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Power TransformerPower Transformer

Thermal RatingsThermal RatingsTim RaymondTim Raymond

2003 IEEE Transmission & Distribution2003 IEEE Transmission & Distribution

ConferenceConferenceDallas, Texas P0! Tuesday, "#$e%#2003Dallas, Texas P0! Tuesday, "#$e%#2003

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Transformer LoadingTransformer Loading

LimitLimit In eneral, transformer ca%acityIn eneral, transformer ca%acity

limited by e'ui%ment ()indin &limited by e'ui%ment ()indin &

oil* tem%eratures+oil* tem%eratures+ Desin limits (0C -ot s%ot* notDesin limits (0C -ot s%ot* not

necessarily o%erational limitsnecessarily o%erational limits

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Nameplate RatingNameplate Rating

ame%late ratin is notame%late ratin is not

necessarily o%erational limitnecessarily o%erational limit

.ased on 30C (/0C ax+*.ased on 30C (/0C ax+*

ambientambient

Continuous load at ratedContinuous load at rated

currentcurrent

Conser1ati1e -ot s%ot tem%Conser1ati1e -ot s%ot tem%

of 0Cof 0C Results in o1erlyResults in o1erly

conser1ati1e ratinsconser1ati1e ratins

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Transformer CoolingTransformer Cooling

Insulatin oil acts as coolin mediumInsulatin oil acts as coolin medium il o) is eit-er naturalil o) is eit-er natural

t-ermosi%-on o) (no %um%s* ort-ermosi%-on o) (no %um%s* orforced o) (%um%s*+forced o) (%um%s*+

4eat exc-aners or radiators transfer4eat exc-aners or radiators transfer-eat from oil to en1ironment-eat from oil to en1ironment

5ans may be used to increase forced5ans may be used to increase forcedcon1ection o1er radiator 6nscon1ection o1er radiator 6ns

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Oil Flow WithinOil Flow Within

WindingWinding

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Core & CoilsCore & Coils

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Transformer CoolingTransformer Cooling

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Thermal ModelThermal Model

$im%le lum%ed %arameter model$im%le lum%ed %arameter model

il tem%erature rise function of total lossil tem%erature rise function of total loss 7indin tem%erature rise function of current7indin tem%erature rise function of current

( )( )

m

iRHSiHS

n

i

RTOiTO

K

R

RK

2

,,

2

,,1

1

=

+

+=

$teady#state Tem%eratures8

2,2,2,2, HSTOAHS ++=

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il t-ermal time constant8il t-ermal time constant8

5or 9 and 59 (9 and 595* t-e t-ermal5or 9 and 59 (9 and 595* t-e t-ermalca%acity, C, e'uals8ca%acity, C, e'uals8

C : 0+02;2 < 7C : 0+02;2 < 7CCCC= 0+0>/ < 7= 0+0>/ < 7Tan?Tan?= @+03/

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( )( )

n

URTOUTO

R

RK

++

=1

12

,, ( ) 1,1,,2, 1 TO

t

TOUTOTOOe

+

=

m

URHSUHS K2

,, =

( ) 1,1,,2, 1 HS

t

HSUHSHSWe

+

=

2,2,2,2, HSTOAHS ++=

To% oil tem%erature rise at time t2:

t=t

4ot s%ot tem%erature rise at time t2:

t=t

4ot s%ot tem%erature at time t2: t=t

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RiskRisk

9ny eneried transformer -as9ny eneried transformer -as

6nite ris? of failure6nite ris? of failure

Ris? increases )it- o%eratinRis? increases )it- o%eratintem%eraturestem%eratures

1erload is %ossible, if ris?s are1erload is %ossible, if ris?s are

considered and mitiated toconsidered and mitiated toacce%table le1elsacce%table le1els

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RisksRisks

T-ree areas of ris?8T-ree areas of ris?8

Fon#term (insulation ain*Fon#term (insulation ain*

$-ort#term (bubbles, oil ex%ansion*$-ort#term (bubbles, oil ex%ansion*

9ncillary com%onents (bus-ins,9ncillary com%onents (bus-ins,

FTCs, CTs*FTCs, CTs*

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Long-term RiskLong-term Risk

Insulation aes at anyInsulation aes at any

tem%eraturetem%erature

Rate of ain increases )it-Rate of ain increases )it-tem%erature (rou-ly doublin fortem%erature (rou-ly doublin for

e1ery !C rise*e1ery !C rise*

EGects mec-anical strent- ofEGects mec-anical strent- of%a%er%a%er

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Inslation !gingInslation !ging

il and %a%er ad1ersely aGectedil and %a%er ad1ersely aGected

by -eatby -eat

il is re%laceable, %a%er is notil is re%laceable, %a%er is not Pa%er aes at any tem%eraturePa%er aes at any tem%erature

Pyrolysis, -ydrolysis, oxidationPyrolysis, -ydrolysis, oxidation

.rea? bonds in cellulose c-ain,.rea? bonds in cellulose c-ain,

decreases mec-anical strent-decreases mec-anical strent-

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9rr-enius Reaction Rate9rr-enius Reaction Rate

9 is reference -ot s%ot9 is reference -ot s%ot 9:"@C for @@C insulation9:"@C for @@C insulation 9:0C for !@C insulation9:0C for !@C insulation

Interate 5Interate 59999o1er time to et e'ui1+ aino1er time to et e'ui1+ ain Fife end %oint diHcult to determineFife end %oint diHcult to determine .ased u%on moisture content of 0+@ and.ased u%on moisture content of 0+@ and

lo) 2lo) 2

+

+= 273273, HSRHSAA

AA

eF

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$ource8 IEEE C@;+"

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"hort-term Risks"hort-term Risks

Reduction in dielectric strent-Reduction in dielectric strent-

due to bubble formationdue to bubble formation

(J/0C*(J/0C* 5unction of moisture content, as5unction of moisture content, as

content, %ressure & tem%eraturecontent, %ressure & tem%erature

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#$$le Formation#$$le Formation

$ource8 EPRI Re%ort RP 2>"#

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#$$le Predi%tion#$$le Predi%tion

2 models a1ailable2 models a1ailable

Com%lex model by Rouse et al+, sumCom%lex model by Rouse et al+, sum

of instantaneous %artial %ressure ofof instantaneous %artial %ressure ofas and )ater 1a%or 1s+ staticas and )ater 1a%or 1s+ static

%ressure%ressure

Em%irical model by T+ A+ ommenEm%irical model by T+ A+ ommen

relates moisture content, asrelates moisture content, as

%ressure and static %ressure to onset%ressure and static %ressure to onset

tem%eraturetem%erature

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#$$le Predi%tion#$$le Predi%tion

Oommen'Oommen'( ) 273-

30

V

P-W1.4495+22.454

6996.7=

1.55!"W0.473#

$%e&WP

'('')eWP

EXP

lnln

7-ere8

bubbleis bubble onset tem%erature

77Pis moisture content of %a%er ( by )t*

P%resis static %ressure

Ais total as concentration ( by 1ol*

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!dditional Risks!dditional Risks

Ex%ansion of oil beyond ca%acityEx%ansion of oil beyond ca%acity

of tan?of tan?

Deterioration of as?ets and sealsDeterioration of as?ets and seals Delamination of com%ositeDelamination of com%osite

insulatin materialsinsulatin materials

Reduced mec-anical strent- ofReduced mec-anical strent- of

conductor bondin materialsconductor bondin materials

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!n%illar( Components!n%illar( Components

.us-ins.us-ins

FTCsFTCs

DETCsDETCs

FeadsFeads

.us-in CTs.us-in CTs

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#shings#shings

il#im%renated, %a%er#il#im%renated, %a%er#

insulated, ca%acitance#insulated, ca%acitance#

raded bus-insraded bus-ins Desined )it- 0@CDesined )it- 0@C

bus-in -ot s%ot limit atbus-in -ot s%ot limit at

rated load and "@C to% oilrated load and "@C to% oil

tem%erature a1erae o1ertem%erature a1erae o1er2/ -ours2/ -ours

1erload is %ossible1erload is %ossible

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#shing %ont'#shing %ont'

1erload ris?s of bus-ins include81erload ris?s of bus-ins include8

Pressure build#u% due to oil ex%ansionPressure build#u% due to oil ex%ansion

Deterioration of as?ets and sealsDeterioration of as?ets and sealsT-ermal deterioration of %a%erT-ermal deterioration of %a%er

insulationinsulation

Increase in dielectric loss, %ossiblyIncrease in dielectric loss, %ossiblyresultin in t-ermal runa)ayresultin in t-ermal runa)ay

Kas e1olution at extreme -ot s%otsKas e1olution at extreme -ot s%ots

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#shing %ont'#shing %ont'

1erload limits81erload limits8

/0C ambient/0C ambient

0C transformer to% oil0C transformer to% oiltem%eraturetem%erature

2x rated bus-in current2x rated bus-in current

@0C bus-in -ot s%ot tem%erature@0C bus-in -ot s%ot tem%erature

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Load Tap ChangerLoad Tap Changer

Desined )it- a contact riseDesined )it- a contact rise

o1er oil of less t-an 20C ato1er oil of less t-an 20C at

+2 times rated load+2 times rated load Desined to brea? t)iceDesined to brea? t)ice

rated current at least /0rated current at least /0

timestimes

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%ont'%ont' 1erload ris?s of FTCs include81erload ris?s of FTCs include8

Increased contact )ear and ablation )it-Increased contact )ear and ablation )it-

increased load durin brea? o%erationsincreased load durin brea? o%erations Increased contact tem%eratures increasesIncreased contact tem%eratures increases

%robability and rate of co?in of contacts%robability and rate of co?in of contacts

(J20C*(J20C*

4i-er o1erloads result in %roloned arcin4i-er o1erloads result in %roloned arcindurin brea? o%eration+ Drain t-e arcdurin brea? o%eration+ Drain t-e arc

across t-e contacts could result in s-ort#across t-e contacts could result in s-ort#

circuitin t-e reulatin )indincircuitin t-e reulatin )indin

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%ont'%ont' 1erload limits81erload limits8

20C contact tem%erature (-i-er is20C contact tem%erature (-i-er is

L, but may result in reaterL, but may result in reatermaintenance*maintenance*

2x FTC rated load current (limit2x FTC rated load current (limit

brea?in o%erations at -i- loadbrea?in o%erations at -i- load

le1el to fe) timesByear*le1el to fe) timesByear*

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OtherOther

DETC similar in ratin to FTCDETC similar in ratin to FTC

Feads can be a concern+ $ame -otFeads can be a concern+ $ame -ot

s%ot limits a%%ly for leads as fors%ot limits a%%ly for leads as for)indins+ 4o)e1er, leads are usually)indins+ 4o)e1er, leads are usually

not limitin+not limitin+

.us-in CTs are limited by transformer.us-in CTs are limited by transformer

to% oil tem%erature+ aintainin a to%to% oil tem%erature+ aintainin a to%oil M 0C s-ould a1oid excessi1eoil M 0C s-ould a1oid excessi1e

tem%eraturestem%eratures

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)*ample Lead +eating)*ample Lead +eating

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Operation Con%ernsOperation Con%erns

DiGerence bet)een tested risesDiGerence bet)een tested rises

and desin limitsand desin limits

9e & loadin -istory9e & loadin -istory Prior o%erational recordsPrior o%erational records

(maintenance -istory*(maintenance -istory*

1erall Condition (moisture, DK9,1erall Condition (moisture, DK9,

oil 'uality*oil 'uality*

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,prating,prating

9dd fans and %um%s9dd fans and %um%s Practical maximumPractical maximum

Caution must be exercised to a1oid lareCaution must be exercised to a1oid lare-ot s%ot radient-ot s%ot radient

Increase sie of -eat exc-anerIncrease sie of -eat exc-aner $ame ca1eats as abo1e$ame ca1eats as abo1e

7ater s%ray durin %ea? loadin7ater s%ray durin %ea? loadin Careful to a1oid excessi1e localCareful to a1oid excessi1e localmec-anical forces and steam may causemec-anical forces and steam may causeas-o1er of bus-insas-o1er of bus-ins

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03 Td 0467 Raymond