lic thesis

THESIS FOR THE DEGREE OF LICENTIATE OF ENGINEERING

Application of Dielectric Spectroscopy forEstimating Moisture Content in Power

Transformers

by

CHANDIMA EKANAYAKE

Department of Electric Power Engineering

CHALMERS UNIVERSITY OF TECHNOLOGY

Göteborg, Sweden 2003

ii

Application of Dielectric Spectroscopy for Estimating MoistureContent in Power TransformersCHANDIMA EKANAYAKE

CHANDIMA EKANAYAKE, 2003

Technical report no. 465LDepartment of Electric Power EngineeringChalmers University of TechnologySE-41296 GöteborgSweden

Telephone: +46 (0)31 – 772 1641Fax: +46 (0)31 – 772 1633E-mail: [email protected]

Chalmers Bibliotek, Reproservice

Göteborg, Sweden 2003

iii

Abstract

Moisture in oil-paper insulated power transformers has an immense effect ontheir performance. Therefore, early identification of moist insulation is vitalfor avoiding failures in transformers. Although chemical analyses of oilprovide direct information on water content, they have to be performed underlaboratory conditions. In contrast, estimations of moisture content usingon-site electrical measurements allow for obtaining this information faster.

The study presented in this report aims at further investigating thepossibilities of using dielectric response measurements for diagnostics ofpower transformer insulation and special attention is paid to frequencydomain spectroscopy (FDS) measurements. FDS measurements wereperformed on various transformers, both under field and laboratoryconditions. In addition, the same measurements were carried out onpressboard samples containing different moisture contents. The results wereinterpreted by means of a so called X-Y model, and the sensitivity of thismodel was also evaluated. Based on the observations of this analysis asimplified model, called the X model, was introduced. In the X model, thepresence of spacers in transformer insulation has been neglected. On theother hand, an additional measurement of the oil sample was introduced forestimating oil conductivity. Derived moisture contents from the X modelwere compared with the corresponding moisture contents obtained from theKarl Fischer analyses of oil samples.

It was revealed that FDS measurements can be used as a reliable tool forestimating moisture content in transformer insulation when a consistentdatabase on dielectric responses of well-defined pressboard samples isavailable. A reasonable agreement between the estimated and the measuredmoisture contents was found. However, the estimates for distributiontransformers were higher than the corresponding values derived from thechemical analyses. Results of the measurements on field installedtransformers belonging to the Ceylon Electricity Board (Sri Lanka) revealedthat most of the transformers used in this study were wet. Therefore,immediate precautions were suggested for avoiding further ageing of theirinsulation.

Key words: oil-paper insulation, Frequency domain spectroscopymeasurements, modelling transformer insulation, moisture, oil conductivity

v

Acknowledgement

I would like to express deep gratitude to my supervisor and examiner Prof.Stanislaw Gubanski for his guidance, encouragement and invaluablecomments on this report. In addition the guidance of Dr. Manjula Fernando,during my stay in Sri Lanka, is highly appreciated.

I specially thank Dr. Janake Ekanayake for encouraging me to continue mystudies in high voltage engineering.

All the field measurements were performed in the Ceylon Electricity Boardpremises. So I greatly appreciate the support of CEB technical staff inMahaweli and Laxapana complexes. Special thanks go to Eng. KamalMularachchi and Eng. Anura Herath for their invaluable support andencouragement.

Prof. Anders Cedergren in University of Umeå helped me to learn KarlFisher titration measurements. I really appreciate your help. Prof. KlauseFröhlich and PhD. student Wolfgang Hribernik gave me an opportunity tomake measurements on pressboard samples in the high voltage laboratory ofETH, Zurich. My special thanks go to them.

I sincerely thank all my department colleagues for your support and keepinga friendly working environment.

My Sri Lankan friends Vishaka, Dhammika, Siri and Chandi provided me agood Sri Lankan environment. Thank you all for everything.

I thank my colleagues Disala and Lilantha for the good friendship andsupport.

I owe my deepest gratitude to my parents, sisters, brother and brother in lawfor their love and support for my studies. At last but not least many thanks goto Charani for her understanding and encouragement, especially when I waswriting my thesis.

I appreciate the finance support for this research through Sida SAREC.

vii

Table of contents

ABSTRACT ............................................................................................................ III

ACKNOWLEDGEMENT ....................................................................................... V

TABLE OF CONTENTS ...................................................................................... VII

1. INTRODUCTION ............................................................................................. 1

2. DIELECTRIC RESPONSE OF INSULATION ............................................. 5

2.1 TIME DOMAIN RESPONSE .............................................................................. 52.2 FREQUENCY DOMAIN RESPONSE ................................................................... 82.3 PRINCIPLES OF DIELECTRIC RESPONSE MEASUREMENTS ............................. 10

2.3.1 Frequency domain measurements...................................................... 102.3.2 Time domain measurements............................................................... 11

2.4 TEMPERATURE DEPENDENCY OF DIELECTRIC RESPONSE............................. 142.5 RESPONSE OF OIL-PAPER INSULATION SYSTEMS ......................................... 15

3. COMPONENTS AND PROPERTIES OF TRANSFORMERINSULATION SYSTEM ........................................................................................ 17

3.1 PAPER AND PRESSBOARD............................................................................ 173.2 MINERAL OIL.............................................................................................. 183.3 OIL IMPREGNATED PAPER INSULATION ....................................................... 193.4 MOISTURE IN TRANSFORMER INSULATION.................................................. 20

3.4.1 Moisture in paper .............................................................................. 203.4.2 Moisture in oil.................................................................................... 203.4.3 Moisture in oil-paper insulation ........................................................ 213.4.4 Source of excessive moisture in transformer insulation .................... 22

3.5 DEGRADATION OF OIL-PAPER INSULATION SYSTEMS .................................. 23

4. CONDITION ASSESSMENT OF POWER TRANSFORMERINSULATION.......................................................................................................... 25

4.1 CHEMICAL AND PHYSICAL ANALYSES......................................................... 254.2 ELECTRICAL TESTS ..................................................................................... 26

4.2.1 Traditional methods........................................................................... 274.2.2 Dielectric response measurements .................................................... 31

5. MODELLING DIELECTRIC RESPONSE.................................................. 35

5.1 DIFFERENT MODELLING TECHNIQUES ......................................................... 35

viii

5.1.1 Debye model with single and distributed time constants....................365.1.2 General response function .................................................................375.1.3 X-Y model ...........................................................................................38

5.2 EFFECT OF DIFFERENT PARAMETERS OF X-Y MODEL ON THE FINAL FDSRESPONSE................................................................................................................45

5.2.1 Influence of oil conductivity σ............................................................455.2.2 Influence of spacers............................................................................475.2.3 Variation of permittivity at 1 kHz (ε´1kHz)...........................................535.2.4 Conclusion of the analyses .................................................................55

5.3 MODELLING USING X MODEL......................................................................555.4 MODELLING USING DISTRIBUTED RELAXATION TIMES ................................57

6. TRANSFORMERS AT THE CEYLON ELECTRICITY BOARD ............59

6.1 INTRODUCTION ...........................................................................................596.2 GENERATOR TRANSFORMERS UNDER STUDY...............................................596.3 MAINTENANCE OF GENERATOR TRANSFORMERS AT CEB ...........................61

6.3.1 Maintenance tasks carried out ...........................................................616.3.2 Information recording ........................................................................636.3.3 Generator transformer replacements .................................................64

6.4 DISTRIBUTION TRANSFORMERS UNDER STUDY............................................64

7. MEASUREMENTS..........................................................................................67

7.1 INSTRUMENTATION FOR DIELECTRIC RESPONSE MEASUREMENTS ...............677.2 FIELD MEASUREMENTS ...............................................................................697.3 LABORATORY MEASUREMENTS...................................................................70

7.3.1 Laboratory transformer......................................................................707.3.2 Oil test cell .........................................................................................717.3.3 Pressboard samples............................................................................727.3.4 Karl Fischer titration measurements..................................................73

8. RESULTS AND DISCUSSION.......................................................................75

8.1 PRESSBOARD SAMPLES................................................................................758.2 DISTRIBUTION TRANSFORMER IN LABORATORY ..........................................77

8.2.1 Modelling using X-Y and X model......................................................778.2.2 Comparison of time domain and frequency domain spectroscopymeasurements.....................................................................................................80

8.3 POWER TRANSFORMERS IN THE FIELD .........................................................828.3.1 Single-phase power transformers.......................................................828.3.2 Three-phase power transformer .........................................................92

8.4 DISTRIBUTION TRANSFORMERS IN THE FIELD ..............................................938.5 LIMITATION ON FDS MEASUREMENTS ........................................................97

9. CONCLUSION.................................................................................................99

ix

10. FUTURE WORK....................................................................................... 101

11. REFERENCES .......................................................................................... 103

Chapter 1: Introduction

1

1. Introduction

Demand for a reliable electricity supply has significantly increased during thelast few decades. Therefore, fault free operation of power systems hasbecome very important. However, due to the high cost of power systemcomponents, especially transformers, it is not economical to replace them inorder to increase the reliability, by considering their age. A relatively largenumber of transformers that are still working in different power systemsaround the world remain in fairly good condition although they have beenused longer than their designed lifetime. Therefore, correct conditionassessment of power transformers is needed before making any conclusionsabout replacements or refurbishment.

Degradation of power transformer insulation, which mainly consists of oiland paper, is the main factor for power transformer failures. Chemicalanalyses and electrical measurements are used for monitoring the conditionof transformer insulation. Among these, chemical analyses provide directinformation on parameters, such as water content, degree of polymerisationof paper, sludge content in the oil, acidity of the oil and quantity of differentgasses dissolved in the oil. However, most chemical analyses must beperformed under laboratory conditions and for some chemical analyses(eg. Chromatography tests) paper samples are needed. On the other hand,electrical measurements are simpler and it is possible to perform themon-site. Because of this simplicity, electrical tests are currently preferred forcondition monitoring of transformer insulation instead of chemical testsalthough they do not provide direct information about the above mentionedparameters.

Traditional electrical tests, such as insulation resistance (IR), polarisationindex (PI) and loss factor (tanδ) provide very little information abouttransformer insulation since they are limited to a single value measurement.To overcome this disadvantage, dielectric response measurements, namelyreturn voltage measurements (RVM), polarisation and depolarisation currentmeasurements (PDC) and frequency domain spectroscopy measurements(FDS), have been introduced for condition monitoring of transformerinsulation, especially for the evaluation of water content in transformerpressboard. In the early stages, RVM was introduced because voltagemeasurements were simpler than measurements of low currents. The othertwo methods, requiring current measurements, were introduced recently due

Chapter1: Introduction

2

to improvements provided by the use of sophisticated electronic devices. Notonly measuring techniques but also the interpretation of results have alsobeen developed. However, for most of the present interpretation techniques, aprior knowledge of the geometrical arrangement of the insulation is required.Most of the power utilities lack this information about transformerconstruction. Therefore, practical difficulties arise when power utilities utilisethese techniques. Hence, there are still needs for improvement in theinterpretation of the results of all these techniques and, therefore, furtherinvestigations are essential [1]. Comparisons with chemical analyses arenecessary for calibrating the correlation between the dielectric response dataand the moisture content in the insulation.

Work presented in this thesis has been carried out to find out the potential forusing frequency domain dielectric spectroscopy measurements andconductivity measurements on oil for assessing the insulation condition, i.e.the moisture content in the paper insulation in power transformers withoutusing information about geometrical capacitance. Transformers used for thisstudy belong to the Ceylon Electricity Board (CEB) in Sri Lanka. A numberof power and distribution transformers of different ages were selected for themeasurements. In parallel to dielectric measurements on transformers,moisture content in the oil and the frequency dependent conductivity of theoil was also measured by means of Karl Fischer titration (KFT) and dielectricspectroscopy measurements of oil samples, respectively. In addition similarmeasurements were performed on well-defined oil-paper samples to get aclear understanding of the influence of different parameters on the dielectricresponse of the oil-paper insulation and, later, results of these measurementswere utilised for estimating moisture contents in transformers selected forthis study.

A short description of each of the chapters in this report is given below.

Chapter 2 This chapter describes the theory behind different dielectricresponse measurement techniques on insulation systems. The influence oftemperature on dielectric response of oil-paper insulation systems isdiscussed as well.

Chapter 3 In this chapter, properties of oil impregnated paper insulationsystems are discussed. The sources and effects of excessive moisture intransformer insulation are also reviewed.

Chapter 4 Here, different methods utilised for condition assessments oftransformer insulation are presented.

Chapter 1: Introduction

3

Chapter 5 This chapter reviews techniques developed for modellingdielectric response of insulation systems. The influence of different parts oftransformer insulation on total response is also discussed. Based on thisanalysis, a simple model for transformer insulation is presented.

Chapter 6 A description of transformers selected for field measurements isgiven in this chapter. Present maintenance tasks carried out by CEB are alsoreviewed.

Chapter 7 Field and laboratory measurements conducted are brieflydescribed in this chapter.

Chapter 8 Results of the measurements are discussed.

Chapter 9 Conclusions.

Chapter 10 Future work proposed.

Publications

• C. Ekanayake, W. Hribernik, S.M. Gubanski, M.A.R.M. Fernando“Calibrating the results of dielectric measurements on field aged powertransformers using oil analyses and similar measurements on well-defined pressboard samples”, Accepted to publish in Nordic InsulationSymposium 2003 (NORD-IS 03), Tampere, Finland.

• C. Ekanayake, M.A.R.M. Fernando, K.Mularachchi, S.M. Gubanski“Diagnostic of power transformer insulation using frequency domainspectroscopy” Ninth annual conference of IEE Sri Lanka, Colombo,September 2002.

• C. Ekanayake, S.M. Gubanski “Correlation between results of dielectricmeasurements and oil analyses for a power transformer” Proc. ofElectrical Insulation Conference and Electrical Manufacturing & CoilWinding Conference 2002, Cincinnati, USA, October 15-17, 2002.

• C. Ekanayake, S.M. Gubanski, K.Mularachchi, M.A.R.M. Fernando,“Diagnostic of power transformers in Sri Lanka: Application of dielectricspectroscopy in frequency domain” Proc. of Electrical InsulationConference and Electrical Manufacturing & Coil Winding Conference2001, pp. 593 - 596, Cincinnati, USA, October 16-18, 2001.

Chapter 2: Dielectric response of insulation

5

2. Dielectric response of insulation

2.1 Time domain response

Maxwell equations, which describe electromagnetic phenomena, are the basisof mathematical modelling of the electromagnetic behaviour of insulation.

0=⋅∇∂∂−=×∇

∂∂+=×∇

=⋅∇

Bt

BE

t

DjH

D ρ

(2.1)

Here, D dielectric displacement, ρ free charge density, H magnetic field,j ohmic current density, E electric field and B magnetic flux density,respectively. However, dielectric materials studied here are assumed to havezero magnetisation. Therefore, in further considerations magnetic propertieswill not be considered.

In addition to Maxwell equations, D and E are interrelated by the propertiesof insulating material.

PED += 0ε (2.2)

Where ε0 is the permittivity of free space and P is the polarisation vector,which is material dependent. The insulating materials considered here areassumed isotropic, homogeneous and linear, which allows for applying theabove equations without considering microscopical electromagneticbehaviour of the materials.

Dielectric displacement D is often linear with the applied electric field E.Therefore, D and E can be interrelated using a proportional constant calledrelative permittivity εr.

ED r 0εε= (2.3)

Moreover, by combining (2.2) and (2.3) P and E can be interrelated as,

Chapter2: Dielectric response of insulation

6

( ) EEP r 00 1 χεεε =−= (2.4)

where, χ is known as the dielectric susceptibility of the material.

Also by using (2.1) and (2.2),

t

P

t

EEJ

∂∂+

∂∂+= 0εσ (2.5)

Here, J is the total current density due to the applied field and σ is the volumeconductivity of the material. Equation (2.5) shows the contribution ofpolarisation to the total current through the insulation.

Polarisation in materials can be due to several mechanisms known aselectronic, ionic, atomic, dipolar, interfacial polarisation and hopping. Ofthese, the first three mechanisms are much faster than the others. The timethese mechanisms take is under 10-13 seconds. In contrast, interfacialpolarisation and hopping are slower, which take about 10-3 - 104 secondsdepending on the material. Therefore, when a constant electric field isapplied, total polarisation of the material varies, as shown in Figure 2.1.

Figure 2.1. Time dependence of polarisation when a constant electric field E0 is applied att=t0.

In Figure 2.1, P∞ represents polarisation due to fast mechanisms and PS is thetotal saturated polarisation of the material (after an infinite time).

t0 t

P=P∞

P=PS

E=E0

E=0

P, E


7

Therefore, polarisation at any given time after t=t0 can be written as,

( ) ( ) ( ) 00 ttforttgPPPtP s ≥−−+= ∞∞ (2.6)

where g(t) is a monotonically increasing function that satisfies the followingconditions,

( )

( ) ( ) 0

0

0&0

1

0

ttfortgtg

and

t

tt

if

iftg

≥≥≥

∞→≤

=

By substituting (2.4) to each polarisation term in (2.6), total polarisation dueto a constant electric field E0 can be expressed as,

( ) ( ) ( ) ( )[ ] 000 1 EttgtP S −⋅−+−= ∞∞ εεεε (2.7)

Where, ε0 and ε∞ are static and high frequency relative permittivities of thematerial, respectively.

Equation (2.7) can be extended to find the polarisation of a linear materialdue to any arbitrary electric field since any arbitrary function can berepresented by sum of infinite number of step functions. By applying thesuperposition principle and using convolution integral polarisation at time tdue to any arbitrary electric field, E(t) can be given as,

( ) ( ) ( ) ( ) τττεεε dEtftEtPt

∫∞−

∞ −+−= )(1 00 (2.8)

Where f(t) is a monotonically decreasing function known as the dielectricresponse function. The first part of (2.8) corresponds to fast polarisationprocesses in the material.

By combining (2.5) and (2.8), the total current density J(t) due to a constantelectric field can be written as follows,


8

( ) ( ) ( )( ) ( ) ( ) ( )

( ) ( ) ( )N

( )tEtfttE

t

dEtftE

t

tEtEtJ

t

++=

∂

−+−∂+

∂∂+=

∞

∞−∞ ∫

32

0

1

00

0

1

δεεσ

τττεεεεσ

(2.9)

As shown in (2.9), total current density comprises three components:

1 – Current density due to conductivity of the material.

2 – Instantaneous current density due to fast polarisation processes.

3 – Current density due to slow polarisation processes.

Moreover, one can see that in the time domain, the behaviour of the dielectric

material is characterised by conductivity σ, high frequency dielectric

permittivity ε∞ and the dielectric response function f(t).

2.2 Frequency domain response

When considering time varying electromagnetic fields, it is possible todescribe them by using a single frequency sinusoidal function. Then timevarying field E(t) can be written as,

( ) tjeEtE ω0= (2.10)

The real part of this function represents the physical electric field.

Making the same assumptions, which we made to derive the time domainresponse and by substituting (2.10) in (2.2) and (2.8), dielectric displacementD(t) can be expressed in the following form,

( ) ( ) ττεεε ωτω deEtfeEtD jt

mtj

m ∫∞−

∞ −+= 00 (2.11)


9

by substituting t0 = t-τ,

( ) ( ) tjm

A

tj eEdtetftD ωω εε 0

0

+= −∞

∞ ∫

(2.12)

Term A of (2.12) is equivalent to the Fourier transform of f(t), which isdefined as frequency dependent electric susceptibility ( )ˆ . Where,

( ) ( ) ( ) ( ) dtetfj tj

o

ωωχωχωχ −∞

∫=′′−′=ˆ (2.13)

Here, χ′(ω) and χ′′ (ω) are real and imaginary components of the complexsusceptibility, respectively. Since both of them are derived from the samefunction f(t), they are interrelated by the so called Kramers-Kronig (K-K)transformation [2].

( ) ( )

( ) ( )dx

x

x

a

dxx

xx

aa

a

∫

∫

−′

∞→−=′′

−′′⋅

∞→=′

022

022

lim2

lim2

ωχ

πωωχ

ωχ

πωχ

(2.14)

In the frequency domain, the current density ω) in a dielectric material dueto an external electric field of Ê(ω) can be written as,

( ) ( ) ( ) ( ) [ ] ( )

( )P

( ) ( )

( ) ( ) ( )ωωεωεωε

ωωχωε

σωχεωε

ωωχωχεεεωωσω

Ejj

Ejj

EjjEJ

B

A

ˆ

ˆ

ˆˆˆ

0

21

00

00

′′−′=

′′+−′+=

′′−′++=

∞

∞

(2.15)

Where ε′ and ε′′ are real and imaginary components of complex permittivity,respectively.


10

Sections A and B of (2.15) represent the capacitive and resistive componentsof the total current, respectively. Resistive current, which is in-phase with theapplied electric field, is coupled with the losses in the material. Term 1 of theresistive current, where the conductivity term is located, is associated withthe conduction or ohmic losses due to free charge movement in the material.Term 2 of the resistive current corresponds to the dielectric losses in themedia, which occur due to the inertia of bound charges when accelerated bythe driving field.

In the frequency domain conductivity σ, high frequency permittivity ε∞ andelectric susceptibility )(ˆ ω characterised the dielectric behaviour ofmaterial.

For deriving equations in both domains, it was assumed that the insulatingmaterial is isotropic, homogeneous and linear. Therefore, equal informationcan be collected using measurements in either time or frequency domain. Onecan transform the information from one domain to another by Fouriertransformation of f(t) or inverse Fourier transformation of )(ˆ ωχ .

2.3 Principles of dielectric response measurements

As described in the previous sections, response measurements can beperformed either in the time domain or the frequency domain. In the timedomain, two measuring techniques known as polarisation and depolarisationcurrent measurements and recovery voltage measurements are utilised. Bothof these techniques provide information on the conductivity σ and theresponse function f(t). In the frequency domain, complex capacitance and theloss factor are measured as a function of frequency. Information on thecomplex permittivity )(ˆ ωε and the conductivity σ are provided.

2.3.1 Frequency domain measurements

In this technique, slow polarisation processes in the insulation are studied bymeasuring current due to a sinusoidal excitation. Since single frequencycomponent is considered at a time, resultant current can be written as follows


11

( ) ( ) ( ) ( )

( ) ( ) ( )( ) ( )ωωω

ωωωω

ωωχωεσωχεωω

UCj

UCjCj

UjCjI

ˆˆ

ˆ

ˆˆ

00

=

′′−′=

′′+−′+= ∞

(2.16)

where C0 is the geometrical capacitance and Û(ω) is the applied voltage.C′(ω) and C′′ ( ω) are real and imaginary components of the complexcapacitance ω). This shows possibility of calculating complex permittivityby measuring magnitude and phase angle of the response current whengeometrical capacitance is known, but it is not sufficient to distinct ohmicand dielectric losses in the insulation. However, at very low frequencies theresistive part is often dominating. When this happens ε′′ (ω) has a slope of –1

in the log frequency scale and ε′(ω) does not vary with frequency. But whenhopping mechanisms are present, it is hard to find this type of behaviour. Theother possible way of separating these two losses is by calculating dielectric

losses using the K-K transformation of ε′(ω). Results of this techniquedepend on the size of the measured frequency window.

When the geometrical capacitance is unknown, the frequency dependent loss

factor tgδ(ω) can be used to present the measured results.

( ) ( )( )ωεωεωδ

′′′

=tg (2.17)

However, in such a case information on important dielectric parameters

( )(ˆ ωχ , σ and ε∞) cannot be obtained since the loss factor is a ratio of real

and imaginary components of complex permittivity.

2.3.2 Time domain measurements

Polarisation depolarisation current measurement

Polarisation and depolarisation current measurements can be used to analysethe slow polarisation processes in insulation materials. When a fixed dcvoltage (U0) is applied across a totally discharged material having thegeometrical capacitance of C0, the resultant current can be given as,


12

( ) ( ) ( ) 0000

0 ttforUCtfttI pol <<

++= ∞ δεεσ

(2.18)

Here, the contribution of the delta function δ(t) is only at t=0. Therefore, thepolarisation current consists of two major components, related to theconductivity σ and the response function f(t).

After a certain time t0, the applied voltage is removed and at the same timethe material is short-circuited. The resultant current due to reorientation ofpolarised species can be expressed as,

( ) ( ) ( ) ( ) ∞<<++−−= ∞ tforUCtttftftI depol 0000 δε (2.19)

Measured polarisation and depolarisation currents have opposite signs, butfor convenience, when they are plotted, only the magnitudes are considered.

As shown in (2.19) depolarisation current does not have a conductivity term.Therefore Idepol can be used to calculate the response function by disregardingthe contribution of the delta function.

( ) ( ) ( )000

ttfUC

tItf depol ++−= (2.20)

When the test object is charged for a sufficiently long time (t0), i.e. at least5 - 10 times as long as the measurement time of depolarisation current,

( ) ( ) 00 >+>> tforttftf (2.21)

Therefore one can assume,

( ) ( )00UC

tItf depol−≈ (2.22)

The response function of many solid dielectrics decreases slowly with timeresulting long measuring times to fulfil the requirement given in (2.21). Insuch cases, (2.20) can be used to compute the response function with suitablenumerical calculations.

Measured polarisation and depolarisation currents can be used to estimate thedc conductivity σ of the measured object from,


13

( ) ( ) ( ) 00000

0 >+−+= tforttftItIUC depolpol εεσ (2.23)

In solid materials, it is difficult to distinguish the influence of conductivityand the dielectric polarisation on Ipol if the charging period is not sufficient. Itis recommended to charge the object until the effect of the dielectric responsefunction has disappeared or f(t+t0) << σ/ε0 , to obtain true conductivity.

Recovery voltage measurement (RVM)

Figure 2.2. Voltage and current variation during RVM measurement.

In this method, time dependent voltage measurements are utilised instead ofcurrent measurements. First, a step voltage U0 is applied across fullydischarged insulation for a period of tc. During this stage, the polarisationcurrent Ipol flows through the insulation. The polarisation of particles having arelaxation time less than tc, and free charge movement cause this current.After the time tc, the material is short-circuited for a period of td. Because ofthe reorientation of the polarised particles, the depolarisation current Idepol

flows. During the period td, species, which have a relaxation time less than td,

are totally relaxed. After td, the short-circuited connection is opened andvoltage across the insulation is measured. Relaxation of remaining polarisedparticles develops a voltage across the insulation. This voltage re-polarises

2tt

R

dt

dU

→=α

chargingshort circuit

open circuit

U=U0

t

Ipol

Idepol

U=UR(t)

tc td tm

U=Umax

t=t2


14

some of the species in the insulation, which yields some difficulties wheninterpreting RVM data because the relaxation of polarisation processes in theinsulation is integrated over the dispersive material. During RVMmeasurement, current through the insulation is zero. Therefore, the followingrelation can be used to interpret the RVM result,

( ) ( ) ( ) ( )

( ) ( )

( ) 0

0

2

20

1000

2

==

∞<<=−

+−−++

∫

∞

ttU

ttfordUtfdt

d

ttftfUdt

tdUtU

R

t

t

R

RR

τττε

εεεσ

(2.24)

Resolving the conductivity σ, high frequency relative permittivity ε∞ and thedielectric response function f(t) from the measured recovery voltage aredifficult compared to the previous methods. The formal way of doing this isby assuming an analytical form of f(t) and then minimising the current givenin (2.24). This methods works well with systems that have a known shape off(t), which can be expressed using a simple parameterised analytical function.

2.4 Temperature dependency of dielectric response

The dielectric response of insulation is not only a function of frequency ortime but is also a function of temperature. However, for most of thematerials, the spectral shape of the response does not change withtemperature, at least over a temperature range during which the structure ofthe material does not alter significantly [2]. This allows for normalising timeor frequency dependent spectra for different temperatures by shifting thecorresponding spectra until they coincide into a single curve, which is calleda master curve [2-4]. The master curve contains more information than asingle temperature measurement since it covers a wider range of frequency ortime span compared to a single measurement. For some dielectric materials, ashift in the spectral function due to a change in absolute temperature from T1

to T2 can be expressed with an Arrhenius factor as,

−−=

1221

11exp),(

TTK

ETTS (2.25)


15

Here, E is the activation energy and K is Boltzman’s constant. Furthermore,for most Arrhenius activated systems, this shift is along the frequency axisand when the frequency is on a log scale it is a constant shift, independent offrequency.

( ) ( )

−=−=

2121

11loglog

TTK

Eshift ωω (2.26)

Where, ω1 and ω2 are any two frequencies corresponding to the samemagnitude of the spectral functions at T1 and T2, respectively.

The master curve technique is applicable for both real and imaginarycomponents of the frequency dependent complex capacitance, since both ofthem contain similar information. However, first the contribution of the high

frequency permittivity ε∞ and dc loss σ/ε0ω should be subtracted.

In the time domain, a corresponding shift of dielectric response function, toobtain the master curve, is along a line with a -1 slope when it is plotted on alog-log scale.

The temperature dependence of the dc conductivity σ can also becharacterised with an Arrhenius factor with the activation energy Edc and the

pre-exponential constant σ0 as follows,

−=

KT

Edcexp0σσ (2.27)

where T is the absolute temperature.

2.5 Response of oil-paper insulation systems

The dielectric response of an oil-paper insulation system is affected by theway the components are combined. Therefore, the dielectric response of suchinsulation systems reflects the properties of each material, as well as thegeometrical arrangement of insulation materials. When the combinedinsulation is subjected to an electrical stress, charge accumulation occurs atthe interface due to the difference in conductivity of the two materials. Thisphenomenon is known as interfacial polarisation or the Maxwell-Wagnereffect [2].


16

The dielectric dispersion of mineral oil can be neglected in the frequencyrange (< 1000 Hz) of interest. Therefore, the dielectric response of oil cansimply be described by a constant permittivity value (εr = 2.2) and the dcconductivity σ, which is dependent on the presence of ionic contaminants inoil. The temperature dependence can be characterised by selecting its ownactivation energy. On the other hand, the response of the pressboard andpaper is characterised by their dielectric response function, which is stronglydependent on the presence of moisture and other ageing products in theinsulation.

Nevertheless, it has been shown that both oil and pressboard dominate in thetotal response at different frequencies [5], when the oil channels are in serieswith pressboard. Dominant influences of pressboard and oil at differentfrequencies on the total response is shown in Figure 2.3.

10-4

10-2

100

102

104

10-4

10-2

100

102

frequency (Hz)

tan

δ

pressboarddominates oil dominates

pressboard and geometrydominates

Figure 2.3. Example of variation of loss factor of oil-paper insulation with frequency anddominant influences.

When the geometry of insulation and the dielectric response of each material,i.e. oil and paper, are known, it is possible to calculate the dielectric responseof the complex structure. This is the basis of some of the modellingtechniques described in Chapter 5.

Chapter 3: Components and properties of transformer insulation system

17

3. Components and properties of transformerinsulation system

This overview is focused on the insulation systems of oil impregnated powertransformers, where Kraft paper is used as the conductor insulation whereaspressboard and ducts filled with mineral oil are mainly used as the maininsulation.

3.1 Paper and pressboard

Kraft paper is a key component of transformer insulation. It is a low cost basematerial obtained from wood pulp with outstanding mechanical and electricalproperties. Paper is composed of 90 % cellulose, 6 – 7 % hemi-cellulose and3 – 4 % lignin [6, 7].

Long cellulose fibres provide high mechanical strength to paper. However toobtain higher density, which gives higher electric impulse strength and higherdielectric permittivity, the length of fibres should be shorter [8]. Therefore, toachieve good electrical properties a compromise is needed at the expense ofweakening mechanical strength. Although the density of cellulose fibres isabout 1.55 g/cm3, the maximum density of paper is about 1.15 g/cm3. Thisreduction in density is due to the porous nature of paper. Because of itsfibrous structure, paper can bear higher mechanical stresses in thelongitudinal direction than in the transverse direction. The longitudinaltensile strength varies between 40 – 150 N/mm2, whereas transverse tensilestrength lies between 25 – 85 N/mm2 [9].

The important dielectric properties of paper, which stand for its insulation

quality, are dielectric permittivity ε, loss factor tanδ and conductivity σ. Thepermittivity of dry paper varies from 1.5 to 3.5 and the loss factor variesbetween 0.003 and 0.004. Dry paper has a very high volume resistivity, i.e.

values between 1015 and 1017 Ωcm can be obtained [10]. However, paper hasto be protected from direct contact with moisture to maintain its gooddielectric properties, due to the high affinity of paper to water.

There are areas in transformers where the electrical and mechanical stressesare high and these cannot be withstood by single layers of paper. Therefore,pressboard is used instead. Pressboard is produced by wet pressing severalpaper layers without any bonding material. The density of pressboard can

Chapter3: Components and properties of transformer insulation system

18

reach up to 1.3 g/cm3 due to its low porosity, compared with that of paper.Also the permittivity of pressboard is higher than the permittivity of paper,i.e. around 4.5 [11].

3.2 Mineral oil

In the majority of power transformers, mineral oil is used as the insulatingliquid due to its wide availability and its low cost compared to otherequivalent products, like silicon oil and organic esters. The main purpose ofusing mineral oil is to impregnate the paper insulation to avoid direct contactbetween paper, air and moisture. The oil also acts as a heat transfer medium,which controls unnecessary rises in the temperature of the apparatus due tolosses in conductors, core and dielectric materials [11, 12].

Transformer mineral oil is refined from hydrocarbons collected during thedistillation of crude petroleum. Characteristics of the refined oil depend onthe relative proportions of paraffinic, napthenic and aromatic hydrocarbons.Of these the aromatic component is chemically unstable compared with thetwo other, due to its unsaturated chemical structure. Therefore, aromaticcomponents in mineral oil have to be minimised in the final product usingappropriate refining methods. Depending on the refining method, theproportions of paraffin, naphthene and aromatics can vary between 40 – 60 %, 30 – 50 % and 5 – 20 %, respectively [12, 13]. All these hydrocarbonshave little or no polarity. Apart from the above named constituents one canfind trace quantities of polar compounds (sulphur, oxygen and nitrogen) andionic species (organic salts) present in mineral oil. These constituents have animmense influence on chemical and electrical properties of the mineral oil[14].

To obtain better performance as a cooling medium and as electricalinsulation, transformer oil must have high dielectric strength, low viscosity,high heat capacity and a low expansion coefficient. It should also be freefrom moisture, gasses, chemical impurities and mechanical contaminants toavoid unnecessary electrical discharges. Therefore, different standardisedmeasurement techniques that measure the above listed parameters, are usedto identify the suitability of the oil for use as transformer oil. Table 3.1 showsthe accepted limits of electrical properties for new mineral-based transformeroil [13].


19

Table 3.1. Electrical properties of new transformer oil.

Property Standard Value

Breakdown strength(~20 °C)

IEC 156 ~30 kV/mm

Dissipation factor(90 °C)

IEC 247 <0.1 %

Relative permittivity(50 Hz)

IEC 247 2.2

Volume resistivity(~20 °C)

IEC 247 100⋅1012 Ωcm

Apart from the above mentioned electrical properties, one of the otherimportant parameters is water solubility in oil, since free water can create aneasy path for electrical discharges. This is discussed in detail in Section 3.4.

3.3 Oil impregnated paper insulation

Paper itself shows modest dielectric performance due to its porous structure.When air is present, the dielectric strength of paper is predominantlydetermined by the gaseous ionisation within the air space. Heat, water andoxygen accelerate the degradation of paper insulation [7]. To control theinfluence of the above mentioned critical factors, it is necessary toimpregnate paper insulation.

The impregnation of paper insulation is a sophisticated process. However,present technology does not allow for achieving 100 % perfect impregnation[8]. Imperfect impregnation may cause damage to the insulation duringservice. This may happen due to the partial discharges through thegas-trapped voids between paper layers or paper fibres.

Most of the electrical properties of impregnated paper, except for thebreakdown voltage, are closely dependent on the electrical properties of drypaper, since the proportionality of oil is small compared with paper and thesetwo materials are chemically inert with each other.


20

3.4 Moisture in transformer insulation

Moisture in transformer insulation is one of the key factors that determinesthe condition of the insulation. Therefore, in the following section moisturedistribution and its influence on transformer insulation are discussed.

3.4.1 Moisture in paper

Water in paper can be found in four states: as water molecules adsorbed tosurfaces, as vapour, as free water in capillaries and as imbibed free water [15,16]. Moisture migration and adsorption in paper is due to the diffusion ofwater molecules through the porous structure of paper and by attraction ofwater molecules to active sites or surface polar groups. Diffusion is quite aslow process that determines the time needed for an equilibrium moisturedistribution. The amount of water in oil impregnated paper mainly dependson the temperature and the water vapour pressure (or the relative humidity) inthe state of the equilibrium. Water content in paper increases with increasingwater vapour pressure while it decreases with increasing temperature.However, there is a significant difference in the sorption of water at a giventemperature and a relative humidity in pressboard and kraft paper, i.e. themoisture content in pressboard is bit higher than the corresponding moisturecontent in paper [16, 17].

To maintain the desired electrical and mechanical properties of paper, it isnecessary to minimise the moisture content in it. During the manufacture oftransformers, paper insulation is dried out, which brings down the moisturecontent to a value between 0.5 – 1 %. During the operation, the moisturecontent in paper has to be maintained below 2.5 % [17, 18]. A 0.5 – 1 %increment of moisture decreases the lifetime of insulation by roughly a factorof 2. Moreover, high moisture content, up to a certain limit, increases thepaper conductivity, which leads to high dielectric losses [19].

3.4.2 Moisture in oil

In mineral oil, water can exist in three states [16, 20]. Most of the waterresides in the dissolved state. Water also exists in the oil as tightly bound tooil molecules, especially in deteriorated oil. In addition to these two states,water in oil can be found as free drops when moisture in oil exceeds thewater saturation limit of the oil. Among the above mentioned three states, themost critical state is the free water state since it provides easy breakdown


21

paths for electrical discharges. Water content in oil is directly proportional tothe relative saturation, up to the saturation level [21]. The saturationsolubility of water in oil, Ws can be expressed in form of Arrhenius ,

−=

T

BAWs exp (3.1)

where the saturation solubility of water in oil, Ws is in parts per million (ppm)and T is the absolute temperature. Coefficients A and B are basically the samefor most unaged transformer oils, but may be slightly different for someproducts due to differences in aromatic contents. Measurements of differentlyaged transformer oils indicated that there were no significant differences inmoisture solubility among differently aged oils [20]. Table 3.2 shows thewater saturation solubility in mineral oil at 20 °C and 100 °C presented bydifferent authors [13, 22, 23].

Table 3.2. Water saturation solubility in oil presented by different authors.

Water saturation solubility in oil (ppm)Temp. (°C)

Oommen [22] Griffin [23] Fofana [13]

20 53 56 45

100 880 777 650

As shown in Table 3.2, the water saturation limit in oil at 20 °C is more than10 times lower than that at 100 °C. Therefore, water droplets can be formedduring a sudden cooling of a transformer, since the rate of diffusion ofmoisture in paper is slow compared with the rate of moisture release from theoil.

Moisture in oil has an immense effect on the dielectric strength of the oil,when the distance between electrodes is shorter than 1 cm. On the other hand,in saturated hydrocarbons, the effect of free water on conductivity is less than10-12 S/m [24, 25].

3.4.3 Moisture in oil-paper insulation

Paper has a higher affinity to water than oil. Therefore, in oil-paper insulationsystems, the moisture resides mainly within the paper part of the insulation.


22

Moisture distribution in oil-paper insulation is highly temperature dependent.At high temperatures, the affinity of paper to water decreases and, at the sametime, the water saturation level of oil increases.

Under stable conditions, that is to say under constant temperature and vapourpressure, moisture distribution between oil and paper is in an equilibriumstate. Several sets of moisture equilibrium curves for oil-paper systems canbe found in the literature [16, 22, 23, 26]. These curves have been derivedusing different measurement techniques, data sources and generatingmethods, which have resulted in differences between the curves. By usingthese curves, it is possible to estimate the moisture content in paper bymeasuring the moisture content in oil at a known temperature. In IEEE std62 - 1995 for estimating the moisture content in pressboard, a correlationmultiplier is introduced instead of equilibrium curves [18]. In all thesetechniques, the estimated moisture content can be biased since it is practicallyhard to reach the equilibrium state in operating transformers. This problemcan be overcome, to some extent by locating moisture sensors in the suitablepositions of pressboard structure.

Bubble formation is one of the critical effects of moisture in oil-paperinsulation, and may occur when the moisture concentration in paper exceeds3.5 % during actual overloading conditions of a transformer [27].

3.4.4 Source of excessive moisture in transformer insulation

Moisture in power transformer insulation continuously increases underservice conditions. There are three main sources that can produce excessivewater in power transformer insulation.

Direct moisture ingress from the atmosphere is the main source of water intransformer insulation. This may take place when the insulation is directlyexposed to air during installation and repair. The other two main ways ofmoisture entering from the atmosphere to transformer insulation are theviscous flow of moisture through poor seals and water migration throughopen breathers (about 0.1-0.2 % per annum) [17, 27].

Residual moisture trapped in thick structural components, such as wood andplastic resin impregnated materials are also the sources of excessive water,since residual moisture can remain in these structures due to their longerdrying time compared with pressboard. Heat generated during the servicedrives this excessive moisture to other thin insulation structures [17].


23

The other main source of excessive water in transformer insulation is thedecomposition of paper. Due to thermal stresses acting on paper insulation,high molecular weight cellulose chains in paper undergo scission reaction inwhich water and furanic compounds are formed as by-products. The amountof water produced from the decomposition of paper varies with the actualcondition of the insulation [7, 28].

3.5 Degradation of oil-paper insulation systems

Current designs of power transformers are optimal in size. Therefore, theinfluence of the condition of transformer insulation on the correct operationof the unit is critical. Changes in electrical, mechanical and chemicalproperties of the insulation due to degradation phenomena during service canlead to severe damage. The rate of degradation of oil-paper insulationdepends on the existing thermal, oxidative, hydrolytic, electrical andmechanical conditions within the transformer [28].

High molecular weight cellulose chains undergo scission reactions duringservice. These reactions decompose the cellulose chains to low molecularweight chains, which yield the deterioration of the mechanical and electricalproperties of paper [7].

Heat, water and oxygen accelerate the decomposition of cellulose; all of theseare present in operating transformers. Degradation is a chemical reaction, itobeys the Arrhenius law of reaction kinetics. However, above 140 °C,degradation is faster due to a change in the degradation mechanism [7, 29].According to several studies, the rate of paper degradation is proportional towater content in the insulation. The presence of oxygen increases thedegradation rate of paper by a factor of 2.5 [7, 29, 30].

Not only the paper but also the properties of oil are affected by the presenceof oxygen. Oxidation of napthenic and parafinic hydrocarbons increases theacid number, whereas oxidation of aromatics remarkably increases the lossfactor [31, 32]. The rate of oxidation of oil highly depends on temperature,the presence of light and catalysts. At elevated temperatures and with thepresence of active metals, such as Cu, Pb and their alloys, rapid oxidation ofoil can be experienced [31].

Ionisation is the other important process that increases the ageing of oil-paperinsulation. During initial processing of oil-paper insulation, it is not possibleto remove whole trapped gases within the insulation. However, during


24

service, these local areas, where air bubbles are trapped, may be stressed byhigh electric fields. The field strength in the bubble can be higher than thebreakdown strength of air and can lead to discharges within the insulation.Oil molecules absorb energy from these discharges and decompose intohydrocarbons and hydrogen. If oil is saturated with gases, this hydrogen maycreate more bubbles, which develop more discharging paths within theinsulation. Therefore, the breakdown strength of insulation decreases withtime. Also discharges initiated in cavities of paper insulation produceconductive carbon particles [12, 32].

Chapter 4: Condition assessment of power transformer insulation

25

4. Condition assessment of power transformerinsulation

The condition assessment of power transformer insulation is essential sincethe degradation of transformer insulation is unavoidable. Mainly chemicalanalyses and electrical measurements are used for the condition assessmentof transformer insulation.

4.1 Chemical and physical analyses

Chemical analyses provide direct information about the actual condition ofthe insulation. Either a paper or oil sample taken from the interested oil-paperinsulation is used for these analyses.

To assess the quality of the paper insulation within transformers, one makesuse of a factor called the degree of polymerisation DP, which represents theaverage number of glucose rings in the cellulose polymer. DP is determinedby measuring the intrinsic viscosity of a paper solution in an appropriatesolvent. Original kraft paper has a DP of about 1100. After factoryprocessing, the DP drops down to a value about 750 and then decreases withtime. A limit of DP equal to 250 is often used as corresponding to the end ofthe lifetime of paper insulation [33]. Apart from DP measurements, ionchromatography tests are also used to characterise the thermal degradation ofpaper insulation by determining various sugars (monosaccharides,polysaccharides and anhydrous sugars) as described in[6]. However, all thesetests require a paper sample, which means that the transformer must be takenout of service, and that steel covers have to be opened. Consequently, takingpaper samples might be deleterious to the transformer.

Oil analyses are regularly used for the condition assessment of transformerinsulation, since obtaining a sample of transformer oil is much easier.Table 4.1 shows different physcio-chemical analyses of transformer oil andthe parameters determined by these tests.

Among the tests shown in Table 4.1, the results of Karl Fischer titration(KFT), dissolved gas analyses and high performance liquid chromatographycan also be used to predict the condition of paper insulation.


26

Table 4.1. Different physcio-chemical analyses on transformer oil.

Test name Parameters / properties

Karl Fischer titration Water content in the oil

Neutralisation number Acidity of the oil

Dissolved gas analyses Amount of different gasses i.e. H2,CH4, C2H6, C2H4, C2H2, CO, O2

High performanceliquid chromatography

Furanic compounds in mineral oil

Colour Deterioration and contaminants

Interfacial tension Interfacial tension of oil againstwater

For finding the amount of moisture content in transformer oil, and thereafterin the paper, coulometric KFT is widely used. This technique provides abetter resolution compared with volumetric titration when measuring lowmoisture contents [34]. The water content in paper is predicted using themoisture equilibrium curves between oil and paper [16]. It is important tokeep the percentage of moisture saturation of oil and the moisture in paperunder 30 % and 2.5 %, respectively. This may help to increase the lifetime ofa transformer [18].

Most of the chemical analyses must be performed under laboratoryconditions, which is a major draw back of these techniques.

4.2 Electrical tests

Electrical measurements, used for the condition assessment of transformerinsulation are simpler compared to the chemical analyses and it is possible toperform them on-site. Because of this simplicity, electrical tests are currentlypreferred for monitoring the condition of transformer insulation rather thanchemical tests. However, electrical tests do not provide direct informationabout the constituents of the insulation system. Therefore, results of theelectrical measurement must be calibrated.

Different electrical tests and their merits and demerits are discussed in thefollowing section.


27

4.2.1 Traditional methods

Insulation resistance, the polarisation index and loss factor are the widelyused traditional on-site electrical tests for assessing the condition oftransformer insulation. Partial discharge measurement is also a traditionalmeasuring technique for on-site and on-line insulation diagnostics, which hasbeen continuously improving.

Insulation resistance (IR)

Insulation resistance measurement is one of the conventional methods usedfor determining the dryness of the transformer insulation. IR is measured byapplying a fixed dc voltage, usually 0.25-5 kV, across the insulation. Theresultant current, which is a combination of capacitive current, absorptioncurrent and conduction current, monotonically decreases. Therefore, it is hardto measure true dc resistance, especially on new transformer insulation wherethe true dc level is reached only after a few hours. Usually the resistance ismeasured after 1 minute from the moment the dc voltage has been applied.The temperature of the insulation influences the IR. It has been observed thatan increase in temperature by 10 °C will approximately decrease the IR byhalf. Therefore, it is important to note the temperature of the insulation whenperforming the IR measurement [18].

In this technique, no absolute values are defined as acceptance limits. Instead,the IR values have to be compared with values from previous measurementsof the same transformer or from measurements of the same type oftransformers in order to evaluate the actual condition of the insulation. IRmeasurements give a good indication of whether or not the insulation is wetand contaminated, but it is rather difficult to identify partially wet insulation.It is recommended to use a guard electrode in IR measurements to avoid theinfluence of unwanted leakages, e.g. leakage current through bushinginsulation. If the measuring system does not provide a guard electrode thenthe surfaces of bushings must be cleaned well before the measurement.Results influenced by leakages can lead to wrong conclusions, especiallywhen the measured IR is low. Another reason for not totally relying on thismethod is that the IR of a poor insulation might appear higher than that of agood insulation, if it is not measured long enough and the total current variesas shown in Figure 4.1. Therefore, when the IR is low, it is recommendedthat other diagnostic tests be performed, too [4, 18, 35].


28

However, among utility companies this method is still very popular becauseof its simplicity and lower equipment cost compared with otherinstrumentation.

Figure 4.1. Variation of current during IR measurement on two different transformers.

Polarisation index (PI)

PI is an extension of IR measurement. In this technique, insulation resistanceis measured at two different times (i.e. at 1 minute and 10 minute intervals)after the voltage has been applied. By definition, PI is the ratio of insulationresistance at 10 minutes to that at 1 minute. PI measured on powertransformer multi-layer insulation is strongly influenced by macroscopicinterfacial polarisation and the geometrical composition of oil andpressboard. However, PI is less temperature dependent than IR, since it is aratio of two values at a certain temperature.

In the present interpretations of PI, it is indicated that PI is greater than unityfor a good insulation system. However, when the charging current of theinsulation varies, as shown in Figure 4.2, it is impossible to distinguishbetween the PI values of good and bad insulation. Transformers TF1 and TF2have very close PI values although the insulation of TF1 is better than that ofTF2. Therefore, the reliability of this technique is limited. It is also notpossible to use this method for predicting the moisture content orconductivity of the insulation.

Itot

TF2

1 min

I1

I2

t

After 1 min.I2 > I1

ThereforeIR2 < IR1

TF1


29

Figure 4.2. Variation of current during IR/PI measurements on two different transformers.

Loss factor (dissipation factor, tanδ)

Loss factor measurement is a traditional electrical measurement used foridentifying power loses in high voltage insulation under an alternatingvoltage.

The loss factor of insulation is defined as the ratio between resistive andcapacitive currents caused by an ac voltage applied across the insulation(Figure 4.3). Subsequently, the total loss of insulating material ischaracterised by the loss tangent.

Figure 4.3. Phasor representation of insulation current.

Usually the loss factor is measured at power frequency, that is to say at 50 Hzor 60 Hz depending on the operational frequency of the system. Nevertheless,in some measuring systems frequencies close to the power frequency are

δ

VIres

Icap

Itot

cap

res

I

I=δtan

Itot

TF1

TF2

1 t (min)10


30

used as the measurement frequency for eliminating power frequencydisturbances while taking on-site measurements. For example in theliterature, we can find an empirical relation between loss factor at 80 Hz and50 Hz for a specific type of current transformer [32]. But this method shouldbe developed further if it is to be applied to other transformers.

In conventional loss factor measurement, a Schering bridge coupled with ahigh voltage ac source is used. Test voltage used in typical field test setsvarying broadly (10 V-12 kV). A voltage lower than the rated voltage of theinstrument is used for loss factor measurement.

Loss factor is sensitive to temperature and varies with frequency. Hence, forloss factor to be a meaningful parameter, it is necessary to provideinformation on the frequency of applied voltage and the temperature ofinsulation during measurements. Apart from this, the loss factor depends onthe geometrical composition of the oil-paper insulation. Because of this, theloss factor of different insulation systems cannot be compared.

When partial discharges occur there is a tip-up in the loss factor versusvoltages curve [36]. The Garton effect is another phenomenon, which on theother hand, reduces the loss factor with increase in test voltage [37].

Partial discharge measurements (PD)

Major sources of partial discharge activity on power transformers are defectscaused by the mechanical deformation of windings, deterioration and ageingof the components and defects of the insulating structure of tap changer.Partial discharges within transformer insulation can be detected using eitherelectrical or acoustic methods. Electrical methods are more sensitivecompared with acoustic methods, but it is rather hard to obtain goodsensitivity with these methods due to other sources of electric noise,especially corona discharges. Acoustic methods are simple and they detectmainly arcing processes within the transformer. A good knowledge of theinternal construction of the transformer is needed to correlate the results ofPD measurements with the deterioration mechanism. One of the advantagesof this technique is that it can be used as an on-line diagnostic tool. Thisallows for identifying the faults and defects of insulation at an early stage[36, 38].


31

4.2.2 Dielectric response measurements

The major disadvantage of the first three techniques (IR, PI and loss factor)described in the previous chapter is that they do not provide sufficientinformation about the condition of insulation, which is necessary for areliable condition assessment. Dielectric response measurements providemore information compared with the above methods. Though thesetechniques were recently introduced for assessing the condition oftransformer insulation in the field they have been used for many years,especially for laboratory measurements.

Polarisation and depolarisation currents (PDC)

Polarisation and depolarisation currents measured between low voltage (LV)and high voltage (HV) windings of oil-paper insulated power transformershave been used for assessing the condition of the insulation [39-42]. Formeasuring the polarisation current, fixed dc voltage (over 500 V) is appliedacross the separately short-circuited LV and HV terminals. Then thedepolarisation current is measured by short circuiting the HV and LVterminals through an electrometer after dc voltage has been removed. Aguard terminal is also exploited for avoiding the influence of leakagecurrents. Presently, computer controlled PDC measuring systems areavailable for on-site measurements [43, 44].

The major advantage of this method is that more information can be gatheredcompared with the traditional IR and PI measurements. In-between twoconsecutive PDC measurements, the end terminals have to be short-circuitedfor a sufficiently long time to reduce memory effects. A simple rule of thumbon time domain measurements is that the short-circuit should last for at leastas long as the previous charging time before starting a new measurement.Very sophisticated noise suppression methods are needed for PDCmeasurements, which is another draw back to using this method as an on-sitediagnostic test.

Recovery voltage measurements (RVM)

As described in Chapter 2, RVM is the other time domain technique forstudying slow polarisation processes in insulating materials. When it isutilised for identifying the condition of transformer insulation, most oftenseveral measurements between HV and LV terminals are conducted fordifferent charging times while the ratio tc/td is maintained at 2 (Figure 2.2)


32

[45, 46]. Typically, a voltage between 500-2000 V is applied as the chargingvoltage.

The typical measurement set-up used for RVM is shown in Figure 4.4.

Figure 4.4. Schematic diagram of RVM.

In a study conducted by Hungarian scientists [45, 46] a variation of thecentral (dominant) time constant used for estimating moisture content inpaper insulation. When tc/td is 2, the central time constant is the charging timetc where the maximum recovery voltage is highest. As mentioned in [45, 46]the central time constant decreases significantly with the ageing of insulation.Although this interpretation is very simple, the predicted moisture content inpaper insulation is often higher than the actual amount [1, 47]. In anotherapproach, secondary time constants, corresponding to subsidiary peaks ofUmax versus tc curve, are used for estimating the moisture content in paper[47]. Here, a plot of the initial slope of the recovery voltage curve α againstthe maximum recovery voltage Umax, which is known as “Guuinic”representation, is used to confirm the oil peak and for identifying thepresence of secondary time constants above the dominant time constant.These subsidiary peaks are considered as reflections of polarisationphenomena of solid insulation [47].

Dielectric spectroscopy in frequency domain (FDS)

In the FDS technique, a known sinusoidal voltage is applied across theinsulation. The measurement is repeated for several frequency sweeps.Usually these measurements are carried out from high frequency to lowfrequency for minimising memory effects. For increasing the reliability ofmeasurements, at least two voltage cycles are applied at each measurementfrequency. Therefore, a rule of thumb is that the total measuring time, whichis a critical parameter for on-site measurements, is equal to four times thelength of the period of lowest frequency [41].

Voltagesource

SW1

SW2

Testobject

Voltmeter


33

The major advantage of FDS measurements is that the inherent smallbandwidth of electronic components is relatively insensitive to interferenceand therefore a high voltage power supply is not necessary. It is also possibleto avoid measurements at power frequency by selecting a suitable frequencysweep. The other advantage is the possibility of utilising three terminalconfiguration, which excludes the unnecessary leakage current [41].

Chapter 5: Modelling dielectric response

35

5. Modelling dielectric response

Although the dielectric response function reflects the condition of insulation,it is rather difficult to identify the actual state of the insulation (e.g. the exactamount of moisture in the insulation) only by observing the relevant curves,especially when the insulation is a complex combination of differentmaterials. This problem can be solved, to some extent, by comparing thedielectric response of insulation under study with the dielectric response ofwell-defined similar types of insulation samples. The dielectric response ofinsulating materials can be expressed as an analytical function of time orfrequency, which offers possibilities for fast and easy mathematicalmanipulations in such comparisons. In addition, when the insulation is acomplex combination of two or more materials, modelling the geometricalarrangement of insulation structure provides better comparisons between theresponses of the insulation and the models.

5.1 Different modelling techniques

Two different approaches to finding an analytical function, which representsthe dielectric response of insulation, have been used in [4, 35, 48-50].

In one of these approaches, called the functional approach, [4, 48-50] ananalytical expression is fitted to a time or frequency dependent dielectricresponse by selecting suitable coefficients for the expression. A few ofwidely used such expressions for modelling slow polarisation processes arediscussed below.

In the other approach, called the equivalent circuit approach, the behaviour ofinsulation is modelled by equivalent RC circuits [35], which is also discussedin the latter part of this section.


36

5.1.1 Debye model with single and distributed time constants

Time and frequency domains dielectric response functions of the classicDebye model are given in (5.1 A) and (5.1 B), respectively.

( ) τ

τε t

etf−∆= (5.1 A)

( )ωτεωχj+

∆=1

ˆ (5.1 B)

where, τ is known as the dielectric relaxation time and ∆ε is the dielectricstrength.

Debye model has been derived under the assumption that dipoles in therelaxing medium do not interact with each other. Therefore, simple Debyebehaviour is not applicable to most materials other than polar liquid [2]. Timeand frequency dependent Debye responses are shown in Figure 5.1 Figures(A) and (B), respectively.

(A) (B)

Figure 5.1. Time and frequency dependency of different dielectric response functions.

t=τ=1/ωp log(t)

log

(f(t

))

Debye

Generalresponse

-n

-1-m

ω=ωp log(ω)

log

(χ))

χ’

χ’

χ’’

χ’’

ω

ω−1

ωm ωn-1

Debye

General

response


37

log (ω)

log

(χ′′(

ω))

totalresponsesingle Debye

loss peaks

log (t)

log

(f(t

))totalresponse

single Debyeresponses

Dielectric responses, which depart from the ideal Debye behaviour, canusually be interpreted by the distribution of relaxation times. Here, the totalresponse is modelled by the addition of single Debye processes as,

( ) ( )∫∞

+=

0 1τ

ωττωχ dj

g(5.2)

where function g(τ) defines the distribution of relaxation times, which isconsidered as a discrete function in most of the interpretations. Graphicalrepresentations of this operation in frequency (loss) and time domains areshown in Figure 5.2.

Figure 5.2. Schematic representation of non-Debye dielectric response by distribution ofrelaxation times.

One could also explain the physical significance of distributed relaxationtimes in solid materials as the presence of inhomogeneities in materials,which can create different surrounding environments for different dipoles [2].

5.1.2 General response function

The “general response” function can be used to model a dielectric response

function, which shows a transition of two processes at t = 1/ωp, as shown inFigure 5.1. In time domain “general response” function can be written as,

( ) ( ) ( ) 1,01

<<+

= + mntt

Atf

mp

np ωω

(5.3)


38

A logarithmic representation of this expression shows two straight lines withdifferent slopes. When t >> 1/ωp, the slope of the line is –(1+m) and forshort times (t << 1/ωp) this slope is -n. An approximate frequency dependentloss corresponding to this function can be written as [2],

( )n

p

m

p

−−

+

≈′′

1

1

ωω

ωω

ωχ (5.4)

The real component of the frequency dependent “general response” functionvaries as shown in Figure 5.1 (B). Values of n can be used to classify thematerial as a charge carrier system (0 < n < 1) or dipolar system (1 < n < 2).The dielectric response of pressboard and Kraft paper is well-defined by“general response” [2].

However, the “general response” function cannot be analytically Fouriertransformed. Therefore sometimes another function, which has the sameasymptotic behaviour as the “general response” function, is used formodelling. This function has an analytic Fourier transformation and its timeand frequency dependencies can be written as,

( )

⋅

−+

⋅=

−−

−−

ntmt te

teAtf

ττττ 1 (5.5 A)

( ) ( ) ( ) ( )( )

−Γ+

+

−Γ−

+

−Γ= −−− n

n

n

n

m

m

i

n

i

n

i

mA

111

1

1

1

1

1ˆ

ωτ

ωτ

τ

ωτ

τωχ (5.5 B)

However, this expression does not have any physical significance [50].

5.1.3 X-Y model

The functional approach is further developed in modelling the powertransformer insulation system by considering its geometrical arrangement[48]. Figure 5.3 shows the typical winding and insulation arrangement of apower transformer. As shown in the figure, the LV winding is usually

0 < m < 10 < n < 2


39

surrounded by the HV winding and these windings are separated by the maininsulation duct that consists of pressboard layers and oil channels. Therefore,response measurements between the HV and LV windings are affected bythis composite insulation system.

In core type transformers, the main insulation consists of cylindricalpressboard barriers in series with oil ducts and spacers as shown inFigure 5.4. The complex geometrical arrangement shown in this figure can besimplified by combining all oil ducts, barriers and spacers separately, whichsimplifies the modelling. Then the main insulation is simplified to the socalled X-Y model, as shown in Figure 5.5 [41, 48].

Figure 5.3. Typical winding configuration of a power transformer.

Yoke

Yoke

Cor

e

LV

win

ding

HV

win

ding


40

Figure 5.4. Cross section of main insulation of a core type transformer.

Figure 5.5. Simplified insulation structure of a core type power transformer.

Here,

duct theofperiphery

spacer theof width totalduct theofwidth

barriers totalof thickness

=

=

Y

X

In real power transformers, X and Y often vary between 0.2 - 0.5 and0.15 - 0.25, respectively [48].

oilspacers

barriers

Y 1-Y

X

1-X

LV

HV barriers

spacers


41

The dielectric response across the X-Y system can be calculated when theindividual dielectric response of oil, spacers and barriers are known. Theresponse of the oil is described using the constant values of dc conductivity σand permittivity εr ( = 2.2). Since both barriers and spacers are formed bypressboard, they can be treated as a single material. The dielectric response ofpressboard has significant dispersion in the time and frequency range ofinterest. Hence, the response of pressboard is described by its dielectricresponse function or susceptibility in time and frequency domains,respectively. Dielectric measurements of well-defined pressboard samples areused for forming a database, where information of relevant responsefunctions with different moisture content is stored [1, 41, 48]. Theseresponses can be well described by the general response function [2, 48]. Thetemperature dependence of oil and paper is treated as described inChapter 2.4. This is characterised by the activation energies of 0.9 eV and0.7 eV for pressboard and oil, respectively [48].

In addition to above information, when X and Y for a particular transformerare known, (5.6) can be used for calculating the corresponding frequencydependent dielectric response at a known temperature for a given moisturecontent in pressboard and the conductivity of oil.

( )

barrieroilbarrierspacer

duct XXY

XXY

T

εεεε

ωε

ˆˆ

1

1

ˆˆ

1,ˆ

+−−+

+−= (5.6)

Where,

( )

ωεσεε

ωεεε

0,

)(ˆ

,ˆˆˆ

Tj

T

oilroil

pressboardbarrierspacer

−=

==(5.7)

Furthermore, the following set of equations can be used for calculating thepolarisation current Ipol of the same insulation system in the time domain.

boilspol IIII =+= (5.8)

sob UUU += (5.9)

Here Is, Ioil and Ib are the polarisation current through spacers, oil andbarriers, respectively. Ub and Uso are the voltages across the barrier and the


42

spacer components, respectively. The respective currents can be calculated byapplying (2.18) to each material separately.

When charging time tc and discharging time td are specified, (2.24) combinedwith (5.9) can also be used for deriving the return voltage spectrumcorresponding to the X-Y arrangement of the considered transformer.

The measurement techniques described in Chapter 4.2.2 can be utilised forperforming FDS, PDC and RVM measurements. Then the results obtainedfrom these measurements are compared with the relevant derived quantitiesusing the X-Y model. Error between derived and measured quantities isminimised by changing the amount of moisture content in the pressboard andthe conductivity of the oil until a best fit is obtained. This allows forpredicting the moisture content in paper and the conductivity of oil in theinvestigated transformer.

In the second modelling approach, each material is represented by anequivalent RC circuit, instead of a single analytical function. An example ofsuch a circuit representing a single material is shown in Figure 5.6.

Figure 5.6. Equivalent RC circuit representing the dielectric response of an insulation.

Here, the dielectric dispersion of this material is characterised by n parallelconnected RC elements. When the geometrical capacitance C0 is known, for along charging time tc the response function f(t) can be written as,

( ) ( )i

n

ii

depol tAUC

Itf τ/exp

10

−=≈ ∑=

(5.10)

CR

R1 R2 R3 Ri

C1 C2 C3 Ci

Rn

Cn

U


43

where,

iii

i

c

ii

CR

t

RCA

=

−−=

ττ

exp11

0

Therefore, using measurements of well-defined insulating samples, it ispossible to calculate the equivalent time constants τi, using a suitablesequential algorithm [35].

The application of this method for the X-Y model is shown in Figure 5.7[35]. In the figure, both barriers and spacers are formed by pressboard.Therefore, when barriers and spacers are connected in series, the equivalentcan be assumed as a single unit (index sb - stands for serial connection ofspacers and barriers).

Figure 5.7. Equivalent RC circuit of the X-Y model.

Roil

CbRb

C0ilRsbn

Csbn

Rb1 Rb2 Rbn

Cb1 Cb2 Cbn

Ipol

Ipol

B

A

Ioil

Ib

Uso

Ub

Isb

Rsb Csb

Rsb1 Rsb2

Csb1 Csb2

Rsbi

Csbi

Rbi

Cbi


44

The capacitance and resistance of oil can be directly calculated whenpermittivity εr and dc conductivity σ are known, see (5.11) and (5.12).

X

YCC roil −

−=1

100 εε (5.11)

−−=

Y

X

CRoil 1

1

0

0

σε

(5.12)

Therefore, for a particular transformer, the geometrical arrangement of whichis known (Co is known), the polarisation current Ipol of the main insulationcan be calculated using the following set of equations (5.13) - (5.17).

dt

UdC

R

UI so

oiloil

sooil += (5.13)

∑=

−++=

n

i

CR

t

bi

bbb

b

bb

bibieR

U

dt

UdC

R

UI

1

(5.14)

∑=

−++=

n

i

CR

t

sbisb

sbsb

sbisbieR

U

dt

UdC

R

UI

1

(5.15)

sobboil UUUandII +== (5.16)

sbbpol III += (5.17)

The calculated polarisation current is fitted with measured current bychanging conductivity and the RC parameters of distributed RC circuits asmentioned in [35]. The same parameters can be used to calculate thefrequency dependent dielectric spectroscopy and the return voltage spectrum,as described earlier in this chapter.

One of the major assumptions made in X-Y modelling is that the dielectricresponse is linear. However, in some cases this assumption may not be true,especially when insulating oil is new. Therefore, it is advisable to keep themeasuring voltage at the minimum possible level [48].


45

5.2 Effect of different parameters of X-Y model on the finalFDS response

The X-Y model is widely used for the diagnostics of transformer insulationdue to its simplicity and the direct relation between the model and theconstruction of transformer insulation [1]. However, the main disadvantageof this approach is the necessity for knowing the geometry of transformerinsulation, which is rather hard to find for utility engineers. Therefore,additional simplification of X-Y model would be useful to enable engineersto study the condition of insulation with little knowledge of its geometricalconstruction. For this purpose, it is essential to study the influence ofdifferent model parameters in the X-Y model on the final response of thesystem. Such analyses are presented in the following section. These analysesare based on the measurements made of the pressboard samples described inChapter 7.

5.2.1 Influence of oil conductivity σ

The conductivity of oil is highly temperature dependent. Therefore, it isnecessary to observe the influence of conductivity values in a wide rangesince, in our study, field measurements were performed in a temperaturerange varying between 25 ºC and 65 ºC. According to IEC 6422, theconductivity of new mineral oil should be less than 16.5 pS/m at 90 ºC [51].By assuming an activation energy of 0.7 eV [48] (2.27) gives thecorresponding conductivity at 25 ºC as 0.1 pS/m. Hence, 0.05 pS/m at 25 ºC(or 1.25 pS/m at 65 ºC) is taken as the lower limit of the conductivity rangeselected. The upper limit of the range is selected as 104 pS/m at 65 ºC (or4·102 pS/m at 25 ºC), which is more than 10 times higher than the acceptancelimit of the conductivity of used oil.

Figure 5.8 shows the influence of oil conductivity on FDS response at 25 ºC ,when the amount of moisture content in paper and the amount of pressboardin the insulation system are kept at a minimum level. In the figure, thevariations of permittivity and loss clearly show the Maxwell-Wagnerbehaviour of a parallel system, which is mainly influenced by the conductionin the oil and, at the same time, the dispersion of the spacer component isnegligible. However, one can observe that when oil conductivity is less than

4·102 pS/m, it has little influence on the permittivity ε´ of the system atfrequencies above 100 Hz.


46

Figure 5.9 shows the influence of oil conductivity at 65 ºC, when the amountof moisture content in paper and the amount of pressboard in the insulationsystem are kept at a maximum level. Due to the significant influence of lowfrequency dispersion in the paper, the influence of oil conductivity on thetotal permittivity of the system is weaker in comparison with the previouscase. In addition, oil conductivity, which lies within the same range, has little

influence on the permittivity ε´ of the system at frequencies above 100 Hz.

10-4

10-2

100

102

104

100

101

102

Frequency (Hz)

ε '

0.05 pS/m4.5 pS/m400 pS/m

10-4

10-2

100

102

104

10-4

10-2

100

102

Frequency (Hz)

ε ''

0.05 pS/m4.5 pS/m400 pS/m

Figure 5.8. Derived real and imaginarycomponents of complex permittivity forthe X-Y model for different values of oilconductivity at 25 ºC, when X=0.2,

Y=0.15 and moisture content=0.2%.

10-5

10-3

10-1

101

103

105

100

101

102

ε '

10-5

10-3

10-1

101

103

105

10-2

100

102

ε ''

1.25 pS/m 112 pS/m 10000 pS/m

1.25 pS/m 112 pS/m 10000 pS/m

Frequency (Hz)

Frequency (Hz)

Figure 5.9. Derived real and imaginary

components of complex permittivity for

the X-Y model for different values of oil

conductivity at 65 ºC, when X=0.5,

Y=0.25 and moisture content=5%.


47

The total permittivity ε´ at 1 kHz under different conditions is given inTable 5.1. Case 1 and Case 2 in the table correspond to the two casesdescribed in Figure 5.8 and Figure 5.9, respectively. This reveals that in bothcases the influence of oil conductivity, which lies within the specified range,has little influence on the ε´ at 1 kHz . The significant difference of ε´ at1 kHz between the two cases considered is mainly caused by the variation ofother parameters.

Table 5.1. ε´ at 1 kHz under different conditions.

Case No. Conductivity σ(pS/m)

ε´ at1 kHz

0.05 2.66

4.5 2.66Case 1

400 2.66

1.25 3.67

112 3.67Case 2

10000 3.68

5.2.2 Influence of spacers

According to previous studies, in a typical power transformer insulation, theperipheral area covered by spacers can be usually vary from 15 % - 25 %[48]. Based upon this information, the maximum influence of spacers on thefinal dielectric response can be encountered when Y equals 0.25. Hence, theinfluence of spacers is examined by comparing the dielectric response of theX-Y system with Y equals 0.25, with the same response with Y equals 0. Theinfluence of spacers is studied under different conditions since the totalresponse is dependent on the amount of barriers, moisture content, oilconductivity and temperature (Figure 5.10 – Figure 5.13). These figures alsoshow the residual curves representing the difference between two responsecurves corresponding to Y = 0.25 and Y = 0.

Table 5.2 describes the way the spacers influence the total dielectric responseunder different conditions, based on the results shown inFigure 5.10 - Figure 5.13.


48

10-4

10-2

100

102

104

10-2

100

102

104

Frequency (Hz)

ε '

Y=0.25Y=0Residual

10-4

10-2

100

102

104

10-4

10-2

100

102

104

Frequency (Hz)

ε ''

Y=0.25Y=0Residual

X = 0.2

X = 0.5

X = 0.2

X = 0.5

X = 0.2 X = 0.5

X = 0.2 X = 0.5

Figure 5.10. Influence of spacers with different amounts of barriers.


49

10-2

100

102

104

100

102

104

Frequency (Hz)

ε '

Y=0.25Y=0Residual

10-2

100

102

104

10-2

100

102

104

Frequency (Hz)

ε ''

Y=0.25Y=0Residual

mc = 0.2 %

mc = 5 %

mc = 0.2 %

mc = 5 %

mc = 5 %

mc = 0.2 %

mc = 0.2 %

Figure 5.11. Influence of spacers with different amounts of moisture content.


50

10-2

100

102

104

10-2

100

102

104

Frequency (Hz)

ε '

Y=0.25Y=0Residual

10-2

100

102

104

10-2

100

102

104

Frequency (Hz)

ε ''

Y=0.25Y=0Residual

σ = 1000 pS/m

σ = 10 pS/m

σ = 10 pS/m

σ = 1000 pS/m

σ = 1000 pS/m

σ = 1000 pS/m

σ = 10 pS/m

σ = 10 pS/m

Figure 5.12. Influence of spacers at different oil conductivity values.


51

10-2

100

102

104

101

Frequency (Hz)

ε '

Y=0.25Y=0Residual

10-2

100

102

104

10-2

100

102

104

Frequency (Hz)

ε ''

Y=0.25Y=0Residual

T = 25 oC T = 65 oC

T = 25 oC

T = 25 oC

T = 25 oC

T = 65 oC

Frequency (Hz)

Figure 5.13. Influence of spacers at different temperatures.


52

Table 5.2. Influence of spacers under different conditions.

State defineparameter

Meanrelative

error (in %)

Permittivity (ε´) at1 kHzFig.

No.

Name Value

Values ofother

parameters

mc-% ;σ-pS/m ; T-ºC

ε´ ε´´Y=0.25

Y=0

Error(%)

5.10 X0.2

0.5

mc – 5

σ (25 ºC ) – 10

T – 25

14

9

20

19

3.1

3.6

2.5

3

19

17

5.11mc

(%)

0.2

5

X – 0.2

σ (25 ºC ) – 10

T – 25

12

19

33

30

2.8

3.1

2.4

2.5

14

19

5.12σ (25 ºC )

(pS/m)

10

103

X – 0.2

mc – 5

T – 25

19

10

30

15

3.1

3.1

2.5

2.5

19

19

5.13T

(ºC)

25

65

X – 0.2

mc – 5

σ (25 ºC ) – 10

19

20

30

35

3.1

3.2

2.5

2.5

19

22

As shown in all four figures (Figure 5.10 – Figure 5.13) within the specifiedranges of all the parameters of interest, the affect of spacers on the shape ofthe final dielectric response curve is not particularly significant. This effectcould be explained by the influence of the more conductive oil component,which is in parallel to the less conductive spacer component. One can clearlyobserve this in Figure 5.12, where dielectric responses with two differentvalues of oil conductivity are plotted. The figure shows that when the oilconductivity increases residual substantially decreases.

In Table 5.1, the column corresponding to the relative error, shows a meanerror introduced by removing spacers from the model. The mean error


53

introduced to the real component of the permittivity is always less than 20 %.However, in the imaginary component, this can reach up to 35 %.

According to the results shown in Table 5.1, the maximum influence ofspacers on the total dielectric response can be experienced at highertemperatures with a minimum level of barriers, high moisture content andlow oil conductivity.

5.2.3 Variation of permittivity at 1 kHz (ε´1kHz)

As described in the Section 5.2.2, oil conductivity has little influence onpermittivity over 100 Hz. Moreover, Figure 5.13 and Table 5.1 show that theeffect of temperature on permittivity at 1 kHz is also insignificant.Furthermore, the last right hand column of Table 5.2 shows the estimatederror of permittivity ε´ at 1 kHz when the spacers are removed. One can seethat this error is always under 22 %.

The other two parameters, which affect ε´ at 1 kHz are moisture content inpaper and the amount of barriers in insulation. Figure 5.14 shows theinfluence of these two parameters with-in the region of interest of eachparameter.

0 1 2 3 4 5

2.5

2.6

2.7

2.8

2.9

3

3.1

Moisture content (%)

ε ' @

1 k

Hz

Mean = 2.45

Mean = 2.59

Mean = 2.76

Mean = 2.94

X = 0.2

X = 0.3

X = 0.4

X = 0.5

Figure 5.14. Variation of ε´ at 1 kHz with moisture content in paper and amount of barriers

when Y=0.


54

As shown in Figure 5.14, any variation of ε´ at 1 kHz with moisture contentis dependent on the amount of barriers. When X = 0.5, the dielectric responseof the barriers has the greatest influence on the total response. This gives a

maximum variation of ε´ at 1 kHz (∆ε´1 kHz) with moisture content, the value

of which is 0.22. Table 5.3 shows a variation of ε´ at 1 kHz as a percentage of

lowest ε´ at 1 kHz for each X value. Therefore, we assume that ε´ at 1 kHzdoes not vary with moisture content for a given X and is equal to the mean

value of ε´ at 1 kHz (Figure 5.14), then the introduced maximum percentageerror should be around 4 % (Table 5.4).

Table 5.3. Percentage variation of ε´ at 1 kHz at different X.

X ∆ε´1 kHz (∆ε´1 kHz * 100)/min(ε´1 kHz)

0.2 0.06 2.5

0.3 0.11 4.3

0.4 0.16 6.0

0.5 0.22 7.8

Table 5.4. Possible maximum percentage error if constant ε´ at 1 kHz (mean) value is

assumed at each X.

X Mean ε´1 kHz Maximum error (%)

0.2 2.45 1.2

0.3 2.59 2.4

0.4 2.76 3.4

0.5 2.94 4.3

The variation of the mean ε´ at 1 kHz with X is shown in Figure 5.15. Thepoints obtained from the calculation are points on a quadratic curve describedby the following expression, in which

2.294.02 ++= xxy (5.18)

This provides an easy way for calculating the mean ε´ at 1 kHz for a given X.


55

0.2 0.25 0.3 0.35 0.4 0.45 0.52.4

2.5

2.6

2.7

2.8

2.9

3

3.1

y = 1*x2 + 0.94*x + 2.2

X

Me

an

ε ' @

1 k

Hz

Mean from Figure 5.14 Fitted quadratic curve

Figure 5.15. Variation of mean ε´1 kHz with amount of barriers.

5.2.4 Conclusion of the analyses

The above analyses reveal that the influence of spacers on the total dielectricresponse of the X-Y system is relatively low and permittivity ε´ at 1 kHz ismainly determined by the amount of barriers. Therefore, when there is a lackof construction details of transformer insulation one can reduce the so calledX-Y model to a simple X model, while keeping the resultant errors within areasonable limit.

5.3 Modelling using X model

The dielectric response of the system shown in Figure 5.16 can becharacterised by the conductivity and permittivity of oil, the moisture contentin paper, the amount of barriers and temperature. Of these parameters, themoisture content in paper and the amount of barriers are the only twounknown, when separate measurements can be carried out to measure theconductivity of oil.


56

Figure 5.16. Further simplified X model of the transformer insulation.

The dielectric response of the X model can be written as shown in (5.19).

( )( )

XTj

XT barrier

0

oil

modelX

ˆ

2.2

1

1,ˆ

ε

ωεσ

ωε +

−

−= (5.19)

Equation (2.27) is used to calculate the conductivity at temperature T fromthe measured conductivity at a known temperature. The dielectric responsemeasurements of pressboard samples at a known temperature are alsotransformed into the temperature T by using (2.26).

In the modelling technique proposed, first a barrier percentage is assumedand then (5.18) is used for calculating the corresponding ε´ at 1 kHz and thisvalue is utilised to calculate the geometrical capacitance of the transformer,as shown in (5.20).

kHz

measuredkHzCC

1

,10 ε ′

′= (5.20)

This provides a path for transforming the measured complex capacitance intocomplex permittivity.

Subsequently, the percentage error between the complex permittivity of themodel and the transformer is minimised by changing the amount of moisture

barrier

oil

X

1-X


57

content in paper. This error minimisation is performed using an optimisationroutine developed in MATLAB software. The same procedure is followed fordifferent X values until the minimum percentage error is obtained.

5.4 Modelling using distributed relaxation times

Another modelling technique that can be utilised to model transformerinsulation without considering geometrical information, is presented below.The complex insulation system is modelled using distributed relaxationtimes. Each Debye relaxation can be represented by an equivalent series RCcircuit. Therefore, the insulation system is denoted as shown in Figure 5.6.

Subsequently, the complex capacitance ω) of insulation can be written as

( )

∑ ∑

∑ ∑

∑

= =

= =

=

+∆

+−+∆

+=

+∆

+−+∆

+=

−

+∆

+=

n

i

n

i i

ii

i

ihf

n

i

n

i i

ii

i

ihf

n

i i

ihf

CGj

CC

jC

jj

CC

1 12222

1 122

0220

1 00

11

11

1ˆ

τωωτ

ωτω

τωωτε

ωεσ

τωεε

ωεσ

ωτεεω

(5.21)

where,

Chf is the capacitance of insulation at highest measurable frequency

∆Ci is the strength of relaxation process corresponding to relaxation time τi

G is the dc conductance of the insulation.

When measured complex capacitance data ( ω)m) are available, thetechnique described in the following section can be utilised to calculateunknown parameters in (5.21).

By considering real components of the measured and modelled complexcapacitances,

( ) hfrealm

n

i i

i CCC

−=+∆∑

=ω

τωˆ

1122

(5.22)


58

The coefficients τi and ∆Ci are determined by a sequential algorithm whereleast square optimisation is implemented. In the first step of this algorithm, ntime constants, which are equally distributed in a logarithmic scale, arechosen. Maximum and minimum τi are selected as the reciprocal of theminimum and maximum angular frequency involved in the correspondingmeasurement. Then,

minmax

maxmin

1

1

ωτ

ωτ

=

=(5.23)

Subsequently, the constrained least square optimisation technique provided inMATLAB 6 is used for calculating the strength of relaxations, ∆Ci. Duringthe optimisation the selected τi values also change within a specified rangefor obtaining a better fitting.

Thereafter, (5.24) is used for determining the dc conductance of insulationusing measured loss ( m imag).

∑= +

∆−=n

i i

iiimagm

CCG

122

2

1ˆ

τωωτ

(5.24)

Chapter 6: Transformers at the Ceylon Electricity Board

59

6. Transformers at the Ceylon Electricity Board

6.1 Introduction

The Ceylon Electricity Board (CEB), a power utility of Sri Lanka, has amonopoly on generation and transmission on the island. About 90 % of thegeneration is owned by CEB and the balance is owned by independent powerproducers. However, distribution is shared with the Lanka ElectricityCompany Limited (LECO) and Local Authorities (LA). More than 15medium scale generator stations (highest capacity is 210 MW), about 30 gridsubstations (mainly 132/33 kV), more than 100 primary substations(33/11 kV) and more than 10 000 low tension substations (33 or11 kV/400 V) are in the CEB system, which annually delivers about6500 GWh of energy. At the end of year 2000, the installed capacity was2000 MW. Until the year 2000, hydro electricity was the major component ofthe capacity, which is over 65 % of the total generation. Lakshapana andMahaweli are the two main complexes that generate 40 % and 51 % of thetotal hydro electricity, respectively. Of these, Lakshapana is the oldest hydrogeneration scheme and started operation in 1952 [52].

In all the above generator stations and grid substations with more than 150power transformers of different rating and voltage levels, differentmanufactures, and of broadly varying age are installed. Also more than 200transformers of rated power less than 10 MVA are installed at the primarysubstations. Numerous locally produced distribution transformers alsooperate in the system.

6.2 Generator transformers under study

In this study, we have mainly focused on generator transformers in theLaxapana and Mahaweli complexes. Table 6.1 provides information on theyear of manufacture and the rated power of all the transformers underoperation in these two schemes. According to the table, the age and ratedpower of these transformers is between 2 - 50 years and 2 – 50 MVA,respectively. During the study period, it was not possible to performelectrical measurements on all these transformers. By utilising possibilitiesarising from the CEB maintenance schedule, the transformers shown inTable 6.2 were selected for electrical measurements.


60

Transformers from T 1 to T 11 are located in 3 generator stations in theLaxapana complex. Others are installed in the Mahaweli complex. Annualplant factors shown in the Table 6.2 are for the year 1999. However, thesevalues do not vary broadly since the annual rainfall, which is almost constantin the area, is the main decisive factor if no major maintenance is involved.

Table 6.1. Generator transformers in Mahaweli and Laxapana complexes.

Year ofmanufacture

Power(MVA)

Phase No. ofTransformers

1963 11 Single 7

1965 18 Single 7

1974 24 Single 7

1985 26/38 Three 1

1987 28.5/38 Three 1

1989 13 Single 3

Laxapanacomplex

~1950 5 Single 6

1975 27 Three 1

1976 50 Three 1

1982 30 Single 9

1984 32 Single 10

1984 34.5 Three 2

1985 27 Single 7

1985 2 Three 2

1990 27 Three 1

Mahawelicomplex

2000 30 Single 1


61

6.3 Maintenance of generator transformers at CEB

6.3.1 Maintenance tasks carried out

Routine tasks

For cheap, reliable, safe and efficient operation of power transformers correctroutine maintenance is desired. This allows for precautions to be taken beforea sudden failure of the unit occurs.

In general, manufacturer’s specify tasks and the performance frequency ofeach task needed to maintain the quality of the transformer. Some of thesetasks depend on the design of the transformer. Therefore, routinemaintenance tasks carried out on CEB power transformers, vary from stationto station. For example, Table 6.3 shows the routine maintenance schedule oftransformer T 12 and other identical generator transformers in the samestation.

Table 6.2. Transformers under study.

ID No. Year ofmanufac.

Manufacturer Power(MVA)

Voltage(kV)

Phase Plantfactor(%)

T 1 – 7 1965 Canadian GE 18 12/132 Single 70

T 8 –10

1963 Le materielelectrique

France

11 11/132 Single 25

T 11 1974 Alsthom 24 12/132 Single 45

T 12 1984 GEC 32.5 12.5/220 Single 45

T 13 1990 Hyosung,Korea

27 12.5/132 Three 50

As shown in Table 6.3, the two major insulation tests carried out on thetransformer insulation system are insulation resistance (IR) measurementsand the dielectric strength of warm oil. The other important test is chemical


62

analysis of the insulating oil, which should be performed annually. However,in CEB this analysis is not performed as an annual test due to a lack ofinternal resources. Instead, the analysis is performed when the othermeasurements indicate inferior quality of the insulation.

On-site insulation improvements

CEB has its own facilities for on-site purification of transformer oil. Usuallythis is done when the results of the above mentioned measurements (IR,chemical analyses) fall outside acceptable range. The purification is mainlyused for removing excess moisture from transformer insulation. In addition,particles of dirt and dissolved gasses are also removed to some extent.However, with the available purification plants, at least 10 days ofcontinuous purification is needed for a significant improvement oftransformer insulation. The purification time depends mainly on the qualityof transformer insulation before purification and the individual specificationsof the purification plants.

Table 6.3. Routine maintenance schedule of transformer T-12.

Frequency of thetask (in months)

Jobs to be carried out

1

Check the drycol operation counter and whether wateris being ejected from the drycol; inspect thetransformer carefully to see whether there is any oilleakage or abnormal vibration conditions; check the oillevel of conservator and bushing; check the generalcleaning of the transformer.

3Check the condition of silica gel at the breather ofstorage oil tank.

12

Measure insulation resistance (IR) between windingsand from each winding to the tank; measure the oildielectric strength while oil is warm; operate the tapchanger in full range (no load); check the operation ofrelays; grease the bearings; measure IR values offan/pump motors; measure motor starting and runningcurrent; measure power factor of the bushing; checkthe condition of the paint; check the condition of motorbearings.


63

6.3.2 Information recording

Keeping stringent records of the collected data and other information isessential for identifying the condition of transformers under operation.However, the recording system used by CEB is not up to contemporarystandards. Therefore, in this study, it was hard to find information on themeasurement history of the all transformers of interest. For example,Table 6.4 shows the past measurement history of transformer T 13.

According to the data shown in Table 6.4, the insulation condition of thistransformer was drastically reducing during the period 1991 - 2002.However, other factors that influence these measurements, such as conditionof the bushings, atmospheric conditions during the measurements, theinstruments used for the measurements are not clearly mentioned. Therefore,it is hard to draw firm conclusions using the results of such measurements.

Table 6.4. Measurement history of transformer T-13 (LT – low voltage winding; HT – high

voltage winding; E – tank).

IR (GΩ) PIDate

LT – E HT – E HT – LT LT – E HT – E HT – LT

1991.11.22 5 15 --- 3 --- ---

1992.07.31 4 5 7 1.8 2 1.4

1993.11.25 1.7 2.1 7 1.5 1.7 ---

1995.05.30 1.5 0.9 4 2.3 1.4 1.9

1995.11.24 2.5 0.9 5 1.2 1.3 1.2

1996.12.03 1.8 0.9 5 2.2 1.5 1.8

1997.12.08 1.3 0.7 3 2.1 1.4 2.3

1999.01.13 3.8 0.6 1.4 2 1.5 1.8

2002.09.16 0.8 0.4 0.9 1.5 1.3 1.8

--- IR is higher than the measurable range of the instrument.


64

6.3.3 Generator transformer replacements

There are only four cases of replacing generator transformers reported in theLaxapana and Mahaweli complexes, as follows below:

• A 27 MVA 12.5/132 kV generator transformer was burnt down in 1989due to a guerrilla attack. Later transformer T 13 replaced it.

• A 30 MVA 13.8/132/220 kV three winding single-phase transformer wasburnt down due to an insulation failure after being in operation for 16years. During investigations performed after the failure it was found thatwhole windings were coated with thick carbon layer. Therefore onepossible reason for this breakdown could be the contaminants producedon load tap changer (OLTC) since in this particular design OLTC and thewindings were immersed in the same oil bath. After the investigations itwas decided to replace it with a new transformer since refurbishment ofthe damaged transformer would be too expensive.

• 13 MVA 11/132 kV three single-phase generator transformers werereplaced by new ones in 1989. Major reasons for replacement were theage (about 50 years) of the transformers and high number of faults noted.

• Six other 5 MVA single-phase generator transformers in the same powerstation will be replaced during an on-going rehabilitation project. Similarto the previous case, the major reasons for this are the age (operationstarted in 1958) of the transformers and high number of faults noted.

6.4 Distribution transformers under study

The CEB distribution system consists of two main categories called mediumvoltage (33/11 kV) distribution and low voltage (415 V) distribution systems.Therefore, most of the distribution transformers operate at 33 kV, 11 kV and415 V. Nevertheless, due to the rapid increase in demand for electricity andfor obtaining a more reliable supply, most of the 11 kV system is currentlybeing replaced by a 33 kV system.

The majority of the distribution transformers are 33/0.4 kV (less than500 kVA) sealed units. There are no specific routine maintenance proceduresfollowed by CEB for these transformers. Once a major fault in a distributiontransformer is identified, the transformer is replaced with a new one and the


65

old one is sent for possible factory maintenance. Table 6.5 shows details ofsome of the distribution transformers selected for this study.

Table 6.5. Some of the distribution transformers under study.

ID No. Year ofmanufac.

Manufacturer Power(kVA)

Voltage(kV)

Vector

DT 1 1974 Mitsubishi 500 33/0.4 Dyn11

DT 2 1981 BBC 200 33/0.4 Znyn11

DT 3 1996 LankaTransformers

250 33/0.4 Dyn11

The effectiveness of the oil purification process carried out on transformerDT 1 was studied by performing an oil analysis on oil samples and FDSmeasurements on the transformer, before and after oil purification.

Of the above listed transformers, transformer DT 3 has never been energised.The other two transformers are still in operation. Due to lack of records, itwas hard to find information about previous measurements or analyses ofthese transformers.

Chapter 7: Measurements

67

7. Measurements

7.1 Instrumentation for dielectric response measurements

During the study presented in this thesis two measurement set-ups wereutilised for dielectric response measurements.

FDS measurements were performed using a commercially available FDSmeasuring system IDA 200. Figure 7.1 presents a schematic diagram of thisinstrument.

Figure 7.1. Schematic diagram of frequency domain dielectric measuring system IDA 200.

Non-grounded measurements are often used for transformer measurements.The tank is connected to the guard electrode. Channels Ch1 and Ch2 in thedigital signal processing (DSP) board are used for measuring the magnitudeand phase of the applied voltage and the resultant current, respectively. Thenthe complex capacitance of the test object is calculated using (2.16).

The main features of the IDA 200 are given in Table 7.1 [53].

V AI

UVoltagesource

Computer withDSP - board

Sample

Z

ElectrometerMeasured voltage

Measured current

Controlvoltage

Voltmeter

Ch1

Ch2

Guardconnection


68

Table 7.1. Features of IDA 200.

Parameter Value

Voltage sources 1 – 0-10 V peak

2 – 0-200 V peaak

Frequency 0.1 mHz – 1kHz

Current 0-50 mA peak

PDC measurements were performed under laboratory conditions. A Keithley6571 electrometer was used for these measurements. Figure 7.2 shows theschematic diagram of a typical PDC measurement set-up.

Figure 7.2. Schematic diagram of PDC measurements.

In the electrometer settings the integration time for A/D conversion of thecurrent measurements was set to 20 ms, which was good enough to reducethe influence of noise, even at the lowest current levels measured. During themeasurements, the tank was isolated from the ground and connected to theguard electrode. The settings of the electrometer were such that the lowpotential terminal of the voltage source was internally connected to the lowend of the electrometer. Also when the voltage source was switched off, itstwo terminals were internally short-circuited. These features allowed forperforming the polarisation and depolarisation current measurements withoutusing any additional switching arrangement.

Electrometer

Voltagesource

SW1

SW2

Testobject

Guardconnection


69

7.2 Field measurements

Field measurements were carried out in Sri Lanka on the transformersbelonging to CEB as specified in Table 6.2 and Table 6.5. The measurementconfiguration used is shown in Figure 7.3.

Figure 7.3. Measurement configuration used in field measurements.

During the measurements, all the terminals of HV and LV windings wereshort-circuited separately. The grounded tank was connected to the guardterminal.

Field measurements were performed at different temperatures (between 30 ºCand 60 ºC) depending on the operational situation. When the measurementswere performed at elevated temperatures, the temperature of the oil wasnoted both at the beginning and at the end of each measurement. In most ofthe cases, the temperature difference was less than 3 ºC. Therefore,throughout this study the starting temperature was considered as themeasurement temperature.

Apart from the FDS measurements, whenever possible oil samples were alsotaken from the transformers for further chemical and electrical analyses. Oilsampling was done according to IEC 60475 [54].

HV

LVTo IDA 200

Voltageterminal

Currentterminal

Guardterminal Tank


70

7.3 Laboratory measurements

7.3.1 Laboratory transformer

The transformer selected for study in a laboratory environment was adistribution transformer (100 kVA, 20 kV/400 V), which was built in 1979by ASEA. It consists of a foiled low voltage winding and an enamel coatedhigh voltage winding. In 2000, the transformer was renovated and the oil waschanged. After the renovation, the transformer was not used until the testsdescribed here were conducted. The insulation of distribution transformersusually differs from what is typical for large power transformers. In thetransformer investigated, the insulation between the low and high voltagewindings consisted of pressboard barriers and glued masonite spacers.

In order to balance the water content in the oil and the paper, the transformerwas heated to above 70 °C for 2 weeks. This was done by continuouslyfeeding the high voltage windings with a current of 2.1 A, which is about70 % of the full load, while the low voltage windings were short-circuited.To minimise the heat losses and to keep the temperature at the level required,the transformer was thermally isolated. The electric resistance of the highvoltage winding was measured to calculate the temperature inside thetransformer.

Thereafter, oil samples were taken from the hot transformer (70 °C) for KFTanalyses and for dielectric characterisation.

For checking the effect of thermal treatment on the insulation, FDSmeasurements were repeated before and after the transformer was heated.These measurements were performed at 20 °C in frequency range10-4 Hz - 103 Hz and the voltage used was 200 Vpeak. Instrumentation wasconnected to the transformer as described in Section 7.2.

PDC measurements of this transformer were performed using the set-updescribed in Section 7.1. The PDC measurements were performed after thetransformer was cooled down to room temperature (20 °C) after heattreatment. A voltage of 500 V dc was applied across the main insulation for aperiod of 7.5⋅104 s for measuring the polarisation current. Then thedepolarisation current was measured for a period of 104 s by short-circuitingthe two winding sets through the electrometer.


71

7.3.2 Oil test cell

A three terminal stainless steel cylindrical oil test cell was used forperforming FDS measurements on the transformer oil-samples. Results ofthese measurements were used to calculate the conductivity of the oil. Theparameters of this cell are given in Table 7.2.

Table 7.2. Parameters of the test cell.

Parameter Value

Volume 45 ml

Electrodes distance 2 mm

Geometrical capacitance 70 pF

Figure 7.4 shows a schematic diagram of the cell used for FDSmeasurements. Before the cell was filled with oil it was cleaned with hexaneand dried. During the measurements the cell was placed on an insulatingplate for isolating the voltage terminal from the ground.

Terminals of IDA 200 instrument were connected to three terminals of thecell as indicated in Figure 7.4. To avoid non-linear effects, the appliedvoltage was limited to 5 Vpeak.

Figure 7.4. Three terminal test cell.

3 2 1

Insulating plate

1- Measuring terminal2- Guard terminal3- Voltage terminal


72

Since there are no relaxation processes in oil within frequency window ofinterest, the conductivity of oil can be calculated according to,

( ) ( ) ωεωωσ 00C

C ′′= (7.1)

where;

σ(ω) is the frequency dependant conductivity of oil,

C′′ (ω) is the imaginary part of complex capacitance at frequency ω,

C0 is the geometrical capacitance of the cell.

7.3.3 Pressboard samples

Impregnated pressboard samples containing different amounts of moisturewere taken for this analysis. These pressboard samples were prepared in thecentral laboratory of the Weidmann company in 1995 and stored in separateoil-filled cylindrical sealed ducts. All of these ducts were placed in the highvoltage laboratory of ETH Zürich, where all the measurements were carriedout in 2002. The same samples were also used for the measurementsdescribed earlier in [35].

Figure 7.5. Schematic diagram of the test cell.

Additionalweight

Voltageelectrode

GuardelectrodeMeasuring

electrode

Pressboardsample

Transformeroil


73

FDS measurements were performed on the impregnated samples to form adatabase on the correlation between the frequency dependence of thecomplex permittivity of pressboard and moisture content. Thesemeasurements were performed in a special test cell (Figure 7.5) using theIDA 200 system. This cell has been described in detail in [35]. All themeasurements were performed at room temperature (between 20 ºC and27 ºC).

The thickness and diameter of all the pressboard samples under study were2 mm and 159 mm, respectively. The diameter of the measuring electrodewas 113 mm, which yielded a geometrical capacitance between the electrodesof 44.4 pF. An additional weight of 2981 grams was placed on the topelectrode to apply equal pressure on the pressboard samples during themeasurements. FDS measurements were performed with 50 Vpeak ac voltage.

7.3.4 Karl Fischer titration measurements

The moisture content in pressboard samples was also measured using thecoulometric KFT technique. In this case, the indirect stripping oventechnique was utilised. In this method, a known weight (about 0.5 g) ofpressboard sample was placed in the oven, which was heated to 140 ºC.Moisture released from the pressboard was led away to the titration vessel bya dry N2 gas flow. The procedure described in IEC 60814 was followed fordetermining the moisture content in the pressboard [55].

For calibrating the moisture content estimated from FDS measurements, KFTanalyses were performed on oil samples. The direct coulometric KFTtechnique was utilised and the methodology described in IEEE 62 - 1995 wasfollowed for interpreting the results [18].

Chapter 8: Results and discussion

75

8. Results and discussion

8.1 Pressboard samples

Complex permittivities derived from the measured complex capacitance ofthe five different pressboard samples are presented in Figure 8.1. Theseresults are normalised at 27 ºC by assuming an activation energy of 0.9 eV.

10-2

100

102

104

100

101

102

Frequency (Hz)

ε '

mc=0.16 %mc=0.9 %mc=1.4 %mc=2.5 %mc=4.5 %

10-4

10-2

100

102

104

10-2

100

102

Frequency (Hz)

ε ''

mc=0.16 %mc=0.9 %mc=1.4 %mc=2.5 %mc=4.5 %

Figure 8.1. Real and imaginary components of complex permittivity as a function offrequency at 27 ºC for pressboard samples containing different amounts of moisture (mc).


76

As shown in the Figure 8.1, permittivity ε′ increases substantially at lowfrequencies with increasing moisture content in the pressboard. Dielectriclosses ε′′ also substantially increase with increasing moisture content. At thesame time, the variation of ε′ with moisture content at 1 kHz is relativelysmall. This behaviour is one of the basic observations used in thedevelopment of the X model, described in Chapter 5.2.

These data were used to form the database for modelling the dielectricresponse according to the X model and for estimating the moisture content inthe insulation of the transformers measured in the field and under thelaboratory conditions. Linear variations of the logarithmic values of both thepermittivity ε′ and the loss ε′′ between two consecutive moisture contentswere assumed for calculating responses corresponding to moisture contentsnot included in the database.

The moisture content in the pressboard samples estimated by KFT analysesare given in Table 8.1. MODS software, provided by the manufacturer of theIDA 200 instrument, was also used for estimating the moisture content inthese samples. This software uses two different databases for modelling; onecalled “cellulose” and another called “pressboard”. The former one is basedon dielectric measurements performed on paper samples, whereas the latterone is based on measurements on pressboard samples. The estimatedmoisture contents using both databases and their percentage difference withthe results of KFT analyses are also given in Table 8.1.

Table 8.1. Moisture content in pressboard samples.

MODS-“Pressboard” MODS-“Cellulose”Sample

No.

mcusingKFT(%)

mc (%) pd (%) mc (%) pd (%)

1 0.16 0.16 0 0.002 -99

2 0.9 0.68 -24 0.2 -77

3 1.4 1.14 -19 0.35 -85

4 2.5 2.73 9 1.5 -40

5 4.5 4 -11 2.6 -19

mc – moisture content ; pd – percentage difference.


77

As seen in the table, the estimates based on the database “cellulose” werealways lower than the estimates based on the database “pressboard”. At thesame time, the difference between the estimated moisture contents from thedatabase “pressboard” and from our own KFT analyses are relatively low(max 24 %).

Since insulation in power transformers is mainly comprised of pressboardand oil, “Pressboard” database of MODS software as well as the databasepresented in Section 8.1 were utilised for modelling the dielectric responsesof power transformer insulation presented in this thesis. However, theconstruction of distribution transformers is often different. Therefore, formodelling the results of measurements of distribution transformers, thedatabase “cellulose” was also used.

8.2 Distribution transformer in laboratory

8.2.1 Modelling using X-Y and X model

Results of FDS measurements obtained before and after the thermal treatmentof the transformer are shown in Figure 8.2. One may notice a small shift ofthe curves to the lower frequency region after the treatment. This shift ismainly seen at frequencies where the contribution from oil conductivitydominates the total dielectric response of an oil paper insulation system. Thecurves were modelled using both the X-Y model (MODS) and the X model.The modelled parameters and the results of oil analyses are shown inTable 8.2.

Table 8.2. Modelled and measured parameters of lab transformer.

“pressboard”

X-Y model

“cellulose”

X-Y modelX model

Oilanalyses

mc σ X Y mc σ X Y mc X mc σ

A 3.2 13 20 22 2.1 13 20 22 -- -- -- --

B 3.2 8.5 20 22 2.1 8.5 20 22 2.4 20 2 4

mc - moisture content in pressboard (%) ; σ - oil conductivity at 20 ºC (pS/m) ; A - before

treatment ; B - after treatment ; X and Y are in (%).


78

10-4

10-2

100

102

104

10-10

10-9

10-8

10-7

Frequency (Hz)

C ' (

F)

before treatmentafter treatment

10-4

10-2

100

102

104

10-12

10-10

10-8

10-6

Frequency (Hz)

C '' (F

)

before treatmentafter treatment

Figure 8.2. Comparisons of real and imaginary parts of the complex capacitance measuredon lab transformer at 20 °C before and after thermal treatment.

Two values for oil conductivity were obtained from the X-Y modelling. Theywere 13 and 8.5 pS/m, from the data corresponding to the measurementsbefore and after the thermal treatment, respectively. This explains the earlierconclusion based on the frequency shift between the two dielectric responsesin Figure 8.2. The conductivity of the oil measured after the thermaltreatment was 4 pS/m. This value is similar, but somehow lower than thevalues derived from the modelling.

Figure 8.3 shows the frequency dependent capacitance C′(ω), loss C′′ (ω) andσ of the oil, sampled from the lab transformer. The results presented were


79

measured at 20 ºC. As shown in the figure, slope of frequency dependent lossis nearly –1, which shows the dominance of oil conductivity in the dielectric

response of oil. Also it can be observed that the capacitance C′(ω) is almostconstant, which indicates the insignificant dielectric dispersion of oil withinthe measured frequency range. These two observations verify the validity ofmodelling the dielectric response of oil with a parallel connected resistor anda capacitor.

10-3

10-1

101

103

10-13

10-12

10-11

10-10

Frequency (Hz)

C ',C

'' (F

) a

nd

σ

(S/m

)

C'

C''

σ

Figure 8.3. Capacitance (C′), loss (C′′ ) and conductivity (σ) as a function of frequency at20 ºC for oil sampled from lab transformer.

The moisture contents estimated by means of MODS are different for bothdatabases used, but identical within each of the databases for Cases A and B.The value obtained from the “cellulose” database is lower and similar to thevalue obtained from oil analyses. The moisture content estimated from the Xmodel is also comparable with the one obtained from oil analyses. Accordingto IEEE std. 62-1995 [18] this value is close to the margin of the acceptancelimit, which is equal to 2.5 %.

As shown in Table 8.2, the barrier contents derived are also similar.However, due to the lack of exact information on the construction of thistransformer it was not possible to compare them with the real barrier content.


80

8.2.2 Comparison of time domain and frequency domainspectroscopy measurements

The FDS response of the lab transformer was modelled using the distributionof relaxation times and later it was used for deriving the corresponding timedomain response.

The distribution of relaxation times was derived in two different ways. In thefirst approach (Distribution 1), we assumed that there were no otherrelaxation processes outside of the angular frequency window used for themeasurements (2.9⋅10-03 rad/s to 6.3⋅103 rad/s). Therefore, all the derived timeconstants were limited to the time span corresponding to this window. In thesecond approach (Distribution 2), the measured data were extrapolated at thelow frequency end for two decades further by assuming that C′(ω) and C′′ (ω)changed proportionally according to the fractional power law.

( ) ( ) nCC −∝′′∝′ 1ωωω (8.1)

where n was set to be equal to 0.62. This allowed for increasing the time spanof the time constants derived by two decades. The distributions of dielectricstrengths and relaxation times obtained using the two approaches are shownin Figure 8.4. Both distributions contain a weak local peak at about 100 s.

The depolarisation currents derived from these distributions are comparedwith the measured depolarisation current, as shown in Figure 8.5. One maynotice that the current calculated from the Distribution 1 deviates from themeasured data at longer times (>100 s). On the other hand, the depolarisationcurrent calculated from the Distribution 2 agrees well with the measured data.This indicates the necessity extrapolations in the low frequency data forderiving correct parameters.


81

10-4

10-2

100

102

104

106

10-13

10-11

10-9

10-7

Relaxation times (s)

∆ C

' (F

)Distribution 1Distribution 2

Figure 8.4. Dielectric strengths of relaxation times derived by the two approaches used.

10-4

10-2

100

102

104

106

10-15

10-13

10-11

10-9

time (s)

De

po

laris

atio

n c

urr

en

t (A

)

Calculated depol. current distri 1Calculated depol. current distri 2Measured depol. current

Figure 8.5. Calculated and measured depolarisation currents.


82

8.3 Power transformers in the field

Results presented in this section are based on measurements performed onthe field transformers owned by CEB in Sri Lanka. Information on thesetransformers is provided in Chapter 6.

8.3.1 Single-phase power transformers

10-2

100

102

104

10-9

10-8

10-7

Frequency (Hz)

C ' (

F)

T4T5T6T7

10-2

100

102

104

10-11

10-9

10-7

C '' (F

)

T4T5T6T7

Frequency (Hz)

Figure 8.6. Real and imaginary components of the complex capacitance measured on powertransformers T4 – T7 as a function of frequency at 30 ºC.

Data for complex capacitance measured on transformers T4 – T7 arepresented in Figure 8.6. These single-phase power transformers are identical


83

in construction, in rating, and in age. They also have a similar number ofoperating hours. All the results presented in the figure were obtained whenthe corresponding generator was shut down for annual maintenance.Therefore, the temperature of these transformers was stable, around 30 ºC,when the measurements were performed. As shown in Figure 8.6, the

dispersion of capacitance C′(ω) at frequencies lower than 1 Hz is relatively

high. In addition, some deviation in C′(ω) between the transformers is alsonoticeable. The measured capacitance of T7 at the lowest frequency is higherthan the corresponding values for the other three transformers. One mayassume that this difference is caused by a higher moisture content in T7. This

can be seen when comparing the results of the oil analyses. Losses C′′ (ω) ofthese transformers have noticeable differences especially within thefrequency range where oil conductivity dominates the total response. Losses

C′′ (ω ) of transformers T4 and T7 are slightly higher than those of the othertransformers. The effect is caused by the higher oil conductivity in theses twotransformers.

Table 8.3. Comparison of modelled FDS results and the results from oil analyses.

Oil analyses X-Y model parametersX model

parametersIDNo.

ST σ mco mc σ mc X Y C0 mc X C0

T1 43 19 35 2.1 20 4 20 25 0.9 3.9 30 1

T2 40 25 36 2.6 22 4 20 25 0.9 3.8 31 1

T3 38 48 56 4.5 54 3.9 21 25 0.9 3.6 32 1

T4 47 190 86 3.9 194 4 21 25 1 4.2 31 1

T5 48 70 82 3.7 77 4 21 25 0.9 4.1 30 1

T6 54 142 140 4.9 150 4 20 25 1 4.1 29 1

T7 35 150 150 10? 182 4.3 20 25 0.9 4.4 28 1

ST – oil sampled temperature; σ - oil conductivity at 27 ºC; mco - moisture content in

oil (ppm); mc - moisture content in paper (%); C0 – Geometrical capacitance (nF).

Figure 8.7 shows the variation of capacitance and loss of transformersT1 - T3, which are also identical in construction to transformers T4 - T7.However, these transformers were measured at elevated temperatures (i.e. at


84

40 ºC and 45 ºC) since they were in operation just before the measurementsbegan.

The measured complex capacitances of all seven transformers were used tomodel their dielectric response. The parameters derived from the MODSsoftware and the X model are listed in Table 8.3. Results of the oil analysesof the corresponding transformers are also shown in the table.

10-4

10-2

100

102

104

10-9

10-8

10-7

Frequency (Hz)

C ' (

F)

T1T2T3

10-4

10-2

100

102

104

10-12

10-10

10-8

10-6

Frequency (Hz)

C '' (F

)

T1T2T3

Figure 8.7. Real and imaginary components of complex capacitance of transformers T1, T2and T3 at 40 ºC, 45 ºC and 40 ºC respectively.

The derived and measured oil conductivity values are compared afterrecalculating their values at 27 ºC by assuming an activation energy of


85

0.7 eV. As shown in Table 8.3, oil conductivity values obtained from MODSand the corresponding measured values are similar.

Moisture contents estimated from MODS are rather close to thecorresponding values estimated from the X model. However, these values aredifferent from the values estimated from the results of oil analyses, especiallyin T1, T2 and T7. Estimating the moisture content in paper based on themoisture content in oil is highly temperature dependent. Therefore, thisdifference can be due to errors in the temperatures of transformer insulationmeasured when oil samples were taken. The results of oil analyses shows a10 % moisture content in transformer T7, which is an extremely high andunexpected result for an operating transformer. These results may be due toan imbalance in moisture distribution between oil and paper during the timethe oil sample was taken, or due to the presence of contaminants in the oil,which react with iodine in the KFT solvent and yield an overestimate ofmoisture content.

The derived parameter X and the geometrical capacitance obtained from theX model are nearly 10 % higher than those obtained from the X-Y model.This higher estimate can be due to the negligence of spacer content in the Xmodel. Due to the lack of construction details, it was not possible to checkthe accuracy of these results.

The estimated moisture content showed that all the transformers testedcontained an excessive amount of moisture. Therefore, CEB has beenrecommended to perform vacuum oil purification on all these units as soon aspossible.

The results of measurements made on three other identical single-phasetransformers (T8 – T10) are presented in Figure 8.8, whereas the estimatedparameters for these transformers are listed in Table 8.4. As seen in

Figure 8.8, the C′(ω) and C′′ (ω) of these three transformers are similar.Therefore, we can clearly state that the current conditions of the insulation inthese three transformers are similar. This statement is further supported bythe parameters derived from the oil analyses. Moreover, the parametersobtained from the modelling are also fairly close.

Figures 8.8 – 8.10 illustrate the measured and the derived permittivities ε′and losses ε′′ for transformers T8-T10. The derived quantities were obtainedfrom the X model. It can be noticed in all three figures, that the measured andthe derived losses do not match well in the frequencies above 100 Hz. Thisdifference can be caused by the negligence of the influence of spacers in the


86

X model. Furthermore, the modelled loss curve of transformer T9 ismoderately higher than the corresponding measured curve within thefrequency range where oil conductivity dominates the total response. Thisdifference is due to the high oil conductivity value assigned to the X model,which was obtained from the measured oil conductivity and by assuming anactivation energy of 0.7 eV.

Table 8.4. Comparison of modelled FDS results and the results from oil analyses fortransformers T8 – T10.

Oil analyses MODS parametersX model

parametersIDNo.

ST σ mco mc σ mc X Y C0 mc X C0

T8 35 51 44 4.2 63 5.2 20 15 0.6 4.7 24 0.7

T9 47 52 108 4.9 57 5 20 15 0.6 4.7 24 0.7

T10 50 47 115 4.6 52 4.5 20 15 0.6 4.3 24 0.7

T12 41 3 42 2 3 2.4 20 15 0.8 1.9 25 0.9

ST – oil sampled temperature; σ - oil conductivity at 27 ºC; mco - moisture content in

oil (ppm); mc - moisture content in paper (%); C0 – Geometrical capacitance (nF).

Figure 8.12 illustrates the modelled and measured results for transformerT12, the age of which is less than 20 years. All the estimated parameters forthis transformer are also shown in Table 8.4. The estimated moisture contentand the oil conductivity of T12 are much lower than estimates for thepreviously considered transformers. As described in the previous case, thedifference between the modelled and measured loss ε′′ curves are mainlycaused by the oil conductivity assigned to the X model.


87

10-1

101

10-8

10-7

Frequency (Hz)

C ' (

F)

T8T9T10

10-1

101

10-10

10-8

10-6

Frequency (Hz)

C '' (F

)

T8T9T10

Figure 8.8. Real and imaginary components of complex capacitance of transformersT8 - T10.


88

10-3

10-1

101

103

100

101

102

mc=4.7 %

X=24 %

Frequency (Hz)

ε '

MeasuredModeled

10-3

10-1

101

103

10-4

10-2

100

102

Frequency (Hz)

ε ''

MeasuredModeled

X=24 %

mc=4.7 %

Figure 8.9. Comparison of modelled and measured permittivity and loss of transformer T8.


89

10-3

10-1

101

103

100

101

102

mc=4.7 %

X=27 %

Frequency (Hz)

ε '

MeasuredModeled

10-3

10-1

101

103

10-4

10-2

100

102

Frequency (Hz)

ε ''

MeasuredModeled

X=27 %

mc=4.7 %

Figure 8.10. Comparison of modelled and measured permittivity and loss in transformer T9.


90

10-3

10-1

101

103

100

101

102

mc=4.3 %

X=24 %

Frequency (Hz)

ε '

MeasuredModeled

10-3

10-1

101

103

10-4

10-2

100

102

Frequency (Hz)

ε ''

MeasuredModeled

X=24 %

mc=4.3 %

Figure 8.11. Comparison of modelled and measured permittivity and loss of transformerT10.


91

10-3

10-1

101

103

100

101

102

mc=1.9 %X=25 %

Frequency (Hz)

ε '

MeasuredModeled

10-3

10-1

101

103

10-4

10-2

100

102

Frequency (Hz)

ε ''

MeasuredModeled

X=25 %

mc=1.9 %



92

8.3.2 Three-phase power transformer

10-4

10-2

100

102

104

100

101

102

mc=3.5 %X=43 %

Frequency (Hz)

ε '

MeasuredModeled

10-4

10-2

100

102

104

10-4

10-2

100

102

104

Frequency (Hz)

ε ''

MeasuredModeled

X=43 %mc=3.5 %


In this study, there were not many possibilities for performing measurementson three-phase power transformers, since such transformers are rarelyinstalled in accessible generator stations. Opportunities for measuring gridtransformers were, at the time, excluded. T13 is the only three-phase powertransformer on which measurements could be carried out. Although thistransformer was installed very recently (1990), its insulation resistance (IR)value has been drastically reduced throughout the past years (see Table 6.4).Furthermore, the records of this transformer contain information that the oil


93

seals were not good enough and oil was leaking through the seals. Therefore,one possible reason for reducing IR was moisture ingress from theatmosphere through these poor oil seals. Therefore, one could expect fairlyhigh moisture content in its insulation.

Figure 8.13 illustrates the measured and modelled permittivity ε′, and loss ε′′of transformer T13. The X model yielded a moisture content of 3.5 % in theinsulation. The corresponding values derived from the X-Y model was 3.8 %.However, the corresponding moisture content derived from the oil analyseswas 2.3 %, which was much lower than the modelled values. At the sametime, the oil conductivity estimated from MODS (11 pS/m) and the measuredoil conductivity (9 pS/m) were simillar. The estimated geometricalcapacitance derived from MODS was 1.5 nF, and similarly, the geometricalcapacitance derived from the X model was 1.6 nF.

These results confirm that the moisture content in this transformer waselevated, although it has been in operation for less than ten years. Therefore,immediate action must be taken to repair the damaged oil seals and, ifpossible, to perform vacuum oil purification.

8.4 Distribution transformers in the field

In the first instance, FDS measurements were performed on the distributiontransformer DT1 before and after the purification procedure. The resultingdielectric response curves are shown in Figure 8.14. One should notice theshifts between corresponding curves, which were mainly caused bytemperature differences during the measurements. The main conclusion fromthe results obtained is that the purification procedure did not improve the oilquality. To check it, additional analyses of the oil samples, taken before andafter the purification, were done. Oil resistivity and other typical parametersof the oil were measured. The temperature dependencies of the resistivitiesare shown in Figure 8.15 and it is clear that the purification did not lead toany improvement of this parameter. Also the other analyses performed at thecompany ABB in Västerås, Sweden showed similar behaviour. The moisturecontent decreased from 26 to 14 ppm, the neutralisation number changedfrom 0.28 to 0.24 mg KOH/g-oil, the breakdown voltage from 76 to74 kV/2.5 mm, the loss factor from 0.519 to 0.476, and colour from 7.0 to6.5. These results confirmed the earlier conclusion based on FDSmeasurements. Estimated parameters derived from MODS, from the X modeland from oil analyses are given in Table 8.5.


94

10-3

10-1

101

103

10-9

10-8

10-7

Frequency (Hz)

C ' (

F)

C' at 25 oC before purification

C' at 35 oC after purification

10-3

10-1

101

103

10-10

10-9

10-8

Frequency (Hz)

C '' (F

)

C'' at 25 oC before purification

C'' at 35 oC after purification

Figure 8.14. Real C′(ω) and imaginary C′′ (ω) components of the complex capacitance oftransformer DT1 before and after oil purification.

Estimated moisture contents derived from the “pressboard” database and theX model were similar. On the other hand these values are much higher thanthe corresponding estimates derived from the “cellulose” database and oilanalyses. This might be due to differences in the internal construction ofdistribution transformers from that of power transformers, and the modelsused were mainly based on the response of oil impregnated pressboardinsulation systems.


95

Table 8.5. Estimated parameters for transformer DT1.

Before purification After purificationMethod

mc σ X Y C0 mc σ X Y C0

cellulose 3.9 150 20 20 0.4 2.9 87 20 20 0.4

MO

DS

pressboard 5.2 170 20 20 0.4 4.4 106 20 20 0.4

X model 5.6 -- 30 -- 0.4 4.2 -- 30 -- 0.4

Oil analyses 3.2 180 -- -- -- 1.8 140 -- -- --

mc - moisture content in paper (%); σ - oil conductivity at 27 ºC; C0 – Geometrical

capacitance (nF).

18

19

20

21

22

23

24

25

26

0,0026 0,0028 0,003 0,0032 0,0034 0,0036 0,0038

After purificationBefore purification

y = 3,2699 + 5812,1x R= 0,99545

y = 2,6634 + 5919,4x R= 0,99849

ln(R

esis

tivity

)

1/T [1/K]

Figure 8.15. Temperature dependence of the oil resistivity in transformer DT1 before and

after purification. The slope corresponds to an activation energy of 0.5 eV.

FDS measurements performed on the transformers DT2 and DT3 revealed adifferent insulation state. Since these two units were sealed transformers, oilsamples were not available for further analyses. Hence, modelling using the


96

X model, in which measured oil conductivity was one of the inputparameters, was also not carried out. Therefore, modelled parameterspresented in Table 8.6 are only from the X-Y model (MODS). FDS results,presented for the purpose of comparison, as ε′(ω) and ε′′ (ω), are shown inFigure 8.16. Geometrical capacitance estimated by MODS was used to derivethese curves from the measured complex capacitance. Much higher lossesand a stronger dispersive character of the real ε′(ω) component of thecomplex permittivity ε*(ω) were found in the insulation of the usedtransformer DT2. This indicates the necessity for taking preventive measuresto improve the state of insulation in this transformer.

Table 8.6. Modelled parameters for transformers DT2 andDT3.

“pressboard”

X-Y model

“cellulose”

X-Y modelIDNo.

mc σ X Y mc σ X Y

DT2 3.1 12 20 20 4 12 20 20

DT3 1.5 0.7 20 20 1.5 0.7 20 20

mc - moisture content in paper (%) σ - oil conductivity at 27 ºC.


97

10-3

10-1

101

103

100

101

Frequency (Hz)

ε '

DT2DT3

10-3

10-1

101

103

10-2

100

102

Frequency (Hz)

ε ''

DT2DT3

Figure 8.16. Dielectric response results for used (DT2) and spare (DT3) transformers.

8.5 Limitation on FDS measurements

It was revealed during this study that in some transformers difficulties appearwhen attempting to perform FDS measurements across the main insulation.The results presented in Figure 8.17 are from the FDS measurements madeon transformer T11.

As shown in the figure, the capacitance measured is extremely low (~40 pF),for an oil-paper insulated power transformer rated 24 MVA. Moreover, themeasured loss C′′ (ω) was negative for frequencies higher than 10 Hz. The


98

results of FDS measurements made on other transformers of the same typewere similar. This type of behaviour can be expected when a grounded metalplate is installed between the LV and HV windings. However, it was notpossible to find information on the internal construction of thesetransformers, which would be necessary for further explanation of thisproblem.

100

101

102

10-11

10-10

Frequency (Hz)

C '

(F)

100

101

102

-3

-2

-1

0

1x 10

-10

Frequency (Hz)

C '' (

F)

Figure 8.17. Measured capacitance C′(ω) and loss C′′ (ω) of transformer T12.

Chapter 9: Conclusion

99

9. Conclusion

The purpose of this study was the further investigation of possibilitiesprovided by the use of dielectric response measurements for the diagnosticsof oil-paper insulating systems. The work was focused on the application offrequency domain spectroscopy (FDS) measurements. The majority of themeasurements were performed on field-installed power and distributiontransformers in Sri Lanka, all of them owned by the Ceylon Electricity Board(CEB). Some of the measurements were performed in a laboratoryenvironment too. The latter were carried out on both transformers andoil-impregnated pressboard samples.

Presently, when using results of FDS measurements for estimating themoisture content in transformer insulation the so called X-Y model isutilised. This model takes into account the complex nature of transformerinsulation, i.e. the presence of barriers and spacers, and for this reason thereis a need for knowing the geometrical design of the transformer in detail.However, power utility companies, such as CEB, usually do not have accessto such data. Subsequently, searching for possibilities to interpret FDS datawith limited information on geometrical design of the insulation wasdesirable. Furthermore, a sensitivity analysis of the X – Y model revealedthat influences of moisture content, oil conductivity and spacer content on thedielectric permittivity of the transformer insulation system at 1 kHz wererelatively low. In addition, it was recognised that the influence of spacercontent on the total dielectric response in the X-Y model was also low. Thesetwo effects were observed when the barrier content was within the range of20 - 50 %, spacer content was within the range of 15 - 25 %, oil conductivitywas between 10 - 400 pS/m at 25 ºC, and moisture content in pressboard wasvaried from 0.2 to 5 %. In the large population of the power transformers, thevalues of these parameters fall within the specified limits. Based on theseobservations, a simplified model, called the X model, was introduced. In theX model, the presence of spacers in the transformer insulation has beenneglected. On the other hand, an additional measurement on oil sample hasbeen introduced for estimating oil conductivity, which is then used in themodelling.

The results presented show that moisture contents estimated by means of theX model were fairly close to the values obtained from the X - Y model.

Chapter 9: Conclusion

100

However, both these estimates were usually a bit higher than the onesobtained from oil analyses. It is therefore necessary to continuemeasurements of the frequency response of pressboard under differentconditions. There is also a need for checking the models against a muchlarger population of power transformers by comparing the modelling resultsof FDS measurements with results of KFT analyses. Such comparisonswould be helpful in improving the quality of the modelling. One should alsocompare the estimated pressboard contents from the X model with the actualbarrier contents when there is potential for obtaining data on the internalconstruction of transformers. This might help to assess the validity of all theassumptions made and the accuracy of derived quantities.

Most of the tested field transformers were identified as wet transformers.Therefore, necessary precautions must be taken to avoid further ageing inthese transformers. Furthermore, CEB is advised to set up a proper andwell-organised database on maintenance and measurement history of all thepower transformers they use.

Chapter 10: Future Work

101

10. Future Work

Based on the conclusions presented in the previous section, we can point outthe following areas for future study which, when completed, will furtherincrease our understanding of the behaviour of oil paper insulation in powertransformers and will improve the quality of the interpretation of diagnosticmeasurements.

• Further frequency domain spectroscopy (FDS) measurements onpressboard samples under broadly varying conditions, such as differentmoisture contents, differently aged and impregnated with oils withdifferent qualities.

• Further measurements of field-installed power transformers forcalibrating the results of dielectric response analyses against results ofdetailed oil analyses.

• Further Study of the effects of thermally driven unequal moisturedistribution in transformer insulation on the results of dielectric responsemeasurements.

• Extending the frequency range used for the measurements, especially foranalysing the response of pressboard at higher frequencies.

• Investigating influences of electrical noise generated by other highvoltage units operating near the transformer under study and theinfluence of parasitic creepage on the results of FDS measurements.Attempts must be undertaken to identify reasons for apparent difficultiesin performing FDS measurements on some types of transformers.

Chapter 11: References

103

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lic thesis

Documents

frequency domain measurements

frequency domain response

time domain response

finance support

high voltage engineering

pressboard samples

high voltage laboratory

water content