IEEE Transformer Life by Taking Corrective Measures Based on Oil Analysis

8/14/2019 IEEE Transformer Life by Taking Corrective Measures Based on Oil Analysis

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lntr0ducti0[Transformer is one of the critical equipment usedin power generation, transmission and distribution.It is considered as a critical equipment because -1) P.remature failure halts the production for indefiniteperiod. 2) Cannot be procured immediately as ittakes longest time to manufacture. 3) Cannot be

kept as ready spare as it blocks huge amount ofworking capital.The standard life of a transformer is 35 years.But there is a possibility of failure of transformerwithin 10 to 15 years ol service mainly due to -1) lmproper lnstallation and Commissioning. 2)lmproper protection coordination 2) Not takingcorrective action based on the periodical oil testreport. 3) Frequent exposure of core and coil to theatmosphere for internal inspection / attending leaksetc. 4) Keeping transformer idle for longer time.

The life of any transformer is the life of its insulation.There are two types of insulations - liquid insulation(Oil)which is replaceable and solid insulation (Paper)which is non replaceable. Hence, solid insulation isthe most critical component of any transformer sinceit decides the life of any transformer.

The deterioration of Solid insulation may takeplace mainly due to Chemical degradation,Thermal degradations, and Electrical stress. All thedegradation process takes up very slowly and it isreflected in the transformer oil. Hence periodical oiltest and taking corrective action based on the oil testreport is essential for enhancing the life of any oilfilled transformer.

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based on oil analysi$

The purpose of the article is to discuss the inferenceof the oil test report so that corrective action canbe taken up in time for enhancing the life of oilfilled transformers.

Translomer oilThe basic function of transformer Oil is 1) To act asa coolant.

2)To

act as insulation. 3)Toprotect

paperfrom chemical attack. 4) To prevent building up ofsludge 5) To be used as Diagnostic Tool.

90

ffi.ltil,4lr+o.di,lnterfacial Tension Ring Method IS: 6104-

1971

Flash Point Pensky Martens lS: 1448(P-21)-1992

Neutralization

\hlue (Acidity)

ChemicalTest 1S: 1448

(P-2)-1e67Di-ElectricStrenght (BDV)

Electrode Type lS: 6972-1gg2

SpecificResistance(Resistivity)

3 Terminat Cell lS: 6103-1971

Moisture ( Water)Content

Karl Fischerluratron

lS: 13567-92

Di-Electricurssrpatton ( tano)

STerminal Cell lS: 6262-1971

Sludge Content ChemicalTest lS: 1866

annex-ADissolved GasAnalysis (DGA)

GasChromatogragh

lS: 10593 &lS:92134-1992

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For achieving these functions, the physical, chemicaland etectrical properties of the oit must il witnin tnerange as specified in relevant standard.Due to continuous servieo nit ^,^^^-,.deteriorated."Jr*"oni,ill'13;r'"rj#::;::r:"7different operating and climatic

""**"r. ,n,. changein oil properties can be ilaced when oiltest shown inthe table I is conducted periodically. Other-than tnesenormal tests, special tests like frra, nrafysis of oiland Dp test of paper are also conductej, as anOwhen required to know the residual fit" oi trun.forre,(Solid insutation) indirecily and Oirecfly.Hence transformer oil is tested mainly for knowing- t. Aging status, l. Dietectric s;i;,'ii.'rnt"rnutstatus (Tabte il)

T-able ll:.Logical sequence for conducting varioustests and their inference

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Aging [roccss of solid insulalionMoisture, oxygen and heat are the main threeingredients which initiesorid insuration (r"bb'l;: H f::XT'#: jJffi,Tprocess, acid, Oxygen, Moisture sludge etc. areformed which changes the chemicalproperties (acidNumber), physical propefties (lFI Sludge content,moisture) electrical properties (BDV Resistivity, Tand)and aging,status (Furan content) of the oil.

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Table lll: Aging process of solid insulation

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Aging shtus ot oilDecomposition / oxidation of oil depends on thexposure of oit to heat, moistu*

"r;-";;en and itause production of Acids. These or,g"ni,"r;"i0, ,r"etrimentat to the insutation sysiem ;;";"; inducecorrosion inside the transformeiwn"n *"i"r'ilpresent.An increase in the acidrty i."n

inJi"ri,li'oirn" ,,rr"f deterioration of the oil with .tr;;" ;;;#evitabteby-product of an acid situation. Hence acid numberdetermines the presence ot invisioie s]ro*j, the oit.The presence of oil decay products and .otrOt" potarcontaminants from sotio'insutaling'iri"n",.

",.oeduces the lnterfaciatrension Gn ;r;';;; tn"r"tor"thereis a definite relationship L"i,r;;;; wrro"r.and IFT with the aging of trrn.f*r"r.-o,ifr, ,1.

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Because of this relationship, it is possible to determine the quality of oil for acceptance (Table lV) on basisColour, Acid Number, IFT and Oil Quality lndex (OQIN)

Aging statu$ ol uolid insulation.The degradation process of solid insulation getsaccelerated with the increase of % moisture in thepaper. The presence of moisture in the solid insulationcan be determined indirectly through water ppm in oilwith the help of graph shown in Fig 2 and the basedon the condition of the paper (Table V) the decisionfor hot oil circulation or Drying up of transformer canbe taken up.

. 1ry*$r$s*'it*fi l$ Mliirr#l fftt" ***1.wtJl#tFig 2 Griffin cuNe fot water equilibrium in cellulose and mineraloil system

Due to chemical decomposition of paper, furancontent consist of l) 5 Hydroxymethyl-2 furaldehydell) Furfuryal Alcohol lll) 2- Furfural (Furaldehyde) lV)2- Acetyfuran (Furyl methyl ketone) V) 5-Methyl-2-furfural (furaldehyde) are produced in the oil. Eachfuran contents signify different condition as shownin the table Vl.

Since the total furan content (addition of all furancontent) in the oil increases with the acceleration ofdecomposition process, the residual life of the paperinsulation can be established indirectly by knowingthe total furan content in the oil. The aging status ofpaper can also be determined directly by conductingDegree of Polymerization (DP) test on paper sampletaken from the subjected transformer and the agingof the transformer can be established directly byknowing the DP value of the paper (Table Vll). Hencebased on the furan content in the oil decision like -

total replacement of oil, conducting DP test and RLAstudy; Refurbishing of solid insulation and disposalof transformer can be taken up.

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PaleYellow

0.03 -0.10 30-45 600 - 1500 Very Good oil (Provides all required functions)

Yellow 0.05 -0.'t0 27-29 270 - 600Good oil {Provides allthe required iunction, a

drop in IFT may signallhe beginning of sludgeand sediments)

BrightYellow

0.11 -0.15 24-27 1N -.274Acceptable oil (Not providing proper cooling andwinding protection, Organic .acids are beginningto coat winding insulation; sludge in insulation

voids is highly probable.)

Amber 0.16 -0,40 18-24 45 - 160 Bad oit

'Brown 0.41 -0.65 14,18 22.44 Efiremely Bad oil

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Due to thermal and electricalfaults, oil and paper getsdecomposed and formed gases (Table lX) known askey gases, which remains dissolved in the oilTable lX

Since different gases are formed at differenttemperature

(Table X), the internal condition ofthe transformer which may lead to the failureof transformer, can be predicted by tracing theamount key gases available in the oil. The methodof tracing the amount bf gases.in the oil with thehelp of gas chromatography is known as DGA(Dissolved Gas Analysis)Table X

ldentilication0l the intemal proilemlnterpreting DGA for the identification of internal fault

in transformer is a difficult task since the popularmethods such as Key gas ratio, Roger Ratio, Duvaltriangle which are normally used for identifying theinternal problem, have some limitation. Hence, forobtaining the better interpretation by applying theabove methods, following points may be kepiin mindbefore taking any decision such as internal inspectionof transformer, tightness checking, changing defectiveparts etc.

. DGA is unreliable if the transformer is de_energizedand has cooled, if the transformer is new, or if it

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is less than 1 to 2 weeks of continuous serviceafter oil ptocessing.Dissolved Gas Analysis to be carried out after 1month of service (Base line DGA) and thereaftertwice in each year ( periodical DGA)A sudden increase in key gases and the rate ofgas production is more important in evaluating atransformer than the amount of gas.Each DGA must be compared to prior DGAs sothat trends can be recognized and rates of gasgeneration established.lf any DGA indicates internal defects, repeat DGAis to be conducted for confirmation.

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AceMene {Grllr}Generation of any amount of acetylene (CrHr)above a few ppm indicates high_energy arcinf.Trace amounts (a few ppm) can o" g"n"iit"d byivery hot thermal fault (500 degrees Celsius (EC) orhigher). A onetime arc, caused by a nearby lightningstrike or a high voltage surge, can also generate asmall amount of CrHr. lf C2H2 is found in tne OeR,orr samples should be taken weekly or even dailyto determine if additional CrH, is being generated.lf no additional acetylene is f5und ,nO ti" tevel isbelow the limit value the transformer may continue inservice. However, if acetylene continuesto increase,the transformer has an active high_energy internalarc and should be taken out of service immediatelyfor internal inspection.

Ethane (CrHu) ethylene {C?H4}Ethane and ethylene are sometimes called ,,hot metalgases." When these gases are being generatedand acetylene is not, the problem found inside thetransformer normally involves hot metal. This mayinclude I). Bad connection in leads, contacts on thetap changer il). Stray ftux impinging on the tank. ilt).Discharging of static electricity to ground. lV). Anunintentional core ground with circulating currents.0xygon {0zl

Oxygen (Or) must be watched ctosely in DGA tests.Many experts and organizations, including EpRl,believe that above 2,000 ppm, oxygen in the oilgreatly accelerates paper deterioration. This becomeseven more critical with moisture above safe levels.Under the same temperature conditions, celluloseinsulation in low oxygen oil will last 10 times longerthan insulation in high oxygen oil. lt is recommendedthat if oxygen reaches .t0,000 ppm in the DGA. &,eoil should be de-gassed and new orygen inhibitorinstalled. Excess amount of orygen can Oe preser,fiin the oil due to rapid degradation process of mper


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insulation or direct exposure of oil to atmospherethrough leaks, conservgtor without air bellows andsilica jel breather.

c0l00 BatioThis ratio is useful to determine if a fault is affectingthe cellulose insulation. Experience has shown thatwith normal loading and temperatures, the rate ofgeneration of CO, runs 7 to 20 times higher thanCO. With a CO, /CO ratio above 7, there is littleconcern. With CO, /CO ratio less than 7 along withsignificant increase in H, CHo, and CrHu indicateoverheating of paper. lf a problem is suspected,take another DGA sample immediately to confirmthe problem. Take the amount of CO, generatedbetween the DGAs and divide it by the amount of COgenerated in that same time to establish the ratio. Anexcellent indication of abnormally high temperatures

and rapidly deterioratingcellulose insulation is a COri

CO under 5. lf the ratio is 3 or under, severe andrapid deterioration of cellulose is certainly occurring..Extreme overheating from loss of cooling or pluggedoil passages will produce'a COrlCO ratio around 2 or3 along with increasing Furans. ln-such case alongwith DGAs, Furans test is also to be conducted foraccessing the residual life of the transformer.

f,tmospholic ga$8$ {llx, ilr, and S0r}ln conservator-type transformers if nitrogen, oxygen,and CO, are found increasing in consecutive DGA,there is a good possibility of oil leak or the oil may have

been poorly processed. There should be fairly lownitrogen and especially low oxygen in a conservator-type transformer. Howevet if the transformer wasshipped new with pressurized nitrogen inside andhas not been degassed properly, there may be highnitrogen content in the DGA, but the nitrogen levelshould not be increasing after the transformer hasbeen in service for a few years. When oil is beinginstalled in a new transformer, a vacuum is placedon the tank which pulls out nitrogen and pulls inthe oil. Oil is free to absorb nitrogen at the oiligasinterface, and some nitrogen may be trapped in thewindings, paper insulation, and structure. ln this case,nitrogen may be fairly high in the DGAs. However,oxygen should be very low, and nitrogen should notbe increasing. lt is important to take an oil sampleearly in the transformer's service life to establish abaseline DGA; then take samples at least annually.The nitrogen and oxygbn can be compared with earlierDGAs; and if they increase, it is a good indication ofa leak. lf the transformer oil has ever been degassed,nitrogen and oxygen should be low in the DGA. lt isextremely important to keep accurate records over atransformer's life; when a problem occurs, recordedinformation helps greatly in troubleshooting.

Po[ulat meth0d$ u$Bd for diagnosing internalproHem through IIGAGas fiatio mBthodsThere are three methods - Dornenburg, Rogersand lEC. ln this method, the value of key gasesratio CH./H, C2HJC2H4and CrHolCrHu are used forinterpreting the internal condition of any transformer.One drawback of the gas ratio methods is that someDGA results may fall outside thb ratio codes andno diagnosis can be given (unresolved diagnosesodes compare gas ratios). ln such condition, valueof COr/CO ratio, presence of O, along with the expertdecision is required for conclusion.

Duvalltiangle methodsln this method, relative proportions of 3 importantgases i.e. CHo, CrHo and CrH, are plotted in atriangular graph (Table Xl) where different zone is

allotted for different type of faults as shown in theLegend. The interpretation of fault is done on thebasis of intersection point of the relative proportionsof 3 important gases in which zone it falls.Table Xl: Duval triangle

Since, there is no area on the triangle for a transformerthat does not have a problem. The triangle will showa fault for every transformer whether it has a fault ornot. Therefore, ihis method cannot be used for allcases. lt can be used in combination with gas ratiomethods (Roger ratio) only when there is a increasingtrend of these three principal gases.

LegendPD = Partial DischargeT1 = Thermal Faulty

Less than 3000 CT2 = Thermal Fault

Between 3000Cand 7000 C

T3 = Thermal FaultGreater than Toooc

D1 = Low EnergyDischarge (Sparking)

D2 = High EnergyDischarge (Arching)

'4do & ,t DT = Mix of Thermalr-*t*6r*b & Electrical Faults

Table Xl : Calculating methods for Duval Triangletr ln order to disptay a DGA result in lhe Triangle. ore must start with the@ncentralions of llE thH gass, (cH4) ' A, {c2Ha} = B and (c'H2) = c' in ppm'O CalculEte the sm of lho* thre values: (C1'14 + C2H4 + C?H2) ' S' in ppm,O Caldlate lhe relatire pDportim of the ttlH gasss, in %:

x 3 % cH4= 100 (Ar's),Y 3 % CrH4= 100 (&/S),z=%ciH?= 10o(c/s).

tl Plot X, Y and Z itr lho DUVAL Trianglell Point of inte6ecrion will be laying in a parlicular zone indicated in Ble trianghtl hbrpret the Hsn lom the owALtrble

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Conclusion .:Transformer is robust and static electrical equipment.Any lnternal / external damage due to impropermaintenance practice does not cause immediatefailure of transformer,

rather it takes longer time totranslate this damages to failure, if it is unioticed andcorrective action is not taken in time. To avoid suchconsequence, maintenance schedule of periodicalelectrical test, oiltesting and analysis and recordingsthese events in history register are followed up strictlyso that required corrective action is initiated as andwhen deviation in test result is observed.There are international standards _ IEC (lnternationalElectro technical Commission / France), ANSI/ASTM(American National Standards lnstitute/RmericanSociety for Testing and Materiats/ USA), EPRI (EtectricPower Research tnstitute/ USA) CiOne (Councitlnternational des Grand Reseaux Electriques /France), IEEE (lnstitute of Electrical and EtectronicEngineers/USA) and national standards _ lS (lndianStandard I lndia), ERDA (Electrical Research andDevelopment Association I lndia), CBlp ( CentralB_oard for lrrigation and powei / lndia), CpRl(Centrat Power Research lnstitute / lndia) *hi.h h"r"specified the limiting values for oil parameters andthe guideline for transformer oil analysis accordingto transformers voltage grade, age, application andenvironmental condition.Though the relevant standards specify the timitingvalues for oil parameters and the quantity (ppm)of dissolved gases, decision for coriective'actionssuch as hot oil circulation, total oil replacement,internal inspection, RLA study etc are'advised totake carefully on the basis of eiperience and historyof the transformers.

REFERENCES:

1. SDMI International(Substation Services)2. LA.R. GRAY Transformer Chemistry Services3. IEEE Std CS7.1O4-1991, .Guide for the tnterpretation ofGases Generated in Oil- lmmersed frr".toirSo ,4. M. Duval, 'A Review of Fzu.hs Detectable by Gas-in-Oll Analysis

i n Transformers,,,IEEE Electricar f nsuf attn rrrf ajJine.5. DISSOLVED GAS ANALYSIS OF MINERAL OIL INSUI.ATING

FLUIDS by Joseph B. DiGiorgio, ph.D.

lnsulotion foilure leods to short circuits.lnsulolion foilure in Electricol wiring os well os devices leodsto shod circuiis ond couses fire hozords. The moior reosonfor insulotion foilure in o properly instolled eleclricolnetwork is tronsienl over vohoges. These over vollogescreoted due io switching or electromognelic inlerferencedetoriote the insulotion of electricol network ond couse short

.circuils. SPD's ore designed to reduce over voltoges due loswitching or even lightning ond prolects your electricolnetwork ogoinsi short circuits ond your building ondvoluobles ogoinst fire.

Transformer Maintenance- US department of interiorbureau of reclamation

IEEE electrical insulatingmagazine - Vol 15,No1,January-February 1999lntroduction to the Half-Century Transformer by theTransformer Maintenancelnstitute, S.D. Myers Co.,2002 11e1.

H8ilg&dNeyvef LQnlte Corfrdian Ltd

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OBO BETTERMANN tndia pvt, LtdPlot No: O-37, Door No: g, Bharathidasan Street, Thiru Nagar. Jaftaoas.Chennai - 600 083. Tamit Nadu, tndia. Tet: + fi aa a3sg gai jril ; ;ffi"in.lxE-mail: [email protected] www.obojndia.com

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IEEE Transformer Life by Taking Corrective Measures Based on Oil Analysis