Transformer Condition Monitoring

8/20/2019 Transformer Condition Monitoring

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J S Sastry

a discussion

on

Condition Monitoring


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Need for

Transformer Condition Monitoring

• Expensive capital equipment

• Not easy to repair or replace

•

Advanced computer aided design practices.• Transformers operation.

• Condition Monitoring imperative.

• A corrective/preventive action can be initiated.


3/55

Transformer Ageing

• Life of transformer depends on te

condition of insulation system !"raft

paper and oil# $it time under impact of%

– Termal stresses

– Electromagnetic stresses

– Electro&dynamic stresses

– Contamination and natural ageing processes

effecting te tensile strengt of paper


4/55

Transformer Ageing

• 'ate of deterioration depends upon te

operating conditions & loading pattern(

incipient faults( maintenance

• Condition monitoring also enables to

estimate te residual life of te

transformer.


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Poor Quality of

Design or Materials or

Production or

Maintenance

Processes

results in …failure of transformers


6/55

Types of failures

)nfant failures% Early life failures are te

result of latent or delivered defects.

& Latent defects are abnormalities tatcause failure( depending on degree of

abnormality and amount of applied

stress.

& *elivered defects are tose tat escape test / inspection $itin te factory

& Tey are directly proportional to total

defects in te entire processes.


7/55

Types of failures

Mid life failures% Tese are results of +

& ,rea" system disturbances & -rong specifications

& oor maintenance


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Types of failures

ld age failures% Tese are results of +

& Ageing of insulation system & -ear 0 tear


9/55

Windings

Terminals

Tank & DielctricFluid

OnloadTapchanger

Magnetic Circuit

OtherAccessories

COMPONENTS CAUSING FAILURE IN

SERVICE(29%)

(29%)

(13%)

(13%)

(11%)

(5%)


10/55

Transformer ,ailures

• Te main causes of transformer

$inding failures are due to%

– Moisture contamination and ageing $ic

cause te transformer internal dielectric

strengt to decrease

– *amage to te $inding or decompression

of te $inding under sort circuit forces – *amage to te busings caused by loss of

dielectric strengt of materials.


11/55

Types of Transformer ,aults

• Mecanical ,aults%

–Tan"( fittings and accessories

–damaged porcelain insulators

–clogged oil piping( damaged gas"ets

–mecanical damage of te $indings.


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• Electrical ,aults%

–Lo$ insulation resistance

–unctured insulation bet$een turns(sections( $indings and various ig

potential points.


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• Magnetic ,aults%

– Abnormal eating

– 1rea"do$n of insulation bet$eencore laminations core fixtures and

clamping

– Non&uniform distribution of magnetic flux.


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'eliability

• 12%3445/6776 2ection 8.6 defines reliability

as 9te caracteristic of an item expressed by

te probability tat it performs required

conditions for a stated period of time:.• 'eliability of a transformer is determined by%

– *esign of transformer

– ;uality of materials

– ;uality of manufacturing processes

– peration and maintenance


15/55

9-y: Condition Monitoring<

Early detection of incipient faultsAvoid catastropic outage

rovide basis for economic repair

decision Maintenance ManagementMaintenance management based on

measurements and trend analysis

Ageing process and residual lifeunder control

)mprove safety to personnel and

environment


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9urpose: =.

• urpose of Condition Monitoring%

– Avoid forced outages

– Minimi>e failures – ptimi>e maintenance costs


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ften used tecniquesTests To detect

*?A Ageing of oil( paper( ot spot( arcing or *

*egree ofpolymeri>ation

Ageing of insulating paper

,urfural Ageing of paper insulation

'@M -ater content 0 ageing of insulation paper

Tan *elta *ielectric loss in insulation system due toaccumulation of polari>ing materials

)' @alue 0olari>ation )ndex

Accumulation of polari>ation materials

,'A *etects pysical movement of $indings

* *eterioration of insulation system/able to detect somelocali>ed defects


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9o$: Condition Monitoring

• Monitoring sceme must be% – 2imple – Lo$ cost –

-itout disruption of po$er • *ata must be focused on results to%

– revent problems – *efine te severity of a problem

– rovide information to ta"e action – rovide on&line and off&line diagnostics – Enable 9trending: of data – Avoid intrusive maintenance


19/55

9-at: to Monitor

• -inding resistance measurements• Capacitance and tan B• )nsulation 'esistance !)'# and

olari>ation )ndex !)# measurements•

il parameters• *issolved ?as Analysis !*?A#• ,urfuraldeyde Analysis• *egree of olymeri>ation !*#

• artial *iscarge !*# Measurements• ,requency 'esponse Analysis !,'A#• 'ecovery @oltage Measurements• Capacitance and tan B for busings


20/55

-inding 'esistance Measurements

• Measure 'esistance of all $indings

• Compare $it factory results

•

)ncrease in 'esistance indicates – Loose oints & Leads to local ot spots and

eventual melting of oints

– -orn out contacts + Leads to contact

erosion


21/55

Capacitance and tan B of $indings

• Measure Capacitance and tan B of eac pair of

$indings and $indings $it respect to eart


• )t indicates ealtiness of insulation system +paper( press&board and oil

• )ncrease in tan B indicates deterioration of

insulation system – Contamination

– Moisture absorption


22/55

)nsulation 'esistance !)'# and

olari>ation )ndex !)# measurements

• Measure )' and ) values of eac $inding in

pairs and $it respect to eart


• Lo$er values indicate poor insulation system

) !'atio of 6D min to 6 min# Condition

Less tan 6 *angerous6.D + 6.6 oor

6.6 + 6.F ;uestionable

6.F + .D ,air


23/55

il arametersTest Acceptable ;uestionable Gnacceptable urpose

1*@ H@

!A2TM *&544#

I 8D 7 & F J F Ability of oil to $itstand electric stress

)nterfacialtension Nm/m!A2TM *746#

I 8.D 86.7 & 5 J 4.7 Measures tensionbet$een oil 0 $aterlayer. Gsed to detectpolar contamination

and insulating ageing

Neutrali>ationNo mgH/g!A2TM *743#

J D.DF D.DK + D.6D D.6D Acidic compoundsproduced by oxidationof oil and degradationof solid insulation

Color A2TM *

6FDD 0 6F3

8.F && 8.F @isual indication ofserious contaminationor degradation

*issipationfactor At FDC

At 6DDDCJ D.6

J .77

D.6 + D.8

8.D + 8.77

D.8D

8.D

ealtiness ofinsulation system


24/55

il arameters

• 'esistivity and Tan B )2 88F )2 65KK

ermissible values

'esistivity om&cm at 7DDc 8F x 6D6 6 x 6D6

Tan B at 7DD

c D.DD D.DF

• Moisture Content%

ermissible limits of $ater in oil%

Less tan 4.F H@ F ppm

4.F + 63F H@ D ppm

Above 63F H@ 6D ppm


25/55

*issolved ?as Analysis !*?A#

• Transformer insulating oils consist of

different ydrocarbon molecules.

• 2plitting some of te ydrocarbon bonds

occur due to electrical and termal faults(forming gases +

– ydrogen

– Metane !C3#

– Etane !CK#

– Etylene !C3#

– Acetylene !C#.


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• Lo$ energy faults !li"e artial *iscarges#sufficient to split $ea" &C bonds result inydrogen as main gas.

• iger temperatures are needed for splittingC&C bonds.

• iger temperatures result in – Etane( metane and etylene at FDDDC. – Acetylene requires temperatures 5DD & 6DDDC.

• Carbon particles form at FDD to 5DDDC and areobserved after arcing in oil or around veryot spots.


27/55


Te condition for formation of "ey gases byte degradation of cellulose and oil %

Material Condition Hey gas

Cellulose vereated 6FDDC C( C and

il vereated 8DDDC O6DDDDC Metane !C3#(

Etane !CK#

Etylene!C3#

rganic acids

il Electrical stress !artial

discarge# Arcing to 6DDDDC

ydrogen !#

Acetylene !C#


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?eneral fault condition Hey gases

Termal conditioninvolving te oil

Metane( Etane( Etylene(and small amount of Acetylene

artial *iscarge ydrogen( Metane and smallamount of Acetylene andEtylene

2ustained arcing ydrogen( Acetylene andEtylene

Termal conditioninvolving te paper

Carbon Monoxide and Carbon*ioxide

Categories of "ey gases and general fault conditions


29/55


Caracteristic gas Concentration !ppm# )ndication

C D o$er discarge

6DD artial discarge

PCxy 6DDD

FDD

Termal fault

Cx 6DDDD Cellulose digradation

?as Concentration ratios

'atio @alue )ndication

C/CK 6 *iscarge

/C3 6D artial discarge

C/C 6D

J 8

Cellulose overeating

Cellulose degradation

C/ !$it C 8D# ,ault gases from LTC

C)?'E Metod !Electra No. 65K( ct 77# & Hey gas concentration and indication%


30/55

Metod recommended by

Electrical Tecnology 'esearc Association

A

1

C *

E , ?

D.6 6.D 3 6D 6DD 6DDD

C3/CK

6DDD

6DD

6D

6.D

D.6

D.D6

C

CK

A% Arc *iscarge

1% *iscarge

C% artial discarge

*% vereating 4DDDC

or eating Q discarge

E% ver eating J 8DDDC

,% ver eating

8DDDC + 4DDDC

?% vereating 4DDDC



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'atio EvaluationC3/ CK/C3 C3/CK C/C3

D D D D )f C3/JD.6( *

oter$ise normal ageing

6 D D D 2ligt overeating belo$ 6FDDC

6 6 D D 2ligt overeating 6FD + DDDC

D 6 D D 2ligt overeating DD + 8DDDC

D D 6 D ?eneral conductor overeating

6 D 6 D Circulating current/overeated oints

D D D 6 ,lasover $itout po$er flo$

D 6 D 6 LTC 2elector brea"ing current

D D 6 6 Arc $it po$er flo$ troug orpersistent spar"ing.

'oger:s Metod

'atios J 6 are designated as 9D: R 'atios 6 are designated as 96:

*?A )EC F77 M t d


32/55

*?A & )EC % F77 MetodCode of range of

'atios

C /C

3

C3 /

C3 /C

K

'atios of Caracteristic gases

J D.6D.6 + 6

6&8 8

D66

6D

DD6

Case No Caracteristic fault Typical examples

D No fault D D D Normal ageing

6 * of lo$ energy density D 6 D *iscarges in gas&filled cavities resulting fromincomplete impregnation or super&

2aturation or cavitation or ig umidity

* of ig energy density 6 6 D As above( but leading to trac"ing or perforationof solid insulation

8 *iscarges of lo$ energy !2eenote 6#

6S D 6S Continuous spar"ing in oil bet$een badconnections of different potential or to floatingpotential. 1rea"do$n coils to eart. 2electorbrea"ing current

3 *iscarge of ig energy 6 D *iscarges $it po$er follo$&troug. Arcing +

brea"do$n of oil bet$een $indings or coils toeart. 2elector brea"ing current.

F Termal fault of lo$ temp.J6FDDC !2ee bite #

D D 6 ?eneral insulated conductor overeating.

K Termal fault of lo$ temp.range 6FD+8DDDC. !2ee note 8#

D D Local overeating of te core due toconcentration of flux. )ncreasing ot spottemperaturesR varying from small ot spots incore( sorting lin"s in core( overeating ofcopper due to eddy currents( bad contacts/oints

up to core and tan" circulating currents

4 Termal fault of medium temp.8DD + 4DDDC

D 6

5 Termal fault of ig temp.

4DDDC !2ee note 3#

D


33/55

*?A & )EC % F77 Metod

Notes%

6. ,or te purpose of tis table( C /C3 to rise from avalue bet$een D.6 and 8 to above 8 and te ratioC3 /CK from a value bet$een D.6 and 8 to above 8 aste spar" develops in intensity.

. )n tis case te gases come mainly from tedecomposition of te solid insulation( tis explains tevalue of te ratio C3 /CK.

8. Tis fault condition is normally indicated by increasinggas concentrations. 'atio C3 / is normally about 6Rte actual value above or belo$ unity is dependent onmany factors suc as design of oil preservation

system( actual level of temperature and oil quality.3. An increasing value of te amount of C may

indicate tat te ot point temperature is iger tan6DDDDC.


34/55


?ases in M Age of transformer in years

D & F K & 6D 66 & 6F 6F

ydrogen 6DD 6DD 6DD 6DD

Carbon *ioxide 7DDD 7DDD 7DDD 6FDDD

CarbonMonoxide

FDD 4DD 6FDD 6FDD

Metane 4D 4D DD DD

Etane 3D 3D DD DD

Etylene 3D 6DD DD DD

Acetylene 6D 6D D FD

Acceptable limits of *issolved ?ases !Trafo&Tec DDK#


35/55


• Limitations of te *?A%

– )t cannot detect te incipient faults

–

@alues measured are te average valuesover a period of time.

• n line monitoring of is quite popular.

• 2ensors ave also been developed foron&line monitoring of C( C and C.


36/55

,urfuraldeyde Analysis

• rolonged action of eat on cellulose paper causesde&polymeri>ation( $ic is aggravated by presence ofmoisture( forming furanic compounds.

• Termal degradation of oil&paper insulation systemyields different amounts of furanic derivatives( temost common being &furfuraldeyde.

• 2uc deterioration of paper on $inding conductor canbe assessed by subecting te paper to various tests.

• -indings of oil filled transformers are not accessible.

Can be accessed after draining oil $ic is a tediousprocess.

• Te furfural derivatives produced due to degradationof paper dissolve in oil.


37/55


• ence( te cemical analysis of transformer oil giveste evidence of canges tat are ta"ing place in te$inding during normal operation.

• Te main advantage of using furan analysis as a

diagnostic tool is tat tese compounds aredegradation products specific to paper and can not beproduced by oil.

• )t as been estimated tat ne$ paper under normal

running conditions $ill generate furfural at te rate of6.4 ng/g of paper/our. Te rate of production increases$it increasing degree of degradation to D.F mg/g ofpaper in about 6DD(DDD ours or 6F&D years.


38/55


• Elevated temperatures or presence ofoxygen or $ater increase tis rate. Telimits of detection of furfural are about

D.D mg/l of oil. • Acceptable levels of furfural in

transformer oil% –

JD.6 mg/l + acceptableR – D.6 mg/l + questionable and

– D.F mg/l + unacceptable.


39/55

*egree of olymeri>ation !*#

• aper insulation of conductor is subectedte maximum operating temperature

• Life of transformer depends on te extent of

degradation of conductor insulation.• * closely relates to te mecanical

properties of paper. Life of paper isconsidered to ave been expired once tensile

strengt of paper drops to approximately FDto KD of initial tensile strengt $ic isreaced $en * reaces a level of nearlyD of te initial degree of polymeri>ation.


40/55


• Computation of te percentage

remaining life is calculated by%

'emnant life U!*&DD#x6DDV

!6DD&DD#

Te ne$ paper as a * of 6DD and

end of life is found to be DD.


41/55


• Anoter metod used is to measure tespecific viscosity of a solution of paper incuprietylene diamine. ,rom tis( teintrinsic viscosity of te solution iscalculated by%

ns !@iscosity of paper solution + viscosityof solvent# / @iscosity of solvent.

Te degree of polymerisation is calculatedfrom te intrinsic viscosity using Martin:stables.


42/55


• Te correlation bet$een te furfural

content and te * value and te

residual life is given by te formulae%

Log !fur# 6.F6 + D.DD8F*

Log !fur# &6.58 Q D.DF5T $ere T is

te residual life.


43/55


Correlation bet$een furan concentration and *

Total furan level !ppb# 'ange of * 'ecommended retestperiod !monts#

6DD 333&6DD 6

6D6&FD 888&338 K

F6&6DDD 84&88 8

6DD6&FDD 64&8K 6

FDD J64 ,ailure li"ely


44/55

artial *iscarge measurements !*#

• artial *iscarge is a locali>ed electricaldiscarge tat only partially bridges teinsulation bet$een conductors.

• )t partially ruptures insulation $itout

complete flasover or brea"do$n.• * is caused by%

– )mproper processing of transformer – resence of moisture –

2olid impurities in oil – ?as bubbles in oil – *elamination of press&boards – @oids bet$een glued components – 1ad connection of electrostatic sields


45/55


• Location of * is by% !)EC% KD4D#

– Acoustic *etection

– Electrical location

• Narro$ band metod or 'adio )nterference

@oltage metod !'ange 6D H>#.

• -ide&band metod measured in pico&coulombs

!pC# !'ange FD to 3DD H>#

• )dentifying location of * needs s"ill

and expertise.


46/55


Classification to be developed to support caution andalarm levels

Critical 2tages and Alarms

*ielectric Condition * Levels CautionLevels

Alarm levels

*efect&free 6D&FD pC ,irst $arning

signal%q FDD&6DDDpC

,irst fault

signal%q FDD pCNormal deterioration J FDD pC

oor impregnation 6DDD + DDD pC

Long term destructiveioni>ation

FDD pC in paper6D(DDDpC in oil

2ignal ofdefective

condition%

q 6DDD +FDD pC

Criticalcondition%

q 6(DD(DDD + 6D(DD(DDDpC

Large !8&F mm in dia#air/gas bubbles in oil

6DDD + 6D(DDD pC

aper moisture upto 8&3and relevant level in oil

DDD&3DDD pC andreduction of * inceptionvoltage by D

* Level and teir Critical 2tages !as per C)?'E -? 6.65#


47/55

,requency 'esponse Analysis !,'A#

• *ielectric faults may be caused bymecanical displacements occurring during

– transportation(

– sort circuit forces or

– inadequate processing during manufacturingcausing srin"age of $indings during service.

• 2uc canges can not be detected by *?A or

any oter tests.

• ,'A is easy to perform in te field and

provide reliable indication of mecanical

condition of transformer.


48/55

,requency 'esponse Analysis !,'A#

• Te transformer is isolated from te systemand te impedance or admittance of te

transformer is measured as a function of te

frequency ! H> or D H> or M>#.

• Tis gives a 9fingerprint: of te transformer.

• Tis test is repeated over time and te

9fingerprints: are compared.


49/55

'ecovery @oltage Measurements

• '@M metod is used to detect te condition ofoil&paper insulation and te $ater content in teinsulation.

• )t relies on te principle of te interfacial polari>ationof composite dielectric materials( i.e.( te build&up of

space carges at te interfaces of oil&paperinsulation due to impurities and moisture.

• A dc voltage is applied to te insulation for a time.Te electrodes are ten sort&circuited for a sort

period of time after $ic te sort&circuit isremoved to examine te rate of te voltage build&upor te polari>ation profile. Te time constantassociated $it tis pea" recovery voltage gives anindication of te state of te insulation.


50/55

'ecovery @oltage Measurements

• Te main parameters derived from tepolari>ation spectrum are te maximum valueof te recovery voltage( te time to pea"value and te initial rate of rise of te

recovery.• Tis metod is very controversial as to its

suitability for direct measurement of moisturecontent in te oil( due to te strong

dependence of te results on te geometry(construction of te insulation system of tetransformer.

C it d t B


51/55

Capacitance and tan B

for 1usings

• Capacitance and tan B measurements of

condenser busings give indication of te

condition of te insulation in te busing.

• A ne$ busing $ill ave a tan B value lesstan D.F

• Capacitance value of busing in service need

to be compared $it factory test result value

for deciding te extent of degradation.


52/55

Capacitance and tan B

for 1usings ig Tan delta !bet$eenD.DD4 and D.D6#%

)ngress of moisture in busing insulation

@ery ig Tan delta !1eyond D.D6#%

ig umidity( 2trong ageing of insulation

Lo$ Tan delta% -ea" otential connection

ig Capacitance% artial 1rea"do$n.


53/55

2uggested periodicity of diagnostic tests

Test eriodicity

)'/)( 1*@( Tan B of oil and$inding capacitance 0 Tan B

ne year

1using capacitance 0 Tan B

*issolve ?as Analysis

-inding resistance

-ater ppm alf year


54/55

To conclude …

•

;uality and 'eliability are a measure of teavailability of te transformer for continuous

operation trougout its stipulated life time.

• )t is igly dependent upon te design and

manufacture of te transformer( its materialsand construction. )t is also dependent on

proper erection 0 commissioning and

subsequent maintenance at site.

• )n essence( it is a function of te interactions bet$een te transformer and system.


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Documents

Transformer Condition Monitoring