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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 10, No. 5; October 2003 903

Review of Modern Diagnostic Techniques for AssessingInsulation Condition in Aged Transformers

Tapan K. SahaSchool of Information Technology & Electrical Engineering

University of Queensland, St. LuciaBrisbane, Queensland-4072, Australia

ABSTRACTCellulosic paper and oil insulation in a transformer degrade at higher operatingtemperatures. Degradation is accelerated in the presence of oxygen and moisture.Power transformers being expensive items need to be carefully monitored through-out its operation. Well established time-based maintenance and conservative re-placement planning is not feasible in a current market driven electricity industry.Condition based maintenance and online monitoring are now gaining importance.Currently there are varieties of chemical and electrical diagnostic techniques avail-able for insulation condition monitoring of power transformers. This paper pre-sents a description of commonly used chemical diagnostics techniques along withtheir interpretation schemes. A number of new chemical techniques are also de-scribed in this paper. In recent times a number of electrical diagnostic techniqueshave gained exceptional importance to the utility professionals. Among these tech-niques polarisationrrrrrdepolarisation current measurement, return voltage measure-ment and frequency domain dielectric spectroscopy at low frequencies are the mostwidely used. This paper describes analyses and interpretation of these techniquesfor transformer insulation condition assessment.

Index Terms — Insulation condition, transformer diagnostics, transformerageing, dissolved gas analysis, furan analysis, degree of polymerization, oil insula-tion, cellulose paper, return voltage, polarisation current, depolarisation current,online monitoring, condition based monitoring.

1 INTRODUCTIONHE insulation system of a power transformerTconsists mostly of hydrocarbon oil and paper. Many

of these power transformers within electric utilities aroundthe world are approaching the end of their design life.Insulation degradation is a major concern for these agedtransformers. Insulation materials in transformers de-grade at higher operating temperatures in the presence ofoxygen and moisture. Practicing engineers currently use anumber of modern diagnostic techniques to assess the in-sulation condition of aged transformers. Among themmoisture analysis in transformer oil, dissolved gas analysisŽ . w x Ž .DGA 1 , degree of polymerisation DP measurementand furan analysis by high performance liquid chromatog-

Ž . w xraphy HPLC are frequently used 2,3 .

Molecular weight studies by single point viscosity mea-surements are of limited value when dealing with a com-plex polymer blend such as Kraft paper as used in trans-

Manuscript recei®ed on 18 No®ember 2002, in final form 29 May 2003.

formers, particularly in cases where the molecular weightdistribution of the paper changes significantly as the

w xdegradation proceeds 4 . In these instances, a new tech-nique, gel permeation chromatography is found to be moreuseful than the viscosity method, because it provides in-formation about the change in molecular weight and

w xmolecular weight distribution 4 . A variety of informationcan be obtained from the X-ray photo-electron spec-

Ž .troscopy XPS , which includes chemical state informa-tion, elemental information, and variation of compositionwith depth, variation of chemical composition spatially onthe surface and thickness of layers. Oxidation changes thecolour of the oil and this colour change can be monitoredby the change in absorbance of the oil. This can com-monly be examined by UV visible spectroscopy. The re-sults from these experiments have been presented in ref-

w x Ž .erence 5 . Fourier transform infrared FTIR and near-Ž .infrared NIR spectroscopy have been used to charac-

terise the ageing of cellulosic paper, with the long termaim of developing a technique to assess the condition of

w xpaper insulation in electrical transformers 6 .

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Saha: Re©iew of Modern Diagnostic Techniques for Assessing Insulation Condition in Aged Transformers904

In recent years, new diagnostic methods have been pro-moted that are complementary to the classical insulationresistance, power frequency dissipation factor and polari-sation index measurements. This is significantly due to theavailability of modern computer controlled instrumenta-tion. These new methods are based on either time or fre-quency domain polarisation measurements. In frequencydomain measurement, a sinusoidal voltage is applied andthe complex dielectric constant is determined from theamplitude and phase of the current flowing through the

w xsample 7 . On the other hand, time domain measure-ments are conducted by the application of a step voltageacross the insulation object. Time domain measurementsbased on polarisationrdepolarisation current measure-ment and return voltage measurement have gained signifi-

w xcant importance over the last ten years 8 . Particularly,there has been growing interest in the condition assess-ment of transformer insulation by the return voltage

Ž . w xmethod RVM 9 .

There are many papers available on these individual di-agnostic techniques. However, a review of all these tech-niques in a single paper is rarely available. Some attemptshave been made in the past to review chemical techniquesw x10 . The author of this paper recently wrote a review pa-per on time domain polarisation measurement techniquesw x11 . In this paper a brief introduction of different chemi-cal diagnostic methods will be first outlined. Then differ-ent interpretation schemes based on chemical diagnosticswill be discussed. This paper will then provide special re-view of modern diagnostic techniques, in particular, cur-rently available polarisation measurement techniques.Most recent emphasis has been directed to techniques ofdetermining moisture content of insulation indirectly bymeasuring RV parameters. The major difficulty still lies inthe accurate interpretation of return voltage results. Thispaper will investigate different thoughts regarding the in-terpretation of RV results for different moisture and age-ing conditions. A number of techniques have been corre-lated with the RV method and hence correlation andcomparison will also be briefly discussed in this paper.

2 CHEMICAL TECHNIQUES2.1 MOISTURE ANALYSIS

Water content in insulation materials increases electricconductivity and dissipation factor and reduces electricstrength. It has been a common practice to measure the

w xmoisture content in oil by the Karl Fischer titration 12method and then estimate the moisture in solid insulation

w xby different equilibrium curves 13,14 . There are severaldirect measurement methods currently reported in the lit-erature. A thin film capacitive humidity sensor was tested

w xfor moisture sensing in transformer oil by Oomen 15 . Thissensor was found to respond well for oil in transformer incold and warm weather conditions. Neimanis et al. inves-

Ž .tigated the near infrared NIR spectroscopy for the de-

termination of moisture content in oil impregnated paperw x16 . Their results showed that NIR spectroscopy alongwith their developed multivariate modelling could result

w xaccurate estimation of moisture. Gupta 17 reported theeffectiveness of NIR spectroscopy technique to detect verysmall changes in moisture content of paper insulation. Anumber of polarisation based dielectric diagnostic tech-niques are also currently in use for indirect moisture anal-ysis of oil-paper insulation system.

( )2.2 DISSOLVED GAS ANALYSIS DGAAmong chemical techniques, dissolved gas analysis

Ž .DGA has gained worldwide acceptance as a diagnosticw xmethod for the detection of incipient faults 18 . Fault

gases are produced by degradation of the transformer oiland solid insulating materials such as paper, pressboardand transformerboard, which are all made of cellulose.The rate of cellulose and oil degradation is significantlyincreased in the presence of a fault inside the trans-former. The important gases produced from the trans-former operation can be listed as follows.

� Ž .Hydrocarbon gases and hydrogen: methane CH ,4Ž . Ž . Ž .ethane C H , ethylene C H , acetylene C H , and2 6 2 4 2 2

Ž .hydrogen H .2

� Ž .Carbon oxides: carbon monoxide CO and carbonŽ .dioxide CO .2

� Ž . Ž .Nonfault gases: nitrogen N and oxygen O .2 2

w xAccording to Emsley et al. 10 a healthy transformerŽshould have less than 0.05 ml of combustible gases hydro-

gen and short chain hydrocarbons: methane, ethane, eth-.ylene, acetylene per 100 ml of oil and insignificant levels

of higher hydrocarbon gases. Measurements on freebreathing transformers show average COqCO levels of2

0.4 mlr100 ml of oil after 15 years. Normally causes offault gases are classified into three categories:

� Corona or partial discharge

� Thermal heating

� Arcing.

Several authors attempted to classify these gases basedon the faults. In the past certain key gases have been cor-related with fault type and the rate of gas production cor-

w xrelated with fault severity. IEEE standard 19 with an ex-tensive literature list discusses key gas method in detailwith relative proportions of gases for the four general typesof fault. It is commonly accepted that hydrogen gas is pro-duced from the corona effect on oil and cellulose. Methaneand ethane are produced from low temperature thermalheating of oil and high temperature thermal heating pro-duces ethylene and hydrogen as well as methane andethane. Acetylene is only produced at very high tempera-tures that occur in the presence of an arc. Low tempera-ture thermal degradation of cellulose produces CO and2

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high temperature produces CO. Low energy electrical dis-charges produce hydrogen and methane, with small quan-tities of ethane and ethylene. Electrical arcing produceslarge amounts of hydrogen and acetylene with minorquantities of methane and ethylene. The most commonlyused gas-in-oil diagnostic methods include the following:Ž .a IEEE C57.104-1991.Ž .b Doernenberg method.Ž .c Rogers’ method.Ž .d IEC 60599.Ž .e Duval’s triangle.

w xThe IEEE standard 19 also reports two ratio methodsbased on five ratios. Where

CH C H C H4 2 2 2 2Ratio1s , Ratio2s , Ratio3s ,

H C H CH2 2 4 4

C H C H2 6 2 4Ratio4s , Ratio5s .

C H C H2 2 2 6

The Doernenberg ratio method is based on Ratios 1, 2,3 and 4. This method is based on special concentrations asshown in Table 1 to determine whether there is any prob-lem and whether there is sufficient generation of each gasfor the ratio analysis to be applicable. Then the ratios inthe order of Ratio1, Ratio2, Ratio3, and Ratio4 are com-pared to limiting values, providing a suggested fault diag-nosis. The procedure is explained in detail in the IEEE

w xstandard 19 and in many other papers. Rogers’s ratioŽ .method uses only three ratios Ratio 1, 2 and 5 . This

method’s flow chart along with key gas limits is also givenw xin the IEEE standard 19 .

In many situations both the Doernenberg method andthe Rogers’ method may provide a ratio that does not fitinto the diagnostic codes. Other analytical methods suchas those based on total dissolved key gas concentration orkey gas method should then be used. Duval and De Pablow x20 discussed the interpretation of gas-in-oil analysis us-ing new IEC 60599 publication and IEC TC 10 databases.In IEC publication 60599 five different fault types are dis-

w xcussed 21 . The major gas ratios have been retained for

Table 1. Concentration of Dissolve Gas.Ž .Key Gas Concentrations ppm

Ž .Hydrogen H 1002Ž .Methane CH 1204

Ž .Carbon Monoxide CO 350Ž .Acetylene C H 352 2Ž .Ethylene C H 502 4Ž .Ethane C H 652 6

diagnosis with new code limits. Additional gas ratios arealso suggested for specific fault cases. More precise defi-nitions of normal and alarm gas concentrations are alsohighlighted. These faults can be reliably identified by vi-sual inspection of the equipment after the fault has oc-

w xcurred in service 20 . Examples include:Ž .a Partial discharge�with possible X-wax formation

and of the sparking type inducing small carbonised punc-tures in paper.Ž .b Discharges of low energy�evidenced by larger

punctures in paper, tracking or carbon particles in oil.Ž .c Discharges of high energy with power follow

through�evidenced by extensive carbonisation, metal fu-sion and possible tripping of the equipment.Ž .d Thermal faults below 300�C�evidenced by paper

turned brownish and above 300�C when paper carbonises.Ž .e Thermal faults above 700�C�evidenced by oil car-

bonisation, metal coloration or fusion.

In IEC 60599 two additional gas ratios have been intro-duced for specific diagnosis. The ratio C H rH is rec-2 2 2

ommended to detect possible contamination from the OnŽ .Load Tap Changer compartment when �3 and the ra-

Žtio O rN to detect abnormal oil heatingroxidation when2 2.�0.3 . Typical values of gas concentrations are also calcu-

Žlated in this publication. Alarm values maximum accept-.able values and typical rates of gas increase for power

w xtransformers are also given in IEC standard. Duval 22mentioned that a significant number of DGA results inservice fall outside the IEEE-IEC codes and could not bediagnosed. Duval proposed his triangle representation forfault diagnosis as shown in Figure 1. According to Duvalw x22 high rates of paper degradation are indicated when

w xFigure 1. Duval’s Triangle 22 .

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the ethylene concentration increases strongly and theCO rCO ratio decreases below a ratio of about 6. At a2

CO rCO ratio less than 2, the probability of failure in-2

creases significantly when the combustible gas concentra-tion exceeds about 200 ppm.

w xLapworth 23 proposed a new approach using the keygas method of presenting DGA results. The relative pro-portions of six combustible gases CH , C H , C H ,4 2 6 2 4

C H , H and CO are displayed as a bar chart to illus-2 2 2

trate the gas signature. The novel aspect of the approachproposed by the author is that this method is used to in-vestigate and illustrate the clear difference that exists be-tween ‘‘normal’’ and ‘‘abnormal’’ results. A scoring systemhas been developed which translates a DGA result into acomposite DGA score reflecting the perceived seriousnessof the signature. Islam et al. described a fuzzy logic ap-proach to develop a computer based intelligent interpre-tation of transformer faults. They have tested the softwareusing 800 DGA cases and successfully demonstrated de-

w xtection and verification of 20 transformer faults 24 . Avast number of literatures are available on the DGAmethod and its interpretation. However, most of these arebased on IEEErIEC methods.

2.3 DEGREE OF POLYMERISATIONMEASUREMENTS

ŽThe solid insulation paper, pressboard, transformer-.board used in transformers is a sheet of material made

from vegetable cellulose. The main source of cellulose fi-bre is wood. In a dry condition, wood contains 40 to 50%cellulose, 20 to 30% lignin and 10 to 30% hemicelluloseand polysaccharides. Cellulose is a linear polymer com-posed of individual anhydrous glucose units linked at thefirst and fourth carbon atoms through a glucosidic bond.The structure of glucose and cellulose is shown in Figure2. The good mechanical properties of cellulose and itsderivatives, on which their utility depends, are due to theirpolymeric and fibrous nature. The number of monomerunits in the polymer is known as degree of polymerisation

Figure 2. Structure of glucose and cellulose.

Ž .DP . Very often, the quality of the cellulose is measuredin terms of its degree of polymerisation by the averageviscometric method. The length of the cellulose chain thusmeasured by the average degree of polymerisation basedon viscosity method will be denoted by DP .V

DP measurement has been used as a diagnostic tool toV

determine the condition of transformers by several work-w xers 13,25�27 . New Kraft paper has an average chain

length of 1000 to 1500. After a long period of service athigh temperature with high content of water and oxygen,the paper becomes brittle, changes colour to dark brownand DP falls to 200 to 250. Sometimes cotton is used asV

an insulating material. Cotton fibre lengths are greaterthan those of unbleached soft wood sulphate cellulose, buttheir diameter is smaller. The average degree of polymeri-sation for new cotton is higher than Kraft paper. For Kraftpaper with a DP of 150 to 200 the mechanical strengthV

of paper can be reduced to 20% of its initial strength andthis point is regarded as the end of life criterion for trans-

w x w xformer insulation 25 . According to reference 25 at DPV

from 900 to 500, the strength of the paper is virtually con-stant but, in the range 500 to 200, it decreases in directproportion to DP . Comprehensive literature is nowV

available based on accelerated ageing of oilrpaper insula-tion. Most of these papers take the paper end of life crite-rion as 50% of its original tensile strength or 200�250 asits degree of polymerisation value. Then a thermal en-durance curve is plotted to predict the remaining life of

w xpaper insulation 28 .w xPahlavanpour et al. 29 presents some experimental in-

vestigation of the thermal ageing of Kraft paper and sug-gests that DP of paper starts decreasing at 120�C. TheV

decrease of DP is faster with increases of temperature,V

reaching the corresponding end of life DP at a tempera-V

ture of 180�C. McNutt describes the importance of insula-w xtion end of life criterion in his paper 30 . He pointed out

that different investigators tend to choose different end-w xpoint DP levels. Bozzini 31 suggested 100�150, LampeV

w x w xet al. 32 used 200, Fabre and Pichon 33 proposed100�200. The author also highlights that many in-servicetransformers have continued to operate with insulation

w xDP levels below 100. McNutt 30 suggested that his ownV

end point DP would be 200.Vw xEmsely et al. 34 showed that the reaction rate at any

time could be assumed to be proportional to the numberof unbroken chain bonds available and their analysis re-vealed an equation in the form

1 1y skt 1Ž .

DP DP®t ®0

where DP sDP at time t, DP s initial DP and ks®t ® ®0 V

Constant.ŽThey related the slope of the kinetic plot that is the

.reaction rate constant k to temperature by the ArrheniusŽ .relationship, plotting log k against reciprocal absolute

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temperature. They also showed that the slopes of the plotsand hence the activation energy of the reaction remainedconstant under a wide variety of experimental conditions.Substituting values of initial and final DP of 1000 andV

Ž .200 respectively and combining with equation 2 they havegiven the equation for transformer remaining life as in

Ž .equation 3 .E

yŽ .R T q273ks Ae 2Ž .

Where Ts temperature in Celsius, RsGas constants8.314 Jrmoler�K, and Esactivation energy, values of Adepend on operating conditions.

136000.004 Tq273

lifes e hours 3Ž .A

w xEmsely et al. 10 provided an excellent review on chem-ical indicators of degradation of cellulosic materials. Theyhave also highlighted advantages and disadvantages of lifeprediction from DP measurements. DP measurementsV V

are easy to conduct and can be easily empirically relatedto insulation condition. On the other hand the mecha-nisms and kinetics of the process are ill defined. The rateof degradation depends on the type of paper and also onits final chemical treatment. The rate of degradation in-creases discontinuously with increasing temperature above

w xabout 140�C. Heywood et al. 35 discussed in detail thefactors affecting the measurement of the average visco-metric DP . They highlight the errors arising due toV

preparation of solutions, measurement of viscosity andcalculation of DP , due to oxidative degradation of theV

solution, inconsistencies in the measurement temperatureand the conversion of intrinsic viscosity of DP using theV

w xMark Houwink Sakurada relationship 35 .w xMoser et al. 36 reported that with an increase of 0.5%

water content in an ageing transformer the DP value ofV

the paper would be halved. Emsley et al. reported a rela-w xtionship 37 between the DP and tensile strength of theV

paper and it is shown that to a first approximation tensilestrength is directly proportional to reciprocal of DP . HillV

w xet al. 38 shows the nonlinear relationship between thetensile strength and the degree of polymerisation DPwŽ .weight average molecular weight as measured by GPCmeasurement. The relationship has been found to be sig-moidal. The tensile strength of the paper decreases slowlywith decreasing DP until a critical average DP of aboutw w

500 is reached. It is also reported that the tensile strengthdepends on the final molecular properties of the cellulosein the paper and is independent of the degradation tem-perature. Some Australian utilities take paper samplesfrom bushing leads and find a good correlation with DPV

at hot spot positions. Some are introducing dummy sam-w xples to be removed for testing at strategic ages 39 .

Ž .Molecular weight MW studies by gel permeation chro-Ž .matography GPC have been found to be more useful

Figure 3. Molecular weight distribution of a new and 25 year oldw xtransformer paper 4 .

when dealing with Kraft paper, particularly in cases wherethe molecular weight distribution of the paper changessignificantly as the degradation proceeds. GPC providesthe most convenient way to obtain a detailed molecularweight distribution for the polymer. As GPC gives the en-tire distribution for the polymer, any small change inmolecular weight during the ageing process of an insula-tion material is easily observable through the chro-matogram. GPC measurement provides information aboutthe change in molecular weight and molecular weight dis-tribution. Prior to molecular weight measurement, the cel-

Žlulose in the paper samples which is the major compo-nent and that responsible for providing the mechanical

.strength of the paper is converted to the cellulose tricar-banilate derivative using the method described previously

w xby Hill et al. 4 . After purification of the cellulose tricar-banilate, it is subjected to molecular weight analysis by gelpermeation chromatography at room temperature usingtetrahydrofuran as the eluent. A typical GPC chro-matogram with molecular weight distributions for a newpaper and a 25 years old transformer paper is shown inFigure 3. Figure 3 shows that the higher the elution vol-ume the lower is the molecular weight.

As the paper is 25 years old, the peak molecular weighthas been significantly reduced and the shape also changessignificantly. The chromatogram of the new cellulose insu-lation paper shows the presence of two components. Onecomponent at lower elution volume or high molecularweight is due to cellulose, while the smaller, lower molec-ular weight component is due to hemicellulose. In thechromatogram of the cellulose insulation paper taken froman aged transformer, the molecular weight of the cellulosecomponent has decreased significantly. The molecularweight distribution of the cellulose has also broadened

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considerably and the peak of hemicellulose has becomealmost indiscernible, suggesting that the hemicellulosecomponent of the paper may have been largely degraded.GPC measurement has been performed to investigate oxi-dation and thermal ageing in controlled accelerated age-ing conditions and has been found to be very sensitive to

w x w xageing 38 . Ali et al. 40 also point out that DP givesV

no more than an approximate estimate of the averagechain length of the cellulose and its variation during age-ing is a very coarse measure of the change occurring. Ac-

w xcording to Ali et al. 40 GPC has the potential to give afar more detailed analysis of molecular weight distributionchanges during cellulose ageing. GPC could provide thebasis of more sophisticated degradation models becausemore complex situations can be accommodated.

2.4 FURAN ANALYSIS BY HIGHPERFORMANCE LIQUID

CHROMATOGRAPHYFurans are major degradation products of cellulose in-

sulation paper and are found in the insulation oils of op-erational transformers. Furan analysis offers a more con-venient method of analysis than direct measurement on

w x w xinsulation paper 41 . Shroff and Stannett 25 reported onthe formation of 2-furfuraldehyde from the degradationof cellulosic insulation papers in accelerated ageing exper-iments and found that an approximately logarithmic rela-tionship existed between the concentration of 2-furfural-dehyde in the oil and the degree of polymerisation of the

w xcellulose in the paper. Burton et al. 42 made an exten-sive study and measured the rate of furan formation ofseveral furan products over a wide range of temperaturesŽ .120�350�C . They also found an approximately logarith-mic relationship between the concentration of 2-furfural-dehyde and the DP of the paper. Unsworth and MitchellVw x2 used HPLC technique to monitor the formation of fu-ran components during ageing of cellulose insulation pa-per at 20, 80 and 110�C. They correlated their results forthe tensile strength of the paper with the concentration of2-furfuraldehyde and observed that the decrease in thetensile strength of the paper corresponded to an increase

w xin the concentration of the furans in the oil. Hill et al. 41developed a kinetic model and found that the concentra-tion of furans in the transformer oil should increase in aparabolic form with degradation time as shown in equa-

Ž .tion 4

F s A N tq Akt2 r2sbtqct 2 4Ž . Ž .Ž .t c 0

where N is the number of cellulose chains present perc

kilogram of paper at time t, A is proportionality constantŽ .and k is the rate constant. Also bs A N and csc 0

Ž . w xAk r2. The authors 41 determined the values of crbexperimentally and compared those predicted from the

Ž .model using the values of k and N obtained from ac 0w xmolecular weight study 4 . They also reported that 5-hy-

Ž .droxymethyl-2-furfuraldehyde HMF and 2-furfural-Ž .dehyde F are present in the oil at significantly greater

concentrations than any other furan components. Fur-furaldehyde is the furan present in greatest amounts. Thisstudy also found that the rate of HMF formation is fasterwith an increase in temperature than the rate of F forma-tion. Emsley et al. reported that production of 2-furfural-dehyde always outweighed that of the other three furansand the main production occurred when the DP droppedV

w xbelow about 400 3 . They also reported that both waterand oxygen increase the rate of formation of ageing prod-ucts, but water is more effective. The most significant risein furfural level occurs below DP 400 as it approachedV

the critical value of 200 where the paper loses all mechan-w xical strength and becomes susceptible to damage 43 .

Kraft paper produces more furans than cotton probablydue to the presence of hemicellulose in the wood based

w xKraft paper 43 . They also found logarithmic correlationof the concentration of 2-furfuraldehyde to the DP ofV

the paper in the temperature range of 120�160�C.w xPahlavanpour et al. 39 report that furan levels are

generally less than 0.1 ppm and these levels may be main-tained throughout a transformer’s life. However in manyolder units levels of up to 1 ppm and in some, up to 10

w xppm are found 39 . The technique for furan testing is de-w xscribed in IEC 44 , but there is no guideline for interpre-

tations. Blue et al. reported a new solid state materialwhich has been specifically designed for furfural detectionand has been successful in detecting small concentrations

w xless than 1 ppm of furan within transformer oil 45 . Thenw xBlue et al. 46 reported the construction of a novel elec-

trotechnic sensor for the determination of furan concen-trations as low as 0.1 ppm.

Chendong et al. found a linear relationship between thefurfural concentration in logarithmic scale and the degree

w xof polymerisation 47 . This was represented by a formula:Ž . Ž .log F s1.51�0.0035 DP . Where F is the furfuralde-V

hyde concentration in mgrL. De Pablo et al. published aw xcomprehensive report on furanic compound analysis 48 .

They reported a similar log-linear relationship between thefurans and DP . They have reported five different rela-V

tionships based on experiments conducted at differentconditions. They have also reported that from more than5000 European transformers a significant number had fur-fural concentrations higher than 1 mgrkg of 2-furfural-dehyde in oil.

w xDe Pablo 49 reported a relation between furfural andŽ .DP as shown in equation 5V

7100DP s 5Ž .V 8.88qFFA

where FFA is the furan concentration expressed as mgrkgw xof oil. Pahlavanpour et al. 50 modified the formula based

on transformer conditions and an assumption that 20% ofwinding paper and the inner paper layer degrade twice as

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fast as the rest of the paper. The revised formula is givenŽ .in equation 6

800DP s 6Ž .V 0.186� FFA q1Ž .

w x ŽDakin’s 51 chemical reaction rate equation K s0BrT .Ae is the most widely used equation for measuring

thermal degradation of transformer insulation. Where K0

is the reaction rate constant and A, B are empirical con-stants and T is the temperature in degrees Kelvin. Mc-

w xNutt 30 published a list of slope values obtained fromdifferent measurements and based on different end of life

w xcriterion. McNutt et al. 52 described the thermal lifeevaluation of a celluloseroil system, a Nomexroil systemand a hybrid Nomexrcelluloseroil system. Nomex is ameta-aramid paperboard used for higher temperature in-sulation systems. Life data for an oilrNomex insulation

Žsystem operating at 240�C conductor temperature 130�C.bulk oil temperature demonstrated a life in excess of 5000

hours. Actual retained tensile strength of the conductorwrap at 5000 hours was 77% of the initial value.

2.5 NEW CHEMICAL TECHNIQUESw xSaha et al. 5,53 reported that during ageing, the sur-

face of the paper which is in direct contact with the trans-former oil undergoes a colour change and becomes darker.This is particularly evident in the case of the samples agedin air. In order to identify the nature of the changes whichare responsible for the change in colour, the paper sam-ples were subjected to analysis by X-ray Photo-electronSpectroscopy, XPS. An analysis of the oxygen and carbonpeaks indicates that there are significant concentrationsof doubly bonded carbons and oxygen present in the sur-face of the air aged samples, which probably arise fromoxidation products from the oil being attracted to the po-lar cellulosic surface of the paper. The presence of thehydrocarbon on the surface of the paper even after exten-sive washing of the paper with the solvent suggests thatthe hydrocarbon degradation products from the oil arechemically bound to the surface. If this is so, then theyare also unlikely to be removed by treatment with hot

Ž .kerosene used in the vapour cleaning technique .

During accelerated ageing in an air environment the oilobviously became oxidised and turned brown in colour.However, during the corresponding studies in a nitrogenenvironment there was no obvious change in the colour ofthe oil. To monitor any changes, the absorbance of the oilwas examined by UV-visible spectroscopy for the samples

w xaged at 145�C 54 . The absorption spectrum is charac-terised by a strong absorption peak in the region below400 nm, with a long absorption tail extending into the visi-ble region which is responsible for the brown colour. Thespectrum is typical of that observed for species containinga sequence of conjugated double bonds or aromaticgroups. There was no evidence of the formation of signif-

icant concentrations of the absorbing species for the sam-ples aged under nitrogen. A sample of oil aged in air inthe presence of atmospheric moisture, but in the absenceof paper or copper also turned brown, showing that thepaper and the copper are not responsible for the forma-tion of the absorbing species and that it arises solely fromthe oxidation of the oil.

w x Ž .Ali et al. 6 reports Fourier Transform Infrared FTIRŽ .and Near-Infrared NIR spectroscopy to characterise the

ageing of cellulose paper. Differences in the spectra ofnew and aged materials were obtained and this providedthe basis of a ‘‘fingerprint’’ method to categorise papersinto different families. Palmer et al. studied two kinds ofspectral responses by Infrared spectroscopy and UV visi-ble spectrophotometry to monitor the variation of ab-

w xsorbance 55 . Based on the spectral absorption character-istic, a novel methodology for on-line transformer healthassessment was developed.

3 ELECTRICAL DIAGNOSTICMETHODS

It is often assumed that when the cellulose insulationŽages in a transformer the dielectric properties dissipation

factor at power frequency and breakdown strengths at both.power frequency and lightning impulse do not change

drastically. Based on this general assumption, very littlesystematic research had been undertaken in the area ofelectrical diagnostic techniques for condition monitoringor for studying the degradation of electrical insulation inpower transformers until 1990. Although insulation resis-

Žtance and polarisation index ratio of 10 minute insulation.resistance to 1 minute insulation resistance have been

used by the electricity utilities for a long time to ascertainthe transformer moisture condition. Partial discharge isan electrical phenomenon that occurs inside a trans-former and the magnitude of such discharges can causeprogressive deterioration and sometime may lead to insu-lation failure. There are vast numbers of papers availableon PD processes, PD patterns and fault mechanisms andare beyond the scope of this paper. A number of re-searchers have worked on the measurement of dielectricstrength of pressboard and paper with different wave

Žshapes power frequency or lightning impulse or switching.impulse or combinations of these . However, in recent

years new diagnostic methods have been promoted com-plementary to the classical insulation resistance, powerfrequency dissipation factor and polarisation index mea-surements. These methods will be presented here with ap-plications.

3.1 TIME DOMAIN POLARISATION[ ]METHODS 11

When a dielectric material is charged with an electricfield the material becomes polarised. The current densityŽ .j t is the summation of the displacement current and the

conduction current. If we expose the insulation to a step

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voltage at time ts0 the charging current density is givenŽ .by equation 7 .

j sE � q� f t 7Ž . Ž .Ž .polar i sat ion o

Ž .where the response function f t describes the fundamen-tal memory property of any dielectric system and can pro-vide significant information about the insulation material.If we consider the case where an insulation system with

Žgeometric capacitance C the geometric capacitance iso

simply the capacitance of the transformer containing no.insulation, just air or vacuum is exposed to a step voltage,

Ž .U , the charging current can be given by equation 80

�i sC U q f t 8Ž . Ž .polar i sat ion o 0 ž /�o

where � is the dc conductivity and � and � are respec-o r

tively the permittivity of vacuum and relative permittivityof the dielectric material. If the step voltage is now dis-

Ž .connected from the insulation, equation 9 gives the dis-charging or depolarisation current.

i syC U f t y f tq t 9Ž . Ž .� 4Ž .de polar i sat ion o 0 ch ar g in g

When a direct voltage is applied to a dielectric for along period of time and then short circuited for a shortperiod, after opening the short circuit the charge boundedby the polarisation will turn into free charges i.e., a volt-age will build up between the electrodes on the dielectric.This phenomenon is called the return voltage. This isshown in Figure 4. We assume that the insulation systemwas initially discharged and that a constant voltage U is0

applied. The current density under this open circuit isŽ .given by equation 10

dD dj t s� E t q s� E t q � � E t q� P tŽ . Ž . Ž . Ž . Ž .Ž .o rdt dt

d d tj t s� E t q� � E t q� f ty� E � d�Ž . Ž . Ž . Ž . Ž .Ho r o ½ 5dt dt 0

10Ž .

During return voltage measurement the circuit beingŽ . Ž .without any supply, j t s0 and equation 10 can be

solved for return voltage. The return voltage can be calcu-

w xFigure 4. Return voltage phenomena 82 .

w xFigure 5. Equivalent circuit of transformer insulation 67 .

lated if the response function, permittivity and conductiv-ity are known. A number of thoughts are currently inpractice to explain the polarisation results. Many of theseare explained with case studies.

w xBognar et al. 56,57 reported on the return voltagemethod and suggested that the insulation structure can berepresented by an equivalent circuit as shown in Figure 5,where C is the geometric capacitance, R is the insulationo

resistance and R -C are substitutes for the individualpn pn

polarisation processes having time constants of t spn

R C . Figure 5 has been used to represent the extendedpn pn

Debye model, which explains relaxation behaviour ofw xoilrpaper insulation. Bognar et al. 56,57 suggest that the

initial slope of the return voltage is directly proportionalto the polarisation conductivity and the maximum value ofthe return voltage is proportional to the intensity of polar-isation phenomena and the absolute value of the returnvoltage maximum value is also influenced by the actual

w xvalue of the insulation resistance. Osvath et al. 58 de-scribe the difference between transformers of widely dif-fering ages and indicate how the return voltage measure-ment could be used for this. The smaller the central time

Ž .constant the time to reach the return voltage peak value ;the worse is the condition of the transformer. This is illus-

w xtrated in Figure 6 58 .w xBognar et al. in the paper 59 compares various meth-

ods for the diagnosis of oilrpaper insulation. Loss factor

w xFigure 6. RV spectra for different ages of transformer 58 .

1070-9878rrrrr1rrrrr$17.00 � 2003 IEEE910


is found to always change due to the consequence of po-larisation and conductive effects. Their findings suggest

Žthat the central time constant charging time to the peak.of the return voltage is fully independent of the conduc-

tivity and of the geometrical capacitance of the trans-former insulation, which totally contradicts the suggestion

w xmade by Gafvert et al. 7 . Temperature inside the trans-former tank is found to be a sensitive parameter for thereturn voltage measurement. At rising temperatures, theRV curve is found to peak at increasingly shorter times.

Gafvert et al. discuss the polarisation process in detailw x w xin reference 8 . Gafvert and Ildstad 60 present mod-

elling of return voltage based on a series combination ofoil duct and paperrpressboard dielectric materials. In theirmodelling work each material is characterised by its con-ductivity and permittivity along with composite dielectricresponse function. They verified their model with a sim-plified structural model of transformer insulation. Theirfindings suggest that the initial rise of the return voltageŽ .i.e. initial slope is determined by relaxation of interfacecharge through the oil gap. They point out that in the caseof short charging and discharging times during the returnvoltage measurement, surface charges completely domi-nate the magnitude of the return voltage. Ildstad et al. in

w xtheir reference 61 , also present a mathematical simula-tion tool to describe the fundamental dielectric processesof conduction and polarisation. They also explain the rela-tion between time domain and frequency domain polari-sation measurements.

w xGafvert et al. in reference 62 describe a comprehen-sive mathematical tool to describe the moisture and age-ing influence on the dielectric properties of transformerinsulation. The authors use the geometrical design of atransformer to analyse the polarisation response of thetransformers insulation. By knowing the appropriate ge-ometry, the composite dielectric constant of oilrpaper in-sulation is calculated based on oil and paper insulation inseries connection. Their findings suggest that the initial

w xFigure 7. Effect of oil conductivity on polarisation current 63 .

current during the polarisation current measurement isproportional to the oil conductivity and the oil conductiv-

w xity can be calculated from this current 63 . The impact ofoil conductivity on polarisation current is demonstrated inFigure 7. The conductivity of the paperboard can be esti-mated from the stationary dc current of the polarisationcurrent. This stationary current was found to be a goodindicator of the moisture content of the paperboard insu-

w xlation as shown in Figure 8 63 .w xIn 62 Gafvert et al. recommend polarisation current

measurement as the preferred time-domain diagnosticmethod since the properties of oil and paper can be sepa-rately assessed from the experimental results. The authorsexplain that return voltage measurement results are con-voluted by two constituents and it is difficult to separate

w xthe oil and paper impacts. Gafvert et al. in reference 7emphasise that return voltage curves are strongly influ-enced by the oil conductivity. The authors present differ-ent return voltage spectrum for three different oil conduc-

w xtivities 7 . The authors suggest that the oil conductivityŽ .not only the moisture content in paper insulation stronglyaffects the central time constant. The authors also showthe influence of paper moisture level on return voltage

w xpolarisation spectra 7 . The authors here highlight theconsistency between polarisationrdepolarisation current

w xmeasurements and recovery voltage measurements 62 .w xCsepes et al. 64 show that the RV method is insensi-

tive to the effects of local reduction in insulation resis-tance. A large change in capacitance changes the absolutevalue of the return voltage while the shape of the curveremained the same. This demonstrates that the character-istic shape of the RV polarisation spectrum is indepen-dent of both shunt resistance and shunt capacitance. Theyrecommend that with a little experience in its evaluations,RVM is also useful for evaluating the uniformity of ageingandror moisture distribution. The RV method is claimedto be insensitive to the choice of test voltage, to externalAC fields and to shunt capacity or resistance, as long asthe shunt values are not changing during the test.

Ž .Figure 8. Effect of paper moisture content paper conductivity onw xpolarisation current 63 .

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Figure 9. Calibration of RV spectra for different moisture levelsw x64 .

w xCsepes et al. 64 explain the practical foundation of theRVM method. Based on comprehensive experiments the

Žauthors suggest that the return voltage maxima expressed.as relative values normalised to charge voltage appear at

lower charge times with increasing moisture content. Theirtesting results also confirm that these maxima move to-ward lower charge times with increasing moisture content

w xas shown in Figure 9. Contrary to Gafvert et al. 7 theauthors claim that the dominant time constant or centraltime constant is not affected by the oil quality, althoughthe presence of multiple polarisation processes can be seenin the slight elevation in the left side of the curve. Domi-nant time constant is found to decrease with increasing

w xageing 65 , just as it is found with increasing moisturecontent. The authors define ‘‘equivalent moisture content’’to explain the effect of both water and ageing products onthe RV spectra. The authors model the RV measurementwith the circuit shown in Figure 5. They suggest that theRV method is also useful for checking the effectiveness ofa drying treatment.

w xUrbani et al. 66 emphasise the significance of RVMfor standard and nonstandard waveshape measurements.When return voltage spectra have unambiguous maxima,a reasonably accurate moisture estimate can be achievedfrom this measurement. On the other hand, for multiplelocal peaks, flat curves and curves with discontinuities theyrecommend that results from other tests and service his-tory can be useful for correct interpretation of results.

w xYu et al. 67 present an extended Debye model to de-scribe the return voltage mechanism. They model the

Žtransformer with eight relaxation time constants as an ex-.tension to that shown in Figure 5 and show a good fit to

w xexplain the return voltage mechanism. Jota et al. 68 pro-pose a mathematical model of the polarisation spectrum.It is shown that from their derived model, some of thehidden RVM peaks could be identified. Using this pro-posed method the parameters of the equivalent circuitcould be determined in tests made in different stages of atransformer’s life. The evolution of the parameter valuescan be correlated with the estimated ones to improve the

knowledge about the effect of moisture ingress and ageingon the insulation time constants. The authors validate theirmodels using RVM tests on a 330r11 kV transformer. It isobserved that capacitance and insulation resistance changedue to thermal ageing and moisture ingress. This methodcan also help to monitor ageing and moisture ingress, oncethe models are formed from the measurements.

w xDer Hohuanesian et al. 69 show that consecutive mea-surements of the polarisation currents of an oilrpaper in-sulation system can be used to quantify its dielectric re-sponse, especially in the low frequency domain. The au-thors suggest that polarisation spectra as quantified by RVmeasurements should not be used to quantify the mois-ture content of the pressboard within a power transformerof unknown construction, as the dielectric response quan-tities of the board are only a portion of the overall re-

w xsponse. Alff et al. 70 find that the relaxation or polarisa-tionrdepolarisation currents are directly proportional tofundamental dielectric quantities, i.e. dielectric responsefunction and dc conductivity. A computer program is de-veloped to calculate polarisation index, return voltagespectrum and complex capacitance using the RC circuit asshown in Figure 5. Finally the authors recommend thatmeasurement and evaluation of polarisation current is verysimple and efficient to quantify the quality of insulationsystems.

w xSaha et al. 71�74 extensively used the return voltagemethod for analysing the condition of aged cellulose insu-lation. Among RV parameters, central time constant isfound to be the most sensitive to ageing. In a recent re-search project, accelerated ageing experiments were per-formed under air and nitrogen environments over thetemperature range 115�145�C. The results are described

w xin the reference 5 . The RV parameters are found to varyat a much slower rate during the ageing of samples in thepresence of nitrogen compared to samples aged under airw x75 .

When paper insulation ages at elevated temperatures,the cellulose chains undergo scission and the chain scis-sion is monitored by the paper molecular weight measure-

w xment using gel permeation chromatography 4 . Themolecular weight is found to drop significantly due tocleavage of the chain between glucose rings in the poly-mer. Correlation between molecular weight measure-ments and RV parameters shows a good correlation forair-aged samples. The findings from nitrogen-aged sam-ples suggest a much lower rate of degradation both fromthe RV and GPC measurements. A number of power anddistribution transformers of different ages were investi-gated with the return voltage measurement technique.Findings suggest that the polarisation parameters mea-

Ž .sured by return voltage RV vary significantly and consis-w xtently with the ageing condition of insulation systems 74 .

The authors also find that geometry of the insulation hasa strong impact on RV parameters. Overall, the author of

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this paper believes that it is very important to consider oilquality and geometry of the insulation structure to accu-rately interpret the dielectric diagnostic results.

The present author is now working on the separation ofaging and moisture by using polarisationrdepolarisationcurrents, decay voltage and RV measurements. Prelimi-nary work has been published in the CIGRE 2002 Paris

w xSession 76 . The authors have investigated a number ofaccelerated ageing experiments at 115�C with known dis-crete moisture levels. The aged samples were then anal-ysed by return voltage, polarisationrdepolarisation cur-rent and molecular weight measurements. Our findingssuggest that for the unaged 5% paper moisture sample,the central time constant is located in the smaller timerange; on the other hand 2% and 3% paper moisture levelunaged sample’s central time constants are located in thelarger time range. It was very interesting to see that therewas little difference in the results for the 2% and 3% pa-per moisture samples.

The amplitude of long term dc polarisation current isvery sensitive to the moisture content in paper insulation.This demonstrates that polarisation current measurementcan be used for assessing the paper insulation moisture

w xlevel. In 76 an attempt has been made to separate theageing and moisture effects on the polarisation measure-ments. The polarisation current has been found to be verysensitive to moisture. The oil and paper conductivitieshave both been found to be sensitive to moisture and age-ing. Similarly the central time constant in RV measure-ment has been found to be sensitive to moisture and age-ing as well. When the higher moisture is produced fromageing, it is understood that the moisture and ageing be-come inseparable. However, the comparison of unagedand aged samples produced some interesting results. Fromthe molecular weight measurements both weight-averageand number-average molecular weights show small reduc-tions for 2% moisture aged samples, while there is a sig-nificant reduction in molecular weight for the 3% mois-ture aged samples.

3.2 FREQUENCY DOMAINPOLARISATION MEASUREMENT

In frequency domain polarisation measurement, dissi-pation factor or tan� is measured as a function of thefrequency of the test voltage. In frequency domain the di-

Ž .electric response function f t is the dielectric susceptibil-ity. The susceptibility and response function are related

Ž .through the Fourier transform given by equation 11w x8,60 . The susceptibility is a complex function of fre-quency and is related to the relative permittivity given by

Ž .equation 12 .

� � yj tX s X y iX s f t e dt 11Ž . Ž . Ž . Ž . Ž .Hs s s0

The sinusoidal voltage is applied to the test object andthe total current through the test object is measured. It is

Ž . Ž .seen from 12 that the conductivity � , the high fre-quency component of the relative permittivity � and the

Ž .electric susceptibility X characterise the behaviour ofthe dielectric material under time harmonic excitation. Ifthe dielectric material is linear, homogeneous andisotropic, the measured information in time or frequencydomain is the same. The frequency dependent ratio ofimaginary and real parts of the complex permittivity is

Ž .called the dissipation factor tan� and this is indepen-dent of the geometry. This makes dissipation factor one ofthe important parameters to study when the geometry ofthe test object is not known.

�� s� y i� s� q X y iX y iŽ . Ž . Ž . Ž . Ž .r r r s s � 0

� � s� q X � Ž . Ž .r s

�� s X qŽ . Ž .r s � 0

�� Ž .rtan�s 12Ž .�� Ž .r

w xGafvert et al. 7 presented dielectric spectroscopy intime and frequency domain for a number of power trans-formers from a nuclear power station. The authors usedlow frequency dielectric spectroscopy, polarisationrde-polarisation current and return voltage spectra measure-ments. Significant differences in RV results are detectedby change in oil quality and moisture content of solid in-sulation. The authors while comparing the time-domainversus frequency-domain diagnostic methods suggest that

Žlow frequency dielectric spectroscopy measurement ofcapacitance and tan� in the frequency range 0.1 mHz to 1

.kHz is the best measurement method for fieldwork. Theirfindings suggest that the RVM method is useful but moresensitive to systematic errors than the other two methods.RVM is a high impedance input method, and hence leak-age current on the bushings can easily corrupt the mea-

w xsurement. Boluis et al. 77 reported that tan� describesthe spectrum of energy absorption by the dielectric. Anageing of the cellulose insulation has an influence on itspolarisation properties, which can be detected by an ex-amination of tan � versus frequency characteristics.

w xHelgeson et al. 78 reported that in the lower frequencyrange, low frequency dispersion can sometimes be mis-taken for conductivity if not measured at sufficiently lowfrequencies. They reported that frequency domain mea-surement is a narrow band measurement and this makes itfairly easy to filter away noise both with analog and digital

w xfilters. Kuschel et al. 79 described the coherency be-tween frequency and time domain diagnostics. The au-thors suggested that for known geometrical information oftransformer insulation structure and assuming a linearsystem good conformity can be achieved between the twotechniques. The application of dc stress in time-domainmeasurement does not produce any partial discharge and

1070-9878rrrrr1rrrrr$17.00 � 2003 IEEE 913


was reported to be not dangerous for oil-paper insulation.On the other hand partial discharge can occur in fre-quency domain measurements at high voltage level.

w xEkanayake et al. 80 reported a study of 105 transformerswith frequency domain dielectric spectroscopy measure-ments. Their findings suggest a good agreement betweenfrequency domain dielectric spectroscopy measurementswith other chemical and electrical analyses.

CIGRE Task Force 15.01.09 recently published an im-w xportant report 81 on dielectric response methods for di-

agnostics of power transformers. They highlight the im-portance and problems in the interpretation of resultsfrom return voltage, polarisationrdepolarisation currentand frequency domain spectroscopy measurements. Theysuggest that all the dielectric response methods reflect thesame fundamental polarisation phenomena in transformerinsulation, the special feature of which is a combination ofoil gaps and solid paperrpressboard insulation. They alsosuggest that the influence of oil gap, i.e. the condition of

Ž .the oil in particular oil conductivity has a significant im-pact on dielectric response and this must be taken intoconsideration while attempting to interpret the moisturecontent of solid insulation. Geometry of the insulationstructure also has an influence on the response but not assignificant as the effect of oil conductivity.

4 CONCLUSIONS ANDDISCUSSIONS

N attempt has been made in this paper to reviewAmodern chemical and electrical diagnostic methods.Firstly traditional chemical methods have been discussedwith currently available interpretation schemes. Amongchemical methods DGA is the most widely used methodfor investigating incipient faults. A number of interpreta-tion techniques are available to analyse fault types. IEEEand IEC have appropriate Standards for the interpreta-tion schemes. The next widely used method for analysingcellulose ageing is furan measurements by the HPLCtechnique. Although no standard is available for the inter-pretation of ageing phenomena, some good literature isavailable on the technique. DP measurement has beenwidely used for monitoring cellulose mechanical strength.The cellulose ageing phenomena and its relation to DPare reasonably well understood. Most of the currently usedtechniques have some drawbacks as well. When trans-former oil is replaced or refurbished, the analysis of gasesand furans in the refurbished oil may not show any traceof degradation although the cellulose may have degradedsignificantly. DP estimation is not possible without collect-ing cellulose samples from the operating transformers. Anumber of new chemical diagnostic techniques are alsopresented in this paper and their usefulness is high-lighted.

Secondly a critical analysis has also been made on themethods of interpretation of polarisation measurements.

A group of papers suggest that RV parameters, in particu-lar central time constant is independent of transformergeometry and is little affected by oil quality. Another groupof researchers explained the RV measurements with thehelp of dielectric theory and polarisation principles. Theirfindings suggest that RV parameters are strongly influ-enced by the geometry of transformer insulation and qual-ity of both paper and oil. They have thoroughly investi-gated the impacts of oil and paper quality separately andput forward a modelling approach to understand thiscomplex behaviour. A number of researchers also pro-posed polarisationrdepolarisation current measurement asthe superior tool because this can monitor oil and paperconditions separately. A third group used frequency do-main polarisation measurements and their findings sug-gest that time domain and frequency domain measure-ments predict the same condition of insulation. We haveinvestigated the impacts of oxidation and thermal ageingphenomena thoroughly by polarisation measurements. Ourexperience suggests that separation of ageing and mois-ture impacts on the RV parameters is the most importantproblem, which is yet to be solved. A number of attemptshave been made by the author of this paper to solve thisproblem and this research is still ongoing.

No single method can be considered as the best diag-nostic method. DGA and furan analysis will remain mostuseful method for practical applications. However, ad-vancement in measurement technologies and data analysistechniques with better computation methods makes thepolarisation based electrical diagnostics more attractive.Polarisation methods are now better understood. It is nowbelieved that if polarisation measurements are performedcorrectly and proper interpretation scheme is used thesenew methods could be very useful for diagnosis purpose.

ACKNOWLEDGMENTSThe author would like to take this opportunity to thank

Emeritus Professor Mat Darveniza, Mr. Zheng Tong Yao,ArProf. David Hill, Dr. Tri Le, Mr. Brian Williams, Dr.Abbas Zargari, Dr. Prithwiraj Purkait, and other staffmembers involved in insulation ageing research projectsduring the last thirteen years at the University of Queens-land for their direct and indirect contributions.

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1070-9878rrrrr1rrrrr$17.00 � 2003 IEEE 915


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