electric insulation magazine 01203017

Dielectric Response Methods forDiagnostics of PowerTransformers

Key Words: Oil-paper insulation, moisture content, recovery voltage, polarization/depolarization cur-rents, dielectric spectroscopy, pancake model, insulation geometry, oil conductivity.

The dryness and aging state of oil-paper insulationis a key factor in both the short- and long-term re-liability of a power transformer, since moisture

has deleterious effects on dielectric integrity and insula-tion aging rates. Today, the water content of the cellulosein an in-service transformer is determined indirectly viamoisture measured from oil samples, according to IEC60422. Moisture distributes unequally between the oiland the pressboard, the greater part residing within thesolid insulation. Because the water concentration in theoil is highly temperature dependent, the measurement ofmoisture in oil is not a particularly reliable indicator ofdryness in the cellulose, particularly for lightly loadedtransformers. Although in principle moisture can also beextracted from pressboard samples and measured, thisapproach is hampered by the impossibility of obtainingsamples from critical locations. Furthermore, a signifi-cant variation in results can arise from different practicesregarding sample storage and preparation.

Most recent attention has been directed to methods ofdetermining moisture content and aging of thepressboard and paper more directly, by measuring the ef-fects of moisture on electrical properties. Rather than thetraditional measurement of power frequency loss angle,recent attention has focused on measuring various dielec-tric response parameters, which characterize someknown polarization phenomena. The three foremosttechniques are:

� Recovery or return voltage measurements (RVM),� Dielectric spectroscopy in time domain, i.e., measure-

ments of polarization and depolarization currents(PDC),

� Dielectric frequency domain spectroscopy (FDS), i.e.,measurements of electric capacitance C and loss factor(tgδ) in dependency of frequency.

Until recently, the RVM technique has been, by far, themost frequently used method. However, there has beenmuch controversy surrounding the technique [1], [2],that has been criticized on various grounds:

� Moisture determinations, as derived by the evaluationmethod used [3], [4] are often much higher than ob-tained by other methods,

� Therecommended interpretation scheme is too simplis-tic, and

� The technique does not take into account dependencieson geometry and oil properties.

On the other hand, there had not been much knowl-edge available on how to interpret the results of the PDCand FDS measurements. The work in this direction hasbeen in progress for only a few years, and more advancedtools are available today [5], [6]. The possibilities of mod-eling the transformer insulation properties, as well as ofcomparing the data obtained by means of the three tech-niques, have been developed.

As a result, CIGRÉ Working Group 12.18.TF 4 re-viewed the opinions of users of polarization techniques

12 0883-7554/03/$17.00©2003IEEE IEEE Electrical Insulation Magazine

F E A T U R E A R T I C L E

CIGRÉ Task Force 15.01.09:S.M. Gubanski (chair), P. Boss, G. Csépes, V. DerHouhanessian, J. Filippini, P. Guuinic, U. Gäfvert, V.Karius, J. Lapworth, G. Urbani, P. Werelius, W. Zaengl

The dryness and aging state ofoil-paper insulation is a key factor inboth the short- and long-termreliability of a power transformer.

and concluded that, although such techniques showedpromise, more work was required to validate the tech-niques and improve the interpretation of results. Subse-quently, Task Force 15.01.09 was set up in 1999 tocompare the three main techniques. Initial work has con-centrated on measurements with all the techniques usedon an insulation model. The model was designed specifi-cally to investigate the influence of insulation geometry ondielectric responses. Comparisons using a mathematicalmodel of polarization behavior were also made. Further-more, the influences of the thermodynamic moisture equi-librium between paper and oil, as well as the oilconductivity, were tested with this model. In parallel, anumber of activities were undertaken in the U.K., Switzer-land, and Sweden, which included measurements with thedifferent techniques on selected power transformers un-der field conditions.

This report summarizes the work performed by TF15.01.09, and presents conclusions regarding the state ofthe knowledge on the applicability of the techniques.

Dielectric Behavior ofOil Impregnated System

Under the assumption that the dielectric material islinear, homogeneous, and isotropic, the information ob-tained in either the time or frequency domain is the sameand results can be transformed from the time to fre-quency domain and vice versa.

In the time domain, the conductivity σdc, the instanta-neous (high-frequency) component of the relativepermittivity, ε∞, and the dielectric response function,ƒ(t), characterize the behavior of a dielectric material[7]. The response function ƒ(t) is the response to a deltapulse. From this, the response, or polarization, P, can becalculated for any excitation shape, E(t) through theconvolution, ( ) ( )P f t E t d= −

∞∫ τ τ0

. It is also worth not-ing that current measurements in the time domain candirectly lead to quantification, or at least estimation, ofσdc and ƒ(t).

The Fourier transform of the response function ƒ(t) al-lows the transformation to frequency domain, and yieldsthe complex susceptibility, ( ) ( ) ( ))χ ω χ ω χ ω= ′ − ′′i . Therelation between the susceptibility and the permittivity is

( ) ( )′ = + ′∞ε ω ε χ ω , ( ) ( )′′ = + ′′ε ω χ ωσε ω

dc0

, and loss fac-

tor ( ) ( )tgδ ε ω ε ω= ′′ ′/ . Therefore, in thefrequency domain, the dc conductivity σdc, thehigh-frequency component of the relativepermittivity ε∞, and the complex dielectric sus-ceptibility ( ))χ ω , characterize the dielectric ma-terial and, as for the time domain, it is possibleto find these parameters by measurements.

The oil-paper insulation system is a com-posite of two different dielectric media, wherean insulating liquid with ionic conduction ismixed with a lower conducting impregnatedsolid (pressboard or paper). It is important to

realize that each has its own dielectric response and,when putting them together, the total response will notonly reflect properties of each material, but also the waythey are combined. When these two media are put intocontact (forming interfaces), charge accumulation occursat the interfaces due to the differences between their elec-trical properties. This kind of polarization is called theMaxwell-Wagner or interfacial polarization (forexamples, see [8], [9]).

The dielectric response of mineral oil is quite simple andcan be characterized by its essentially frequency-independ-ent permittivity ( ))ε ω = =const 2 2. , and volume conduc-tivity σdc, whereas its dielectric response ƒ(t), can beneglected. Pressboard, on the other hand, is characterizedby a large dielectric response, ƒ(t). This response is stronglydependent on moisture content and aging products. If bothcomponents, i.e., oil and paper, are sandwiched, it is possi-ble to calculate the resultant behavior based on modelingprocedures. One can calculate the dielectric response for acomposite insulation if the geometry and the response func-tion for each material are known.

Modeling of Dielectric ResponseTo make a precise moisture estimation of the oil-paper

insulation in a transformer, one needs a library contain-ing data on dielectric properties ε∞, σdc, and ƒ(t), ofwell-characterized materials (oils and impregnatedpressboard) at different humidity content. This informa-tion is needed for calculating the dielectric response ofthe composite duct insulation, and for comparing it withresults of the measurements. It is important to note that,depending on the coupling of the transformer windings,different combinations of insulation may influence themeasurement.

In a core type transformer the main duct insulationusually consists of a number of cylindrical shells ofpressboard barriers, separated by axial spacers (see Figure1(a)).

For modeling purposes, it is sufficient to represent theinsulation structure by the relative amount of spacers andbarriers in the duct. We define a parameter, X (see Figure1(b)), as the ratio of the sum of all the thickness of the allthe barriers in the duct, lumped together, and divided bythe duct width. The spacer coverage, Y, is defined as the

May/June 2003 — Vol. 19, No. 3 13

Y

X

1-Y

1-XOil

Barrier

Spacer

(b)(a)

Figure 1. Section of an insulation duct of a power transformer with barriers andspacers (a) and schematic representation of the barrier content and the spacercoverage in the insulation duct (b).

total width of all the spacers divided by the total length ofthe periphery of the duct. In a transformer, the barrierswill typically fill 20 - 50 percent of the main duct, and thespacers will fill 15 - 25 percent of the circumference.

From the library of data on material properties and thegeometry of the composite system, the PDC, RVM, andFDS responses can be derived. In the time domain (PDCand RVM methods), the calculation is based on theknown response function ƒ(t) and its dependency ontemperature and humidity. In the frequency domain (FDSmethod), the composite dielectric permittivity, εduct, ofthe insulation duct is calculated as

( )ε ω

ε ε ε ε

,TY

X X Xduct

spacer barrier oil barrie

1=− +

+− +1 1X

- Y

r (1)

where the permittivities of the oil, spacers, and barriersare also complex quantities dependent on frequency,temperature, and humidity.

Measuring MethodsThe measurement of polarization and depolarization

currents (PDC) following a dc voltage step is one way inthe time domain to investigate the slow polarizationprocesses. Both polarization current, ip(t) as well as de-polarization current, id(t) contain information about thedielectric response function, ƒ(t), provided the test ob-ject is charged for a sufficiently long time. It is easier touse the depolarization current id(t) where there is no dccurrent present. Many solid dielectric materials have di-electric response functions that decrease slowly withtime. A rule of thumb says that the test object should becharged for at least 5 to 10 times as long as it is depolar-ized in order to get a depolarization current that is pro-portional to the dielectric response function. It is alsopossible to estimate the dc conductivity, σdc, of the testobject from the measurements of the polarization anddepolarization currents.

The recovery voltage method (RVM) is another timedomain method used to investigate slow polarizationprocesses. First, applying a step dc voltage for a specifiedtime period charges the object. During this period, thepolarization current flows through the test object. After-wards the test object is short-circuited (grounded) for acertain time (usually shorter than the charging time) andthe depolarization current flows. When the short-circuit-ing (grounding) period is finished, the recovery voltageUR(t) is measured under open-circuit conditions. Thesource of the recovery voltage is relaxation of the remain-ing polarization in the insulation system, giving rise to aninduced charge on the electrodes. It is intricate to calcu-late σdc, ε∞, and ƒ(t) from a given recovery voltage timedependence. Therefore, the so-called polarization spec-trum has been introduced. The polarization spectrum isestablished by performing a series of recovery voltage

measurements with stepwise increasing charging time tc

and short-circuit time tg, usually with the ratio tc/tg = 2.For each sequence the peak of the recovery voltage URmax

and the initial rate of rise of the recovery voltage dUR/dtare recorded and plotted versus the charging time used.

An equivalent method in the frequency domain to in-vestigate the polarization is to measure the responses fromsinusoidal excitations at different frequencies (FDS). Onthis basis, the complex relative permittivity ( ))ε ω at the fre-quency of the applied field, assuming a capacitive test ob-ject, can be found. It is important to note that theimaginary part of the complex relative permittivity ( )′′ε ω(loss part) contains both the resistive (dc conduction)losses and the dielectric (polarization) losses, and that at agiven frequency it is impossible to distinguish between thetwo. However, at very low frequencies the resistive partwill often dominate. In this case, the imaginary part ( )′′ε ωof the complex relative permittivity, has a slope of ω-1 andthe real part ( )′ε ω is constant. Based on this, the conduc-tivity σdc of the test object may be calculated. Another wayof presenting the measured information of a FDS is to usethe loss factor ( ) ( ) ( )tgδ ω ε ω ε ω= ′′ ′/ .

Today, all the three techniques are used for diagnosticsof the insulation of power transformers, either with com-mercial available instruments [3], [10], [11] or with“home-made” test set-ups. As an alternative to the sim-plistic and questionable interpretation of the polarizationspectrum still supplied and recommended by the manu-facturer of the commercial RVM instrument, some usershave instead introduced their own evaluation proce-dures. The manufacturers of the PDC and FDS instru-mentation offer today more advanced interpretationtools based on extended linear models.

Pancake ModelA so-called pancake model of transformer insulation

was developed for a systematic study of the factors thatinfluenced the dielectric properties of a composite oil-pa-per insulation. The model allowed a systematic study ofthe following factors: (i) test voltage level, (ii) insulationgeometry, and (iii) oil quality (conductivity). Figure 2shows the model design.

The pancake model was built using commonly usedpaper, Kraft Thermo 70, and Nynas Nytro 10GBN insu-lation oil. The model contained eight pancake-shapedcoils (A-H) with ducts containing different amount of oiland pressboard between them (from 85/15 to 0/100oil/pressboard ratio).

Comparative measurements with the different dielec-tric response methods were performed at three differentoccasions:

� Stage 1—shortly after delivery of the model in the sum-mer of 1999.

� Stage 2—after heating the model to 65 °C for twomonths (November/December 1999).

14 IEEE Electrical Insulation Magazine

� Stage 3—shortly after replacing the original oil (NynasNytro10GBN)witha lessconductingoil (ShellDialaD).

In parallel with the dielectric response measurements onthe model, dc conductivity and loss factor at 50 Hz of the oilused were measured simultaneously at CNRS in Grenoble,France and at Trench, based in Basel, Switzerland.

Some typical and instructive results from the PDC,RVM, and FDS measurements will now be presented.Figure 3 shows examples of the measured relaxation (po-larization and depolarization) currents for the geometri-cal arrangement of a duct containing 85/15 oil/pressboard ratio. The moisture content of the insulationstudied was estimated to be below 1 percent, using amodeling procedure similar to the one previously de-scribed. It can clearly be seen how exchanging the oil withone of lower conductivity reduced the polarization anddepolarization currents early in the measurement while,

for longer times dominated by the pressboard, the cur-rents remained unchanged. The results of RVM measure-ments on different geometrical arrangements and atdifferent times represented as the polarization spectraand the so-called Guuinic representation, in which theinitial slope is plotted against the peak recovered volt-ages, are shown in Figures 4(a) and 4(b). The interpreta-tion, as provided by the software of the commercial RVMinstrument [3], suggested a moisture content of about 2percent. Examples of the FSD results, as obtained in thefirst stage of the study, are shown in Figures 5(a) and 5(b).We see that without an oil duct there is no loss factorpeak. Based on these results, the moisture content in thepressboard was evaluated to be below 1 percent. Also,this evaluation was—similar to the PDC measurements—based on the model presented previously. The modelingalso allowed an estimation of the oil conductivity, whichagreed well with results of separately performedmeasurements according to IEC 61620.

Stage 2 was carried out in order to elucidate the originof the discrepancy in moisture evaluation between theRVM measurements and the other methods. The modelwas kept at 65 °C for two months prior to the measure-ments. The results indicated that the oil conductivity wasthe main cause of the experienced difference. To clarifythis further, the oil in the pancake model was exchangedwith new oil (Shell Diala D) with a much lowerconductivity and loss.

The oil change led to a significant modification of themeasured responses. This shows how important takingthe oil conductivity into account is when interpreting re-

May/June 2003 — Vol. 19, No. 3 15

10

Air

1

4

InsulatedConductors

3

5 89 7 6

≈ 1560 mm

Porcelain

CopperConductors

2

Figure 2. Cross-section of the pancake model. The marked elementsare: 1—tank, 2—oil, 3—bakelite plate, 4—bakelite rod, 5—cop-per plate, 6—pressboard, 7—pancake coil, 8—spacer, 9—copperband, 10—connection (bushing).

10–8

10–9

10–10

10–11

1 10 100 1000 104

Time [s]

Cur

rent

[A]

Ip Before Oil Change

Id Before Oil Change

Ip After Oil Change

Id After Oil Change

Figure 3. Polarization and depolarization currents (PDC) as measuredon the pancake model before and after a change of oil; measured geo-metrical configuration was 85/15 oil-pressboard ratio. Solid lines rep-resent the depolarization currents and dashed lines the polarizationcurrents.

sults from the dielectric response measurements onoil-paper insulation systems. A complete documentationof these investigations will be published separately at alater date.

To summarize the results, one may say that dielectricresponse measurements on oil-paper insulation are sensi-tive not only to the state of the paper, but also to the oilconductivity and the geometry of the test object. Asshown, a risk for errors in the interpretation of the RVMdata exists when these parameters are disregarded, result-ing in overestimates of the water content. This may be of

importance, especially when making measurements onnew or relatively new transformers in which the oil con-ductivity is unusually high. In a new transformer, usuallythere will not be a moisture balance between the oil andthe pressboard. The oil is comparatively wetter and will,with time, dry out due to moisture absorption in the cel-lulose. This results in a decrease in the conductivity of theoil. Furthermore, results from measurements on trans-formers with unconventional design (nontypicaloil/paper ratios in the insulation) should also be treatedwith great care.

Field MeasurementsIn parallel with the investigations on the pancake

model, measurements were also made on real transform-ers. At the National Grid Company (NGC) in the U.K.,the objective was to investigate the influence of oil condi-tion on the dielectric responses to find how this might in-terfere with any estimation of the water content of theinsulation [12]. RVM measurements made before and


103

102

101

100

85/15 Oil-Pressboard Ratio


Max

ofR

etur

nV

olta

geU

Rm

ax[v

]

10–2 10–1 100 101 102 103 104

Charging Time t [s](a)

102

101

100

10–1

85/15 Oil-Pressboard Ratio0/100 Oil-Pressboard Ratio

Initi

alS

lope

dU/d

t[V

/s]

100 101 102 103

Max Return Voltage URmax [v](b)

Figure 4. RVM polarization spectra (a) and dUR/dt plot (b) for twodifferent geometrical arrangements of the coils in the pancakemodel measured at 2000 V during the first stage of the study.

10–7

10–8

10–9

10–4 10–3

10–3

10–2

10–2

10–1

10–1

100

100

101

101

102 103

Cap

acita

nce

[F]





Frequency [Hz](a)

10–4 10–3 10–2 10–1 100 101 102 103

Frequency [Hz](b)

Loss

Fact

or,t

anδ

Figure 5. Capacitance (a) and loss factor (b) as a function of fre-quency of different geometrical arrangements of the coils in thepancake model measured in the first stage of the study.

during an attempted dry-out of a 29-year-old 400/132 kV240 MVA auto-transformer have cast doubt on the valid-ity of the algorithm used to estimate the moisture contentfrom measured RVM polarization spectra. A very impor-tant influence of oil condition was demonstrated. A lim-ited number of measurements have also been made byNGC with the FDS and PDC techniques. Not surpris-ingly, oil condition is shown to have a significant impacton the responses obtained from these as well.

As a result, an alternative interpretation is proposed,which resolves many previous anomalous moisture deter-minations. The moisture content in the solid insulation isno longer estimated from the position of the dominantpeak in the polarization spectrum. Instead, the polariza-tion spectrum is examined for evidence of any subsidiarymaxima away from the dominant time constant. TheGuuinic representation has been found to be a useful aidin this process, in particular for confirming that the domi-nant time constant corresponds to the oil peak (narrow“nose”) and assessing if there is any sign of polarizationactivity above the dominant time constant. Anysubdominant maximum would likely stem from polariza-tion phenomena in the solid insulation. The moisturecontent is then estimated from this corresponding timeconstant, using the published calibration curves.

In Sweden, ABB and Vattenfall used the three measur-ing methods on four generator step-up transformers(20/400 kV 500 MVA), installed at the Ringhals nuclearpower station on the west coast of Sweden [13]. Two ofthe transformers were installed in 1973. They have openconservators and the oil was changed once during theirservice. Two other transformers were installed in 1977.They have closed conservators and the oil has never beenchanged. Distinct differences in the dielectric responsespectra were found between the two groups. The resultswere interpreted and simulated using the modeling pre-sented previously, and taking into account the materialsproperties as well as the design of the transformer. Withthis approach, all three methods ranked the insulationquality equally. Also, all three types of measurementswere sensitive to the dielectric properties of both the oiland the board, and could easily detect small changes inthe conductivity of the oil.

Conclusions and GuidelinesThe results of work of TF 15.01.09 presented in this

article confirm that the dielectric response measurementsprovide valuable information on the state of oil-paper in-sulation in power transformers, particularly on the mois-ture content. All of the compared dielectric responsemethods (RVM, FDS, PDC) reflect the same fundamentalpolarization and conduction phenomena in transformerinsulation, the special feature of which is a combinationof oil gaps and solid insulation.

The dielectric measurements described here confirmthat, due to the influence of oil gaps, the condition of the

oil—specifically its conductivity—has a significantimpact on dielectric response. This must be taken into ac-count when attempting to estimate moisture contents inthe solid insulation from the results of all three methods.

Regarding the geometry, it has an influence on the re-sponse, but not as significant as the effect of the oil con-ductivity. For the tested sizes of oil gaps, which are typicalfor transformer insulation, it is primarily the existence ofthe gaps rather than their detailed dimensions that has themain impact on the results of the measurements.

For the RVM technique, the old interpretation—basedonly on a simple relationship between the dominant timeconstant of the polarization spectrum and the water contentin cellulose—is not correct. An improved interpretation re-quires taking into account the complete measured curve. Analternative qualitative interpretation has been proposedthat resolves some previous anomalous conclusions.

Mathematical modeling provides a link between themeasured responses of the three methods applied andshows how responses are affected by oil conductivity (re-sistivity), moisture content, and object geometry. Knowl-edge of the dielectric properties of the insulationcomponents is needed for this purpose. Such modeling isrecommended today for interpretation of the results ofall methods.

Before operational decisions concerning life manage-ment of transformers can be made with confidence fromthe indications of the dielectric response techniques, fur-ther validation is required. There is a particular need toverify the estimates of water content determined by thedielectric response techniques by comparison with basicchemical measurements. The influences of different typesof pressboard/paper and aging products (beyond the ef-fects of oil conductivity and moisture) on dielectricresponse have yet to be determined conclusively.

Since all three methods use different measurements,they could have, in principle—and appear to have inpractice—their own strengths and weaknesses. Theseneed to be assessed further before any one can be recom-mended over the others.

References

[1] A.J. Kachler, R. Baehr, W.S. Zaengl, B. Breitenbauch, and U.

Sundermann, “Kritische Anmerkungen zur Feuchtigkeitsbestimmung

von Transformatoren mit der “Recovery-Voltage-Methode,”

Elektrizitätswirtschaft, Jg. 95, Heft 19, pp. 1238-1245, 1996.

[2] A.J. Kachler, “Ageing and moisture determination in power

transformer insulation systems. Contradiction of RVM methodology,

effects of geometry and ion conductivity,” 2nd International Workshop

on Transformers, Lodz, Poland, November 24-27, 1999.

[3] Tettex Instruments AG, “Polarisation spectrum analysis for diagnosis

of insulation systems,” Information, vol. 29, 1992, TI 29-d/e-04. 92.

[4] A. Bognar, L. Kalocsai, G. Csepes, E. Nemeth, and J. Schmidt,

“Diagnostic tests of high voltage oil-paper insulating systems (in

May/June 2003 — Vol. 19, No. 3 17

particular transformer insulation) using dc dielectrometrics," in Proc.

1990 CIGRÉ Conference, Paris, France, paper 15/33-08, 1990.

[5] U. Gäfvert, G. Frimpong, and J. Fuhr, “Modelling of dielectric

measurements on power transformers,” in Proc. 1998 CIGRÉ

Conference, Paris, France, paper 15-103, 1998.

[6] V. Der Houhanessian, “Measurement and analysis of dielectricresponse in oil-paper insulation systems,” Ph.D. thesis, SwissFederal Institute of Technology, ETH No. 12832, Zurich, 1998.

[7] A.K. Jonscher, Dielectric Relaxation in Solids. London, U.K.:

Chelsea Dielectric Press, pp. 36-52, 1983.

[8] J.C. Maxwell, A Treatise on Electricity and Magnetism, vol. 1, 3rd ed.

Oxford, U.K.: Clarendon Press, reprint by Dover, pp. 450-464, 1981.

[9] K.W. Wagner, “Erklärung der dielektrischen Nachwirkungsvorgänge

auf Grund Maxwellacher Vorstellungen,” Archiv für Elektrotechnik,

vol. II, no. 9, pp. 371-387, 1914.

[10] Electrical Insulation Diagnostic System: PDC-Analyser-1 MOD,

Alff Engineering, Gomweg 7, CH-8915 Hausen am Albis,

Switzerland, ([email protected]).

[11] Insulation Diagnostics Spectrometer IDA, Programma Electric AB,

Eldarv. 4, SE-187 75 Täby, Sweden.

[12] J. Lapworth and R. Heywood, “The determination of the dryness

of transformer insulation: Recent NGC experience with

polarisation tests,” in Proc. Transformer ‘01 Conference, Bydgoszcz,

Poland, 2001, pp. 84-94.

[13] U. Gäfvert, L. Adeen, M. Tapper, P. Ghasemi, and B. Jönsson,

“Dielectric spectroscopy in time and frequency domain applied to

diagnostics of power transformers,” in Proc. 6th Intern. Conf. on

Properties and Applications of Dielectric Materials (ICPADM),

Xian, China, 2000, pp. 825-830.


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