #apital #ost re#o%ery depre#iation rate and the pro it to bemade ea#h year" +rans ormer appli#ations su#h as two winding autotrans ormer step-up or step-down et#" in luen#e the trans ormer rating" Cther a#tors su#h as e,pe#tedpower demand growth and load a#tor ha%e a large in luen#e in determining thetrans ormer rating"

6SA-6 -='* #lause 9"& de ines the rating as ollows" 3the rated load in =V"A togetherwith any other assigned #hara#teristi#s su#h as %oltage #urrent and re1uen#yassigned by the manu a#turer" It shall be regarded as a test rating that de ines an outputthat #an be taken rom the apparatus under pres#ribed #onditions o test and within thelimitations o this standard7" 4urther @Basis o =V"A is de ined in se#tion >" "

IEEE standard 6;>"& " *- *&* #lause 2"2;; states the rating as ollows:- 3+he rating oa trans ormer #onsists o a %olt-ampere output together with any other #hara#teristi#ssu#h as %oltage #urrent re1uen#y power a#tor and temperature rise assigned to it bythe manu a#turer" It is regarded as a rating asso#iated with an output that #an be takenrom the trans ormer #onditions and limitations o established standards7"

IEEE standard 6;>"& "**- *&* #lause ;"9"& des#ribes the rating as ollows" 3+he ratedDV"A o a trans ormer shall be the output that #an be deli%ered or the time spe#i ied atrated se#ondary %oltage and rated re1uen#y without e,#eeding the spe#i iedtemperature-rise limitations under pres#ribed #onditions o test and within the limits oestablished standards7"

IE6 "& "**) where @rated DV"A is @the output that #an be deli%ered at rated se#ondary%oltage A##ording to that method allowan#e or %oltage drop has to be made in thedesign so that the ne#essary primary %oltage #an be applied to the trans ormer7"

I trans ormers are pur#hased per IEEE standard the ele#tri#al lo#ation o the taps (inFV in GV in series winding in #ommon winding et#") appli#ation o the taps ( or input%oltage lu#tuations or or output %oltage regulation) and whether the trans ormers areoperated as step-down or or step-up determine di erent =V"A ratings or the input andoutput #ir#uits (FV and GV)" 6onsidering this some users make the #al#ulations

themsel%es and spe#i y the #urrents or ea#h winding" +his is pre erred sin#e i le t to themanu a#turers ea#h manu a#turer #ould #al#ulate the #urrents di erently"

=any users ha%e standardi ed the trans ormer =V"A ratings used in their systems" +hisis based on o%erall e#onomy (resulting in ewer spare trans ormers) and the le,ibility omo%ing the trans ormers to di erent stations in their system" Site lo#ation andtransportation limitations #an limit the =V"A rating o the trans ormer"


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Input and output (FV and GV) %oltage ratings are determined by the e#onomi#s o powertransmission based on the total =V"A to be transported length o transmission lines et#"

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" +o a%oid undesired #ir#ulating #urrent in the tertiary delta the %oltage or thetertiary must be same or all the units" I the tertiary %oltage on the two healthyunits is 22"&DV then spe#i y 22"&DV and not 22DV"

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WINDING CONNECTIONS+hree phase trans ormers ha%e windings #onne#ted in wye delta or ig- ag inside thetank" +hese #onne#tions or single phase trans ormers are made outside the tanks"hen a neutral is re1uired then it is e#onomi#al to #hoose a wye or a ig- ag" I aneutral is re1uired on a delta side then a grounding trans ormer is needed"

$eutrals usually ha%e a lower insulation le%el #ompared to the lines" Su#h windings #anbe e#onomi#ally designed with graded insulation" +o redu#e no-load loss and end#learan#es to the #ore8 other a#tors permitting su#h windings #an be designed as#enter- ed windings" 6ompared to the taps in delta taps at neutral result additional #ostsa%ings in the tap #hangers tank si e oil 1uantity et#"

hen both FV and GV are wye #onne#ted a stabili ing winding (buried tertiary) may be

needed or suppression o third harmoni#s and or neutral stabili ation" +he determininga#tors are based on the type o #ore (#ore type trans ormers with three-phase threelimbed #ores does not re1uire a stabili ing winding but some users spe#i y a stabili ingwinding) the system and the neutral #onne#tion in the system" +his o ten leads to a#hoi#e to ha%e one o the main windings delta #onne#ted" Prote#tion #osts should be#onsidered in determining whether the FV or the GV should be delta #onne#ted"

In #ertain #ases when the G+6 is ele#tri#ally #onne#ted to the low %oltage winding(#onsidering that the %oltage o this winding is not high) it #an be #onsiderably #heaperto use delta #onne#tion or the low %oltage winding" A delta #onne#ted winding has alower #urrent by H2 and this #ould a%oid the use o a series trans ormer to redu#e the#urrent in G+6" Fowe%er this has some disad%antages" I the insulation le%el o thiswinding is high then the #ost ad%antage #ould be lost" As well %ery ew three-phaseG+6s or delta #onne#tions are a%ailable" Instead three single-phase G+6s are to beused" In su#h a #ase shi ting the G+6 on to the wye-#onne#ted FV needs to bee,plored" Based on how the trans ormer is operated this #ould make the trans ormer a%ariable lu, design"

By #onne#ting the windings in wye. ig- ag rather in wye.delta the need or a groundingtrans ormer on the delta #an be eliminated" C ten in a wye. ig- ag trans ormer it ise#onomi#al to limit the single line to ground ault #urrent to not e,#eed the three-phaseault #urrent by #onne#ting a neutral grounding rea#tor o suitable rea#tan#e in the ig-ag neutral" ithout the grounding rea#tor single line to ground ault #urrents in awye. ig- ag trans ormer will be high #ompared to that o wye.delta trans ormer with thesame parameters (same =V"A and positi%e se1uen#e impedan#e)" A wye. ig- ag

trans ormer #osts about < more than a wye.delta trans ormer" In general a wye. ig-ag trans ormer with a grounding rea#tor #osts less #ompared to a wye.delta trans ormer with a grounding trans ormer" As it is di i#ult and #ostly to ha%e taps on a ig- agwinding8 the taps are normally on the wye winding" In a step-down trans ormer when theig- ag winding is the output winding users pre er to ha%e taps on the ig- ag winding"+his is be#ause8 taps on the input winding to #ompensate or regulation will make thetrans ormer a %ariable lu, unit" 0isad%antages o the %ariable lu, taps are dis#ussedunder 3+ypes o +aps7"


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5sers should be aware that when there is no delta in the trans ormer (wye.wye orwye. ig- ag) the harmoni#s will low in to the system" A method to pre%ent the harmoni#slowing in to the system is dis#ussed under Ve#tor ?roup"

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VECTOR GROUP6SA-6 -='* #lause 9"& states as ollows: 3Angular displa#ement (o a polyphasetrans ormer)--the time angle e,pressed in degrees between the line-to-neutral %oltageo the re eren#e identi ied high-%oltage terminal (F&) and the line-to-neutral %oltage othe #orresponding identi ied low-%oltage terminal (J&)" $ote: +he pre erred #onne#tionsor polyphase trans ormers are those whi#h result in the smallest possible phase-angledispla#ement measured in a #lo#kwise dire#tion rom the line-to-neutral %oltage o there eren#e identi ied high-%oltage terminal (F&)" +hus standard three-phase trans ormersha%e angular displa#ements o either ero or 2*K 6lo#k system (o phase displa#ement)--a method e,pressing the phase displa#ement between two windings by analogy to thehands o a #lo#k"7

=any standards and te,t books show multiple %e#tor group #onne#tions" +he sameangle shi t between FV and GV #an be a#hie%ed by %arious %e#tor groups" Cne o #lo#k%e#tor #an be a#hie%ed by the Ld& 0y& or L & groups" +wel%e o #lo#k %e#tor #an bea#hie%ed by the Ly* 0d* or 0 *" Ve#tor group has to be #are ully sele#ted be#ause ithas a bearing on trans ormer #ost and on the system operation" +his is be#ause ea#h%e#tor group has a di erent ero se1uen#e impedan#e resulting in di erent ault#urrents"

Cnly #ertain types o #ores are re#ommended or the wye.wye #onne#tion" +he book@+rans ormer Engineering by G"4" Blume states the ollowing on wye.wye #onne#tion[ ]"

3L-L #onne#ted trans ormers e,#epting three phase three legged #ore type units arenot #apable o supplying an appre#iable single-phase load rom line to neutral owing tothe a#t that the #orresponding primary #urrents o su#h loads lowing through theprimaries o the unloaded phases magneti e them" +his statement is true or L-L#onne#ted single-phase units and shell-type three-phase units" 6ore-type units howe%ermay on a##ount o the interlo#king o the magneti# lu,es in the three legs gi%e tolerablygood results under #onditions o single-phase loads rom line to neutral or unbalan#edele#trostati# #harging #urrents" L-L #onne#tion with grounded neutral in single-phaseand shell-type three-phase units is dangerous on a##ount o the possibility o resonan#ein the third harmoni# with the line #apa#itan#e" It should ne%er be used7"

In a delta.delta #onne#ted trans ormer a neutral or grounding is not a%ailable" hentwo GVs are re1uired some users are spe#i ying wye. ig- ag. ig- ag" 5sers should#onsider spe#i ying wye.delta. ig- ag (Foeppner trans ormer)" In these trans ormers aneutral is a%ailable" Sin#e there is a delta the third harmoni#s will stay in the trans ormer and do not low in to the system"

hen line %oltage is high and the #urrent is small then #onne#ting this winding in wye ise#onomi#al #ompared to #onne#ting in delta" +his is be#ause the phase %oltage M (line%oltage.2)" hen the line %oltage is small and the #urrent is large then to #onne#t thiswinding in delta is e#onomi#al" +his is be#ause phase #urrent M (line #urrent.2)" 5sers


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should #onsider system #osts system re1uirements and dis#uss with trans ormermanu a#turers be ore stating in the spe#i i#ations whi#h side should be delta whi#h sideshould be wye and the %e#tor group"

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INSULATION LEVELS6SA-6 -='* #lause 9"& de ines Gightning Impulse Ge%el as ollows: 3the pres#ribedpeak %alue o the lightning-impulse withstands %oltage ( ull wa%e)" $ote: +he lightning-impulse le%el (GIG) has been #ommonly re erred to as the @BIG "7

Below are more de initions rom 6SA-6 -='* #lause 9"&"

Swit#hing impulse le%el (SIG) - the pres#ribed peak %alue o the swit#hing impulsewithstands %oltage ( ull wa%e)"

5ni orm insulation (o a trans ormer winding) - the insulation o a trans ormer windingwhen all its ends #onne#ted to terminals ha%e the same power re1uen#y withstand%oltage"$onuni orm insulation (o a trans ormer winding) - the insulation o a trans ormer windingwhen it has an end intended or dire#t or indire#t #onne#tion to ground and is designedwith a lower insulation le%el assigned to this ground or neutral winding end"

Insulation le%els should be based on insulation #oordination o the system" Standardsgi%e di erent insulation le%els (BIG le%els) or the same system %oltage" Cne e,ample isor 9;DV8 6SA-6 -='* gi%es our di erent BIG le%els ;*DV ;*DV and';*DV" 4or 2*DV IEEE 6;>"& "**- *&* gi%es our di erent BIG le%els ;*DV;DV and '**DV" Some #onsultants who write spe#i i#ations or users o ten spe#i y

higher BIG le%els than needed" +hese #onsultants say that by spe#i ying higher BIG

le%els they are helping their #lients to pro#ure better trans ormers" Insulation le%els (BIGle%els) ha%e a large impa#t on trans ormer #ost and the losses" As su#h it isre#ommended that the users to spe#i y only the needed BIG le%els and not to o%erspe#i y them"

Some users eel that more reliable trans ormers #an be pur#hased by spe#i ying higherBIG le%el or the bushings than the windings" Sin#e the bushing is the irst #omponent tosee the impulse some users spe#i y one or two higher BIG le%el or the bushings thanwhat is a#tually needed" In su#h trans ormers windings #ould ail be ore the bushingsbe#ause the BIG le%el o the bushings is higher than the windings" +his is %ery #ostlybe#ause #ompared to the #ost o the winding8 the #ost o the bushing is small" Also torepla#e the windings is #ostly and time #onsuming #ompared to the bushings"

=any te#hni1ues (interlea%ed #ounter shield turns et#") to uni ormly distribute theimpulse %oltage in the windings ha%e been de%eloped and all the trans ormermanu a#turers are using them su##ess ully" Also manu a#turers ha%e reliable #omputerprograms to #al#ulate the impulse %oltage distribution in the windings" As su#h there isno need to o%er spe#i y the BIG le%els or windings"

Sin#e re1uests or 1uotes are @ ree some users ask or alternate 1uotes ortrans ormers with di erent BIG le%els" Cne e,ample is or ;**DV units where three


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alternate 1uotes were re1uested" +he irst one was &9 ;DV BIG or windings and&9 ;DV BIG or bushings" +he se#ond alternate was &9 ;DV BIG or windings and& **DV BIG or bushings" +he third alternate was & **DV or windings and & **DV orbushings"

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INSULATION LEVELS (continue !C ten not enough time is gi%en to prepare the tenders" +he designer o the trans ormermanu a#turer has to make two separate designs with di erent BIG le%els or windingsand three separate outline drawings to #o%er all the alternates" In su#h #ases i notenough time is gi%en there is a possibility that the tenders may not be e,a#tly what isneeded by the user" Preparation o tenders is a signi i#ant #ost to the manu a#turers"E%en or house repairs (plumbing heating air #onditioning repairs et#") the #ontra#torso ten #harge to gi%e estimates" I the users were to pay at least or the alternati%es to#o%er a part o the #ost in#urred by the manu a#turers then users will get e#onomi#altenders meeting their system re1uirements" As in house repair work #ustomers need notpay this #ost to the su##ess ul bidder"

Gightning arrestors design has impro%ed to gi%e better prote#tion8 so many standards nolonger re1uire #hopped wa%e tests during impulse tests" 4ront o wa%e testingre1uirements ha%e also been eliminated by almost all o the standards" 5sers shoulddis#uss this with their insulation #oordination engineers and prote#tion engineers be orespe#i ying a #hopped wa%e test or a ront o wa%e test and they should be spe#i ied onlyi needed by the system" Some users spe#i y a / -4 -6 -4 impulse test se1uen#ebe#ause in their e,perien#e this se1uen#e enables easier ailure dete#tion"

Some users #ompare physi#al #learan#e distan#es between windings between windingsand ground to e%aluate di erent bids" Some users use these #learan#es or initial#omparison o the bids8 and in the inal e%aluation the stresses in oil and in solidinsulation are #ompared" 6omparison o the total distan#es is not an e e#ti%e pra#ti#esin#e this does not #orre#tly determine the %oltage stresses in oil and in solid insulation"+he more appropriate pra#ti#e is spe#i ying separately the stress limits in oil and in solidinsulation or power re1uen#y swit#hing surge and impulse %oltages" Somespe#i i#ations ha%e stated these limits" +here are many ways to design the insulationsystem to #ontrol these stresses"

Some spe#i i#ations spe#i y more tests higher test le%els and stri#ter toleran#es thanthose stated in the standards" Gower #osts #an be attained by not de%iating rom thestandards unless it is spe#i i#ally needed by the system"

Some users spe#i y the BIG o the FV neutral to be the same as the line terminal e%enthough the neutral will be solidly grounded in operation" +his will signi i#antly in#rease

#ost" +he #ost will be urther in#reased i there an G+6 in the FV neutral"6ertain types o #ir#uit breakers (S4< and %a#uum interrupters) or when swit#hing#ertain types o loads (solid state loads) #an produ#e a mu#h steeper %oltage wa%e thanwhat is spe#i ied in the standards" +rans ormers #an ail ( ailures had o##urred due topart winding resonan#e high %oltage stresses in the winding et#") i the windings are notdesigned or these %oltage wa%es" I the breaker is going to produ#e a di erent wa%eshape than what is spe#i ied in the standards then the wa%e shape to whi#h thetrans ormer should be designed and tested must be #learly stated in the spe#i i#ations"


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Based on system needs i the user wants the impulse test #onne#tions to be di erentrom those stated in the standards then it must be stipulated in their spe#i i#ations" Cnee,ample is when FV line is impulse tested on an autotrans ormer whether the GV linemust be solidly grounded or grounded through impedan#e"

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TERMINALS4ollowing de initions are rom 6SA-6 -='* #lause 9"&:

+erminal--#ondu#ting element intended or #onne#ting a winding to e,ternal #ondu#tor"Gine terminal--a terminal or #onne#tion to a phase #ondu#tor o a system"$eutral terminal--(a) or polyphase trans ormers and polyphase banks o single-phasetrans ormers terminal (s) #onne#ted to the neutral point o a star-#onne#ted or ig ag-#onne#ted winding and (b) or single-phase trans ormers the terminal intended or#onne#tion to a neutral point o a system"

4ollowing de inition is rom 6;>"& " *- *&* #lause 2"99':

+erminal--(A) A #ondu#ting element o e1uipment or a #ir#uit intended or #onne#tion toan e,ternal #ondu#tor" (B) A de%i#e atta#hed to a #ondu#tor to a#ilitate #onne#tion withanother #ondu#tor"

In the spe#i i#ations users should spe#i y all the ele#tri#al #hara#teristi#s and physi#aldetails o the terminals" A ew are listed below"

&" +ype o bushings (#ondenser type bulk oil type et#")"" Insulation le%el o line and neutral bushings"2" C%er%oltage"9" C%erloads";" +ype o terminal (oil-to-air oil-to-oil oil-to-S4< et#")"

"& "&*- *&* #lause ;"& indi#ates the segments)"&*" +erminal arrangement and markings (/e : 6;>"& "&*- *&* #lause ;" 4igure 2)"&&" Standards to whi#h to #omply (6SA.EE=A6 or IEEE.A$SI or IE6)& " $eutral bushing #urrent rating (same or hal o line #urrent)"&2" Spe#ial details (Bus-du#t or any other spe#ial re1uirements)"&9" 6onne#tion (bottom-#onne#ted or draw-lead.rod)"&;" /e1uirement o 0oble #onne#tors and bus #onne#tors"&" /e1uirement o #ap tap and.or potential tap"

& " Por#elain or #omposite bushings"&'" Bushings terminals heights rom the trans ormer base spa#ing et#"*" =inimum e,ternal #learan#es (line to line and line to neutral) i non-standard"&" Inter#hangeability with e,isting spares or with bushings o another standard"" Crder o bushings arrangement (F& F F2 or F2 F F& et#") i non-standard"2" Paper resin #ore (P/6) or paper oil #ore (PC6)"

9" Swap-out #apability without draining main tank oil"


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Bushings built to IEEE.A$SI standards are less e,pensi%e and ha%e a shorter lead-timethan bushings built to 6SA.EE=A6 standards" As su#h many users in 6anada arespe#i ying the bushings built to IEEE.A$SI standards" I a bushing built to 6SA.EE=A6standards has to be repla#ed with a bushing built to IEEE.A$SI standards it #an bedone by using the adapters whi#h #an be pur#hased rom bushing manu a#turers"

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TERMINALS (continue !Bushing manu a#turers in 6anada are also relu#tant to supply bushings built to6SA.EE=A6 standards" 5sers should #onsider this be ore spe#i ying the standard orthe bushings" 5sers should also #onsider IE6 bushings be#ause o ten pri#es o IE6bushings are lower than 6SA.EE=A6 and IEEE.A$SI bushings"

I users want any tests other than the standard routine tests on the bushings then thesemust be spe#i ied in the spe#i i#ations" +his is be#ause the trans ormer manu a#turersare usually not e1uipped to do tests on the bushings themsel%es" +hese tests must bedone by bushing manu a#turers who would also supply the test #erti i#ates"

hen oil-to-oil or oil-to-S4< bushings are spe#i ied the trans ormer manu a#turer willinstall a #hamber with additional oil-to-air bushings to #ondu#t a#tory tests on thetrans ormer" In su#h #ases it is re#ommended that the user inali e the test set-up andthe tests at the design re%iew stage"

+o #lean the oil sur a#e o the por#elain spe#ial pre#autions and spe#ial materials arere1uired" 5ser should obtain these details rom the trans ormer manu a#turer" Someusers spe#i y that gaskets must be suitable or high temperature (e," Viton gaskets)"Suitability o these gaskets at #old temperatures should also be spe#i ied sin#e somegasket materials per orm well at high temperatures but poorly at #old temperatures"

A ew other points to #onsider are the #urrent ratings o the bushings in a bus du#t oile,pansion %olume ade1uate or the entire operating temperature range in#ludingo%erloads a need or e,ternal #orona shields et#" +he internal #orona shield should bededi#ated to the intended bushing model and the trans ormer" Internal #orona shieldsshould not be #hanged or used on another model bushing or in another trans ormerwithout the appro%al rom the trans ormer manu a#turer"

Spe#i y when the bushings are lo#ated inside isolated-phase bus du#ts or surroundedby the trans ormer s sound en#losure along with all the re1uired details or #orre#tsele#tion and mounting o the bushings" Some o the details to in#lude are the height othe bushing terminal rom the trans ormer base spa#ing langes et#" I the minimum air#learan#es o bushings needed are more that those stated in the standards then thesemust be gi%en in the spe#i i#ations" In many instan#es these air #learan#es depend onthe model o the bushing terminal #onne#tors" I a parti#ular bushing #atalogue numberis needed then it should be spe#i ied in the spe#i i#ations"

6lause 2"; o 6SA-6 -='* spe#i ies Gow-Ambient +emperature Goad 6apability" It issuggested that users %eri y this #apability in the +ender /e%iew =eeting" 5sers shouldalso ensure that the turrets o the terminals are properly designed to enable any gasese%ol%ing rom trans ormer oil and insulation to ind their way to the gas a##umulationrelay"


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hile working as a maintenan#e engineer with a user the author has e,perien#ed moreproblems on terminals than on the windings" +here ore e,tra #are is re#ommended inspe#i ying the details or the terminals and during the appro%al re%iew o the drawings"

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ACCESSORIES6;>"& " *- *&* #lause 2"9 de ines a##essories as ollows 3de%i#es that per orm ase#ondary or minor duty as an adNun#t or re inement o the primary or maNor duty o aunit o e1uipment7"

A #omprehensi%e list o a##essories is gi%en below: Cil preser%ation system (#onser%ator nitrogen pressuri ed et#") Breather (normal sili#a gel maintenan#e ree sili#a gel #ryogeni# prin#iple et#") =o%ing a#ilities (li ting Na#king rolling sliding et#") ?as a##umulation relay Cil le%el indi#ator Cil temperature indi#ator inding temperature indi#ator ( +I) 4iber opti#s temperature sensors 4ans Pumps Val%es (drain sampling ilter radiator et#") ?rounding (#ore tank ele#trostati# shields magneti# shunts et# ) Surge arresters and #ounters Sudden pressure relay 0eluge Cn-line monitoring systems Pressure relie de%i#e E,plosion-resistant tank 4all arrest systems

0ue to time #onstraints a##essories are not des#ribed in detail" I user is amiliar withthe #onstru#tion o the a##essories how they operate and how they un#tion then it willbe %ery use ul in the maintenan#e o the trans ormers" +he ollowing will help inpreparation o the trans ormer spe#i i#ations"

Is an air bag re1uired in the #onser%atorO I re1uired then must it be o any

parti#ular makeO Spe#i ying the temperature limits" Is a parti#ular make onitrogen pressuri ed system re1uired and i so what should the pressure limitsbeO

Is a spe#i i# make o breather re1uiredO Should it be maintenan#e reeO 0oes the time #onstant o the +I mat#h with the time #onstant o the windingsO

+his is %ery important on Cil 0ire#ted) C0 #ooled trans ormers" I a spe#i i# type or make o a##essory is needed then this should be spe#i ied"


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I re%enue metering 6+s are needed spe#i y their a##ura#ies and need or#erti i#ation"

I any pro%isions (on-line monitoring systems arrestor bra#kets et#") are re1uiredspe#i y the parti#ulars like si es types o %al%es mounting bra#kets details et#""

By dis#ussing all the parti#ulars o the a##essories in the tender re%iew meeting most#on li#ts #an be a%oided" +his may also help in de#iding on whi#h bidder to pla#e theorder"

-&;-

T"PES OF COOLING4or oil-natural #ooling systems (C$A$ C$A$.C$A4 et#") 6A$.6SA-6 -='* andIEEE 6;>"& "**- *&* spe#i ies


16/57

4or the normal well-type +I the time #onstant is about 9; minutes" 4or windings thetime #onstant is about 9 to &* minutes" 4or C0 #ooled trans ormers a +I with e1ual orlower time #onstant than the windings should be #hosen" +he IEEE +ask 4or#e on +Ihas done e,tensi%e work on this topi# and the author re#ommends re erring to theirwork" +his in ormation is a%ailable at www"trans ormers#ommittee"org

4or water #ooled trans ormers #hemi#al analysis o the water should be pro%ided in thespe#i i#ations" Supply %oltage a%ailable at the station must be in#luded in thespe#i i#ations or the #orre#t sele#tion o ans pumps G+6 motor and other a##essories"

-&"& "**- *&*: 3+he trans ormer shall be #onne#ted or and energi edat rated %oltage re1uen#y and at no-load" $oise-#ontributing elements o thetrans ormer su#h as pumps and ans shall be operated as appropriate or the ratingbeing tested" hen it is impra#ti#al or undesirable to in#lude the appropriate #oolinge1uipment the sel -#ooled sound le%el may be #orre#ted or #ooling noise #ontribution i suitable #orre#tions are a%ailable and it is mutually agreeable to those #on#erned"7 It issuggested also re er to #lause &2"2"9 o 6;>"& "'*- *&*"


17/57

6lause &2"&"& o 6;>"& "'*- *&* states: 3Audible sound rom trans ormers originatesprin#ipally in the trans ormer #ore and transmits through the diele#tri# luid and.orstru#tural supports to the outer shell and.or other solid sur a#e where it radiates asairborne sound" In some situations the windings may be a noise sour#e under ratedload #onditions but this noise is not in#luded in this standard"7

hen a uture sound le%el redu#tion en#losure is re1uired the spe#i i#ations should#learly state the work to be per ormed by the manu a#turers su#h as ( langes height romthe trans ormer base details o the langes pressure relie mounting details et#")"

-&>-

TAPS (#$n%e& 'oc$tion etc)!+he un#tion o the %oltage taps is to keep the output %oltage #onstant" G+6 and 0E+6stand or Goad +ap 6hanger and 0e-energi ed +ap 6hanger respe#ti%ely"

0e initions below are rom IE6 "& " *- *&*"+ap (#lause 2"99&) A #onne#tion brought out o a winding at some point between its e,tremities to permit#hanging the %oltage #urrent or ratio"

=ost aspe#ts o taps are #o%ered in detail in the ollowing tutorials" +he tutorials will helpusers in determining the tap range lo#ation et#" +he tutorials #an be a##essed atwww"trans ormers#ommittee"org or in ?oogle"

3+APS7 C#tober < **9 IEEE +rans ormers 6ommittee =eetings in Gas Vegas$e%ada 5SA by V" Sankar"

3+APS I$ A5+C+/A$S4C/=E/S7 C#tober ; *&* IEEE +rans ormers 6ommittee=eetings in +oronto 6anada by 0r" +omas Dali#ki and V" Sankar"+he ollowing #lauses are rom 6;>"& "&*- *&*"6lause 9";"& Figh-%oltage winding taps or de-energi ed operation:I spe#i ied de-energi ed tap #hanger (0E+6) the ollowing our high-%oltage ratedkilo%oltampere taps shall be pro%ided" "; and ; abo%e rated %oltage and "; and; below rated %oltage" hen a load tap #hanger (G+6) is urnished per 9";" the high-%oltage 0E+6 may not be re1uired"
http://www.transformerscommittee.org/http://www.transformerscommittee.org/


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6lause 9";" +aps or G+6 trans ormers:hen an G+6 trans ormer is spe#i ied G+6 e1uipment shall be urnished in the low-%oltage winding to pro%ide appro,imately&* automati# regulation o the low-%oltage winding %oltage in appro,imately *"< ; steps with &< steps abo%e and &DV to ;2DV in 2 steps " ithsu#h spe#i i#ations the manu a#turers #an o er bids either with #oarse. ine tapsor with re%ersing taps"

Cther #onsiderations on the taps are gi%en below"

(a) /eliability and #ost o allowing #onne#ting non-linear elements (=CVs) a#rossthe taps %erses other design pra#ti#es without non-linear elements (=CVs)"

(b) A number o spe#i i#ations indi#ate a pre eren#e or a spe#i i# G+6 manu a#turer"It is suggested lea%ing this #hoi#e to the trans ormer manu a#turers" I neededask or e,tra warranty on the tap #hangers o other makes"

(#) 6ompared to tie-in resistors with tie-in #apa#itors the ar# generated during#hangeo%er swit#h operation is more intense" But with the tie-in #apa#itors thelosses in tie-in elements is almost ero" 6onsider these in inali ing the bids"

(d) As the #apitali ation #ost o i,ed losses is o ten %ery high allow themanu a#turer the option to 1uote with a potential swit#h or with tie-in resistors"

(e) 0o not spe#i y that only tank-mounted tap #hangers are a##eptable" In some#ases tank-mounted tap #hangers are more #ostly than in-tank tap #hangers"

( ) Some spe#i i#ations state that the sele#tor #onta#ts should not be in the main-tank s oil" +his results in a #ostly option with no use ul bene it" Very ew tankmounted tap #hangers are a%ailable and the sele#tion is limited by insulationle%el step %oltage #urrent et#"

(g) Spe#i i#ations should spe#i y whether the taps are /6B$ (/edu#ed 6apa#ityBelow $ormal) or 46B$ (4ull 6apa#ity Below $ormal)"

(h) hen the re%ersing taps (either or 0E+6 or or G+6) are ele#tri#ally in the FV#ir#uit and physi#ally below the FV windings the tapping winding will e,perien#eree bu#kling or#e" hereas when the linear taps (in#ludes #oarse. ine taps) are


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ele#tri#ally in the FV #ir#uit and physi#ally below the FV windings the tappingwinding will not e,perien#e ree bu#kling or#e"

(i) /e#ommend not to spe#i y that the G+6 winding should be a separate windingbe#ause linear taps (in#luding #oarse. ine taps) #an be e e#ti%ely situated in thebody o the main winding" C ten this will be a more e#onomi#al design as#ompared to a design with the G+6 as a separate winding" /e#ommend allowingthe manu a#turer to de#ide the lo#ation o taps based on per orman#e and #ost"(N) hen a bid re1uires an G+6 with #urrent splitting obtain G+6 manu a#turer s#on irmation on the appli#ation" +his is be#ause in #ertain appli#ations balan#ingtrans ormers #ould be re1uired" Also #he#k with the trans ormer manu a#turerthat the windings are designed or #urrent splitting appli#ation"

- &-

T"PES OF TAPS IE6


22/57

per turn #hanges rom tap to tap" Spe#i i#ations should in#lude the ollowing:

(a) the #ategory o %oltage %ariation (%ariable lu, taps)"(b) whi#h winding has the taps the tap range and number o tap

steps"(#) whether the taps are /6B$ or 46B$"(d) whether the taps are to ha%e e1ual turns (%oltage o the tap stepis based on the %olts per turn on that tap) or whether they are to

ha%e appro,imately e1ual %oltages" hen the G+6 is o lineartype the manu a#turer #an adNust the turns per tap to obtainappro,imately e1ual %olts per step" hen the G+6 is o

- -

T"PE OF TAPS (continue ! re%ersing type or o #oarse. ine type it is not possible to obtain

appro,imately e1ual %oltage tap steps"(e) It should be agreed between the user and the manu a#turer on

whi#h tap the no-load loss is guaranteed"

2" =i,ed regulation (IE6 #ategory 6b"V"V 6ombined Voltage Variation)" A portion o the taps a#t as #onstant lu, taps and the remaining portion o the taps a#t as %ariable lu, taps" (a) the #ategory o %oltage %ariation (6b"V"V)" (b) whi#h winding has the taps the tap range and number o tap steps" (#) whi#h tapping is the @ma,imum %oltage tapping with the #orresponding tapping %oltages" (d) whi#h tapping is the @ma,imum #urrent tapping with the

#orresponding tapping #urrents" (e) IE6


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GV FV %ariable

4or a similar +able or autotrans ormers re er the tutorial 3+aps in Autotrans ormers7 by0r" +omas Dali#ki and V" Sankar on IEEE web pagewww"trans ormers#ommittee"org

In all systems the input %oltage the load and the power a#tor will be #onstantly#hanging" So almost all trans ormers are being operated as mi,ed regulation taps(6b"V"V")" Cperating engineers must #learly understand the #hanges in the trans ormerparameters based on how the trans ormers are operated" Impedan#e ault #urrent andsound le%el %aries or remains #onstant rom tap to tap based on whether the taps areoperated as V4VV taps or 64VV taps"

- 2-

T"PES OF TAPS (continue !I the user has pre eren#es or spe#ial re1uirements on the ollowing then they must bestated in the spe#i i#ations with all the re1uired details to design the trans ormer"

&" /esistor type G+6"" /ea#tor type G+6"

2" +ank mounted G+6"9" In-tank G+6";" Va#uum type G+6"" Pre%enti%e autos (G+6 rea#tor)"" Series +rans ormers"

'" 6ompensating +rans ormers"&*" $on-linear de%i#es"&&" +ie-in resistors or potential swit#h"& " E,tra #urrent rating on G+6 (&"; p"u" p"u" et#")"&2" Parti#ular make o the G+6"&9" 6apability o the G+6 to meet Gow-Ambient +emperature Goad 6apability"

4or reliability users should #he#k the type and design o the tapping winding leadsbring-out or lead e,its et#" hen e##entri# winding arrangement is used to bring-out theleads it should be #he#ked that the #oils are radially supported with no sla#kness"

Some other #onsiderations on the taps are gi%en below"

&" hen the #urrent in the tap winding is high the use o a series trans ormer is a#ommon method to redu#e the #urrent in the G+6" In these #ases re%ersing taps willredu#e the series trans ormer DV"A to almost hal o that whi#h would be re1uired withlinear taps (in#ludes #oarse. ine taps)" In su#h #ases re%ersing taps #ould be moree#onomi#al as #ompared to linear taps or #oarse. ine taps"

" +ap #hanger rating should be ade1uate or low-Ambient +emperature Goad 6apability"

2" In general no thermal measurements will be done on the tapping winding itsel " Sousers should #he#k at the 0esign /e%iew how the #al#ulated a%erage gradient and
http://www.transformerscommittee.org/http://www.transformerscommittee.org/


24/57

hot-spot gradient o the tapping winding #ompare to the main windings" 5sers shouldmake sure that i the a%erage gradient or hot-spot gradient o the tap winding is higherthan the main windings then these are used or the #ooling #al#ulations"

9" =any ailures ha%e o##urred when the supports o the tap leads are inade1uate tosupport their weight" 4ailures ha%e also o##urred when the #learan#es (distan#es) romthe tap leads o one phase to the tap leads o another phase and.or the #learan#es(distan#es) rom the tap leads to the ground were inade1uate"

- 9-

IMPEDANCE+he ollowing de initions in 6;>-& " *- *&* are related to impedan#e"

2"&> Impedan#e 0rop: +he phasor sum o the resistan#e %oltage drop and the rea#ti%e%oltage drop"$ote: - 4or trans ormers the resistan#e drop the rea#tan#e drop and the impedan#edrop are respe#ti%ely the sums o the primary and se#ondary drops redu#ed to thesame terms" +hey are determined rom the load-loss measurements and are usuallye,pressed in per unit or in per#ent"2"2;' /ea#ti%e drop: +he #omponent o the impedan#e %oltage drop in 1uadrature withthe #urrent"2"2>< /esistan#e drop: +he #omponent o the impedan#e %oltage drop in phase with the#urrent"

0uring preparation o the spe#i i#ations the ollowing in ormation on the impedan#e will

be o help"&" In a #onsiderable number o spe#i i#ations while the per#ent impedan#e is alwaysstated the =V"A base at whi#h the impedan#e applies is not stated"" In some spe#i i#ations impedan#e in ohms is stated but whether the ohms arere erred to FV side or GV side is not stated"2" It is not possible to make generali ations about impedan#e magnitude %ersus #ost"Statement like a lower (or a higher) impedan#e results in redu#ed #osts are notne#essarily true" +his depends on many a#tors like trans ormer =V"A two-windingtrans ormer or an autotrans ormer impedan#e %alue #apitali ation #ost o the lossesshort-#ir#uit or#es et#"9" hen the impedan#e is low the short-#ir#uit #urrents will be high" hen the

impedan#e is high8 regulation eddy losses in windings and stray losses in the tank endrames et#" will be high";" Be ore adopting the impedan#e %alues suggested in the standards (+able 2B o6A$.6SA-6 -='* or +able 2 on 6;>"& &*- *&*) users should #he#k the breaker#apa#ity short-#ir#uit #urrent limitations at the station and the regulation re1uirement"


25/57

the bids #annot be e%aluated e1uitably sin#e the trans ormer #ost will %ary with theimpedan#e %alue">" In almost all the spe#i i#ations impedan#e at rated tap only is stated" I the impedan#eon an e,treme tap is low then the short-#ir#uit #urrent #ould e,#eed the systemlimitation" It is suggested that impedan#es on the tap e,tremes also be spe#i ied"" +he physi#al lo#ation o the tap winding whether the taps are 64VV or V4VV and thetap range determine the impedan#e %alues on e,treme taps"'" It will be bene i#ial to users i they dis#uss the impedan#e re1uirements withmanu a#turers at the time o preparation o the spe#i i#ations"&*" Impedan#e is in%ersely proportional to the s1uare o the %olts.turn" 5sers should beaware o the #hange in impedan#e when 64VV taps are used as V4VV taps"&&" 6I?/E Tuly &'>2 paper[15] is a %ery good re eren#e on how the autotrans ormerimpedan#e #hanges based on ele#tri#al #onne#tion o the taps on physi#al lo#ation othe tap winding the tap range and the ratio o FV.GV %oltages"

- ;-

CAPITALI*ATION OF LOSSES

3IEEE Goss E%aluation ?uide or Power +rans ormers and /ea#tors7 6;>"& *-&''& is a%ery good re eren#e that should be re%iewed while preparing the spe#i i#ations" +heollowing is rom this ?uide:-

@+he purpose o this ?uide is to pro%ide a method o establishing the dollar %alue o theele#tri# power needed to supply the losses o a trans ormer or rea#tor" 5sers #an usethis loss e%aluation to determine the relati%e e#onomi# bene it o a high- irst-#ost lowlosses unit %erses one with lower irst-#ost and higher losses" =anu a#turers #an use thee%aluation to optimi e the design and pro%ide the most e#onomi#al unit to bid andmanu a#ture" +he e%aluated #ost o losses also enables a user to #ompare the o eringo two or more manu a#turers to aid in making the best pur#hase #hoi#e among#ompeting trans ormers and rea#tors" Goss e%aluation also pro%ides in ormation to a

user or establishing the optimum time to retire or repla#e e,isting units with modern low-loss trans ormers or rea#tors "

5sers will get the lowest li etime #ost by pro%iding the ollowing loss e%aluation %alues inthe spe#i i#ations"

&" $o-load loss in U.D "" Goad loss in U.D at a spe#i ied =V"A (Base =V"A or ma,imum =V"A)"2" Au,iliary loss in U.D (losses o pumps ans et#")"

IEEE ?uide 6;>"& *-&''&gi%es the detailed des#ription and a worked out e,ample oe%aluation o the bids" +he ?uide des#ribes 0emand Portion (#ost o installing system

#apa#ity in dollars per Dilowatt) and Energy Portion (Present Value o the energy that willbe used by one Dilowatt o loss during the book-li e o the trans ormer #on%erted todollars per Dilowatt)"

+he ollowing will be help ul to users in preparing the spe#i i#ations"

&" In many spe#i i#ations U.D or load loss is stated but the =V"A re eren#e is notstated" In a bid or a > .' =V"A)" +hough the irst #ost o the


26/57

trans ormer was U&** *** #heaper than the other bidders the order was lost be#ausethe user e%aluated the bids with the load loss e%aluation at top rating (& *=V"A)" +hee%aluated #ost was U& ; *** more than the nearest #ompetitor"

" Some spe#i i#ations state that the losses are not e%aluated" +his may be due torestri#tions on #apital spending" In su#h #ases the users will lose out on the li e #y#le#ost" 0ue to the #ustomer re1uirements o ree bu#kling #riteria sound le%el et# o ten itis not possible to redu#e the irst #ost e%en when losses are not e%aluated"

2" Some users buy spare trans ormers with no loss e%aluation" Based on how long thespare unit will operate in the system the user #ould lose or make money"

9" Some spe#i i#ations gi%e a D F rate and a ew other details but not U.D " 4rom thedata in the spe#i i#ations there is no guarantee that all the bidders will #ome-up with the

- ; 6" +he spe#i i#ationsshould ollow the standards unless there is a spe#ial reason that the user #an pro it byde%iating rom the standards"

" 0uring the design stage or during the manu a#turing stage the manu a#turer #ould#ome-up with an idea to redu#e the losses" C ten to implement su#h an idea needs e,tramaterials or e,tra labor or both" 5nless this e,tra #ost is shared by the user themanu a#turer has no in#enti%e to #ome up with ideas that will sa%e money or the user or to implement them" A manu a#turer #ame up with a solution on regulators to redu#e loadloss on hal o the taps up to


27/57


28/57

" =ethod o #onne#tions e"g" isolated phase bus et#"" +he #hara#teristi#s o ea#h system short-#ir#uit apparent power le%el and the ratiobetween the ero se1uen#e impedan#e and the positi%e-se1uen#e impedan#e"

+he ollowing will help in pro#uring reliable and e#onomi#al trans ormers"

&" In many spe#i i#ations it is stated that ault #urrent #al#ulations should be based onin inite bus on all terminals" Alternati%ely some spe#i y system impedan#es per aspe#i i# standard (6SA-6 -='* 6;>"& "** et#")" Sin#e system impedan#e %aluesstated in the standards are low it is ad%isable or the user to spe#i y the system

impedan#es at the trans ormer station" System impedan#es will help to redu#e the short-#ir#uit #urrents (espe#ially on trans ormers with low impedan#es)"

- -

SHORT-CIRCUIT WITHSTAND (continue !hen inter#hangeability with other stations is needed it is re#ommended spe#i ying thesystem impedan#e whi#h is lowest o all the stations rather than in inite bus"

" A short-#ir#uit test is not #ommon be#ause it is %ery e,pensi%e" 4or users to ha%e#on iden#e that the trans ormers will withstand all types o aults the users ha%e todepend on manu a#turer s design #al#ulations (e1ui%alent #ir#uits to #al#ulate ault#urrents o -set %alues amp-turn balan#e et#") and manu a#turing pra#ti#es (#lampingstru#ture drying pro#ess si ing o the #oils e,posure time during the tanking o #ore #oils et#")" In most #ases by the time the trans ormer e,perien#es a ault with ullasymmetry and ma,imum pre- ault %oltage the warranty would ha%e e,pired" So it is%ery important or the user to know the philosophies ea#h manu a#turer adopts in#al#ulating the short #ir#uit #urrents or#es and stresses in the windings and thewithstand strengths o the windings"

2" 4or three-#ir#uit trans ormers in many spe#i i#ations whether the in- eed is rom onesystem or rom two systems is not stated" Per 6;>"& "**- *&* or three-#ir#uittrans ormers the in- eed is by more than two sets o un aulted terminals rated greaterthan 2; o the terminal DV"A o the highest #apa#ity winding" 4or the three-#ir#uittrans ormers i user re1uirement is di erent rom 6;>"& "**- *&* then spe#i i#ationsshould spe#i y whether the in- eed is rom one system or rom two systems"

9" In many spe#i i#ations or the autotrans ormers it is spe#i ied that the tertiary has tobe brought-out to supply station loads (usually a ew DV"A)" hen the tertiary is brought-out then it has to be designed to withstand a three-phase ault on its own terminals" +hiswill in#rease the tertiary winding si e #onsiderably or a 6urrent Gimiting /ea#tor willha%e to be used" In many #ases it will be e#onomi#al to the users to supply the stationloads rom another sour#e su#h as SSV+ (Station Ser%i#e Voltage +rans ormer)"

;" Almost all spe#i i#ations state that inner windings should withstand ree bu#kling"hen the G+6 is ele#tri#ally in the FV winding or in the series winding and the G+6winding is physi#ally lo#ated inside o the FV winding or o the series winding it may


29/57

e,perien#e ree bu#kling or#e depending on the type o taps" I the taps are o linear or#oarse. ine type then the tap winding will not e,perien#e any ree bu#kling" I the tapsare o re%ersing type then the tap winding will e,perien#e ree bu#kling"


30/57

9" 6onstru#tion and withstand strength o the edge strip on heli#al #oils ( or press-boardthe withstand strength %aries #onsiderably depending on whether the or#es are on thesur a#e or on the thi#kness)";" ?rade and strength o material used or #lamping rings" I openings are made in the#lamping ring or leads bring-out this should be #onsidered in withstand strength"" Support o the #oils to the #ore"" ithstand strength o the tank (espe#ially or e,plosion-resistant tank)"'" 0e%i#es to relie%e the tank pressure during short #ir#uit (to relie%e the pressure ast toa%oid the bursting o the tank)"&*" 0esign o #urrent limiting rea#tors in the tertiary"& " 0esign and %alue o neutral grounding rea#tor.resistor"&2" 0esign o #lamping stru#ture (e,ternal tie rods lit#h plates internal tie rods et#") andits ade1ua#y to withstand short #ir#uit or#es and also or li ting the #ore #oilsassembly"&9" Cn trans ormers with 0E+6 and G+6 taps #al#ulations o the or#es on all 0E+6 tappositions #ombined with G+6 rated tap position and both G+6 e,treme tap positions"

-2*-

SPECIAL RE+UIREMENTS Along with the regular trans ormer parameters (=V"A %oltages insulation le%elsimpedan#e loss e%aluation %alues taps details et#") the spe#i i#ations should in#ludedetails o many other spe#ial re1uirements"

&" Ambient temperatures i di erent rom the standards (6SA-6 -='* 6;>"& "**-*&* et#")"

" Spe#i i#ations that 1uote IEEE standards rom the southern 5nited States #an obtain

e#onomi#al trans ormers by spe#i ying that an in#rease in =V"A rating at #oldtemperatures (up to -2*K6 per 6;>"'&-&'';) is not re1uired" +his is be#ause the ambienttemperatures in southern states will ne%er rea#h su#h negati%e temperatures"

2" 6SA and IEEE standards generi#ally spe#i y the #ontinuous o%er-%oltage re1uirementas &&* at no-load and as &*; under load" Some spe#i i#ations state &2*#ontinuous o%er-%oltage (not #lear at no-load or under load)" 5sers should understandthat this o%er-%oltage re1uirement will in#rease the trans ormer #ost #onsiderably" Someusers spe#i y su#h high o%er-%oltage re1uirement thinking that they #an obtain better1uality trans ormers" Some users spe#i y high o%er-%oltage re1uirement with the opinionthat when the G+6 is on the input side and i the G+6 #ontrol #ir#uits mal un#tion then theG+6 motor #ould runaway to the minimum tap position" Pro%en #ontrol #ir#uits area%ailable to a%oid su#h mal un#tions at a mu#h lower #ost" Spe#i ying the o%er %oltagere1uirement or the trans ormers dire#tly #onne#ted to the generators is %ery importantbe#ause during the load reNe#tion the trans ormers will see mu#h higher o%er-%oltagethan spe#i ied in 6SA-6 -='* or 6;>"& "**- *&*" 9" 6SA-6 -='* spe#i ies a pre- ault %oltage o &"& p"u" 5nless there is a system needusers should not o%er-spe#i y this re1uirement"


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;" =ost on-load tap-#hangers operate down to - ;K6 ambient temperature with high-%is#osity oils like Voltesso 2;" ith low-%is#osity oils like Petro 6anada Guminal G+6operation below - ;K6 is permissible without heaters" +ype o oil ambient temperatureand low temperature #ut-out setting are to be #orre#tly #oordinated"


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" Some spe#i i#ations pro%ide o%erload magnitude and duration su#h as8 &">; p"u" orW hour &"< p"u" or hours &"&; p"u" or hours et#" +hese spe#i i#ations do not saywhether these o%erloads o##ur one a ter the other or at di erent times" Also thespe#i i#ations do not say how many times in a year these o%erloads o##ur" 0+E6 andG+6 manu a#turers need this in ormation in si ing the tap #hanger"

2" Almost all the spe#i i#ations ha%e the ollowing senten#e based on #lause '" " o6;>"'& irrespe#ti%e o whether the o%erload pro ile and all the rele%ant data is in#ludedin the spe#i i#ations: @An#illary e1uipment should not restri#t loading to le%els belowthose permitted by the insulated #ondu#tor and other metalli# part hot spots " heno%erload in ormation is not stated in the spe#i i#ations ha%ing this #lause in thespe#i i#ations is irrele%ant"

9" +he spe#i i#ations must in#lude #omplete in ormation per #lause '"> o 6;>"'&"(a) Goad" (&) +wo step load #y#le approa#h" Prior steady-state load per#ent o ma,imum nameplate rating"

=a,imum load per#ent o ma,imum nameplate"0uration"

-22-

OVARLOADS (continue ! ( ) Goad #y#le o%er a 9 hour period"(b) Ambient temperature K6

(&) 6onstant per load #y#le approa#h" ( ) Variable o%er the load #y#le or load #y#le approa#h"(#) +ype o loading planned or emergen#y long-time or short-time"(d) Gimiting oil temperature"(e) Gimiting hottest-spot temperature"( ) Statement that an#illary #omponents shall not limit the loading #apability"

In almost all the spe#i i#ations with /6B$ taps it is not stated how /6B$ taps are to be#onsidered or o%erloads"

Goading #apability at low temperatures&" 6lause 2"; o 6SA-6 -='* states that the trans ormers (e,#ept ?S5 units) shall be#apable o an in#rease in loading or ea#h degree that the daily a%erage ambienttemperature is lower than that o ;K6 down to a minimum a%erage ambient temperatureo *K6"

" 6lause


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It is ad%isable that the users should in#lude in the spe#i i#ations that written #on irmationrom 0E+6 G+6 and bushing manu a#turers that they meet the abo%e low temperature#apability re1uirement" Cn-load tap #hanger rating is mainly dependent on swit#hingDV"A and bushing rating at low temperatures is di erent to that o the trans ormer"

Some spe#i i#ations or autotrans ormers are not #lear whether the o%erload re1uirementapplies to the tertiary or not" Some spe#i i#ations are not #lear whether the o%erloads areappli#able or step-down #ondition step-up #ondition arithmeti# loading o tertiary loadand output load %e#torial loading o tertiary load and output load or simultaneousloading o tertiary load and output load et#" Cn high %oltage autotrans ormers manyusers #onne#t rea#tors to the tertiary" +hese spe#i i#ations should be #lear whether therea#tors are #onne#ted or dis#onne#ted during the o%erloads" 0uring o%erloads somespe#i i#ations also limit top oil temperature and hottest-spot temperature the same asthat o normal load &*;K6 and & *K6 respe#ti%ely" 5sers should reali e that they are Nustbuying a larger =V"A rated trans ormer"

=any spe#i i#ations state one ambient temperature or summer o%erloads and oneambient temperature or winter o%erloads" +he ambient temperature is mu#h less during

the e%ening and the night #ompared to that during the a ternoon" Cnly a ewspe#i i#ations state the hourly ambient temperature in 9 hour period" /e#ommend-29-

OVERLOADS (continue !spe#i ying hourly ambient temperature8 be#ause trans ormers to this spe#i i#ation will#ost less than the trans ormers with the same temperature or 9 hour period"

+o obtain a lower operational #ost some spe#i i#ations state that additional ans.pumpsare to be pro%ided or use during o%erloads" In#lusion o this senten#e in thespe#i i#ations is re#ommended be#ause o ten the pumps are pro%ided to meet theo%erloads"6;>"'&-&''; does not re#ommend any %alue or loss o li e" I the user wishes to restri#tthe loss o li e per o%erload #y#le then it should be stated in the spe#i i#ations"

Spe#i i#ations should de ine whether the o%erload is Planned loading beyond nameplaterating or Gong-time emergen#y loading or Short-time emergen#y loading and gi%elimiting %alues or insulated #ondu#tor hottest-spot temperature other metalli# hot-spottemperature (in #onta#t and not in #onta#t with insulation) and top-oil temperature i theyare di erent rom the %alues in +able o 6;>"'&-&'';

I the user wishes to per orm the o%erload test then it must be stated in thespe#i i#ations" Sometimes manu a#turers test e1uipment #an be a limitation to per ormthe o%erload test" It is re#ommended that the user and the manu a#turer agree on how toper orm the o%erload test in the 0esign /e%iew meeting" ?as-in-oil le%els duringo%erload test will be higher than those during the heat-run test" In the 0esign /e%iewmeeting user and the manu a#turer should agree on a##eptable gas-in-oil le%els duringthe o%erload test"

In the 0esign /e%iew meeting it should be #he#ked that magneti# shields are notsaturated during the o%erloads" Also the need or splitting the last #ore step or other


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&*" All internal leads and terminals not permanently #onne#ted shall be identi ied withnumbers or letters in a manner that permits #on%enient re eren#e to pre%ent #on usionwith terminal and polarity markings"&&" 6ontains no dete#table le%el o P6B (less than ppm) at the time o manu a#ture"& " Standard number in#luding the year o the standard to whi#h the trans ormer inmanu a#tured"&2" Impedan#es on e,treme tap positions"&9" Va#uum withstand #apability o #onser%ator and all oil #ir#ulating parts"&;" In addition to the main rating plate plates with identi i#ation and #hara#teristi#s oau,iliary e1uipment (bushings tap #hangers spe#ial #ooling e1uipment et#")" 5sers should #onsider in#luding the ollowing under what is to be in#luded on the ratingplate"

&" 0iagram showing the lo#ation o emergen#y man-holes"" 0iagram showing the lo#ation o maNor %al%es (drain ilter et#")"2" here appli#able to show the ollowing"(a) +ie-in resistors in#luding their rating and the manu a#turer s name"(b) +ype make and serial number o G+6 rea#tors"(#) 6urrent limiting rea#tors in#luding their impedan#e %alues and the manu a#turer sname"

-2 -

RATING PLATE (continue !9" 0iagram showing the type and lo#ation o all arrest system";" Cn nitrogen pressuri ed trans ormers minimum and ma,imum pressure settings o theregulator"" Go#ation o stati# #ylinders when used (on the #ore under G+6 winding et#")"" A statement that the %oltages and #urrents marked are based on no-load and are notduring the load"'" Qero se1uen#e impedan#e"&*" Serial number make and #omplete type designation o G+6 and 0E+6 tap #hangers"&&" =ake serial number %oltage ratio and rating o series trans ormer and.or#ompensating trans ormer when used"& " 6onstru#tion designation #ore type or shell type"&2" 6ore details single two three our i%e or se%en legged"&9" Pro%ision or uture sound en#losure in#luding the type o en#losure"&;" =a,imum #urrent in #ommon winding on autotrans ormers with loaded tertiary andduring the step-up operation"&" +ype o oil (Voltesso 2; Guminal et#")& " P6B #ontent in oil (less than ppm) at the time o irst illing"


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-2'-

ALTERNATIVESEn#ouragement by users is the key or the birth o inno%ati%e ideas" I the users had saidin the spe#i i#ations that step-lapped #ores shielded windings et#" are not a##eptablethese te#hnologies would not ha%e e%ol%ed" A ew de#ades ba#k some spe#i i#ationsused to state that the #ore must be built with the top yoke"

It is un ortunate that some spe#i i#ations state that 3$o alternati%e will be #onsidered7"Some spe#i i#ations ha%e stated that 3no prototype eatures will be a##epted7" Somespe#i i#ations ha%e stated 3Bidders are en#ouraged to 1uote alternati%es to thespe#i i#ations detailing the bene its and meeting all the re1uirements o the userpro%ided the main bid is per the spe#i i#ations7"

Some users ha%e pur#hased these alternati%es with lower losses on all the tap positionsbelow the normal tap (linear taps instead o re%ersing taps) with better %oltage #ontrollower short-#ir#uit or#es et#" Some users ha%e not pur#hased these bene i#ialalternati%es be#ause they are not e,a#tly identi#al to the trans ormers they had pro#uredin the past" 4or the health o this industry the users should gi%e in#enti%es and work withthe manu a#turers or impro%ements in design and manu a#turing"

=any users are o the opinion that trans ormer de%elopment works (redu#tion o lossesimpro%ed reliability et#") is the sole responsibility o the manu a#turers" =any engineersworking or users ha%e %ery good ideas and these should be nourished" +he bestde%elopments o##ur when users and manu a#turers work as a team rather keeping theirdistan#e as the buyers and the suppliers"

5sers should #onsider the alternati%e without the main bid when the alternati%e meetsthe system re1uirements stated in the spe#i i#ations" C ten there is no time to design


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and #ost the main bid and the alternati%e" Preparation o two bids (one or the main bidand the other or the alternati%e) is #ostly and time #onsuming or the manu a#turers"

Some o the hurdles the te#hni#al department (or spe#i i#ations writing ?roup) o theuser a#es to #onsider the alternati%es without the main bid are: #on%in#ing thepur#hasing department #on%in#ing the maintenan#e department te#hni#al e%aluation oalternati%es et#" Some o the users ha%e #on1uered these hurdles" 5nderstand romthese users that it is not di i#ult and hope that all the other users will also #on1uer thehurdles"

Some spe#i i#ations ask or too many alternati%es" +his is be#ause the user does notneed to pay money or any number o alternati%es" +he author has seen spe#i i#ationsasking or ; alternati%es with di erent impedan#es di erent insulation le%els di erent=V"As et#" +his is not air" +he users should #onsider the di i#ulties o themanu a#turers"

-9*-

ALTERNATIVES (continue !+o redu#e the number o alternati%es users should intera#t with the manu a#turers whilewriting the spe#i i#ations to determine the e,a#t parameters o the trans ormers" Almostall the spe#i i#ations ha%e a #lause stating that the pur#haser has a right to reNe#t all thebids without gi%ing any reason" Some users e%en a ter asking or 1uotes withalternati%es will not pla#e an order" 5sers should reali e that preparation o the bidsre1uire #onsiderable te#hni#al resour#es to the manu a#turers" Preparation o the bidsalso adds #onsiderable #ost to the manu a#turers" As an e,ample the #ost orpreparation o a bid or the phase-shi ting trans ormers was about U&** *** dollars orea#h bidder" hen all the bids are too e,pensi%e or the budget allo#ated to the proNe#tthen it is understandable that user not pla#ing the order" In su#h a situation the usershould in orm the reason or not ordering to all the bidders"

In an en1uiry the user re1uirement was 2 impedan#e at a gi%en =V"A" An alternatewith < impedan#e at the same =V"A is also asked" 5nderstand that the user thoughtthat it is not possible to design a trans ormer with 2 impedan#e at that =V"A" I the user had dis#ussed with the manu a#turers prior to issuing the spe#i i#ations themanu a#turers would ha%e sa%ed time and money in preparing a main bid and analternati%e"In another en1uiry the main bid with FV wye . GV delta and the alternati%e withFV delta . GV wye were asked" +he taps are on GV or GV %ariation" hen GV is wye the#urrent in GV is high and a series trans ormer has to be used" hen GV is delta theseries trans ormer is not needed" +his made GV wye option %ery e,pensi%e as seriestrans ormer #ost and losses are to be in#luded" 5sers should do some home-work whilewriting the spe#i i#ations and ask or the alternati%es that are bene i#ial"


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Some users ask or bids on &* to * di erent trans ormers (all the ratings in theirsystem) but pla#e the order only on a ew ratings" +his a%oids users going or bids whena need arises or a di erent rating trans ormer thus sa%ing them money" Alsounderstand that this is or bulk orders to #o%er ;-years re1uirements" Suggest that atleast on#e a year users and manu a#turers should dis#uss ea#h other s di i#ulties"

I the users are not sure about getting &** o the budgeted unds and may ha%e toredu#e the #apital #ost then the spe#i i#ations #an ask or a bid with loss e%aluation andan alternati%e with no loss e%aluation"

-9&-

INTERESTING CLAUSESIt is suggested not to use the ollowing #lauses in the spe#i i#ations as either they areambiguous or will add #onsiderable #ost with almost no bene it to the user"

&" I any te#hni#al in ormation is missing rom the spe#i i#ations then it should be perindustry standard ( hat is the industry standardO here it is de inedO)"

" +he windings o ea#h trans ormer shall be made o su#h materials and assembled in amanner best suited or the parti#ular appli#ation ( hat is meant by best suited and howto measure itO)"

2" Best materials should be used (Are windings to be o sil%er instead o #opperO)"

9" Ade1uate barriers shall be pro%ided between the #ore and the low %oltage windingbetween the low %oltage winding and the medium %oltage winding and the medium%oltage winding and the high %oltage winding (Ade1uate or whatO Fow to determinewhat is ade1uateO)"

;" In-tank type G+6 is a##eptable but the sele#tor #onta#ts must not be in trans ormermain oil (+his is %ery #ostly" $ot ha%ing the di%erters in trans ormer main oil is logi#al")"


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*" All #omponents in#luding the tap #hangers shall pro%ide utmost reliability (0o notstate that is not measurable" Spe#i y a test)"

&" 0uring the ser%i#e there should be no abnormal deterioration o insulation (It wouldbe meaning ul i normal and abnormal are de ined)"

" +he trans ormers shall #on orm in all respe#ts to high standards o engineeringdesign and workmanship ( here are the high standards de inedO)"

2" E1uipment meeting with the re1uirements o other authoritati%e internationalstandards whi#h ensure e1ual or better per orman#e than the standards mentionedabo%e shall also be #onsidered (It will be interesting to know i the user was the bidderwhat the user wound ha%e done when su#h #lauses are in the spe#i i#ations)"

9" +he impedan#e %oltage at prin#ipal tap and rated =V"A shall be stated in the order($o impedan#e %alue is stated in the spe#i i#ations issued to get the bids" 5ser shouldreali e that to prepare a bid the impedan#e is re1uired")"

;" $on-magneti# materials used shall be o established 1uality ( here this established1uality is and who established itO)"

-92-

GLOBALI*ATION$owadays users and manu a#turers are re1uently lo#ated in di erent #ountries ando ten on di erent #ontinents" 0i erent languages #ould be#ome barriers in#ommuni#ations between the engineers on both sides" /epair #ost and time to repair areimportant in sele#ting the bidder or the new trans ormers" 0ue to the abo%e #on#ernsthe reliability o trans ormers is gaining more awarenessX&


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Spe#i i#ations are the irst and the most important tool the users ha%e in pro#uringreliable trans ormers at e#onomi#al pri#es" Be ore globali ation lo#al manu a#turerswere supplying trans ormers to the lo#al users" 0ue to many years o a#1uaintan#ebetween the lo#al users and the lo#al manu a#turers the manu a#turers ully understoodthe user re1uirements" $ow the spe#i i#ations must re le#t all o the system needs" Asmany bidders in the global market are not luent in English the spe#i i#ations should bein simple English with globally known terminology #lear to understand and with noambiguities" Some users in lo#ations where the main business language is not Englishissue the spe#i i#ations in their lo#al language" +o obtain #ost e e#ti%e bids it issuggested that these users issue the spe#i i#ations in English also"

A #onsiderable number o users ha%e not #hanged their spe#i i#ations or many years"E%en when the users are aware that slight modi i#ations to the spe#i i#ations gi%e#onsiderable bene its they are relu#tant to #hange" +his trend should #hange or thebene it o all" hen a #lause in a standard restri#ts the e#onomi# bene its then the usershould make a suitable #hange to the #lause in the standard be ore 1uoting the #lause inthe standard in the spe#i i#ation"

-99-

GLOBALI*ATION (continue !=anu a#turers in the #ountries who adopt IE6 standards ha%e many years o e,perien#ein designing trans ormers with linear taps and #oarse. ine taps" 5sers in these #ountriesha%e good e,perien#e in operating these trans ormers" 6on%ersely users that adoptIEEE standards mostly #all or the trans ormers with the re%ersing taps" By modi ying the

spe#i i#ations su#h that the bids #an be o any type o taps will bring great e#onomi#bene its to the users" +he tap #hanger or #oarse. ine and or re%ersing taps is the samee,#ept or a #onne#tion" Be#ause o this tap #hanger manu a#turers do not #all theme#hanism in these tap #hangers re%ering swit#h or #oarse. ine swit#h" In both thesetypes o tap #hangers they #all it #hangeo%er swit#h" Present %a#uum type tap #hangersno longer re1uire a minimum leakage indu#tan#e between the #oarse and the ine tapwindings"

Another e,ample is based on the work done by IE6" 5sers and the manu a#turers inmany #ountries ha%e established that internal non-linear de%i#es (=CV de%i#es) arereliable" Based on some un ortunate in#iden#es some spe#i i#ations restri#t the use othese de%i#es" +he author has e,perien#e with these de%i#es as a user and as adesigner" It is re#ommended remo%ing su#h restri#tions to obtain more reliabletrans ormers and also to get the e#onomi# bene its" /egular updating o thespe#i i#ations based on the work done in other #ountries will bring #ost sa%ings andmore reliable trans ormers"

In the present globali ation market along with all the system re1uirements it is ad%isablethat the spe#i i#ations in#lude user maintenan#e pra#ti#es and the sa ety re1uirements"Spe#i i#ations should also #o%er the problems related to the transport" Along with thetransport restri#tions to the spe#i ied station restri#tions to mo%e the trans ormer in


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;" +ime rom initiation to #ompletion o one tap o the G+6"" G+6 is in whi#h windingO (+o be asked when the #hoi#e o winding is le t to the bidderand not when the spe#i i#ation spe#i ies G+6 is in whi#h winding)"" Both no-load and load losses at *K6 and also both the losses at ;K"'" Goad loss at base =V"A and at ma,imum =V"A"

0ata sheets should not ask or in ormation the user has in the spe#i i#ation" Also thein ormation asked in the data sheets must be understandable" +here are in#iden#eswhere the data sheets ask the in ormation on whi#h the users themsel%es are not #lear"

0ata Sheets in some spe#i i#ations are in ele#troni#s orm only and these spe#i i#ationsstate that 0ata Sheets must be submitted in ele#troni#s orm only" C ten these 0ataSheets ha%e dropdown menu and %ery little spa#e to ill-in the data" +hese 0ata Sheetswill not allow the bidders to ill the in ormation #orre#tly and ully" 5sers should take e,tra#are in preparation o Ele#troni# 0ata Sheets"

A ter initial e%aluation o the bids i re1uired more in ormation #ould be asked rom one

or two inalists be ore the +ender /e%iew =eeting"0ata sheets #annot substitute the +ender /e%iew =eeting" Purpose o the data sheets isor initial e%aluation o the bids" +ender /e%iew =eeting is strongly re#ommended"

-9"& "'*- *&*"(a) Per 6SA-6 -='* swit#hing surge test is a routine test or trans ormers 2< DV andabo%e" 4or %oltages below 2< DV i re1uired the swit#hing surge test should be #alled"(b) Per 6;>"& "**- *&* swit#hing surge test is a routine test or trans ormers 29;DV andabo%e"2" 6SA-6 -='* #alls or one hour e,#itation run at &&* rated %oltage a ter #ompletiono the diele#tri# tests8 6;>"& "**- *&* does not #all this test"9" 6;>"& "**- *&* spe#i ies the ollowing tests and 6SA-6 -='* does not"


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(a) 0iele#tri# test or low %oltage #ontrol wiring asso#iated au,iliary #ontrol e1uipmentand #urrent trans ormer se#ondary #ir#uits"(b) Cn trans ormers with both 0E+6 and G+6:(i) /atio tests at all #onne#tion positions o the tap #hanger or de-energi ed operationwith the G+6 on the rated %oltage position"(ii) /atio tests at all G+6 positions with the tap #hanger or de-energi ed operation on therated-%oltage position"(iii) Impedan#e and load loss measurements at si, #ombinations o 0E+6 G+6 tappositions per +able &' on page ;*";" In 6SA-6 -='* and in 6;>"& "**- *&* ollowing tests are not spe#i ied"(a) /atio test between the windings that will be permanently paralleled"(b) S4/A"(#) C%erload test"

=any spe#i i#ations ask or the sound le%el at load with ull e,#itation %oltage applied" Inthe a#tory it is not possible to do su#h a test" 0uring load loss measurement sound le%elprodu#ed by shields and windings is measured" 0uring these tests the lu, density inthe #ore will be %ery low and does not gi%e the sound le%el produ#ed by the #ore at ulle,#itation %oltage" Sound le%el produ#ed by the #ore is measured at no-load during thismeasurement sound le%el produ#ed by the shields and the windings is %ery small" 4romthe sound le%els measured rom the abo%e two tests there is no uni%ersally agreedmethod to #ompute to obtain sound le%el at load with ull e,#itation %oltage applied"

-9>-

TESTS (continue ! As su#h the user and the manu a#turer should agree to the method pre erably in the0esign /e%iew to #ompute these tested %alues to obtain the sound le%el at load with ulle,#itation %oltage applied"

+o determine whether sound en#losures are to be installed or not some users spe#i ysound le%el tests with the trans ormer de-energi ed with all the ans and the pumpsrunning" $either 6SA-6 -='* nor 6;>"& "**- *&* spe#i ies these tests" I re1uiredthe spe#i i#ations should #learly state these tests with a des#ription o how they shouldbe per ormed"

Cn three-phase trans ormers the standards #all or a three-phase indu#ed test" Isingle-phase indu#ed test is also re1uired then this should be stated in thespe#i i#ations"

In autotrans ormers when the FV is impulsed whether the GV line terminals are to be

grounded through surge impedan#es or solidly grounded is not #lear in the standardsand had disagreements between users and manu a#turers" In the 0esign /e%iewmeeting all o the tests #onne#tions should be dis#ussed and agreed upon"

6SA-6 -='* and 6;>"& "**- *&* list the routine and design tests" I the user wishesto per orm some design tests as routine tests then these tests should be stated in thespe#i i#ations" Also i the user wants tests on a##essories then they should be listed inthe spe#i i#ations"


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6;>"& "'*- *&* des#ribes how the tests are to be per ormed" 6;>"& "'*- *&* alsogi%es partial dis#harge limits" I the user re1uirements are di erent to those in6;>"& "'*- *&* then the spe#i i#ations should #learly state what is needed"

+ests be ore shipping tests a ter re#ei%ing and tests be ore energi ation are di erent todi erent users" As su#h suggest stating these tests in the spe#i i#ations"

In the 0esign /e%iew meeting the user and the manu a#turer should dis#uss all thetests to be per ormed test e1uipment limitations e,tra tests the user want to per ormthat are not spe#i ied in the spe#i i#ation test #onne#tions et#" and #ome to anagreement" +his sa%es time and aggra%ations during the testing"

Some spe#i i#ations ha%e stated the a#tions to be per ormed by the manu a#turer whenthe trans ormer ails during the a#tory tests (in%estigation #orre#ti%e a#tion retestinget#")" +his is a good pra#ti#e and suggests ha%ing this in all the spe#i i#ations"

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TENDER AND DESIGN REVIEWS+he purpose o the tender and design re%iews should be to enable the user and themanu a#turer to work together to produ#e a trans ormer that meets all o the systemre1uirements to maintain trans ormer reliability throughout its operational li e and supplythe trans ormers per the #ommitted deli%ery dates"+ender and design re%iews #ost money and time or both users and manu a#turers"Based on the =V"A rating o the trans ormer #ompli#ation o the trans ormer bidders listet#" the user should de#ide whether to in#lude the re1uirement or +ender and 0esign/e%iew meetings in the spe#i i#ations"

Spe#i i#ations should #learly state the re1uirements o the +ender re%iew and the 0esign/e%iews" +his is important be#ause the bidder #an in#lude them in their #osting" =anyspe#i i#ations are stating the re1uirement o the 0esign /e%iews and also what will bedis#ussed in the 0esign /e%iew" Very ew spe#i i#ations are stating the re1uirement othe +ender /e%iew" A +ender /e%iew is mu#h more important and essential than a0esign /e%iew" +his is be#ause the manu a#turer s designers were gi%en %ery little timeto do a tender design" =any users are o the opinion that with #omputers designs aredone in a ew minutes" +hey are allowing %ery little time to submit a bid" C ten there is notime or the bidders to obtain #lari i#ations to the spe#i i#ations" $ormally the sameanalysis and #he#ks as in a produ#tion design are not done or a tender design" =anytimes the interpretation o the spe#i i#ations by the user and the bidder is di erent"5sually the +ender /e%iew meeting is done in the user s o i#e" +his gi%es an opportunityor the manu a#turer to meet user s system operation prote#tion and stations designengineers and to know how the trans ormers are being operated" +his also gi%es the


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#han#e to know why #ertain re1uirements are spe#i ied in the spe#i i#ations and todis#uss the e#onomi# alternati%es" In the +ender /e%iew meeting suggest dis#ussingthe ollowing:

&" 6ore details:(a) 6al#ulated and measured losses and e,#itation #urrent on re#ently manu a#turedunits with the same grade o #ore steel o the units o the bid"(b) 4lu, density at ma,imum system %oltage #al#ulated ma,imum sur a#e temperatureand ma,imum temperature in any part o the #ore (inside the #ore et#")"(#) 0i eren#e in losses and the e,#itation #urrent be ore and a ter the diele#tri# tests onre#ently manu a#tured units with the same grade steel o the units o the bid"(d) ?eneral design philosophy in #ore #lamps design"" Proposed winding types or trouble- ree operation throughout the li e o thetrans ormer"2" +hermal and short-#ir#uit withstand #hara#teristi#s o the materials proposed"9" Go#ation o taps and the e e#ts on trans ormer parameters based on how the taps areused in the operation";" Variation o impedan#e o%er the tap range"" =anu a#turing and testing #apabilities and limitations"" Assembly and pro#essing pra#ti#es o #ore and #oils" Also pro#essing pra#ti#es o oil"'" /e#ent (past i%e years) shop ailures in%estigations and how #orre#ted"&*" Past deli%ery delays and re#ent tra#k re#ord"

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TENDER AND DESIGN REVIEWS (continue !&&" Philosophy o design and manu a#ture to withstand short-#ir#uits or#es"& " +hermal and o%erload #al#ulations"&2" ]uality and inspe#tions"

&9" Dnowledge and e,perien#e o ere#tion #onsultant"&;" /e#ent ield problems and how they are re#ti ied in#luding ser%i#e a ter the warranty"

Cn#e the order is pla#ed it is di i#ult or the user to in luen#e any #hanges to te#hni#aland non-te#hni#al issues" 5sers ha%e e,perien#ed that to in#lude a eature or to modi ya parameter a ter the order is pla#ed is o ten #ostly and #auses delay in deli%ery" I athorough +ender /e%iew has been done then only a ew dis#repan#ies may ha%e to beresol%ed at the 0esign /e%iew"

$ormally a 0esign /e%iew is done Nust be ore ordering the materials" As su#h usersshould insist on only the meaning ul #hanges when essential" +he user should not allowthe manu a#turer any 1ui#k i, to maintain the deli%ery" +his o ten a e#ts the reliability"

hen an e,ternal #onsultant is employed by the user to #ondu#t the 0esign /e%iew the#onsultant should not be biased and insist on #hanges based on the #onsultant spre%ious e,perien#e" $ew design pra#ti#es and new produ#tion methods will be e%ol%ing#ontinuously" I the user wants to employ a #onsultant or the 0esign /e%iew then it is%ery strongly re#ommended that the same #onsultant must be present at the +ender/e%iew also" +hen the #onsultant #an ask all the te#hni#al #lari i#ations in the +ender/e%iew and re#ommend the best bidder to pla#e the order" ith this %ery ewdisagreements will o##ur at the 0esign /e%iew" It is important that the user should not


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insist on #hanges based upon the pra#ti#es o another manu a#turer that had suppliedthe trans ormers"

In the 0esign /e%iew meeting it is suggested to dis#uss the ollowing" +he ollowing maynot be needed at the time o preparation o the spe#i i#ations but are in#luded as theywill be use ul to the users at later stages"

&" =agneti# #ir#uit--4lu, density in legs yokes outer legs et#" at ma,imum system %oltage"--6ore #onstru#tion (type o step.lap)" 6ore #onstru#tion in split #ore and lu, density atdi erent se#tions"--6al#ulations o leakage lu, entering the #ore and the method that will be adopted toeliminate o##urren#e o undesired hot-spots"--?rade o #ore steel the supplier and laminations sur a#e insulation details"--6ore #lamping stru#ture design espe#ially to minimi e the #ir#ulating #urrents in theseparts"--6ore temperature (on the sur a#e and inside #ore) at ma,imum lu, density in ser%i#e"--$umber and method o #onstru#tion o #ooling du#ts" Cil low in the du#ts to pro%ide

#ooling to a%oid ormation o hot-spots"--?rounding details o the #ore and other parts"--6ore li ting method a ter #ompleting the sta#king o the #ore (hydrauli# tables or othermethods)"--=ethod o #ore and yokes binding (resiglass tape dowels et#")"

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TENDER AND DESIGN REVIEWS (continue !" indings leads e,its arrangements o leads and their #onne#tions--+ype o #oils (disk heli#al et#") and their suitability or the appli#ation"--Physi#al lo#ations o taps and its impa#t on impedan#e swing a#ross the tap rangeshort-#ir#uit or#es and impulse %oltage distribution"--+ype o winding #ondu#tors (6+6 with or without paper #o%ering magnet wire singletwin triple et#")"-- hen epo,y bonding is used ( or 6+6 twin #ondu#tors et#") then the temperaturerating o the epo,y is to be #he#ked (not below the winding hot-spot temperature)"--6urrent densities #ooling a%erage and hot-spot gradients"--6ondu#tor (or strand or 6+6) dimensions and eddy losses"--=ethod to #ontrol the impulse %oltage distribution (shielded interlea%ed et#") ormulaein the impulse %oltage distribution program os#illations stresses and strengths--Stress in oil and stress in solid insulation o the radial gap between the windings"--Stati# shields stati# #ylinders stress rings et#: #onstru#tion lo#ation and intended use"--6ooling (no pumps with pumps oil dire#ted into the windings oil %elo#ities to a%oidstati# buildup hot spot #al#ulations et#")"--4or C0 #ooled units #oordination o time #onstant o windings and the time #onstant o winding temperature indi#ator"--Geads e,its (#urrents indu#ed in other parts gradients and insulation withstand)"--Sele#tion #riteria on number o non-linear de%i#es and the method o their assembly"--Toints (Noints in the winding and the Noints in e,ternal #onne#tions)"--6al#ulation o short-#ir#uit or#es (o -sets modeling asymmetry a#tor et#")"


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--6oils supports (number o supports radial and a,ial details o the supports et#") andthe #lamping pressures or li ting and or short-#ir#uits"--0etails o the #ounter shield interlea%ed et#"--$umber and type o transposition and their lo#ations in the windings"-- inding #ondu#tor supplier and insulation details (to go through the pur#hasingspe#i i#ation details o the winding #ondu#tors and the insulation)"

2" E,ternal to the #oils--0esign o #urrent limiting rea#tors"--0esign o series trans ormer"--0esign o tap #hanger rea#tor (pre%enti%e auto)"--0esign o #ompensating trans ormer"--0esign and mounting details o #ore shunts #lamp shunts and tank shields"--Geads layout to a%oid lasho%er to other parts me#hani#al stability and pre%ention oindu#ed #urrents in to other parts"--0e le#tors to #hannel the gasses in to the gas relay"

9" A##essories--+ap #hanger type make sele#tion to meet the spe#i i#ations (o%erloads step #apa#ityBIG withstand %alues et#") tie-in resistors or potential swit#h suitability o the in#rease inthe rating at low temperature e e#ts o physi#al lo#ation o the taps et#"--Bushings type make sele#tion to meet the spe#i i#ations (o%erloads #reepagedistan#e suitability o in#rease in rating at low temperatures et#")"-- +I (winding temperature instrument) type make et#" to meet the spe#i i#ations" 4orC0 (oil dire#ted in to the windings) #ooled units to #he#k the #orrelation o its time

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TENDER AND DESIGN REVIEWS (continue !#onstant with the time #onstant o the windings" IEEE +I +ask 4or#e has done %erygood work on this and is a%ailable atwww"trans ormers#ommittee"org--Cn-line monitoring de%i#es"--6+s (#urrent trans ormers) design #ooling meeting the o%erloads et#"--Pressure relie de%i#es and other sa ely de%i#es"--Go#ations o bushings #oolers ans #ontrol bo, #onser%ator tap #hanger et#"--Go#ation o all arrest systems"--0esign o gaskets langes et#"

;" =anu a#turing and pro#essing--6ore: type o #ore #on iguration ma#hinery or #utting the laminations and or theassembly burrs shorting o the laminations air gaps during the assembly li ting a terthe assembly rela#ing the top yoke rigidity o the #ooling du#ts splitting o outermost#ore pa#kets #ore tightness grounding et#"-- indings: tightness during the winding maintaining the tightness a ter remo%

Documents

Preparation of Transformer Spec