Download - 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.
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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
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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
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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.
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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
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Windings
Terminals
Tank & DielctricFluid
OnloadTapchanger
Magnetic Circuit
OtherAccessories
COMPONENTS CAUSING FAILURE IN
SERVICE(29%)
(29%)
(13%)
(13%)
(11%)
(5%)
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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.
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Types of Transformer ,aults
• Mecanical ,aults%
–Tan"( fittings and accessories
–damaged porcelain insulators
–clogged oil piping( damaged gas"ets
–mecanical damage of te $indings.
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Types of Transformer ,aults
• Electrical ,aults%
–Lo$ insulation resistance
–unctured insulation bet$een turns(sections( $indings and various ig
potential points.
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Types of Transformer ,aults
• Magnetic ,aults%
– Abnormal eating
– 1rea"do$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
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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
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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
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-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
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Capacitance and tan B of $indings
• Measure Capacitance and tan B of eac pair of
$indings and $indings $it respect to eart
• Compare $it factory results
• )t indicates ealtiness of insulation system +paper( press&board and oil
• )ncrease in tan B indicates deterioration of
insulation system – Contamination
– Moisture absorption
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)nsulation 'esistance !)'# and
olari>ation )ndex !)# measurements
• Measure )' and ) values of eac $inding in
pairs and $it respect to eart
• Compare $it factory results
• Lo$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
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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
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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 $ater in oil%
Less tan 4.F H@ F ppm
4.F + 63F H@ D ppm
Above 63F H@ 6D ppm
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*issolved ?as Analysis !*?A#
• Transformer insulating oils consist of
different ydrocarbon molecules.
• 2plitting some of te ydrocarbon bonds
occur due to electrical and termal faults(forming gases +
– ydrogen
– Metane !C3#
– Etane !CK#
– Etylene !C3#
– Acetylene !C#.
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*issolved ?as Analysis !*?A#
• Lo$ energy faults !li"e artial *iscarges#sufficient to split $ea" &C bonds result inydrogen as main gas.
• iger temperatures are needed for splittingC&C bonds.
• iger temperatures result in – Etane( metane and etylene at FDDDC. – Acetylene requires temperatures 5DD & 6DDDC.
• Carbon particles form at FDD to 5DDDC and areobserved after arcing in oil or around veryot spots.
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*issolved ?as Analysis !*?A#
Te condition for formation of "ey gases byte degradation of cellulose and oil %
Material Condition Hey gas
Cellulose vereated 6FDDC C( C and
il vereated 8DDDC O6DDDDC Metane !C3#(
Etane !CK#
Etylene!C3#
rganic acids
il Electrical stress !artial
discarge# Arcing to 6DDDDC
ydrogen !#
Acetylene !C#
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*issolved ?as Analysis !*?A#
?eneral fault condition Hey gases
Termal conditioninvolving te oil
Metane( Etane( Etylene(and small amount of Acetylene
artial *iscarge ydrogen( Metane and smallamount of Acetylene andEtylene
2ustained arcing ydrogen( Acetylene andEtylene
Termal conditioninvolving te paper
Carbon Monoxide and Carbon*ioxide
Categories of "ey gases and general fault conditions
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*issolved ?as Analysis !*?A#
Caracteristic gas Concentration !ppm# )ndication
C D o$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/ !$it C 8D# ,ault gases from LTC
C)?'E Metod !Electra No. 65K( ct 77# & Hey gas concentration and indication%
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Metod recommended by
Electrical Tecnology 'esearc Association
A
1
C *
E , ?
D.6 6.D 3 6D 6DD 6DDD
C3/CK
6DDD
6DD
6D
6.D
D.6
D.D6
C
CK
A% Arc *iscarge
1% *iscarge
C% artial discarge
*% vereating 4DDDC
or eating Q discarge
E% ver eating J 8DDDC
,% ver eating
8DDDC + 4DDDC
?% vereating 4DDDC
*issolved ?as Analysis !*?A#
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*issolved ?as Analysis !*?A#
'atio EvaluationC3/ CK/C3 C3/CK C/C3
D D D D )f C3/JD.6( *
oter$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
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*?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
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*?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.
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*issolved ?as Analysis !*?A#
?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#
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*issolved ?as Analysis !*?A#
• 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.
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,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 $ic is a tediousprocess.
• Te furfural derivatives produced due to degradationof paper dissolve in oil.
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,urfuraldeyde Analysis
• ence( te cemical analysis of transformer oil giveste evidence of canges tat are ta"ing place in te$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.
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,urfuraldeyde Analysis
• 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.
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*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 $en * reaces a level of nearlyD of te initial degree of polymeri>ation.
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*egree of olymeri>ation !*#
• Computation of te percentage
remaining life is calculated by%
'emnant life U!*&DD#x6DDV
!6DD&DD#
Te ne$ paper as a * of 6DD and
end of life is found to be DD.
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*egree of olymeri>ation !*#
• 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.
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*egree of olymeri>ation !*#
• 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 $ere T is
te residual life.
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*egree of olymeri>ation !*#
Correlation bet$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
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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$een glued components – 1ad connection of electrostatic sields
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artial *iscarge measurements !*#
• Location of * is by% !)EC% KD4D#
– Acoustic *etection
– Electrical location
• Narro$ 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.
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artial *iscarge measurements !*#
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#
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,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.
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,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.
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'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 $it tis pea" recovery voltage gives anindication of te state of te insulation.
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'ecovery @oltage Measurements
• Te main parameters derived from tepolari>ation spectrum are te maximum valueof te recovery voltage( te time to pea"value and te initial rate of rise of te
recovery.• Tis metod is very controversial as to its
suitability for direct measurement of moisturecontent in te oil( due to te strong
dependence of te results on te geometry(construction of te insulation system of tetransformer.
C it d t B
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Capacitance and tan B
for 1usings
• Capacitance and tan B measurements of
condenser busings give indication of te
condition of te insulation in te busing.
• A ne$ busing $ill ave a tan B value lesstan D.F
• Capacitance value of busing in service need
to be compared $it factory test result value
for deciding te extent of degradation.
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Capacitance and tan B
for 1usings ig Tan delta !bet$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$n.
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2uggested periodicity of diagnostic tests
Test eriodicity
)'/)( 1*@( Tan B of oil and$inding capacitance 0 Tan B
ne year
1using capacitance 0 Tan B
*issolve ?as Analysis
-inding resistance
-ater ppm alf year
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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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