W ater in transformersappears as an un-wanted substance,

decreasing insulation breakdownstrength and shortening trans-former life dramatically. Highwater content in paper may re-sult in bubbling, the formationof free water and an increasedrisk of dielectric breakdown. Wa-ter also causes accelerated paperaging, corrosion of core/tankand progressive consumption ofoil additives.

Water contamination intransformers comes from threemain sources: residual moisturein the “thick structure elements”,ingress from the atmosphere,and the aging decomposition ofcellulose and oil.

Residual moisture in a newtransformer after factory dry out,prior to transportation, is expect-ed to be less than 1%, with a tar-

get value of 0.5%. Excessiveresidual moisture of 2-4% can re-main in some thick insulationcomponents, particularly plas-tics1. During service, this mois-ture gradually seeps into the oil,increasing the water content inthe thin insulation structure1.

Atmospheric water is themain source of transformer con-tamination. Three mechanismsare acting here: the absorptionof water from the direct expo-sure of the insulation to the air(installation and repair works),the ingress of moisture in theform of molecular (Knudsen)flow due to the difference in thewater concentration in the at-mosphere and in the oil in thetank, and the viscous flow of wetair into the transformer causedby the difference between the at-mospheric pressure and the pres-sure in the tank1.

The molecular flow of mois-ture is practically negligible. Themain mechanism of water pene-tration is the viscous flow of wetair through “poor sealing”, i.e.through the sealing points ofbushings, the explosion ventand the cooling circulation loopetc., due to the pressure gradi-ent. Large amounts of rainwatercan be pumped into the trans-former in a very short period oftime (just a few hours) if there isimproper sealing and a rapiddrop in pressure1.

Methods and instrumentation formoisture measurementThe term “moisture” in thetransformer industry is com-monly used to indicate waterwhich is absorbed in the paperor dissolved in the oil. Occasion-ally, the terms “water” or “watercontent” are used as an alterna-tive way to describe the samesubstance.

Water in transformers can befound in different parts of theinsulation system. It can accu-mulate in solid insulation, bedissolved in oil, or be found inthe form of liquid water at thecore or bottom of a transformer.Depending on the media, differ-ent methods and instrumenta-tion are used for moisture meas-urement. For example, to evalu-ate water content in oil, polymerrelative humidity/relative satura-tion sensors are frequently used.By measuring the relative satura-tion of oil one can easily deter-mine its absolute moisture con-tent expressed in parts per mil-lion (ppm). In this case the watersolubility characteristic for thespecific oil must be known inadvance. Being a real-time effi-

18 160/2002

V. G. DavydovMonash University Australia

O. RoizmanIntellPower Australia

Moisture assessment in power transformers

Lessons LearnedThe need to know the water content of paper accurately underall operating conditions is essential for the safe operation oftransformers. To improve existing methods of determining thewater content of paper, a concept of “water-in-paper activity”has been developed. The water solubility parameter is of paramount importance for the accurate evaluation of watercontent. A stand-alone software application, the Transformer Moisture Monitor (TMM), is recommended for moisture assessment. The development of a new classification procedureprovides a means of ranking dry and wet transformers.

cient on-line method, the mois-ture-in-oil measurement has at-tracted a lot of attention in re-cent years. A number of com-mercially available sensors havebeen tested in different condi-tions using the Monash test rig.

Another method for ppmdetermination is a techniquebased on the well-known KarlFischer titration reaction. How-ever, this method is not a real-time one and requires appropri-ate chemical instrumentation forits implementation. It is alsoprone to a high level of error andits results are often difficult tointerpret. A timely and accuratemeasurement of the oil tempera-ture should be made when ob-taining the oil sample.

There is also a group ofmethods based on the measure-ment and analysis of dielectricproperties of insulation. Diagno-sis with these methods is basedon the fact that paper and oilchange their dielectric propertieswhen moisture levels increase.Therefore, by measuring para-meters such as the dissipationfactor, polarization and depolar-ization currents (PDC) and re-turn voltages (RVM) in the fre-quency and time domains, it ispossible to relate these changesto water concentration in a pa-per-oil insulation system. How-ever, with most of these meth-ods it is difficult – if not impos-sible – to distinguish the effectsof moisture on the insulationfrom the effects of aging causedby other processes. This creates alot of confusion over the relia-bility of dielectric responsemethods. Interpretation of theresults still remains more artthan science.

Water content of paperinsulation

One of the aims of the moistureassessment of oil-filled powertransformers was to evaluate thewater content of its paper insula-tion. The term “paper insula-tion” is a generic term that de-scribes solid cellulose material,such as paper, pressboard, woodand cotton. Another term, “wa-ter content of paper (WCP)”, isused to quantify the amount ofwater present in paper insula-tion. Traditionally, the termWCP refers to a ratio betweenthe mass of water and mass ofdry non-oily paper, expressed ina percentage. The mass of wateris calculated from the differencebetween the masses of wet anddry paper. The paper is usuallydried out in an oven prior to themeasurement of the dry papermass. The traditional term WCPgives a clear meaning for non-oil-impregnated paper.

For oil-impregnated paperthe WCP can still be measured,in this case using the Karl Fisch-er titration method. Followingthe measurement of water massin the Karl Fischer apparatus, theoily paper is degreased and onlythen placed in an oven for dry-ing. To obtain an accurate meas-urement, the thickness of thepiece of paper or strip of press-board should not exceed a fewhundred microns. However, fora paper-oil insulation systemwith both thin and thick paperinsulation the situation formeasuring the WCP is com-pletely different.

In a study conducted atMonash University under anEPRI, USA sponsored project, anew method of moisture assess-

ment in operating transformerswas developed, based on a water-in-paper activity concept. Theparameter of water-in-paper ac-tivity is used to access moistureconditions in both new and oldtransformer insulation systems.Another term, “active water con-

tent of paper (WCPA)”, was alsointroduced. The term WCPA re-flects the water available in thetransformer insulation for ex-change between paper and oil. Anew moisture equilibrium chart(shown in Figure 1) relating theparameter of water-in-paper

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Figure 1. Moisture Equilibrium Curves relating water-in-paper activity toactive water content of paper for temperatures from 0 to 100 °C.

Figure 2. Moisture Equilibrium Chart as published in “MoistureEquilibrium Charts for Transformer Insulating Drying Practice”.2

➤

activity to the WCPA was devel-oped.

Numerically, water-in-paperactivity (Awp) is equal to the equi-librium percentage of relativesaturation (RSeq) divided by 100.

There are a number of waysto evaluate the WCPA of paperinsulation in a paper-oil insula-tion system. One of them is ap-parent from Figure 1. To use thecurves of Figure 1 we need to es-tablish equilibrium conditionsin the system and measure theparameter of Awp using a relativesaturation (RS) sensor.

Another way to evaluate theWCPA of paper insulation in apaper-oil insulation system is touse a moisture equilibrium chart,similar to the well-known mois-ture equilibrium chart publishedin 2 and shown in Figure 2.

The chart in Figure 2 givesthe relationship between the wa-ter content of oil (WCO) ex-pressed in ppm, the temperatureand the “% Moisture in Paper”,which, in fact, for thin paper in-sulation is equal to the WCPA.Again, to evaluate the WCPA, weneed to establish equilibriumconditions in the system andthen measure the WCO usingthe Karl Fischer apparatus. How-

ever, we must measure one moreparameter in this case, prior tothe evaluation of the WCPA.This parameter is called the “wa-ter solubility of oil S(T)”, or, asin Figure 2, the “Solubility Lim-it, ppm”.

The moisture equilibriumchart in Figure 2 was developedfor new oil. The figures aroundthe chart show the water solubil-ity parameter S(T) for new oil attemperatures from 0 to 100 °Cin 10 °C intervals.

Assessment of moisturein an operating transformerIt is known that an operatingpower transformer is never inmoisture equilibrium. Thus, thetwo moisture equilibrium chartsin Figures 1 and 2 cannot be ap-plied directly when assessingmoisture in the field. To addressthis, a method for the moistureassessment of transformer insu-lation has been developed. Themethod is based on the Monashwater-in-paper (WIP) algorithm,which incorporates the follow-ing three steps:

- Evaluation of true oil rela-tive saturation in the trans-former

20 160/2002

Figure 3. Chart for off-line moisture diagnostics of transformers containingnew oil.

- Evaluation of the water-in-paper activity (Awp) for the trans-former oil

- Evaluation of the active wa-ter content of paper (WCPA) as afunction of Awp and temperature.

A software application, theTransformer Moisture Monitor(TMM), has been developed onthe basis of the WIP algorithm.The TMM uses neuro-fuzzycomputing to evaluate the con-sistency of the moisture sensoroutput, to access the WCPA, andto alert the user when insulationconditions require attention.The TMM acquires data fromthe Vaisala HMP228 Humidityand Temperature Transmitter.The probe of the transmitter hasboth moisture and temperaturesensing elements at its tip.

New classification procedure for off-linemoisture diagnosticsThe research conducted atMonash University has shownthat, for the accurate moistureassessment of transformer insu-lation, the continuous monitor-ing of a number of parameters,including transformer load, tem-perature and oil relative satura-tion, is required. The measure-ment of water solubility charac-teristic of oil in the transformeris also required.

A utility may operate tens oreven hundreds of transformers.However, not all of the trans-formers require continuousmonitoring. How is it possible toidentify a transformer with mois-ture concerns for further moni-toring? There is a demand for aclassification procedure, whichwould rank transformers bymoisture levels.

A traffic light approach is theEPRI adopted way of rankingthe health of plant equipment.Red, yellow and green colorswould indicate the ‘wet’, ‘re-quires attention’ and ‘dry’ statesof moisture in transformers.

A new classification proce-dure was developed to rank,from a population of powertransformers, critical transform-ers in terms of moisture for fur-ther continuous monitoring.The new classification procedureaims to improve the effective-ness of the management of trans-former life.

The new classification proce-dure is based on a method ofmoisture assessment that evalu-ates the WCPA and was brieflydescribed in the section above. Itis available in the form of achart, shown in Figure 3. Valuesof WCO in ppm represent theresults of Karl Fischer (KF) meas-urements and values of the tem-perature T represent the temper-ature of the oil at the moment ofoil sampling. In Figure 3, valuesof WCPA are distinguished bycolor. Green corresponds to aWCPA of less than 1%, yellowcorresponds to a WCPA of be-tween 1% and 2% and red corre-sponds to a WCPA of more than2%. The numbers in the squaresrepresent the diffusion time con-stant for moisture across a 1-mmpressboard plate exposed to oilfrom both sides. The 1-mmpressboard was chosen to reflectthe approach of the water-in-pa-per activity applied to the trans-former ranking and classifica-tion.

At an oil temperature below45 °C and WCO of less than 20ppm, the moisture diffusiontime is too long for the reliablediagnosis according to the pro-posed procedure. To reflect thisfact the bottom left corner of thegraph in Figure 3 is shadowed.

The classification procedurechart in Figure 3 is valid for oilwith a water solubility character-istic close to that of new oil. Inthe previous section of this pa-per it was demonstrated that thewater solubility characteristic ofnew and aged oil can differ sig-nificantly. This means that for

the accurate ranking of agedtransformers, evaluation of theoil’s water solubility characteris-tic would be required.

In light of the new know-ledge presented here, the tradi-tional approach, which states: “Ifthe WCO exceeds 20 ppm, thetransformer requires further at-tention, if the WCO is less than20 ppm no action is required,”must be reconsidered. From Fig-ure 3 it follows that atWCO=20-25 ppm a transformerwith new oil would be consid-ered “probably wet” if the oilsample was taken at a tempera-ture of below 55 °C. However, ifthe oil sample was taken at atemperature of above 80 °C,with the same WCO=20-25ppm, the transformer would beconsidered “probably dry”. Toprove or reject the conclusion,one more sample of oil shouldbe taken in the number of hoursshown in the related square onthe chart. This example illus-trates the importance of themeasurement of oil temperatureduring the oil sampling, the im-portance of the second oil sam-pling, and the importance of thelength of time between the twosampling events.

It is apparent that knowledgeof the WCO only, gained from asingle Karl Fischer measure-ment, is insufficient for the ac-curate assessment of transformermoisture condition. Knowledgeof the temperature of the first oilsample, the dynamics of mois-ture and temperature gainedfrom the second oil sample, andthe water solubility of the oil arealso required.

ConclusionsOil type, age and flow rate con-tribute to the measurement andsubsequent accuracy of moistureassessment in a paper-oil insula-tion system. Sensor positioningalso contributes to the accuracyof moisture assessment.

The parameter of the watersolubility of oil is required forthe validation of the moisturesensor installed in the trans-former. Water-in-paper activitywas found to be a factor in deter-mining the state of dryness in apower transformer.

A stand-alone software ap-plication called “TransformerMoisture Monitor” (TMM) isproposed for the moisture as-sessment of a transformer.

A new classification proce-dure that aims to identify criti-cal units for further continuous

monitoring is suggested for theranking of power transformersby the level of moisture. Red,yellow and green colors wouldindicate the ‘wet’, ‘requires atten-tion’ and ‘dry’ states of moisturein transformers. The classifica-tion procedure presented is onlyvalid for oil with a water solubil-ity characteristic close to that ofnew oil. For the accurate rankingof an old transformer, the watersolubility characteristic of thetransformer’s oil would need tobe evaluated. �

References:1. V. Sokolov and B. Vanin, “Experience

with In-Field Assessment of the WaterContamination of Large PowerTransformers”, Proceedings of EPRISubstation Equipment DiagnosticsConference VII, New Orleans, 1999

2. T. V. Oommen, “Moisture EquilibriumCharts for Transformer InsulatingDrying Practice”, IEEE Trans. onPower Apparatus and Systems, Vol.PAS-103, 10, 1984, pp.3063-3067

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T he HMP228 Transmitter provides on-linemeasurement of moisture and temperature

in transformer oil. The measurement gives con-tinuous information and enables better mainten-ance against transformer failures.

Vaisala’s microprocessor-based HMP228Transmitter is accurate, reliable and fast. The ba-sic model of the HMP228 Transmitter calculatesthe average water solubility in mineral trans-

former oil. The water solubility in oil is tempera-ture dependent: water solubility increases astemperature raises. In addition to traditionalppm-output, the transmitter measures water ac-tivity. The water activity (aw) indicates directlywhether the oil is too moist. The aw measurementis independent of the type, aging or temperatureof the oil. �

HMP228 Moisture and Temperature Transmitter for Transformer Oil

The determination ofmoisture in oil is an

essential part of acomprehensive

transformermaintenance

program.