relaxation phenomena in oil filled transformers

Investigation of ‗Off-Line‘ Relaxation Phenomena in Oil Filled Transformers

1

INVESTIGATION OF ‘OFF-LINE’ RELAXATION

PHENOMENA IN OIL FILLED TRANSFORMERS

A THESIS

Submitted in partial fulfilment of the

requirements for the award of the degree

of

MASTER OF ENGINEERING (RESEARCH)

In

SCHOOL OF ENGINNEERING SYSTEMS

By

VEERENDRA LINGAMANENI

B.E (EEE)

Faculty of Built Environment and Engineering

Queensland University of Technology

BRISBANE – 4001 (AUSTRALIA)

May, 2010


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Queensland University of Technology

CANDIDATE’S DECLARATION

I hereby certify that the work which is being presented in the thesis, entitled

―Investigation of Off-Line Relaxation Phenomena in Oil Filled Transformers” in

partial fulfilment of the requirements for the award of the degree of Master of

Engineering and submitted in the School of Engineering Systems of the University is

an authentic record of my own work carried out under the supervision of

Prof. Gerard Ledwich and Prof. Birlasekaran Sivaswamy, School of Engineering

Systems, Queensland University of Technology, Brisbane.

The matter presented in this thesis has not been submitted by me for the

award of any other degree of this or any other University.

(Veerendra Lingamaneni)


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ACKNOWLEDGEMENT

This thesis is a living testimony to the numerous contributions of a galaxy of

distinguished personalities whom I had the good fortune of being associated with. I

deem it an honour and duty to acknowledge all help I received from these luminaries.

I am awed and overwhelmed as I bow to my most revered ‗mentor‘ Prof. Gerard

Ledwich, Chair in Power Engineering, Faculty of Built Environment and

Engineering, QUT. There is much for me to learn from his artistic touch to academics

and his meticulousness. It is an eternal honour to have worked as his student for such

a long spell. His support, personal guidance, thought provoking discussions and

encouragement helped me glide through the upheavals, which are inevitably in-built

into a research work.

Words desert me when I rise to offer my humble respects to my second guide,

Prof. Birlasekaran Sivaswamy, Faculty of Built Environment and Engineering,

QUT. ―It is the master who makes things easy‖ holds true for him. It is a great honour

to work under his supervision. His fathomless knowledge always turned a pearl of

advice to satiate my academic inquisitions. The support and kindness that he has laid

on to me is much appreciated.

I heartily thankful to the QUT for the scholarship they provided to me. Without their

support, it may not possible to conduct this research in a smooth fashion.

I avail the privilege to pour on paper, my regards to my parents,

Venkatarao Lingamaneni and Bhaskaramba Lingamaneni. Their blessings, love,

care, inspiration, seen and unseen blessings kept me sailing through the storms.

The active support provided by my revered wife Jyothsna Lingamaneni, and my

revered brother Srimanth Lingamaneni who has been a parent, a guide, a patron

also a trouble saver is acknowledged.


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I thank all the souls who helped me in this herculean task.

Finally, I thank God, for all the blessings that he has showered on me and helped me

to achieve my true potential in this temporal world.

Veerendra Lingamaneni


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ABSTRACT

Power transformers are one of the most important and costly equipment in power

generation, transmission and distribution systems. Current average age of

transformers in Australia is around 25 years and there is a strong economical tendency

to use them up to 50 years or more. As the transformers operate, they get degraded

due to different loading and environmental operating stressed conditions. In today‘s

competitive energy market with the penetration of distributed energy sources, the

transformers are stressed more with minimum required maintenance. The modern

asset management program tries to increase the usage life time of power transformers

with prognostic techniques using condition indicators. In the case of oil filled

transformers, condition monitoring methods based on dissolved gas analysis,

polarization studies, partial discharge studies, frequency response analysis studies to

check the mechanical integrity, IR heat monitoring and other vibration monitoring

techniques are in use.

In the current research program, studies have been initiated to identify the degradation

of insulating materials by the electrical relaxation technique known as dielectrometry.

Aging leads to main degradation products like moisture and other oxidized products

due to fluctuating thermal and electrical loading. By applying repetitive low

frequency high voltage sine wave perturbations in the range of 100 to 200 V peak

across available terminals of power transformer, the conductive and polarization

parameters of insulation aging are identified. An in-house novel digital instrument is

developed to record the low leakage response of repetitive polarization currents in

three terminals configuration. The technique is tested with known three transformers

of rating 5 kVA or more. The effects of stressing polarization voltage level,

polarizing wave shapes and various terminal configurations provide characteristic

aging relaxation information. By using different analyses, sensitive parameters of

aging are identified and it is presented in this thesis.


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TABLE OF CONTENTS

Title Page No

CANDIDATE’S DECLARATION 2

ACKNOWLEDGEMENT 3

ABSTRACT 5

TABLE OF CONTENTS 6

LIST OF FIGURES 11

LIST OF TABLES 13

NOMENCLATURE AND ACRONYMS 14

CHAPTER 1: INTRODUCTION 15

1.1 MOTIVATION 15

1.2 OBJECTIVES 19

1.3 OVERVIEW OF THE THESIS 19

1.4 SUMMARY 21

CHAPTER 2: LITERATURE REVIEW ON RELAXATION

PHENOMENA 22

2.1 OIL FILLED TRANSFORMER 22

2.2 TRANSFORMER FAILURE 23

2.3 CONDITION MONITORING 25

2.3.1 FREQUENCY RESPONSE ANALYSIS 28

2.3.2 RECOVERY VOLTAGE METHOD 29

2.3.3 PARTIAL DISCHARGE MONITORING 31


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2.3.4 TEMPERATURE MONITORING 32

2.3.5 VIBRATION MONITORING 33

2.3.6 CURRENT MONITORING 33

2.3.7 BUSHING AND CT MONITORING 33

2.4 ONLINE OIL MONITORING 34

2.4.1 COMBUSTIBLE GAS MONITORING 35

2.4.2 MULTI GAS MONITORING 35

2.4.3 OIL QUALITY MONITORING 36

2.5 RELAXATION PHENOMENA 37

2.5.1 INSULATION RESISTANCE MEASUREMENT 38

2.5.2 POLARISATION AND DEPOLARISATION CURRENT

MEASUREMENT 38

2.5.3 DIELECTROMETRY METHODS 39

2.6 SUMMARY 41

CHAPTER 3: DEVELOPED INSTRUMENTATION AND TEST

ARRANGEMENTS 42

3.1 RELAXATION INSTRUMENTATION 42

3.2 DEVELOPED RLAXATION INSTRUMENT 44

3.2.1 FUNCTION GENERATOR 45

3.2.2 LEAKAGE CURRENT RESPONSE MEASURING SYSTEM 46

3.2.3 DATA ACQUISITION AND STORAGE 46

3.3 DEVELOPED RELAXATION INSTRUMENT SPECIFICATIONS 49

3.4 TESTED HV TRANSFORMERS 50

3.4.1 POLARISATIONINDEX 50


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3.4.2 OIL/(INSULATION+CORE+WINDING)RATIO 51

3.4.3 RESISTANCE OF THE WINDING 51

3.5 RELAXATION TESTS 53

3.5.1 EFFECT OF TERMINALS 54

3.5.2 EFFECT OF PERTURBING VOLTAGE 55

3.6 PROCEDURE TO PERFORM OFFLINE RELAXATION TESTS 55

3.7 SUMMARY 55

CHAPTER 4: EXPERIMENTAL RESULTS 56

4.1 MEASUREMENTS 56

4.2 SIGNAL CONDITIONING 57

4.3 TPICAL RESULTS 58

4.4 CONSOLIDATED RESULTS 62

4.4.1 VARIATION OF PEAK CURRENT MAGNITUDE WITH

FREQUENCY 62

4.4.2 VARIATION OF LEADING PHASE SHIFT WITH

FREQUENCY 64

4.5 SUMMARY 67

CHAPTER 5: ANALYSIS 68

5.1 THEORY OF RELAXATION PHENOMENA 68

5.2 VARIATION OF IR(f) AND IC(f) WITH FREQUENCY 70

5.3 VARIATION OF ADMITTANCE WITH FREQUENCY 73

5.4 VARIATION OF TAN (δ) WITH FREQUENCY 75

5.5 EFFECT OF VOLTAGE ON LOSS FACTOR 77


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5.6 EFFECT OF VOLTAGE ON REAL AND IMAGINARY

ADMITTANCE 79

5.7 SUMMARY 82

CHAPTER 6: DISCUSSION 85

6.1 TEST ARRANGEMENT 86

6.1.1 DEVELOPED DIELECTROMETRY INSTRUMENTATION 86

6.1.2 THE TESTED TRANSFORMERS 87

6.1.3 COMPUTER INTERFACE 87

6.2 RATIO OF SINE WAVE RESPONSE CURRENT AT TWO

EXTREME FREQUENCY RANGE LIMITS 88

6.3 TREND OF CURRENT VARIATION WITH FREQUENCY 89

6.4 TREND OF LEADING PHASE SHIFT VARIATION WITH

FREQUENCY 90

6.5 TREND OF RESISTIVE AND CAPACITIVE CURRENT VARIATION

WITH FREQUENCY 91

6.6 TREND OF REAL AND IMAGINARY ADMITTANCE VARIATION

WITH FREQUENCY 92

6.7 TREND OF TANδ VARIATION WITH FREQUENCY 93

6.8 EFFECT OF VOLTAGE ON TANδ AND ADMITTANCE 94

6.9 SUMMARY 94

CHAPTER 7: CONCLUSIONS AND SCOPE OF FUTURE WORK 96

7.1 CONCLUSION 96

7.2 SCOPE OF FUTURE WORK 99


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REFFERENCES 101

APPENDIX-A 109

APPENDIX-B 112


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LIST OF FIGURES

Figure Title Page

Fig 1.1 An ideal Power transformer 17

Fig 2.1 Life prediction using Condition monitoring data 26

Fig 3.1 Developed Lab view graphical program 47

Fig 3.2 Typical screen control, generated and captured outputs as seen in

Computer screen front panel 48

Fig 3.3 Flow diagram 48

Fig 3.4 Developed Sine wave Relaxation Instrument 49

Fig 3.5 General Layout of Connections (A, B and G are the three terminals) 50

Fig 3.6 Tested Transformers T1, T2 and T3 52

Fig 4.1 Test on T1 in 2 second period with tank grounded. Perturbation

voltage in blue is to be multiplied by 352 and the response

current in red is to be multiplied by 6.14µA. 57

Fig 4.2 Relaxation response of T1 in periods of 0.67s and 66.7s with

141VpSine 58

Fig 4.3 Relaxation response of T1 in periods of 0.67s and 66.7s with 176VpSine 58









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Fig 4.11 Relaxation current response of T1 at different voltages 63



Fig 4.14 Relaxation leading phase shift response of T1 at different voltages 65

Fig 4.15 Relaxation leading phase shift response of T2 at different voltages 65

Fig 4.16 elaxation leading phase shift response of T3 at different voltages 66

Fig 5.1 Relaxation IR and IC current response of T1 at different voltages 71



Fig 5.4 Real and imaginary admittance response of T1 at different voltages 73



Fig 5.7 Variation of loss Factor with frequency at different voltages in T1 76



Fig 5.10 Variation of loss Factor with reference to 176V level 79

Fig 5.11 Variation of Real admittance with reference to 176V level 81

Fig 5.12 Variation of Real admittance with reference to 176V level 82


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LIST OF TABLES

Table 2.1 Condition monitoring techniques for oil filled transformer 27

Table 2.2 Common In-service Oil Diagnostics 34

Table 3.2 Name plate details of SWER Transformers 53

Table 3.3 Terminals Connections 54

Table 4.1 Multiplication Factors 57


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NOMENCLATURE AND ACRONYMS

Symbol Notation

SWER Single Wire Earth Return

SMPS Switch Mode Power Supply

TBM Time-Based Maintenance

CBM Condition-Based Maintenance

DLF Dielectric Loss Factor

PD Partial Discharge

DGA Dissolved Gas Analysis

DC Direct Current

AC Alternate Current

HV High Voltage

LV Low Voltage

CM Condition Monitoring

RVM Recovery Voltage Method

FRA Frequency Response Analysis

PI Polarisation Index

IR Insulation Resistance

CT Current Transformer

OLCM On Load Current Method

Hz Hertz

V Volt

A Ampere

F Farad


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CHAPTER 1

INTRODUCTION 1.1 Motivation

The restructuring of the electric energy business toward competition, is being of great

concern throughout the world. It moves electrical energy business towards a

deregulated global electricity market and posing new scenarios in power systems

planning and operation through a deregulated global electricity market. In a typical

deregulated environment, energy is going to be considered as a ―product‖ and its

delivery to the users as a separate ―service‖. Hence the service provided should ensure

that the electric energy providers will maintain a standard level of security, reliability

and power quality to the users. This puts the electric utilities under severe stress to

reduce operating costs, enhance the availability of the generation, transmission and

distribution equipment and improve the supply of power and service to customers.

The most important risk experienced in the power distribution is a catastrophic failure

which may result in outages for longer period of time [1]. Hence one of the easiest

ways to improve the reliability and avoid catastrophic failure is to detect incipient

faults in the electrical equipment, which can predict failures ahead of time and suggest

necessary corrective actions to be taken to prevent outages and reduce down times.

With modern educational platform of management combined with basic engineering

qualifications, asset management programs based on scientific principles are getting

into the industry [1].

Transformer is a device which converts one form of energy to another form of energy.

Transformers are one of the most important pieces of costly equipment in power

systems. Transformers represent a high capital investment in any substations at the

same time as being a key element determining the loading capability of the station

within the network. With appropriate maintenance, including insulation

reconditioning at the appropriate time, the technical life of a transformer can be in

excess of 60 years [2]. The end of life, however, can be strategic or economic.

Quantitative risk based approach can be used to aid costly investment decisions

involving transformer life, otherwise made from a subjective viewpoint [3]. Ever

since commercialization of electrical transmission began, number of transformers has


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been installed and a great percentage of them are in constant service delivering

electrical power to big cities and rural areas throughout the world for more than 40

years. A constant threat which prevails with these old transformers operating in the

deregulated power systems is the risk of experiencing a catastrophic failure ultimately

which is associated with considerable costs. Also under deregulated competitive

energy market most of the transformers are operating close to or over their operating

limits, it further increases their chances of imminent failure due to poor operational

practices. Economics and concern for the environment (as new transformers mean use

of additional environmental resources, capital, and problem with the recycling of old

transformers) no longer permit the easy replacement of transformers. Hence by

detecting the developing faults in transformers, a catastrophic future failure can be

avoided by good maintenance techniques [4]. Thus, incipient fault detection and

proper maintenance in transformers will increase asset value, prevent forced outages

with related consequential unexpected legal costs, and make existing transformers

work longer. The diagnosis should provide the following asset management

information: (i) Information on the condition of the insulation, (ii) the limits on

further use in present condition, (iii) the necessary measures to improve the condition,

(iv) the aging rate under current loading conditions and (v) life expectancy of

transformer. All these factors are predominantly dependent on the physical behaviour

of the insulation, winding and core materials, the quality of design and manufacture

and the conditions of use [4].

The different types of industrial transformers are as follows [5].

(i) Power transformer

(ii) Instrument transformer

(iii) Pulse transformer

(iv) RF transformer

(v) Audio transformer

The different types of materials used in the transformer are as follows [5].

(i) Laminated core

(ii) Toroidal

(iii) windings

(iv) Oil /air/gas cooled transformer

(v) Copper


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(vi) Paper and other insulating materials

Fig 1.1 Example of a typical Power transformer

Fig 1.2 Example of a typical Power transformer

A typical 3φ power transformer is shown in Fig.1.1 with core and windings immersed

in oil kept in a sealed tank, its HV bushings and external oil drum showing oil level

and operating temperature. Fig. 1.2 shows the operational layout in substation. The

power transformer is a static power transfer apparatus, involving no continuously

moving parts except for cooling motors and tap changers, used in electric power

systems to transfer power between circuits through the use of electromagnetic

induction. The word, power transformer is used to those transformers used between


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the generator and the transmission & distribution circuits and are usually rated at

500kVA and above [7]. The purpose of a power transformer in converter mode

Power Supplies is to transfer power at HV DC efficiently. In doing so, the

transformer also provides important additional capabilities [8].

• The primary to secondary turn‘s ratio can be established to efficiently

accommodate widely different input/output voltage levels.

• Multiple secondaries with different numbers of turns can be used to achieve

multiple outputs at different voltage levels.

• Separate primary and secondary windings facilitate high voltage input/output

isolation, especially important for safety in off-line applications

Oil/paper structure is the typical configuration of transformer insulation and they

undergo long term aging due to gradual physical and chemical degradation subjected

to electrical and thermal stresses in-service. The decomposed products from insulation

aging are solid, liquid and gaseous impurity species [9]. Moisture and ageing strongly

influence the dielectric properties of oil/paper insulation system of power transformer.

To assess the reliability of insulation it is necessary to know the conditions of the oil

and the paper. In recent years, new methods to assess insulation systems have been

suggested in addition to the classical insulation resistance and Power frequency loss

factor measurements [10]. Dielectric diagnostic measurements based on polarisation

and depolarisation current measurements [65] and return voltage measurements [62]

have gained significant importance over the last ten years [10]. Large number of

power transformers around the world is approaching towards the end of their design

life [11]. They are very expensive to replace especially in the existing city

environments; however, most of these installed transformers are still in good

condition and could be used for many more years. Clearly determining their reliable

operational conditions would be of tremendous importance to the electricity industry.

Well-established time-based maintenance (TBM) by experienced staff as well as

conservative replacement planning is not feasible in the current competitive market

oriented electricity industry. Condition based maintenance (CBM) and online

monitoring are gaining importance now [13]. A variety of electrical, mechanical and

chemical techniques are currently available for insulation testing of power

transformers [3]. Most of these techniques have been in use for many years, such as

the measurement of insulation resistance (IR), dielectric loss factor (DLF), partial


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discharges (PD), interfacial polarization (IP). Sampling a small quantum of oil from

operating transformer, conditional indicators like oil insulation quality, moisture

content, and dissolved gas analysis (DGA) are estimated [3].

In the relaxation test technique with DC and AC voltages, different voltage levels in

the range from 50V to 10 kV are being used in the industry to identify different

developing faults [51][60-67]. No industrial standard has come up until this stage.

There are many industrial test methods used to identify aging in oil filled transformer

and those methods are reviewed in chapter 2. In this research, I used ‗dielectrometry‘

relaxation technique which can be carried at the University research environment with

the available facilities. It is more reliable, economical and less cost effective

technique and it can provide a lot of research data for processing and scientific

interpretation. The technical details on the developed instrumentation are explained in

the chapter 3 and the analyses are presented in the rest of the chapters.

1.2 Objectives

To develop an industrial technique for testing and identifying the sensitive aging

conditional parameters in different types of field oil filled transformers. With that

points in view, the four identified objectives are listed below:

1. Develop the instrumentation for industrial use

2. Test at QUT HV laboratory with three known aged transformers and collect

the relaxation responses.

3. Analyse the data to identify aging and extract conditional aging parameters for

future industrial transformer degradation model development.

4. Identify the sensitive parameters of aging with voltage, frequency and tested

terminals.


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1.3 Overview of the Thesis

This thesis is divided into 7 chapters to present the contributions with clarity as

follows:

Chapter 1: Introduction

The chapter details the motivation to take up this research work, the identified

objectives after the literature survey, the overview of different chapters‘ content and

summary.

Chapter 2: Literature Review on Relaxation Phenomena

Critical review on oil-filled transformer, different observed failures, and different

condition monitoring methods, procedure to predict the remaining life, different

international standards for testing, relaxation phenomena, and the existing commercial

instrumentation for such relaxation studies is presented. This survey enabled to

formulate the research plan and plan on some novelty on testing procedures to identify

aging in oil-filled transformers.

Chapter 3: Developed Instrumentation and test arrangements

In this chapter, I present the hardware and software details on the developed

dielectrometry instrumentation for relaxation studies on oil-filled power transformers.

The existing details on commercial relaxation techniques, details of test transformers,

the test layout to interface the transformer and instrument and the operational

procedure to get reproducible test data are briefed. It summarizes the technical

specifications of the developed instrumentation, initial characterisation of aged

transformers and the planned tests.

Chapter 4: Experimental Results

This chapter reports the typical measured results on three aged transformers. The

study is carried by varying (i) the voltage magnitude of exciting sine polarisation

wave, (ii) the frequency of the repetitive sine wave, and (iii) the test terminals. It

briefs the calibration procedure in using the instrument, signal conditioning methods

for relaxation data extraction, typical extracted sinusoidal relaxation responses,

consolidated current magnitude and phase shift in the tested frequency range and the

summary.


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Chapter 5: Analysis

This chapter briefs the basic theory needed for the analysis. Various analysed results

with a view to identify aging in oil filled transformers are presented. It analyses the

trend of different analysed parameters with aging. The effect of polarising voltage

magnitude on changing tanδ and real and imaginary admittances is presented.

Chapter 6: Discussion

This chapter discusses the outcomes of this project with reference to different

objectives. It discusses on the developed instrumentation and the trends of different

parameters with aging, voltage and the tested terminals of transformer.

Chapter 7: Conclusions and scope of Future Work

This chapter concludes with the summary of findings with respect to the identified

objectives and lists the topics of future research areas which can be carried further.

1.4 Summary

This chapter 1 presents the motivation to take up this project, the identified objectives

of this research problem, and the overview of presented seven chapters.


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CHAPTER 2

LITERATURE REVIEW ON RELAXATION PHENOMENA

This research on relaxation phenomenon of oil filled transformers deals with the study

of dielectric insulation characteristics. A literature survey on oil-filled transformer,

various causes of transformer failure, different condition monitoring methods and the

available relaxation measuring instruments is presented in this chapter 2.

2.1 Oil Filled Transformer

Transformers are costly essential elements of electrical power systems. In a given

electrical system, transformers are commonly used to change the voltage and current

levels, to establish electrical isolation and impedance matching and to interface the

different measuring instruments. In particular, power and distribution transformers

form a vital link between power generation, transmission and distribution of different

electrical system. In the power system applications, the single-phase and three-phase

transformers with ratings up to 5OOkVA are defined as distribution transformers,

whereas those transformers with ratings over 500kVA at voltage levels of 69 kV and

above are defined as power transformers [15].

A transformer is a static electromagnetic machine. It consists of a primary winding

and a secondary winding linked by a mutual magnetic field [16]. Ferromagnetic cores

built from insulated silicon steel laminations are employed to develop tight magnetic

coupling and high coupling flux densities. The complete transformer assembly is

surrounded by a suitable electrical insulating medium. The oil filled transformer

insulation is made of pressboard insulation immersed in transformer oil. The studied

single wire earth return (SWER) transformers had one/two low voltage (LV)

winding/s with two/ four insulated terminals, one high voltage winding (HV) with two

insulated terminals, one 11 kV rated porcelain bushing connected to the high voltage

terminal, and a sealed metal tank housing all the windings, core, oil and bushings.

Both LV and HV windings with the respective solid insulation are kept immersed in

transformer oil within the tank. It should be noted that the press-board solid insulation

is impregnated with transformer oil under vacuum condition. New transformer oil

and paper insulation will have a minimum quantum of water content. As the


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transformer is put into operation, the loading heats up the insulation and unloading

tracks back the insulation to lower operating temperature. In general, the transformer

is operated with a temperature rise of about 60ºC from the ambient temperature. The

paper and oil have different moisture content at different temperatures. The different

types of cooling used in power transformer are (i) air cooled, (ii) oil filled, (iii) fan

cooled and (iv) water cooled. The life expectancy of transformers, regulators, and

reactors at various operating temperatures is not accurately known, but the procedure

to determine loss of life is considered to be conservative [16]. Aging or deterioration

of insulation is a function of time and temperature. Since in most apparatus the

temperature distribution is not uniform, that part which is operating at the highest

temperature will ordinarily undergo the greatest deterioration. Therefore, it is usual to

consider the effects produced by the highest temperature "hottest spot‖ [14]. The

insulation system of a power transformer is understood as the complete internal

assembly of different dielectric insulating materials. This includes the different

insulating parts and supporting structures that cover the winding wires and also the

insulation from the core, winding and tank. Such insulation systems are fabricated

following different basic principles [17]. The TrafoStar class of ABB power

transformers used in high and extra high system voltages use oil- and cellulose

insulation, mainly arranged in a barrier-type structure [17].

2.2 Transformer Failure

As mentioned above, the transformer is a complex arrangement of coils around a steel

core with the primary purpose of utilizing magnetic induction to change voltage

levels. Components such as magnetic core, primary and secondary windings, cooling

oil and paper insulation are liable to failure.

The transformer failures that most frequently arise in practice are [16]:

1. Failures in the magnetic circuits, i.e. core, yokes, and adjacent clamping

structure;

2. Failures in the windings;

3. Failures in the dielectric circuit, i.e. in the oil and major insulation;

4. Structural failures like clamping of windings and core laminations.


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Most transformer failures occur due to faulty manufacture, short circuit faults, and

abnormal transient or sustained over load operating conditions, premature insulation

failure, and accelerated aging [16]. These transformer faults can be divided into two

main classes:

1. Internal faults, and

2. Overloads and other externally occurring different stress conditions.

Internal faults are faults between adjacent turns or parts of coils, or faults to ground on

terminal or on parts of windings. Overloads and externally applied operating

conditions include over-current, over-voltage, external short circuits and reduced

system frequency [20]. A study of the breakdown records of modem transformer

which occurred over a period of years shows that between 70-80% of the number of

failures are caused internal winding faults [21]. These winding faults are due to the

degradation of the insulation system. The purpose of electrical insulating materials is

to insulate components of a transformer from each other and from ground, and at the

same time providing mechanical support for the components. Degradation means a

reduced insulation quality, which tends to cause a breakdown in the dielectric strength

of the insulation.

During the operation of the transformer, a strong electric field is applied to the

dielectric material. It can result in the aging and deterioration of the insulation. The

relevant factors generally recognized as causing the aging and deterioration of an

insulation include thermal stresses, electrical stresses, mechanical stresses, moisture

and so on [22]. Thermal stresses are caused by the internal heating due to current

overloads combined with rise in ambient temperatures. Under the normal operating

conditions, high voltage gradients will be below the breakdown voltage that does not

cause the detectable aging. However, at elevated temperatures electrical stresses may

act to further accelerate material degradation. Mechanical stresses are caused by

wrong assembly configurations, bad manufacturing techniques and vibration

generated due to short-circuit or over-voltage phenomena in power network. Moisture

is another major cause for the dielectric breakdown properties. It can form a

conductive path on the surfaces of material or react with the material to cause

chemical degradation.


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The structure of a dielectric may be altered significantly during the aging process, and

these changes will affect the electrical properties of the dielectric even before

insulation failure occurs. The relaxation processes within the dielectrics change with

aging to create characteristic dielectric losses. As the structure of the dielectric

molecules alters during aging, the dielectric characteristics and electrical properties

change. Eventually, the aging and degradation process of a dielectric may lead to a

complete dielectric failure [20]. For a transformer, the deterioration of the insulation

between turns results in the dielectric breakdown resulting an internal short-circuit

fault. The period before the short-circuit but with the dielectric in a degraded state is

referred to as incipient fault.

To avoid this failure in transformers, the dielectric conditions should be monitored.

Apart from that, mechanical conditions are also monitored and the entire process of

checking the various conditional trends is called ‗condition monitoring‘ [20].

2.3 Condition Monitoring

Condition monitoring is the process of monitoring a single parameter or many

parameters of degrading conditions in machinery. It is a major part of predictive

maintenance. The use of conditional monitoring allows maintenance to be scheduled,

or other actions to be taken to avoid the consequences of major failure. Predictive

Maintenance helps to predict the possible time of failure. CM systems can only

measure the deterioration level. It is typically much more cost effective than allowing

the machinery to fail [23]. Industry uses different types of CM techniques like

Recovery voltage method (RVM) [62], Frequency response analysis (FRA) [21],

Dielectrometry [60] [64], and Partial discharge (PD) etc [22]. Fig.2.1 shows the life

prediction of transformer through oil and paper degradation data, relaxation

measurement using RVM technique and from historical failure rate data. The Table2.1

lists the different condition monitoring techniques and the best method suitable for

diagnosing different classified known faults within the transformer.

http://en.wikipedia.org/wiki/Predictive_maintenance




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Fig 2.1 Life prediction using Condition monitoring data [24]


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Table 2.1 Condition monitoring techniques for oil filled transformer [24]

There are a number of different techniques available to identify the developing faults

in the transformer by external measurements. Some of them are as follows: (i)

Frequency response analysis (FRA) (ii) Recovery voltage method (RVM) (iii) Partial


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discharge monitoring (iv) Temperature monitoring (v) Vibration monitoring (vi)

Current monitoring and (vii) Bushing and CT monitoring.

2.3.1 Frequency Response Analysis (FRA)

Information about transformer mechanical condition can be extracted from the

transformer winding frequency responses on individual winding or between coupled

windings[21][23]. The winding behaves as a complex RLC network and its transfer

function represents according to the system theory the characteristic behaviour of a

linear shift invariant system. Aging or deformation will cause small changes in the

geometry of the winding leading to changes of the corresponding localized

capacitances and inductances and consequently to a change in the FRA result.

Different methods exist in order to determine the transfer function of a transformer

winding [25].

High Voltage Impulse (HVI) – time domain method

Low Voltage Impulse (LVI) – time domain method

Frequency Sweep Analysis – frequency domain method

Both HVI and LVI techniques are based upon application of an impulse voltage

across the transformer terminals and measurement of current across output terminals.

The transfer function between different terminals namely input and output windings

and windings on the limb can be calculated to know the information about the

transformer impulse response distribution. High Voltage impulse as the name

suggests relies on application of a few single high voltage impulses in the range of kV

[26] across the winding.

While the LVI method relies on application of low voltage impulses with the

magnitude in the range of 100 mV to 10 V peak of either polarity in a cyclic way

across the winding. HVI method suffers from poor frequency spectrum of the input

signal and is unable to detect minor changes in the winding. The advantage of LVI is

that it allows the adjustment of steepness of the applied impulse in order to obtain a

wider frequency band [27].


29

FRA measures the impedance or transferred impedance of the transformer winding. It

is measured as a function of the frequency by applying a low-voltage sinusoidal test

signal with variable frequency in the range from 100 Hz to 10 MHz. The signals are

measured at discrete frequencies to determine amplitude and phase of the transfer

function for the desired full frequency range. Final aging or degradation assessment of

the test is based on verification of repeated recorded signals with original recorded

signals during installation. FRA is free from superimposed environmental noises,

however relatively longer duration of time is required to finish all the frequency

segmented measurements in different windings of the transformer [27]. Frequency

response analyser detects mechanical and electrical changes of the core and the

winding assembly of power transformers by using the SFRA method without de-

tanking the structure. SFRA stands for Sweep Frequency Response Analysis.

Winding or core defects can be identified after faults, mechanical shocks or

transportation using this external measurement. It offers a valuable opportunity to

improve the reliability of transformers, to reduce maintenance costs and, most of all,

to avoid expensive unexpected outages [36].

2.3.2 Recovery Voltage Method

Moisture gets formed due to degradation of paper insulation. Moisture gets distributed

between paper and oil insulation. Due to ageing and thermal loading, the distribution

of moisture content varies [29]. Chemical technique – Karl Fischer technique may

lead to many inaccuracies in determining moisture content in oil and then predicting

proportional moisture content in paper at that temperature. Polarisation technique

using relaxation characteristics of polar components like water is being used in the

industry as they are non-invasive [28]. To detect ageing or moisture content it is

necessary to analyse low frequency part of polarisation spectrum of dissipation factor.

A tanδ measurement using frequency domain technique known as low frequency

dielectrometry can identify the degradation. But finding a sinusoidal high voltage

source in the frequency range of 0.001 Hz is very difficult and the instrumentation is

expensive [28] [64] [66]. Industry uses the simple time domain method known as the

recovery voltage measurement (RVM) [24] [62] [65].


30

It was found that dc measurement of insulation resistance (IR) and the estimation of

Polarisation index (PI) do not provide the complete information on polarisation

process [29] [61] [63]. Cases were reported where electrical motors having good PI

were found to have contaminated windings and also motors having poor PI had no

problems in the winding insulation [29]. To resolve this, dc absorption technique with

one thousand seconds charging time followed by discharging test was developed to

identify the degradation [30]. Recovery Voltage Method for transformer seems to be

developed from this test. In RVM, insulation is charged for a number of known time

intervals and then shorted to ground for pre-decided short time interval. The charging

voltage varies from 50 V to 2000 V. Then the shorting is removed and the recovered

voltage is then measured using high impedance circuit across the insulation after

open-circuiting all the connected terminals. The dominant polarisation time constant

is estimated from RVM time domain spectra which are related to degradation [62].

The diagnosis is done by comparing the initial slope, the maximum of the return

voltage and the time at which the maximum of the return voltage occurs in the

complete spectrum of measurements. Good correlation has been reported with

moisture content [52].

Charging current is given as the sum of the polarisation current and the conduction

current. Polarising current is dependent on material property and state of ageing. The

polarisation of dielectric can be expressed as sum of various slow polarisation

phenomena like ion migration, slow relaxation and interfacial polarisation. Care must

be taken in the interpretation of results of RVM, in particular the relative effects of

moisture, ageing and temperature [32].

This RVM technique is known as time domain based dielectric relaxation technique

and a costly microprocessor based commercial portable instrument is released into the

market around 1995 [62]. Until today, no standard is brought in to use this method in

fixing the voltage level and in the interpretation of the results. Since it involves with

high impedance voltage measurement, this test is done on a clear day with proper

good quality cables. It is learnt that time domain industrial recovery voltage

measuring unit (RVM) takes about 5 hours of test time for one set of measurements

[32].


31

2.3.3 Partial Discharge Monitoring

Partial discharge measurement is the most effective method to detect developing

incipient faults in the electrical system [31]. As the electrical insulation in a

transformer begins to degrade and breakdown, there are localized discharges within

the electrical insulation. Each discharge deteriorates the insulation material by the

impact of high-energy electrons, thus causing chemical reactions. During these

discharges, ultrahigh frequency waves are emitted. Most incipient dielectric failures

will generate numerous partial discharges before the catastrophic electrical failure.

Partial discharges may occur only before failure but may also be present for years

before any type of failure. A high occurrence of partial discharges can indicate voids,

cracking, contamination, or abnormal electrical stress in the insulation. Because of

this importance, the on-line partial discharge measurements are used in diagnosing

potential catastrophic failures in an operating transformer.

PD couplers/sensors to detect frequencies in the range of 1 to 1500 MHz are used in

the industry [31]. Also UHF waves produced by the partial discharge in oil/paper

insulation generate pressure waves that are transmitted through the oil medium. Low

frequency piezoelectric sensors can also be used to detect these waves [32]. These

sensors can be placed on the outside of the tank to detect the acoustic wave impinging

on the tank. The advantage of partial discharge sensors is the ability to estimate the

actual location of insulation deterioration using multi sensors. By placing several

partial discharge sensors around the transformer tank, it becomes possible to pinpoint

the exact location of the discharges [32]. Most often the deterioration occurs at the

start of the coils near the high voltage side of the transformer. The disadvantage to

partial discharge sensors is that they are greatly affected by the electromagnetic

interference in the substation environment. Therefore, signal processing techniques

are often used to improve the signal to noise ratio in order to make the measurements

effective [33].

Recently on-line partial discharge detection technique based on a fibre optic sensor

has been developed. In this technique, a laser diode transmits light into a fibre optic

coupler that has the light propagated across an air gap inside a self-contained


32

diaphragm, lined with reflective gold. The reflected light combines with the small,

reflected wave inside the fibre optic coupler to produce an interference pattern that

differs as the air gap changes. In this way, the acoustic waves produced by partial

discharges can be detected [31].

2.3.4 Temperature Monitoring

Thermography is effective for checking many different transformers quickly to see if

there is any outstanding hot spot problem externally [36]. Excessive generated heat

due to different faults results in rise in temperature in oil, the bushings, pumps and

fans which is an indicator of developing faults in the transformer. Also high surface

temperature distribution at the top of main tank has been known to indicate oil

deterioration, insulation degradation, and water formation [36]. Increase in operating

temperature deteriorates the winding insulation and the dielectric properties of the

mineral oil and other insulation begins to degrade increased rate. The accelerated

chemical reactions deteriorate the insulation at much faster rate. Most common sites

of temperature monitoring on transformer are on the top end of windings and core of a

transformer as the heated oil tries to move up. Thermal sensors mounted on the top

end of core are used for protection and monitoring purposes. The advantages of

temperature sensors are that they are simple, cheap and reliable.

Focused infrared based camera is used to measure the external hot-spot temperature

and its distribution from a very safe distance. They are able to detect temperature

gradients on external surfaces of the transformer and can locate easily the overheating

of bushing or fan bank or tank surface heated area. On-line monitoring of temperature

can be achieved with thermocouples placed externally on the transformer and

windings, and can provide real-time temperature variations with load at various

locations on the transformer. Expensive distributed fibre optic temperature sensor for

power transformer condition monitoring is available in the market [31].

Transformer models for predicting temperature of the oil and winding have been

developed to identify hot-spot location [33]. Accurate complex temperature

distribution models using Kalman filters and adaptive Hopfield networks have been

developed which need input from external thermal detectors [33, 34]. The models are


33

found to be pretty accurate in predicting thermal state of a tested known transformer

after some training with known data.

2.3.5 Vibration monitoring

Vibration monitoring deals with the mechanical malfunctions in a transformer and it

senses the emitted frequency spectrum in the range from 10Hz to 3 kHz. Most

mechanical failures associated with insulation structure clamping are identified. In

transformer, cellulose insulation on the coils of the transformer shrinks with aging.

The shrinkage causes loosening of the clamping pressure on the coils and can lead to

short-circuit between turns [35] [37]. Short circuits often can lead to the catastrophic

failure of the transformer and is the most common cause of transformer breakdowns.

Recently, demand has increased for low-noise power transformers as well as large-

capacity and small-size power transformers. To reduce transformer noise, it is

necessary to reduce vibration of their iron cores, which is caused by magnetostrictive

forces of silicon iron insulated laminations. The vibration measurement is used to

diagnose loose structure within power transformers and the test is called the on load

current method (OLCM). It can acquire the fundamental frequency component of the

core vibration signal without testing the transformer at the open-circuited condition

[37].

2.3.6 Current Monitoring

Load current on the primary, secondary, and tertiary coils can be used to access the

state of the transformer. Imbalance current in the transformer is an indicator of

developing problem or impending failure [36]. Consumption of additional current by

cooling system or drop in current levels of cooling systems is an indication of fan

bank failure. Similar monitoring is done on tap-changer drive motor current.

2.3.7 Bushing and CT Monitoring

Other accessories in the transformer are bushings, load tap changers (LTC), and

cooling system. Any faults in insulation accessories will lead to catastrophic failure of


34

insulation [42]. Similar to the insulation around the transformer coils, there are also

layers of foil and oil impregnated insulation that surrounds the transformer oil-filled

bushings and current transformers (CTs). There is a small amount of charging current

that flows when the system is in operation. Changes in this charging current can

indicate degradation in the geometrical structure of the insulation. As the insulation

degrades, carbon deposits can short circuit some of the layers and increase the stress

on the remaining layers. This leads to a decrease in the capacitance and the charging

current changes. Eventually, the remaining layers of insulation cannot take the

increased voltage stress and the system fails, often catastrophically [32]. Some of the

causes of bushing failures include changing dielectric properties with age, oil leaks,

design or manufacturing flaws, or the presence of moisture in oil. Sensors have now

been used to monitor the health of bushings. The InsAlert monitoring probe from

Square D Co, the Intelligent Diagnostic Device (IDD) for bushings [32] and current

transformers from Doble [32] have the ability to detect abnormalities and possible

failure conditions in the bushings and CTs.

2.4 Online Oil Monitoring

Experience has shown that most internal transformer condition problems can be

detected through oil analysis [1]. There are a number of diagnostics standards that are

commonly applied to in-service transformer oil samples (Table 2.2). Of these, a few

tests are carried continuously by online oil monitors [14]. These monitors can be

roughly grouped into three categories: 1. Combustible Gas Monitoring, 2. Complete

Multi-Gas Monitoring, and 3. Oil Quality Monitoring.

Table 2.2: Common In-service Oil Diagnostics [22]

Test ASTM

Designation

Comments

Colour

D1500 Increase in colour indicates

deterioration or contamination

Visual Examination D1524 Cloudiness or sludge should be

investigated

Dielectric Breakdown

Strength

D877

D1816

Measure oil‘s ability to insulate.

Sensitive to contaminants, moisture

Power Factor

D924 Detects polar contaminants

Dissolved Gas Analysis

D3612 Detects and identifies incipient faults


35

Interfacial Tension D971 Detects polar contaminants and

oxidization

Neutralization Number D974 Measures acidity of oil indicator of

deterioration

Specific Gravity

D1298 Can detect contamination

Moisture Content

D1533 Moisture can damage insulation

2.4.1 Combustible Gas Monitoring

Some of the commercially available combustible gas monitors are as follows: GE

Hydran [40] and Morgan Schaffer Calisto [14]. The GE Hydran method passes the

transformer oil over a special membrane, and hydrogen and other combustible gasses

permeate through the membrane to be sensed by selective gas sensors. It is calibrated

to the proportional dissolved gas content of the oil. Hydran method is sensitive to

nature of combustible gas and the method is not capable of distinguishing different

types of gases. In the absence of ability to distinguish, it‘s difficult to know exactly

the concentration of different individual gases and the exact cause of defect. In fact

it‘s possible that different combination of individual gases might result in same value

of the current. The only way to overcome this problem is whenever reading on oil-gas

monitor changes, extract an oil sample and send it to the chemical laboratory for

immediate quantitative gas distribution analysis using costly gas chromatography

techniques [40].

Morgan Schaffer Calisto method [14] was developed in early 1980‘s. This method

extracts hydrogen using capillary tube probe from the transformer oil after diffusing it

through polymer baffles. The advantage (or disadvantage) of this method is that it‘s

insensitive to other combustible gasses. The concentration of extracted hydrogen is

measured using a thermal conductivity detector which relies on the thermal

conductivity of hydrogen gas volume.

2.4.2 Multi Gas Monitoring

Multi gas monitoring relies upon the use of different integrated circuits (ICs) that have

been developed to measure the concentration of dissolved gases and moisture content


36

in the transformer oil. Some of the examples of commercially available combustible

gas monitors are Serveron Online Transformer Monitor [56].

Serveron Online Transformer Monitor is one of the most sophisticated measurement

techniques and is commonly used instrument for online monitoring of multiple gases.

It is capable of recreating the laboratory gas chromatograph in real time [58]. It can

monitor and measure eight gases simultaneously and is also capable of measuring

moisture concentration [22]. The advantages of Serveron monitor are that it can

replace manual sampling, off-site laboratory analysis and capable of detecting sudden

changes with load. The accuracy of the gas estimation is better than the laboratory

dissolved gas analysis [42]. The disadvantage of Severon monitor is that it is costly

method for diagnosis. Kehnan Transfix Monitor [20] uses photo acoustic

spectroscopy (PAS) to measure concentrations of different gases and moisture. The

principle of operation of Kehan Transfix Monitor relies on absorption of infra red

light by gas, which heats the gas. The sudden heating makes the gas to expand

suddenly producing a sound wave (or thunder). Different dissolved gasses absorb

different wavelengths of the electromagnetic radiation which can be used to identify

different gases. The intensity of the sound is proportional to the concentration of the

gas. The advantages of this method lie in PAS technique, which unlike gas

chromatography, does not require any carrier gas or calibration. Once again the

disadvantage of Kehan Transfix Monitor is the higher price of the monitor [20].

2.4.3 Oil Quality Monitoring

Dissolved gas analysis (DGA) is performed in the industry for more than 60 years as

the method is non-invasive and it can be carried in on-line mode by sampling the oil.

Most of the incipient faults at high stressed area generate 9 different gases which get

dissolved partly in oil and also diffuse to the sampling location. By proper sampling,

most of the incipient faults in the oil may be detected by gas component analysis [22].

This method monitors the dissolved gases in the transformer oil in on-line mode to

assess the condition of the transformer.

Oil Quality Monitoring is an inexpensive method of monitoring quality or purity of

the oil based on its dielectric strength. A Dielectric breakdown strength test

electrically stresses the oil to the point of failure. This can be the most accurate


37

measure of the quality or purity of the oil and its ability to perform the job of

electrical insulation in the transformer [44]. However, destructive nature of these tests

makes it unsuitable for repeated testing and online analysis. Most common method of

online Oil Quality Monitor is the Weidmann Centurion [42]. This method uses special

high speed technology to limit any breakdown energy from damaging the oil. The

advantage of Online Quality Monitoring test is that it‘s an inexpensive method and

can be easily deployed on any transformer equipment. However, the disadvantage of

Online Quality Monitoring test is that it is sensitive to moisture, carbon and metallic

particulates, fibrous and other impurities, and any burning or degradation of the oil in

the tested localised oil volume. Moisture content in oil and paper at an operating

temperature is a very strong indicator of the health of paper insulation in a

transformer. Moisture in oil is measured by a specialized integrated circuit(IC) which

is in contact with the oil. The IC heats the oil to a constant known temperature [20]

and senses the moisture content.

Other non-destructive techniques [48] such as time-domain dielectric spectroscopy,

laser intensity modulation method and pulsed electro acoustic method are used for

research and diagnosis of insulation degradation but these popular techniques have no

practicality as engineering tools to manage the aging apparatus. A number of these

space charge ‗‗dielectric response‘‘ measuring techniques have been reviewed by

Ahmed [49].

2.5 Relaxation Phenomena

Since it is planned to research the relaxation phenomena in oil filled transformers,

literature survey is carried on this relaxation phenomena. In the study of dielectric

systems, the analysis of the dielectric losses associated with relaxation phenomena

can identify the aging effects [44] [45]. Non-destructive relaxation phenomenon in

power equipment was studied to extract the condition indicators [46-48]. The basic

theory is that when dielectric material is subjected to low electric perturbation, E (t)

thereby avoiding any destructive or non-linear effects, dipoles in the dielectrics

becomes excited. It induces new delayed response polarization, P (t) due to electronic,

ionic, dipolar and interfacial polarization processes [45] [46]. The change in

polarisation and the resistive leakage currents with E (t) is effectively used to


38

diagnose the aging in power apparatus. There are costly commercial relaxation

instruments which are being used to estimate the degradation and moisture content in

oil-filled insulation without opening the tank of transformer[52][60][62][64]. They

evaluate the condition of transformer from the external measurements after isolating

all the terminals.

2.5.1 Insulation Resistance Measurement

The classical insulation resistance measurement with time is used to test the

degradation. On application of DC voltage, the dielectric gets polarised and the

current supplied by the source to the dielectric reduces gradually. In simple terms, the

insulation resistance increases slowly depending on the dielectric relaxation

behaviour. Industry uses the variation of insulation resistance with time to identify the

degradation [63]. It estimates the ratio of insulation resistance measured at 10 minutes

to the insulation resistance measured at 1 minute after the application of DC voltage.

This ratio is called polarisation index (PI) and the status of insulation is classified into

four levels. A ratio around 1 or less than 1 needs immediate attention for servicing the

insulation. A dc voltage level from 100 V to 10 kV is used to different HV insulation

systems. A number of different commercial instruments are available for use [61].

Before the application of dc voltage, the terminals are shorted for more than 15

minutes to drain any trapped embedded charges in the insulation. This is a time

domain method of measurement for a period of about 10 minutes.

2.5.2 Polarisation and depolarisation current measurement (PDC)

The polarisation and depolarisation current measurement is used to detect aging of the

insulation in a non-destructive manner [2] [4] [9] [41] [48] [51] [65]. The PDC

measurement is a transient current measurement technique. It is simple, but current

varies significantly depending on the condition of insulation. The order of current

magnitude and the rate of change in current with time are different for different

insulation systems. The accuracy of the measurement of current magnitudes and times

is important for signal processing as it contains relaxation information. In PDC, the

measured current due to polarization and depolarization will be comparable to

background noise, especially in a generating station.


39

A voltage in the order of 100 V to 2000 V is applied to the insulation and the

dielectric will be polarised. A decreasing polarising current with time is recorded and

the system will be polarised until there is not a significant change of polarising current

with time. This time may be from 1s to a few hours depending on the condition of

insulation and the change of current will be from a few amps to pico amps. Once a

steady state is reached, the power supply will be removed and the terminals will be

shorted to record the depolarisation current. Commercial instrument based on [51] is

released into the market around 2000 and a number of companies offer this

technology now at a competitive price. It is mathematically proved; the depolarisation

response behaviour will be the dielectric response function [46].

The advantage of PDC is that its test procedure is simple and the measurement

duration is considerably shorter than RVM. The interpretation of the measured results

in both RVM and PDC is very difficult as the measured dielectric response contains

much information pertaining to the interfacial and dipolar relaxation mechanisms.

This technique is also not standardised due to many uncertainties in measurement and

the level of voltage to be used even though, the technique is available from 2000. One

PDC measurement takes about three hours of test time. This method is in use in the

industry to identify the degradation trend.

Detailed presentation on RVM technique is presented in section 2.3.2. All these dc

time domain methods are simple to use but an extensive care must be made during the

measurements to get reproducible results in such a long period of measurements.

In next section, the literature survey on relaxation measurement in frequency domain

is presented.

2.5.3 Dielectrometry methods

The relaxation measurement is done in frequency domain extensively. The historical

condition monitoring technique uses 50 Hz high voltage testing using Schering bridge

[46]. The insulation is tested at rated voltage by including the apparatus in a Schering

bridge loop which measures the ratio of in-phase 50 Hz current to quadrature

capacitive 50 Hz current. That ratio is known as loss factor or tanδ. A factor of 0.001


40

is considered as good insulator. It is an expensive test and it needs very good

technical expertise in HV testing.

Low voltage and low frequency domain measurements are used by different dielectric

scientists [12] [54] to characterise the insulation. The team from ABB Sweden and

Swiss Federal Institute of Technology used this technique to characterise the

transformer insulation [67]. The outcome of this student project is converted into a

commercial dielectrometry instrument IDA202 around 2000 by Programma [64].

Even though this instrument is expensive and it is able to characterize the insulation

from a frequency of 1 µHz to 1000 Hz automatically with a sinusoidal voltage source

of peak magnitude around 150 V. A few other companies like GE and Omicron [60]

started manufacturing this type of dielectrometry commercial instrument with an

extensive software support. This is now being used in the industry widely. HV

versions of this technique are also in the market [66]. It can generate very low

sinusoidal frequency in the range of 0.1 to 0.01 Hz from 25 kVac to 200 kVac. It is

normally used to test the polymer HV cable.

This technique is used to determine the moisture content in oil-paper insulations of

power transformers, CT's and VT's, bushings and power cables by analysing the

dielectric response. This method can be applied to low and high frequencies, but it

needs a lot of measuring time for very low frequencies [60].

After the literature survey, it is understood that aging can be identified using the

relaxation instrument. It is found that all the existing instruments can be used only in

‗Off-line‘ mode of testing. QUT is interested in coming up with ‗On-line‘

monitoring techniques for power transformers. It is planned to develop our own

instrumentation, as the ultimate objective of the QUT condition monitoring projects is

to come up with on-line relaxation measuring technique for power transformers. The

initial development is concentrated on developing a reliable and portable ‗Off-line‘

relaxation measurement unit to identify aging with known transformers. For industrial

site measurements with a number of field transformers, the period for testing is

planned to be about 5 to 10 minutes for one set of measurement. Digital

instrumentation is planned to generate drift-free controllable low frequency signals

and to record data for further analysis. Reported analysis will be carried out with the

test data to identify the aging parameters.


41

2.6 Summary

In this second chapter, the existing knowledge for this relaxation study on

transformers is reviewed. As a new entrant to this research field, literature survey is

made on oil-filled transformer, possible developing faults, different condition

monitoring techniques and relaxation phenomena in transformer insulation. In section

2.5, a survey of existing commercial relaxation instrumentation is made and the

research plan is formulated. The novel contribution of this work is to investigations

on relaxation measurement by varying polarising voltage level and by changing

terminals of the transformer terminals to identify the aging trend on three known

power transformers are planned.


42

CHAPTER 3

DEVELOPED INSTRUMENTATION AND TEST

ARRANGEMENTS

As the main objective of this study is to identify relaxation phenomena with aging in

oil filled transformers, it is planned to develop relaxation instrumentation in low

sinusoidal frequency domain for such studies. Once the test arrangement is developed,

it is planned to carry a series of relaxation measurements on oil-filled transformers for

different analysis. This chapter reports on the developed relaxation instrumentation,

its specifications for use, the test layout and the details of the tested transformers.

3.1 Relaxation Instrumentation

Literature studied the relaxation phenomena with DC and AC voltages at different

voltage levels. No industrial standard has come up until this stage in fixing the

frequency range and magnitude of polarisation voltage. The relaxation measurement

as described in section 2.5 can be carried in time or frequency domains.

Time domain methods:

DC: With DC voltage, age old ‗megger test‘ is in use [19] [61] [63]. DC voltage

levels up to 5 kV of either polarity are being used depending on the voltage rating of

the tested apparatus. Any insulation resistance (IR) value greater than 100 M is

accepted for safe operation. An improvement in this technique is the measurement of

variation of IR with time. Depending on the polarisation behaviour of insulation, IR

will vary with time. Insulation quality factor known as ‗Polarisation Index‘ (PI) is

defined as the ratio of IR measured at 10 minutes and IR measured at 1 minute time

after the DC voltage energisation. A four level deciding factors like : >3 – very good;

>2 – good; >1 – ok; < 1 – immediate attention. It is widely in use as it is a simple test

and it needs less costly instrumentation. The procedure to carry the test is not

particularly complicated. The only requirement is that the injected charge should be

discharged completely before and after carrying of the test. The safety procedure

should be followed during HV application.


43

An improvement in this technique is called the polarisation and depolarisation current

measurements (PDC) [51]. In this test, the insulation will be energised for a period

until the variation in current (known as polarisation current) is negligible with time.

After that, the DC source is isolated and the insulation will be shorted through an

ammeter and the current known as ‗depolarisation current‘ will be recorded with time.

The measured current will be very low in the range of µA to pA, and it will be

changing very fast with sampling time. With industrial ambient noise, the

measurement to a period up to 1 hour or more requires a good knowledge of leakage

current measurement. No standard is set so far on the applied DC voltage level and

polarity of the applied voltage. The commercial instruments provide a voltage level up

to 2 kV [65]. The variation of depolarisation current with time is analysed and the

trend is fitted to electrical RC equivalent circuits to identify the degradation. It should

be noticed that the injected DC polarisation current should be completely discharged

before and after the test as the trapped charge may induce degradation. The technique

is a time consuming test and it requires good skills to identify the defects.

For high rating apparatus especially on polymer cables, another DC voltage technique

known as recovery voltage method (RVM) is in use [50][62]. The details on this

technique are described in section 2.3.2. In this the insulation of the apparatus is

charged for a certain period, shorted for nearly half of the charging period and then

the terminals will be completely open-circuited. The injected charge with slow

relaxation time will relax slowly and charge the geometrical capacitance of the

terminals slowly. This rises the voltage at the terminals and is known as ‗recovery

voltage‘ (RV) and then it decays slowly. This test will be repeated with various

charging times. By measuring the peak of RV and the initial slope of rise with

different charging time, various degradation factors are identified. It is time

consuming test with the requirement of very high input impedance to measure RV.

Charging, discharging and open-circuited timings are to be controlled. The

interpretation of the responses and the evaluation procedures are a bit complicated. A

properly trained person alone can carry this relaxation measurement.


44

In all the DC tests, ambient conditions play a major role by introducing the leakage

phenomena externally. Also, the injected charge due to uni-polar voltage should be

completely discharged.

Frequency domain methods:

AC: AC test is the more preferred test in the industry as it polarises and depolarises in

the repeated cyclic periods. HV Schering bridge test at 50 Hz is used to identify the

development of loss mechanism for more than 70 years [46]. It estimates the loss

angle – tan which is nothing but the ratio of in-phase to quadrature current

components of AC insulation current. It is normally tested at the rated voltage of the

apparatus or as specified by the user.

An extension of that principle is known as ‗dielectrometry‘ which is widely used by

material scientists to characterize the dielectric materials. It used a voltage in the

range of 10 V with frequency range from 1 Hz to 1 MHz [12]. ABB group used this

idea to identify the degradation in oil-filled transformers. They used a voltage level

around 110 Vac with a frequency range from 1 µHz to 1000 Hz to suit the industrial

test time requirement [67]. The instrument is very expensive but it is bipolar

measurement.

It is planned to use that technique as this may be extended for future ‗on-line

relaxation measurement‘. With the limited budget, an attempt is made to develop in-

house QUT relaxation instrumentation to generate low frequency drift-free AC sine

waves and measure the response relaxation currents in the order of a few nA.

3.2 Developed Relaxation Instrument

After a series of planning, a drift free high voltage amplifier with a gain of 20 to

generate AC voltage in the range of 200 V with a frequency response from 0.001 Hz

to 10 Hz is designed and tested for use. The designed HV amplifier is tested with

normal analog function generator and digital oscilloscope. It is found that many of the

analog function generators were drifting with varying asymmetrical characteristics in


45

the tested low frequency ranges. Hence, (i) A low frequency function generator to

generate distortion free programmable sine wave shape, (ii) a low leakage response

current measuring system and (iii) simultaneous digital data acquisition and storage of

function generator sine wave form and the corresponding leakage response current are

designed and tested for use. The details are presented below:

3.2.1 Function generator

Controllable waveform is generated using the hardware NI6009 [68] which are

interfaced with bench or lap-top computer using the USB interface. Necessary lab

view control software is developed.

Hardware – NI 6009 and HV amplifier:

The hardware NI 6009 can interface with eight analog input channels (AI), two analog

output channels (AO) and twelve digital input/ output channels (DIO). It contains a

32-bit counter for any timing applications. Other technical features are listed in Table

3.1. The generated waveform from one of the analog output channel is interfaced to

the buffer electronics and HV amplifier to generate sine wave with a peak magnitude

in the range from 90V to 200 V. The generated control low voltage waveform is

interfaced to one of the analog input channel to record the control waveform.

Software:

A lab view graphical program shown in Fig. 3.1 is developed to generate sine wave.

Desired frequencies and the magnitude of control voltage can be keyed in the control

panel shown in Fig. 3.2. By activating the lab view buttons, the control waveforms

can be generated in sequence. Screen1 at the top in Figure 3.2 shows the input control

sine wave and left side panels show the keyed in frequencies, evaluated peak values

and input control voltage level.


46

3.2.2 Leakage Current response measuring System

Using current to voltage converter, leakage current response from transformer is

converted to measurable voltage level. Another analog input channel is used to record

the proportional response current simultaneously with control voltage waveform

sampling. A manually controlled 3 range amplification is used to record a current

level from 1A to 100 µA with a maximum resolution of 30 nA without noise.

Screen 2 at the bottom in Figure 3.2 shows the control sine voltage wave form in

white and the corresponding leakage current response in red.

3.2.3 Data acquisition and storage

The developed software can record the proportional perturbation sine wave voltage

and the corresponding proportional response current waveform digitally. The control

flow is shown in Fig. 3.3. Depending on the keyed in frequency value, the sampling

internal clock rate is set. The control panel can be used to generate continuous sine

wave or single burst of 3 sine waves with the same frequency. The 3 sine waves are

generated to identify any initial transients in the measured initial relaxation responses

for analysis. The digital control signal and the corresponding response are

continuously recorded and stored as per the set control panel frequencies in a loop.

The execution can be terminated at any time using the stop button shown in the

control panel.


47

Fig 3.1 Developed Lab view graphical program


48

Fig 3.2 Typical screen control, generated and captured outputs as seen in computer

screen front panel

Fig 3.3 Control flow diagram

NI USB-6009 card, interfacing electronics, HV amplifier and leakage current

measuring system are housed in an earthed instrument box shown in Figure 3.4. A

three terminals measurement is done to eliminate the leakage current through the


49

grounded paths as shown in Fig. 3.5. The HV transformer in the cage is connected

with two long shielded coaxial XLPE insulated leads to the interfacing instrument box

kept on the work table. The developed instrument can be interfaced with the

computer through USB connection. The applied HV level can be monitored using a

DMM. The instrument is calibrated for current ranges and phase shifts using known

resistive loads. Using a manually controlled knob, the desired current amplification is

selected.

Fig 3.4 Developed Sine wave Relaxation Instrument

3.3 Developed Relaxation Instrument Specifications

• Controllable HV output to transformer test terminals: +/-90V to +/-200V.

• Measured leakage current in 3 ranges: R1—0 to 100µA

(6 digit accuracy in 5V range) R2– 0 to 10µA

R3– 0 to 1µA

Frequency range: 0.001Hz to 10Hz

Frequency steps: Five steps or desired number of steps for a sweep.

Waveforms: Sinusoidal, triangular and square

Operation: Continuous or discrete with 3 sine waves of 3000 sampled points

Requirement of Lab view Development System for field testing: No

Requirement of laptop or bench top computer: Yes

Approximate storage area for one sampling: 50kB

Power Consumption: less than 10 W


50

Size of the instrument without computer: 30x20x10 (in cms)

Weight: < 500 gm

Fig 3.5 General Layout of Connections (A, B and G are the three terminals)

3.4 Tested HV Transformers

Since the study is connected with relaxation measurement on the oil-filled

transformers, a sincere attempt is made to collect a few transformers of similar

construction and rating so that the relaxation characteristics can be compared with

aging. It is found that Queensland utilities use a significant number of single wire

earth return transformers (SWER) and they have to be evaluated at the field at a later

date. With that objectives in mind, a brand new SWER transformer T2 is purchased.

The local utilities provided other two aged transformers (T1 and T3) with name plate

age of 30 years and 15 years respectively. The details of those tested transformers are

provided in Table 3.2 and the photograph in Fig.3.6 shows the tested three

transformers. All the transformers are SWER type and they had three terminals. The

high voltage winding is abbreviated as ‗H‘, the low voltage winding is abbreviated as

‗L‘ and the tank is abbreviated as ‗T‘. To evaluate aging and degradation, it is planned

to study the effects of terminals and the effects of perturbing voltage levels. Before

starting the experiments, a preliminary analysis is made on the existing conditions of

the tested transformers.

3.4.1 Polarisation Index

Using Hioki insulation tester [61], PI is determined across the low and high voltage

windings of the transformers. The measured indexes are listed below:


51

[T1]: 1.28

[T2]: 3.49

[T3]: 1.34

From the readings, it is estimated the transformer, T1 is aged more than T3. New

transformer, T2 is fairly in good condition with a value of 3.49 and it is significantly

more than PI readings of T1 and T3.

3.4.2 Oil/( Insulation+ core + winding) weight ratio

[T1]: 19.08/86.92 kg

[T2]: 42.24/97.76 kg

[T3]: 33.18/106.32 kg

The ratio of oil to other components weight ratio varied as 20% for [T1], 30% for [T2]

and 20% for [T3]. It should be noted the new transformer [T2] is rated for 10 kVA

while the other two are rated for 5 kVA only. It clearly suggests that oil plays a

significant role in controlling the polarisation and heat transfer characteristics.

3.4.3 Resistance of the windings

[T1]: HV/LV - 425/1.2 ohms

[T2]: HV/LV - 165/0.5 ohms

[T3]: HV/LV - 1120/1.5 ohms

The above readings suggest that the operating temperature of the winding and oil in

loaded conditions can be estimated using the variation of HV winding resistance. The

change in LV winding resistance with temperature will be significantly low.


52

Fig 3.6 Tested Transformers T1, T2 and T3


53

Table 3.2 Name plate details of tested SWER Transformers

Terms

Transformer 1(T1) Transformer 2(T2) Transformer 3(T3)

Age

30 years old New transformer 15 years old

KVA

5 10 5

Volts HT

12,700 19,100

Volts LT

250 250

Amps HT

0.394 0.262

Amps LT

20 20

IMP 3.5% 3.3% 4.12%

Temp rise 60Deg 65Deg 55Deg

Cooling ON ON ON

Frequency 50Hz 50Hz 50Hz

Resistance HT/LT

425/1.2Ohms 165/0.5 Ohms 1120/1.5 Ohms

Height/Width

55.5/106 cms 55/130 cms 61/105 cms

Oil

21.68 L 48 L 37.7 L

Total wt

106 Kg 140 kg 139.5 Kg

Manufacture

PWA Electrical Industries

ABB Transformer PWA Electrical Industries

3.5 Relaxation Tests

The HV laboratory test floor is a grounded metal sheet.

Test objects:

The test transformers kept in the HV laboratory are insulated from ground so that

floating or insulated H, L and T terminals are available for independent tests. The

transformer terminals are grounded for an hour before starting the relaxation

measurement. Safety procedure is followed by keeping the test transformer in the


54

grounded faraday cage. The test transformer terminals are connected to the relaxation

instrument by two long shielded cables.

Relaxation Instrument:

The instrument is kept on the external work bench and is interfaced with a computer.

Five discrete frequencies are used to scan the desired frequency range.

Three voltage levels are used to understand the role of voltage magnitude on the

relaxation behaviour.

The instrument is calibrated with known resistance loads to estimate the phase errors

in the tested frequency range and to determine the conversion factors on voltage level

(V) and the three current (I) ranges.

Two sets of tests are planned as indicated below:

3.5.1 Effect of terminals

The transformer had three metal terminals to connect. The used abbreviations

are as follows: T- Tank; L – Low voltage winding; H – High voltage winding;

A – High voltage output from the instrument; B – High voltage return terminal

to the instrument and G – system ground.

Table 3.3 lists the connected terminals in Test1, Test2 and Test3.

Table 3.3 Terminals Connections

HV Unit output A B G

Test 1 /Transformer L H T

Test 2 /Transformer T H L

Test 3 /Transformer L T H


55

3.5.2 Effect of perturbing voltage

The effects of voltage on relaxation responses are evaluated by varying voltage level

in the range from +/-141V to+/-195V. Three voltage levels: +/-141V, +/-176V, +/-

195V are selected for the relaxation measurements.

3.6 Procedure to Perform Offline Relaxation Testing

All the transformer terminals are short-circuited and grounded for a minimum

period of 60 minutes before starting the measurements.

The transformer terminals are connected as per Table 3.3.

By selecting the lowest frequency measurement, the current range is set.

The data is collected at the selected five frequencies for each transformer

under three different voltages (+/-141V, +/-176V, +/-195V).

After the test, all the three transformer terminals are shorted to ground after

removing the connections to the instrument.

All the recorded readings are converted by using multiplication conversion

factors. Conditioning of data in the form of noise removal, single sine wave

extraction, and phase correction is carried as post data conditioning, and the

cleaned data is stored for further analysis.

3.7 Summary

With the above procedure and experimental layouts, reproducible relaxation results on

oil filled transformers are obtained in QUT high voltage laboratory. Since the aging

status is approximately known, the relaxation responses can be compared with aging.

No published relaxation papers are available by varying the terminals and perturbation

voltage levels. Novelty of testing by varying the terminals and voltage levels to link

with aging phenomena is introduced in this program. All the results are analysed

further for identifying sensitive parameters with aging on oil-filled transformers. The

developed instrument is also tested for reliable field operation without having lab

view software platform.


56

CHAPTER 4

EXPERIMENTAL RESULTS

In this chapter, the relaxation results obtained on three oil-filled power transformers

are reported. The oil filled transformer consists of low voltage winding and high

voltage winding mounted on iron core, and the entire structure is kept immersed in oil

kept in a sealed metal tank. The windings are insulated from core and tank with oil

immersed paper insulation. The external metallic contact terminals available for

electrical measurements are low voltage winding, high voltage winding and metallic

tank. The insulation gets deteriorated due to thermally induced electrical loading and

ageing. The status of aging in insulation is evaluated by means of dielectrometry

condition monitoring technique using the developed sine wave relaxation

instrumentation.

4.1 Measurements

Since three contact terminals are available for insulation measurements in the tested

transformers, three terminals measurement is carried out by connecting two terminals

to the instrument with other terminal shorted to system ground. Three sets of results

are obtained in the tested frequency range. Since the developed instrument can vary

the voltage level, the measurements are also carried by varying the voltage in 3 levels.

As a result, nine measurements are made. Since the test will introduce transients, the

measurement at each frequency is carried for 3 continuous cycles. At each frequency,

the total number of sampled digital points for the measured 3 continuous cycles is

3000. Five frequencies in the range of 15 MHz to 1.5 Hz are selected for the

measurements. The test periods are as follows: 2, 10, and 20, 100 and 200 seconds for

3 cycles. Hence each set of measurement will take (2+10+20+100+200 = 332

seconds) about 5 to 6 minutes of test duration. Each set of measurement will record

proportional perturbation sine voltage, response current and time of each of 3000

sampled points in separate data files. The stored data are analysed using the developed

Mat lab programs.


57

4.2 Signal conditioning

A typical recorded result is shown below in Figure 4.1. The used abbreviations are as

follows: ‗TG‘ means tank grounded and the insulation measurements are made across

low voltage winding (L) and high voltage winding (H). The distortion of the current

response (TG) shown in red in the first cycle can be seen. The recorded last cycle of

measurement with 1000 sampled points is taken for analysis. The multiplication

factors for current range are tabulated in Table 4.1. Occasional transients are recorded

and it is eliminated using signal processing. The magnitudes of voltage and current,

and phase shift between them are preciously measured. A leading phase angle on the

response current with respect to perturbation voltage is obtained in all the cases.

Table 4.1 Multiplication Factors

Range 1 Range 2 Range 3

Current 61.4 µA 6.14 µA 0.614 µA

0 0.5 1 1.5 2-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

Time in seconds

Vo

ltag

e in

V;C

urr

en

t in

A

T1 176

TG

V

Fig 4.1 Test on T1 in 2 second period with tank grounded. Perturbation voltage in

blue is to be multiplied by 352 and the response current in red is to be multiplied by

6.14µA.


58

4.3 Typical results

By extracting single sine wave, the relaxation characteristics are analysed and

compared. The low and high voltage windings are abbreviated as ‗L‘ and ‗H‘

respectively while the tank is abbreviated as ‗T‘. TG means with the tank grounded

the insulation current response measurements across ‗L‘ and ‗H‘ with perturbation

sine voltage is measured. The three peak sine voltage levels 141V, 176V and 195 V

are used for the studies. The recorded measurements on the tested three transformers

at the two extreme periods 0.67s and 66.7s are presented below in Figures 4.2 to 4.10.

Transformer 1 (T1)

0 0.1 0.2 0.3 0.4 0.5 0.6-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

Time in seconds

Vo

ltag

e in

V;

Cu

rren

t in

A

T1 141

TG

LG

HG

V

0 10 20 30 40 50 60-0.015

-0.01

-0.005

0

0.005

0.01

0.015

Time in seconds

Vo

ltag

e in

V;

Cu

rren

t in

A

T1 141

TG

LG

HG

V

Fig 4.2 Relaxation response of T1 in periods of 0.67s(Vx705;Ix1.25e-5) and

66.7s(Vx14100;Ix1.25e-5) with 141VpSine

0 0.1 0.2 0.3 0.4 0.5 0.6-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

Time in seconds

Vo

ltag

e in

V;

Cu

rren

t in

A

T1 176V

TG

LG

HG

V

0 10 20 30 40 50 60-0.03

-0.02

-0.01

0

0.01

0.02

0.03

Time in seconds

Vo

ltag

e i

n V

; C

urr

en

t in

A

T1 176V

TG

LG

HG

V




59

0 0.1 0.2 0.3 0.4 0.5 0.6

-1

-0.5

0

0.5

1

Time in seconds

Vo

ltag

e in

V;

Cu

rren

t in

A

T1 195

TG

LG

HG

V

0 10 20 30 40 50 60-0.06

-0.04

-0.02

0

0.02

0.04

0.06

Time in seconds

Vo

ltag

e in

V;

Cu

rren

t in

A

T1 195

TG

LG

HG

V



With T1, as the period of sine wave is increased the peak magnitude decreases

significantly. At 141V, the ratio with HG is : 0.38/0.014=27; with TG is:

0.37/0.008=46; with LG is 0.16/0.005=32. The leakage current is minimum with LG.

As the voltage is increased, the peak current magnitude increases.

At 176V, the ratio with HG is : 0.78/0.022=35; with TG is: 0.76/0.018=42; with LG is

0.32/0.015=21. The leakage current is minimum with LG.


0.45/0.015=30. The leakage current is minimum with LG.

The ratio indirectly indicates approximately the ratio of capacitive to resistive

leakage current responses. It is maximum with TG configuration at all voltage levels.

T1 is 30 years old transformer as per the name plate details.

Transformer 2 (T2)

0 0.1 0.2 0.3 0.4 0.5 0.6

-1

-0.5

0

0.5

1

Time in seconds

Vo

ltag

e in

V;

Cu

rren

t in

A

T2 141

TG

LG

HG

V

0 10 20 30 40 50 60-0.015

-0.01

-0.005

0

0.005

0.01

0.015

Time in seconds

Vo

ltag

e i

n V

; C

urr

en

t in

A

T2 141

TG

LG

HG

V




60

0 0.1 0.2 0.3 0.4 0.5 0.6

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Time in seconds

Vo

ltag

e in

V;

Cu

rren

t in

A

T2 176

TG

LG

HG

V

0 10 20 30 40 50 60-0.03

-0.02

-0.01

0

0.01

0.02

0.03

Time in seconds

Vo

ltag

e i

n V

; C

urr

en

t in

A

T2 176

TG

LG

HG

V



0 0.1 0.2 0.3 0.4 0.5 0.6-3

-2

-1

0

1

2

3

Time in seconds

Vo

ltag

e in

V;

Cu

rren

t in

A

T2 195

TG

LG

HG

V

0 10 20 30 40 50 60-0.04

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

Time in seconds

Vo

ltag

e i

n V

; C

urr

en

t in

A

T2 195

TG

LG

HG

V

Fig 4.7 Relaxation response of T2 in periods of 0.67s(Vx97.5;Ix1.22e-5) and


T2 is fairly NEW transformer. As the period of sine wave is increased the peak

magnitude decreases significantly. At 141V, the ratio with HG is : 0.05/0.004=12;

with TG is: 1.05/0.012=87; with LG is 0.2/0.006=33. The leakage current is minimum

with HG.



0.5/0.012=42. The leakage current is minimum with HG.

At 195V, the ratio with HG is : 0.4/0.01=40; with TG is: 3/0.031=97; with LG is

0.7/0.018=39. The leakage current is minimum with HG.

The ratio was maximum with TG configuration at all voltage levels.


61

Transformer 3 (T3)

0 0.1 0.2 0.3 0.4 0.5 0.6

-1

-0.5

0

0.5

1

Time in seconds

Vo

ltag

e in

V;

Cu

rren

t in

A

T3 141

TG

LG

HG

V

0 10 20 30 40 50 60

-0.5

0

0.5

Time in seconds

Vo

ltag

e i

n V

; C

urr

en

t in

A

T3 141

TG

LG

HG

V



0 0.1 0.2 0.3 0.4 0.5 0.6

-2

-1

0

1

2

Time in seconds

Vo

ltag

e in

V;

Cu

rren

t in

A

T3 176

TG

LG

HG

V

0 10 20 30 40 50 60

-1

-0.5

0

0.5

1

Time in seconds

Vo

ltag

e in

V;

Cu

rren

t in

A

T3 176

TG

LG

HG

V



0 0.1 0.2 0.3 0.4 0.5 0.6-3

-2

-1

0

1

2

3

Time in seconds

Vo

ltag

e in

V;

Cu

rren

t in

A

T3 195

TG

LG

HG

V

0 10 20 30 40 50 60

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

Time in seconds

Vo

ltag

e i

n V

; C

urr

en

t in

A

T3 195

TG

LG

HG

V

Fig 4.10 Relaxation response of T3 in periods of 0.67s(Vx97.5;Ix1.23e-5) and

66.7s(Vx325;Ix1.23e-5 ) with 195VpSine

T3 is 15 years old transformer and it was used extensively used for continuous HV

testing. As the period of sine wave is increased the peak magnitude decreases by half.

At 141V, the ratio with HG is : 1.2/0.45=2.7; with TG is: 0.7/0.15=4.6; with LG is

0.35/0.1=3.5. The leakage current is minimum with LG.



62

At 176V, the ratio with HG is : 2.49/0.9=2.7; with TG is: 1.56/0.28=5.5; with LG is

0.8/0.22=3.6. The leakage current is minimum with LG.

At 195V, the ratio with HG is : 2.95/0.82=3.6; with TG is: 2.1/0.42=5; with LG is

1/0.25=4. The leakage current is minimum with LG.

It is maximum with HG configuration at all voltage levels.

The responses clearly suggest that the current responses retain the wave shape but it

leads with respect to voltage. The current peak magnitude decreases if the test sine

wave period is increased. With respect to terminals of test, the patterns of responses

vary. More leakage current is obtained with HG on T1 and T3. With T2, high leakage

current is obtained with TG.

4.4 Consolidated results

The measurements are done at 5 selected frequencies by varying voltage level and

terminals. Since the significant changes occur in the peak sine wave magnitudes and

the phase shift, they are extracted and plotted in the following figures from Figure

4.11 to Fig.4.16.

4.4.1 Variation of peak current magnitude with frequency

Figure 4.11 shows the variation of peak current magnitude with frequency on T1. It

increases with increase in frequency and applied peak voltage magnitude. With LG

configuration, minimum leakage current is obtained.


63

10-2

100

0

0.2

0.6

1

1.4

x 10-5

Frequency in Hz

Resp

on

se s

ine p

eak C

urr

en

t in

AT1 141V

TG

LG

HG

10-2

100

0

0.2

0.6

1

1.4x 10

-5

Frequency in Hz

Resp

on

se s

ine p

eak C

urr

en

t in

A

T1 176V

TG

LG

HG

10-2

100

0

0.2

0.6

1

1.4x 10

-5

Frequency in Hz

Resp

on

se s

ine p

eak C

urr

en

t in

A

T1 195V

TG

LG

HG

Fig 4.11 Relaxation current response of T1 at different voltages

10-2

100

0

0.5

1

1.5

2

2.5

3

3.5

4x 10

-5

Frequency in Hz

Resp

on

se s

ine p

eak C

urr

en

t in

A

T2 141V

TG

LG

HG

10-2

100

0

0.5

1

1.5

2

2.5

3

3.5

4x 10

-5

Frequency in Hz

Resp

on

se s

ine p

eak C

urr

en

t in

A

T2 176V

TG

LG

HG

10-2

100

0

0.5

1

1.5

2

2.5

3

3.5

4x 10

-5

Frequency in Hz

Resp

on

se s

ine p

eak C

urr

en

t in

A

T2 195V

TG

LG

HG


Figure 4.12 shows the variation of peak current magnitude with frequency on T2. It

increases with increase in frequency and applied peak voltage magnitude. With HG

configuration, minimum leakage current is obtained. More current magnitude is

recorded in T2 in comparison with T1.


64

10-2

100

0

0.5

1

1.5

2

2.5

3

3.5

4x 10

-5

Frequency in Hz

Resp

on

se s

ine p

eak C

urr

en

t in

A

T3 141V

TG

LG

HG

10-2

100

0

0.5

1

1.5

2

2.5

3

3.5

4x 10

-5

Frequency in Hz

Resp

on

se s

ine p

eak C

urr

en

t in

A

T3 176V

TG

LG

HG

10-2

100

0

0.5

1

1.5

2

2.5

3

3.5

4x 10

-5

Frequency in Hz

Resp

on

se s

ine p

eak C

urr

en

t in

A

T3 195V

TG

LG

HG


Figure 4.13 shows the variation of peak current magnitude with frequency on T3. The

current increases with increase in frequency and applied peak voltage magnitude.

With LG configuration, minimum leakage current is obtained. More current

magnitude is recorded in T3 in comparison with T1.

4.4.2 Variation of leading phase shift with frequency

Figure 4.14 shows the variation of leading phase shift between the perturbation

voltage and the measured response current. With T1, the phase shift increases with

increase in frequency. At high frequency, it tends to be more capacitive and at low

frequency, it is more ohmic. A variation of 22º to 89º is obtained. More ohmic

behaviour is observed with HG configuration. The effect of voltage magnitude is

significant in the lowest frequency range. The voltage magnitude increases the phase

shift suggesting the increase in capacitive current in relation to resistive current.


65

10-2

100

20

30

40

50

60

70

80

90

Frequency in Hz

Ph

ase

Sh

ift

in d

eg

ree

s

T1 141V

TG

LG

HG

10-2

100

20

30

40

50

60

70

80

90

Frequency in Hz

Ph

ase

Sh

ift

in d

eg

ree

s

T1 176V

TG

LG

HG

10-2

100

20

30

40

50

60

70

80

90

Frequency in Hz

Ph

ase

Sh

ift

in d

eg

ree

s

T1 195V

TG

LG

HG

Fig 4.14 Relaxation leading phase shift response of T1 at different voltages

10-2

100

10

20

30

40

50

60

70

80

90

Frequency in Hz

Ph

ase S

hif

t in

deg

rees

T2 141V

TG

LG

HG

10-2

100

10

20

30

40

50

60

70

80

90

Frequency in Hz

Ph

ase S

hif

t in

deg

rees

T2 176V

TG

LG

HG

10-2

100

10

20

30

40

50

60

70

80

90

Frequency in Hz

Ph

ase S

hif

t in

deg

rees

T2 195V

TG

LG

HG



voltage and the measured response current on NEW transformer T2. With T2, the

phase shift increases with increase in frequency. At high frequency, it tends to be

more capacitive and at low frequency, it is modulated with capacitive and ohmic


66

current responses. A variation of 50º to 89º is obtained. More ohmic behaviour is

observed with HG configuration. The effect of voltage magnitude is significant in the

lowest frequency range and the effect of change is less than observed in T1. The

voltage magnitude increases the phase shift suggesting the increase in capacitive

current with voltage in relation to resistive current.

10-2

100

10

20

40

60

80

Frequency in Hz

Ph

ase S

hif

t in

deg

rees

T3 141V

10-2

100

10

20

40

60

80

Frequency in Hz

Ph

ase S

hif

t in

deg

rees

T3 176V

10-2

100

10

20

40

60

80

Frequency in Hz

Ph

ase S

hif

t in

deg

rees

T3 195V

TG

LG

HG

TG

LG

HG

TG

LG

HG



voltage and the measured response current on 15 years old transformer T3. With T3,

the phase shift increases with increase in frequency. At high frequency, it is

modulated with capacitive and ohmic current responses. At low frequency, it is more

ohmic. A variation of 8º to 70º is obtained. More ohmic behaviour is observed with

HG configuration. The effect of voltage magnitude is significant in the lowest

frequency range and the effect of change is less than observed in T1. The voltage

magnitude increases the phase shift suggesting the increase in capacitive current with

voltage in relation to resistive current.


67

4.5 Summary

Relaxation current responses are recorded on three oil-filled transformers with sine

wave perturbation peak voltage magnitudes in the range from 145V to 195V. A

frequency range from 15 MHz to 1.5 Hz is selected so that each set of measurements

will take about 5 to 6 minutes. The tested transformers had three external terminals

for the measurements: (i) high voltage winding (H), (ii) low voltage winding (L), and

(iii) metallic tank (T). Three terminals measurements are undertaken to evaluate

insulation across selected two terminals at a time. The current magnitudes from new

T2 and old T3 are significantly more in comparison with old T1.

In all the cases, the increase in frequency increases the current magnitude. The

increase in voltage also increases the current magnitude. The current magnitude with

low voltage winding grounded (LG) is less than the values obtained on grounding the

high voltage winding (HG) and tank (TG) in T1 and T3; In the case of T2, HG current

is less. The change in phase shift with frequency is significant in T1 and T2 in all the

terminals configurations and changes in applied sine wave voltages. Further analysis

is carried out to quantify the parameters to relate with ageing in the next chapter.


68

CHAPTER 5

ANALYSIS

In this chapter, the relaxation results obtained on three oil-filled power transformers

are analysed to extract sensitive parameters of aging. The section 5.1 presents the

theoretical basis for the requirement of analysis connected with the relaxation

phenomena. The section 5.2 presents the variation of extracted in phase current (IR)

component and 90º phase shifted capacitive response current (IC) component with

reference to frequency, voltage and terminal connections. The section 5.3 analyses

the variation of real and imaginary admittance components with reference to

frequency, voltage and terminal connections. The section 5.4 evaluates the variation

of tanδ with reference to frequency, voltage and terminal connections. After knowing

the trend of aging on all the three transformers, the effect of polarisation voltage

magnitude is studied. The section 5.5 reports the effect of voltage drop from 176V to

141V, and also the effect of voltage rise from 176 V to 195V on loss factor variation

with reference to 176V record. The section 5.6 evaluates the effect of similar voltage

change on percentage change of real and imaginary admittances.

5.1 Theory of Relaxation Phenomena

The basic theory of relaxation phenomena is that when dielectric material is subjected

to low electric voltage, E (t) thereby avoiding any destructive or non-linear effects,

dipoles becomes excited. It induces new delayed response polarisation, P (t) due to

electronic, ionic, dipolar and interfacial polarization processes [12] [45-46]. The net

induced dielectric flux density, D (t) can be expressed by (1).

tPtEtD 0 (1)

The time dependency of P (t) and D (t), however, will not be the same as that of E (t),

as the different polarization processes will have delays. P (t) is the increase in

polarisation due to the polarisation characteristics of the insulation and it varies as a


69

product of the susceptibility (χ) of the material and the applied voltage, E (t). The

equation (1) is rewritten as (2).

tEttEttEtD r 000 ) (2)

The factor, r is known as the dielectric constant. The measured r for transformer oil

is around 2.2 [48] while for composite insulation as around 3.5 [48]. The dielectric

constant of water is around 80.

On applying electric field E (t), both free and bonded charges will give rise to a

current flow, I (t). The movement of the free charges leads to a leakage current in

phase with the voltage, and the magnitude relies on dielectric conductivity 0 and

electrical stress E (t).

The electric displacement D (t) represents the (positive and negative) electric charges

(Q) per unit area as deposited at the outer surface of the electrodes which are the

origin of all electric field lines from sources and sinks. The so-called ‗displacement

current‘,ID(t) released from the voltage source as necessary to maintain the area

charge density at the electrodes is then only governed by ID(t) = dQ/dt, if Q is the sum

or integral of all charges deposited on each of the electrodes [44][46] [54]. Hence the

bonded charges contribute to the displacement current being a sum of the vacuum or

geometry based displacement current and polarisation current due to dielectrics.

The response current, I (t) is a sum of conduction and total displacement currents

shown in (3).

dt

tdDtEtI 0 = IR(t) + jxIC(t) (3)

By applying sinusoidal voltage, E(t), the response current can be measured. The in

phase component of the current, IR(t) represents the leakage or resistive or energy

dissipating component. While, the quadrature component, IC(t) represents the

capacitive or energy storing component. High quality dielectric materials do not have

any leakage component and the ratio of IR(t) with IC(t) is known as loss factor. It is

expressed in terms of phase shift as per equation (4).


70

Loss Factor = tan (δ) = tan (90º-Φ) = tI

tI

C

R (4)

For very good dielectrics, tan (δ) will be less than 0.001.

In this study, the instrumentation measures the response current in the sinusoidal

frequency (f) plane with period from 0.67s to 66.7s. The next section determines the

corresponding IR(f) and IC(f) in the frequency range down from 1.5Hz to 15 MHz.

Then corresponding admittance can be estimated for different applied voltages. The

variation of admittance and loss-factor with frequency can be related with aging on

each tested transformer.

5.2 Variation of IR(f) and IC(f) with frequency

The polarisation response current followed the sinusoidal perturbation voltage wave

shape with leading phase shift in all the cases. It suggests that the polarisation

response depends on resistive and capacitive components of current which follows the

theoretical equation (3). The variation of peak current magnitude with frequency is

shown in Figures 4.11 to 4.13. As the frequency is increased, the current magnitude

increases significantly. The corresponding phase shifts are shown in Figures 4.14 to

4.16. As the frequency is increased, the phase shift tends to move towards 90º. The in

phase resistive (IR) component is the current component multiplied by the cosine of

measured phase shift angle. The capacitive (IC) component is the current component

multiplied by the sine of measured phase shift angle. By separating each component

of the current, the trend of aging behaviour can be identified with its physical

reasoning. The used abbreviation is as follows: ‗TGIR‘ means tank grounded

configuration in phase (iR) current component. Similar abbreviation is followed on

LG, HG, IC, 141V, 176V, and 195V, T1, T2 and T3.


71

10-2

100

10-8

10-7

10-6

10-5

Frequency in Hz

Resp

on

se p

eak IR

an

d IC

in

A

T1 141V

TGIR

TGIC

LGIR

LGIC

HGIR

HGIC

10-2

100

10-8

10-7

10-6

10-5

10-4

Frequency in Hz

Resp

on

se p

eak IR

an

d IC

in

A

T1 176V

TGIR

TGIC

LGIR

LGIC

HGIR

HGIC

10-2

100

10-7

10-6

10-5

10-4

Frequency in Hz

Resp

on

se p

eak IR

an

d IC

in

A

T1 195V

TGIR

TGIC

LGIR

LGIC

HGIR

HGIC

Fig 5.1 Relaxation IR and IC current responses of T1 at different voltages

Figure 5.1 presents the estimated the current magnitudes on old transformer T1. In

most of the cases, both current components increase with increase in frequency. As

the voltage is increased, the current magnitude increases. In general, capacitive

component is found to be more than the resistive component. The capacitive current is

maximum under TG and HG configurations. The resistive current is maximum under

HG configuration.


72

10-2

100

10-8

10-7

10-6

10-5

10-4

Frequency in Hz

Resp

on

se p

eak IR

an

d IC

in

A

T2 141V

10-2

100

10-8

10-7

10-6

10-5

10-4

Frequency in Hz

Resp

on

se p

eak IR

an

d IC

in

A

T2 176V

10-2

100

10-8

10-7

10-6

10-5

10-4

Frequency in Hz

Resp

on

se p

eak IR

an

d IC

in

A

T2 195V

TGIR

TGIC

LGIR

LGIC

HGIR

HGIC

TGIR

TGIC

LGIR

LGIC

HGIR

HGIC

TGIR

TGIC

LGIR

LGIC

HGIR

HGIC


Figure 5.2 presents the estimated the current magnitudes on new transformer T2.

Both the current components increase with increase in frequency. As the voltage is

increased, the current magnitude increases. In general, capacitive component is found

to be more than the resistive component. The capacitive and resistive current

components are maximum under TG configuration.

10-2

100

10-7

10-6

10-5

10-4

Frequency in Hz

Resp

on

se p

eak IR

an

d IC

in

A

T3 141V

TGIR

TGIC

LGIR

LGIC

HGIR

HGIC

10-2

100

10-7

10-6

10-5

10-4

Frequency in Hz

Resp

on

se p

eak IR

an

d IC

in

A

T3 176V

TGIR

TGIC

LGIR

LGIC

HGIR

HGIC

10-2

100

10-6

10-5

10-4

Frequency in Hz

Resp

on

se p

eak IR

an

d IC

in

A

T3 195V

TGIR

TGIC

LGIR

LGIC

HGIR

HGIC


73


Figure 5.3 presents the estimated the current magnitudes on old transformer T3.

Both current components,IR and IC increase with increase in frequency. Fast rate of

change is observed with the capacitive component. As the voltage is increased, the

current magnitude increases. In general, IC is found to be less than IR in the low

frequency range. In the high frequency range, IC is more than IR. The capacitive

current is maximum under TG and HG configurations. The resistive current is

maximum under HG configuration. It follows the response of old transformer T1.

5.3 Variation of admittance with frequency

The admittance can be computed by dividing IR and IC by the applied voltage, E(t).

Real part of the admittance (AR) represents the conductance or reciprocal of

resistance. Imaginary part of the admittance (AC) represents the reciprocal of

capacitive impedance. AC is proportional to the product of capacitance and frequency.

10-2

100

10-10

10-9

10-8

10-7

Frequency in Hz

Resp

on

se A

R a

nd

AC

in

Mh

os

T1 141V

TGAR

TGAC

LGAR

LGAC

HGAR

HGAC

10-2

100

10-11

10-10

10-9

10-8

10-7

Frequency in Hz

Resp

on

se A

R a

nd

AC

in

Mh

os

T1 176V

TGAR

TGAC

LGAR

LGAC

HGAR

HGAC

10-2

100

10-10

10-9

10-8

10-7

Frequency in Hz

Resp

on

se A

R a

nd

AC

in

Mh

os

T1 195V

TGAR

TGAC

LGAR

LGAC

HGAR

HGAC

Fig 5.4 Real and imaginary admittance responses of T1 at different voltages

Figure 5.4 presents the trend of admittance on old transformer T1. AR and AC

increase with increase in frequency and applied voltage level. The effect of voltage is


74

significant in the lowest tested frequency range. Reactive admittance is more than the

conductive admittance. The capacitive admittance is maximum under TG and HG

configurations. The conductive admittance is maximum under HG configuration.

10-2

100

10-10

10-9

10-8

10-7

Frequency in Hz

Resp

on

se A

R a

nd

AC

in

Mh

os

T2 141V

TGAR

TGAC

LGAR

LGAC

HGAR

HGAC

10-2

100

10-10

10-9

10-8

10-7

10-6

Frequency in Hz

Resp

on

se A

R a

nd

AC

in

Mh

os

T2 176V

TGAR

TGAC

LGAR

LGAC

HGAR

HGAC

10-2

100

10-10

10-9

10-8

10-7

10-6

Frequency in Hz

Resp

on

se A

R a

nd

AC

in

Mh

os

T2 195V

TGAR

TGAC

LGAR

LGAC

HGAR

HGAC


Figure 5.5 presents the trend of admittance on new transformer T2. AR and AC

increase with increase in frequency and applied voltage level. A marginal increase is

observed with increase in voltage. Reactive admittance is significantly more than the

conductive admittance. The capacitive admittance is maximum under TG

configuration. The conductive admittance is also maximum under TG configuration.


75

10-2

100

10-9

10-8

10-7

Frequency in Hz

Resp

on

se A

R a

nd

AC

in

Mh

os

T3 141V

TGAR

TGAC

LGAR

LGAC

HGAR

HGAC

10-2

100

10-9

10-8

10-7

10-6

Frequency in Hz

Resp

on

se A

R a

nd

AC

in

Mh

os

T3 176V

TGAR

TGAC

LGAR

LGAC

HGAR

HGAC

10-2

100

10-9

10-8

10-7

10-6

Frequency in Hz

Resp

on

se A

R a

nd

AC

in

Mh

os

T3 195V

TGAR

TGAC

LGAR

LGAC

HGAR

HGAC


Figure 5.6 presents the trend of admittance on old transformer T3. AR and AC

increase with increase in frequency. The effect of applied voltage level is more in the

low frequency range. Variation of reactive admittance with frequency is significantly

more than the conductive admittance. The capacitive admittance is maximum under

TG and HG configurations. The conductive admittance is more than the capacitive

admittance in the low frequency range. Maximum resistive conductance is obtained in

HG configuration.

5.4 Variation of tan(δ) with frequency

Loss factor, tan(δ) can be computed using equation 4 in the frequency plane. The

computed tan (δ) variations with frequency for three tested transformers are shown in

Figures 5.7 to 5.9 respectively.


76

10-2

100

0

0.5

1

1.5

2

2.5

Frequency in Hz

Lo

ss F

acto

r

T1 141V

10-2

100

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Frequency in Hz

Lo

ss F

acto

r

T1 176V

10-2

100

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Frequency in Hz

Lo

ss F

acto

r

T1 195V

TG

LG

HG

TG

LG

HG

TG

LG

HG

Fig 5.7 Variation of loss Factor with frequency at different voltages in T1

For the old transformer T1, the loss factor decreases with increase in frequency and

perturbation voltage level. Maximum tanδ is obtained in HG configuration.

10-2

100

0

0.2

0.4

0.6

0.8

Frequency in Hz

Lo

ss

Fac

tor

T2 141V

10-2

100

0

0.2

0.4

0.6

0.8

Frequency in Hz

Lo

ss

Fac

tor

T2 176V

10-2

100

0

0.2

0.4

0.6

0.8

Frequency in Hz

Lo

ss

Fac

tor

T2 195V

TG

LG

HG

TG

LG

HG

TG

LG

HG


77


For the new transformer T2 also, the loss factor decreases with increase in

frequency. The decrease of loss factor with perturbation voltage level is less than T1.

The maximum loss factor is obtained with 141 V at the lowest tested frequency.

Maximum tanδ is obtained in HG and LG configurations.

10-2

100

0

2

4

6

8

Frequency in Hz

Lo

ss

Fac

tor

T3 141V

TG

LG

HG

10-2

100

0

2

4

6

Frequency in Hz

Lo

ss

Fac

tor

T3 176V

TG

LG

HG

10-2

100

0

1

2

3

4

Frequency in Hz

Lo

ss

Fac

tor

T3 195V

TG

LG

HG


The maximum loss factor of about 8 is obtained with 141 V at the lowest tested

frequency in old transformer T3. The loss factor decreases with increase in

frequency like all the tested transformers. The decrease of loss factor with

perturbation voltage level is more significant at the lowest tested frequency.

Maximum tanδ is obtained in HG configuration.

5.5 Effect of voltage on loss factor

Since perturbation voltage level is found to alter admittance and loss factor, the

percentage variation of those parameters is estimated with reference to 176 V level

measurements. Figure 5.10 presents the percentage variation from 176V level due to

141 V and 195 V readings on three transformers. The loss factor increases if the


78

perturbation voltage level is decreased as can be seen by the dotted lines. The loss

factor decreases if the perturbation voltage level is increased.

T1 – With the old tranformer T1, the loss factor is maximum around 2.4 at the lowest

test frequency in Fig. 5.7. The percentage change due to 176V to 141V change

(shown as dotted lines in Fig.5.10) is around 12 to 25%. The configurations HG and

LG resulted in more changes at the lowest test frequency. While the percentage

change due to 176V to 195V change (shown as solid lines in Fig.5.10) is less than

10% at the lowest test frequency. The change is almost same in all the three

configurations.

T2 – With the new tranformer T2, the loss factor is maximum around 0.85 at the

lowest test frequency in Fig.5.8. The percentage change due to 176V to 141V change

(shown as dotted lines in Fig.5.10) is around 10%. The configurations TG and LG

resulted in more changes at the lowest test frequency. While the percentage change

due to 176V to 195V change (shown as solid lines in Fig. 5.10) is less than 40% at

the lowest test frequency. The change is more with TG configuration.

T3 – With the old tranformer T3, the loss factor is maximum around 8 at the lowest

test frequency in Fig. 5.9. The percentage change due to 176V to 141V change

(shown as dotted lines in Fig.5.10) is around 55%. The configurations HG and LG

resulted in more changes at the lowest test frequency. While the percentage change

due to 176V to 195V change (shown as solid lines in Fig. 5.10) is less than 38% at

the lowest test frequency. The change is more with TG and LG configurations. With

increase in frequency, the percentage change decreases.

On comparing the three transformers, T3 is found to have significant change at the

lowest test frequency. Maximum change in LG and HG configurations suggest that

both low and high voltage windings may be undergoing severe degradation in T3.


79

10-2

100

-50

0

50

100

Frequency in Hz

Perc

en

tag

e c

ha

ng

e o

f L

F w

.r.t

176

V

T1 Loss Factor/Voltage

TG141

TG195

LG141

LG195

HG141

HG195

10-2

100

-50

0

50

100

Frequency in Hz

Perc

en

tag

e c

ha

ng

e o

f L

F w

.r.t

176

V


TG141

TG195

LG141

LG195

HG141

HG195

10-2

100

-60

-40

-20

0

20

40

60

Frequency in Hz

Perc

en

tag

e c

ha

ng

e o

f L

F w

.r.t

176

V


TG141

TG195

LG141

LG195

HG141

HG195

Fig 5.10 Variation of loss Factor with reference to 176V level

5.6 Effect of voltage on real and imagninary admittance

Variation of real and imaginary admittance with voltage is analysed in this section

5.6. By taking the response at 176V as base, the changes due to 141V and 195V are

analysed.

T1- Real (AR) and imaginary admittance(AC) variation for old transformer T1 is

shown in Fig.5.4. With increase in voltage and frequency, both components of

admittance increase. By changing the voltage from 176V to 141V, percentage

decrease in real admittance (shown as dotted lines in Fig.5.11) is around 52%. The

configuration LG resulted in more changes at the lowest test frequency. While the

percentage change of real admittance due to 195V to 176V change (shown as solid

lines in Fig. 5.11) is less than 80% at the lowest test frequency. The change is more

with HG configuration. With increase in frequency, in many cases the percentage

change decreases.

The behaviour of imaginary admittance is interesting. It increases significantly with

voltage at the lowest tested frequency in Fig. 5.4. By changing the voltage from 176V

to 141V, percentage decrease in imaginary admittance (shown as dotted lines in


80

Fig.5.12) is around 64%. The configuration LG resulted in more changes at the lowest

test frequency. While the percentage change of imaginary admittance due to 195V to

176V change (shown as solid lines in Fig. 5.12) is less than 90% at the lowest test

frequency. The change is more with HG configuration. With increase in frequency,

the percentage change decreases.

T2- Real (AR) and imaginary admittance(AC) variation for new transformer,T2 is



decrease in real admittance (shown as dotted lines in Fig.5.11) is around 52%. The

configuration HG resulted in more changes. While the percentage change of real

admittance due to 195V to 176V change (shown as solid lines in Fig. 5.11) is less

than 85% at the lowest test frequency. The change is more with LG configuration.

With increase in frequency, in many cases the percentage change increases.




Fig.5.12) is around 60%. The configuration HG resulted in more changes at the lowest

test frequency. While the percentage change of imaginary admittance due to 195V to

176V change (shown as solid lines in Fig. 5.12) is less than 75% at the lowest test

frequency. The change is more with TG configuration. With increase in frequency, in

many cases the percentage change decreases.

T3- Real (AR) and imaginary admittance(AC) variation for old transformer,T3 is



decrease in real admittance (shown as dotted lines in Fig.5.11) is around 44%. All the

configurations resulted in more changes. While the percentage change of real

admittance due to 195V to 176V change (shown as solid lines in Fig. 5.11) is less

than 61% at the lowest test frequency. The change is more with HG configuration.

With increase in frequency, in many cases the percentage change decreases.


81




Fig.5.12) is around 62%. The configurations HG and LG resulted in more changes at

the lowest test frequency. While the percentage change of imaginary admittance due

to 195V to 176V change (shown as solid lines in Fig. 5.12) is less than 120% at the

lowest test frequency. The change is more with HG configuration. With increase in

frequency, in many cases the percentage change decreases.

With all the three transformers, the change in real admittance with T3 is less. The

change in imaginary admittance on voltage with old transformers T1 and T3 is more.

10-2

100

-200

-150

-100

-50

0

50

100

150

Frequency in Hz

Perc

en

tag

e c

ha

ng

e o

f re

al

A w

.r.t

. 17

6V

T2 Admittance/Voltage

10-2

100

-100

-80

-60

-40

-20

0

20

40

60

80

Frequency in Hz

Perc

en

tag

e c

ha

ng

e o

f re

al

A w

.r.t

. 17

6V


10-2

100

-100

-50

0

50

100

Frequency in Hz

Perc

en

tag

e c

ha

ng

e o

f re

al

A w

.r.t

. 17

6V


TGR141

TGR195

LGR141

LGR195

HGR141

HGR195

TGR141

TGR195

LGR141

LGR195

HGR141

HGR195

TGR141

TGR195

LGR141

LGR195

HGR141

HGR195

Fig 5.11 Variation of Real admittance with reference to 176V level


82

10-2

100

-80

-60

-40

-20

0

20

40

60

80

100

120

Frequency in Hz

Perc

en

tag

e c

ha

ng

e o

f Im

ag

. A

w.r

.t.

17

6V


10-2

100

-140

-120

-100

-80

-60

-40

-20

0

20

40

60

80

Frequency in Hz

Perc

en

tag

e c

ha

ng

e o

f Im

ag

. A

w.r

.t.

17

6V


10-2

100

-80

-60

-40

-20

0

20

40

60

80

100

120

Frequency in Hz

Perc

en

tag

e c

ha

ng

e o

f Im

ag

. A

w.r

.t.

17

6V


TGC141

TGC195

LGC141

LGC195

HGC141

HGC195

TGC141

TGC195

LGC141

LGC195

HGC141

HGC195

TGC141

TGC195

LGC141

LGC195

HGC141

HGC195

Fig 5.12 Variation of Imaginary admittance with reference to 176V level

5.7 Summary

In Fig. 5.1, (T1) the variation of IC : (a) with frequency is around 50 to 90;

(b) with voltage is around 4

(c) with terminals is around 2

the variation of IR : (a) with frequency is around 4 to 9;



In Fig. 5.2,(T2) the variation of IC : (a) with frequency is around 15 to 100;





(c) with terminals is around 3 to 6

In Fig. 5.3, (T3) the variation of IC : (a) with frequency is around 80 to 110;






83


In Fig. 5.4, (T1) the variation of AC : (a) with frequency is around 47 to 100;



the variation of AR : (a) with frequency is around 2 to 8;



In Fig. 5.5, (T2) the variation of AC : (a) with frequency is around 17.5 to 167;



the variation of AR : (a) with frequency is around 2.8 to 23.3;



In Fig. 5.6, (T3) the variation of AC : (a) with frequency is around 16.7 to 40;



the variation of AR : (a) with frequency is around 1.1 to 5;



In Fig. 5.7, (T1) the variation of LF : (a) with frequency is around 8 to 24;

(b) with voltage is around 1.7 to 2.4

(c) with terminals is around 1.2 to 2.4

In Fig. 5.8, (T2) the variation of LF : (a) with frequency is around 4.35 to 12

(b) with voltage is around 0.7 to 0.85

(c) with terminals is around 0.3 to 0.85

In Fig. 5.9, (T3) the variation of LF : (a) with frequency is around 5.8 to 17;

(b) with voltage is around 4 to 8

(c) with terminals is around 2.5 to 8

In Fig. 5.10, the variation of LF: (a) with frequency is around -40 to 100;

(w.r.t. 176V to 141V &

w.r.t. 176V to 195V) (b) with voltage is around 50 to 100

(c) with terminals is around -40 to 100

In Fig. 5.11, the variation of AR: (a) with frequency is around 150 to -50

(w.r.t. 176V to 141V &


84

w.r.t. 176V to 195V) (b) with voltage is around 50

(c) with terminals is around 60 to -50

In Fig. 5.12, the variation of AC: (a) with frequency is around 120 to -60;

(w.r.t. 176V to 141V &

w.r.t. 176V to 195V) (b) with voltage is around 60

(c) with terminals is around 40 to -60


85

CHAPTER 6

DISCUSSION

In this chapter, the measured relaxation phenomena in oil filled transformers are

discussed with a view to identify ageing tendency. The oil filled transformer

insulation is made of pressboard insulation immersed in transformer oil. The studied

single wire earth return (SWER) transformers had one/two low voltage (LV)

winding/s with two/ four insulated terminals, one high voltage winding (HV) with two

insulated terminals, one 11 kV rated porcelain bushing connected to the high voltage

terminal, and a sealed metal tank housing all the windings, core and bushings. Both

LV and HV windings with the respective solid insulation are kept immersed in

transformer oil within the tank. From the name plate details, the weight of oil

insulation varied from 20% to 30% of total weight of the transformer. It should be

noted that the press-board solid insulation is impregnated with transformer oil under

vacuum condition. New transformer oil and paper insulation will have a minimum

quantum of water content. As the transformer is put into operation, the loading heats

up the insulation and unloading tracks back the insulation to lower operating

temperature. In general, the transformer may be operated with a temperature rise of

about 60ºC (as per name plate details) from the ambient. The paper and oil have

different moisture content at different temperatures [52]. Because of that, moisture

migration occurs between the insulation due to fluctuating operating temperatures and

degrades the oil. In addition, moisture may leak through weak transformer sealing

arrangements. Localised high electrical stresses may create hot-spots leading to

partial discharges [32] [55]. Because of all the different phenomena, it is found that

the acidity level and oxidation rate within the insulation increased [22] [60] and the

paper polymer strength, and oil breakdown strength would be reduced. The leakage

current normally increases leading to more heating of transformer [65].

In this study, dielectric relaxation measurements are carried out to identify ageing of

three known transformers of more or less similar rating and construction. Since the

objective of this study is related with the industrial asset management of oil filled

transformers, an industrial grade dielectric instrumentation is designed and tested for

use.


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6.1 Test arrangement

The test arrangement consists of the developed dielectrometry instrumentation, the

test objects of three ‗SWER‘ transformers with known aging level and a lab-top based

controller to generate the control waveform, to store the current response and to

analyse the relaxation characteristics of insulation.

6.1.1 Developed dielectrometry instrumentation

From the literature survey [60][64], dielectric sine wave measurements in the

frequency range from 1 µHz to 1000 Hz with a perturbation peak sine wave

magnitude of 141V are used to diagnose the degradation or moisture content in

different oil filled power apparatus. Also, it is learnt that time domain industrial

recovery voltage measuring unit (RVM) takes about 3 hours of test time for one set of

measurements [50][62]. It is found that all the existing instruments can be used only

in ‗Off-line‘ mode of testing. QUT is interested in coming up with ‗On-line‘

relaxation measurement unit. My initial development is concentrated on developing a

reliable and portable ‗Off-line‘ relaxation measurement unit to identify aging with

known transformers. For industrial site measurements, the period for testing is limited

to about 5 to 6 minutes. Since it is a digital instrumentation, the output is drift-free to

any lowest frequency level of measurement. The data can be stored with the desired

number of sampled points, and the number and desired number of frequencies can be

programmed in. Chapter 3 briefs the developed hardware and controlling software.

Two coaxial shielded cables with Teflon insulation are used to connect the

transformer.

The developed instrumentation can generate bipolar sine or any other programmed

wave with a peak voltage magnitude in the range from ±50 V to ±200 V. The

maximum current output from the unit can be 1 mA and the response current can be

recorded in 3 ranges with 6 digits resolution with multiplication factors of 1, 10 and

100. A current level of 30 nA can be recorded. The instrument is calibrated with the

known resistive load. The respective multiplication factors to convert recorded

voltage signals to the real applied HV and the response currents are tabulated in Table

(4.1). The high voltage generator introduced a phase shift error and the instrument is


87

calibrated to determine the introduced phase error with frequency in all the current

ranges.

After checking the reproducibility of the results, the measurement procedure is

standardised. A frequency range of 15 MHz to 1.5 Hz is selected so that one set of 5

discrete frequency measurements in that frequency range can be completed in 5 to 6

minutes by using the program. At each selected frequency, three consecutive cycles of

same period are recorded. The first and second cycles are found to have distorted

current responses. The last third cycle of measurement is taken for the analysis.

6.1.2 The test transformers

The tested ‗SWER‘ transformer has three metallic terminals. The insulation between

low voltages winding (L), high voltage winding (H) and tank (T) are tested by taking

two terminals at a time with other terminal grounded. Normally, the high voltage

winding gets degraded due to significant stresses.

Before taking any relaxation measurement, the terminals L, H and T of the

transformers are shorted to ground for about 15 to 30 minutes so that any residual

trapped charge across the insulation can be drained significantly. The trapped charge

can degrade the insulation and may not result in reproducible current response for any

voltage perturbation. T1 and T3 are aged as per the name plate details for 30 years

and 15 years respectively. T1 insulating oil was replaced about 3 years back. T3 was

used extensively for continuous HV testing. T2 is aged only for about six months and

is fairly new transformer. The dc test measurement between low and high voltage

windings indicated the polarisation index of T1, T2 and T3 as 1.28, 3.49 and 1.34. As

per the maintenance rules, T1 and T3 should be inspected for any degradation. The

transformers are kept on the insulated platform so that the tank is not grounded by the

supporting structure.

6.1.3 Computer interface

A lab-top computer loaded with control software communicates with dielectrometry

instrumentation to generate the desired sine waveform, and to store the proportional

HV perturbation voltage signal and response current signal with time of sampling.


88

Mat lab software routines are used to extract single voltage wave and the

corresponding response current wave shown in Figures 4.1 to 4.10. The various

analysis routines to extract aging tendency are developed.

The relaxation measurements are carried by varying the peak voltage magnitude of

sine wave and by varying the metallic terminals, L, H and T of the ‗SWER‘

transformers.

6.2 Ratio of sine wave response currents at two extreme frequency range limits

All the current responses lead the perturbation sine wave voltage suggesting that the

dielectric media behaves like an RC element. Figures 4.2 to 4.10 show the

proportional voltage and current responses with TG, LG and HG configurations. As

the frequency is reduced, the response current magnitude decreases. A frequency

change of 100 times (1.5 Hz to 15 MHz) results in current reduction of 42 to 55, 21to

32, 24 to 35 in TG, LG and HG configurations respectively with T1. With the new

transformer, T2 the corresponding current reductions are 87 to 97, 33 to 42, 12 to 40

in TG,LG and HG configurations respectively. The changes are more on TG

configuration. The corresponding current reductions with T3 are 4.6 to 5.5, 3.5 to 4,

2.7 to 3.6 in TG, LG and HG configurations respectively.

The change in current magnitude with change in frequency is more with new

transformer, T1. It is significantly less with transformer, T3. It suggests that T3 is

aged more than T2 and T1 across low and high voltage insulation.

Following the same reasoning, the insulation of low and high voltage windings with

respect to tank of T3 is in very bad condition.

In T2, new transformer‘s low voltage winding insulation with reference to tank is in

bad condition compared to T1. High voltage winding insulation with reference to

tank of T2 is better than the corresponding high voltage winding insulation to ground

of T1.


89

As the perturbation voltage magnitude is increased, the response current magnitude

also increases.

6.3 Trend of current variation with frequency

Figures 4.11 to 4.13 show the trend of current variation with frequency. In all the

transformers, the current increased with increase in frequency and applied

perturbation voltage.

In the old transformer T1, the observed peak current variation with voltage lied in the

range (0.18-0.58) x10-5

A with a ratio of 3.2; (0.45-1.3) x 10-5

A with a ratio of 2.9;

and (0.45-1.3) x 10-5

A with a ratio of 2.9 with the tested configurations of LG, TG

and HG respectively. The observed ratio of variation at the two extreme frequencies is

discussed in section 6.2.

In the new transformer T2, the observed peak current variation with voltage lied in the

range (0.3-0.8) x10-5

A with a ratio of 2.7; (1.25-3.7) x 10-5

A with a ratio of 3; and

(0.1-0.55) x 10-5

A with a ratio of 5.5 with the tested configurations of LG, TG and

HG respectively.

In the old transformer T3, the observed peak current variation with voltage lied in the

range (0.5-1.25) x10-5

A with a ratio of 2.5; (0.9-2.6) x 10-5

A with a ratio of 2.9; and

(1.4-3.55) x 10-5

A with a ratio of 2.5 with the tested configurations of LG, TG and

HG respectively.

New transformer showed a maximum current variation in the range of (0.55 to 3.7) x

10-5

A with a good sensitivity to voltage in the ratio of 2.7 to 5.5. The old transformer

T3 showed a maximum current variation of (1.25 to 3.55) x 10-5

A with a voltage

variation ratio of 2.5 to 2.9. The transformer T1 followed the characteristic of T3.


90

6.4 Trend of leading phase angle variation with frequency

Figures 4.14 to 4.16 show the variation of phase shift with frequency, voltage and

tested terminals. Monotonically increasing phase shift with increase in frequency and

voltage is seen in all the tested cases.

At 141V, old transformer, T1 had the phase variation of 56º(32º to 88º), 55º (27º to

82º) and 56º (23º to 79º ) with TG, LG and HG configurations for a frequency

increase of 100 times. At 195V, for the same frequency change, the phase variation

reduced to 49º (40º to 89º), 47º (34º to 81º) and 49º (30.5º to 79.5º) with TG, LG and

HG. With an increase in voltage, more polarisations are observed in all the tested

configurations.

At 141V, new transformer, T2 had the phase variation of 25º (62º to 87º), 38º (49º to

87º) and 31º (50º to 81º) for a frequency increase of 100 times with TG, LG and HG

configurations. At 195V, for the same frequency change, the phase variation reduced

to 14º (75º to 89º), 32º (54º to 86º ) and 26º (54º to 80º) with TG, LG and HG

configurations. With increase in voltage, more polarisations are observed in all the

tested configurations.

At 141V, old transformer, T3 had the phase variation of 50º (13º to 63º), 49º (9º to

58º) and 40º (8º to 48º ) for a frequency increase of 100 times with TG, LG and HG

configurations. At 195V, for the same frequency change, the phase variation reduced

to 50º (21º to 71º), 42º (18º to 60º) and 38º (15º to 53º) with TG, LG and HG

configurations. With increase in voltage, more polarisations are observed in all the

tested configurations.

The new transformer, T2 had phase variation in the range of 25º to 38º at 141V which

reduced to 14º to 32º at 195V.

The old transformer T3 had phase variation in the range of 40º to 50º at 141V which

reduced to 38º to 50º at 195V. T1 had phase variation in the range of 55º to 56º at

141V which reduced to 47º to 49º at 195V. With ageing, the range swings to more

phase shift with reduced variation with increase in voltage.


91

6.5 Trend of resistive and capacitive currents variation with frequency

Section 5.2 analyses the variation of in-phase resistive (IR) current and 90º phase

shifted capacitive (IC) current with frequency. Both the currents increase as the

frequency and applied perturbation magnitude are increased.

In old transformer T1, the rate of increase in IC with frequency is slightly more than

the rate of increase in IR with frequency. A frequency change of 100 resulted in

increased change of IC by 80 to125, 50 to 100, 43 to111 under TG, LG, and HG

configurations. The corresponding changes with IR are 1.88 to 2.7, 5 to 6.7 and 5 to

7.5. The increase in voltage resulted in more current for all the cases in the entire

frequency range. More polarisation current is recorded in TG and HG configurations.

More leakage current response is observed in HG configuration.

In new transformer T2, the rate of monotonic increase in IC with frequency is more

than the rate of increase in IR with frequency. A frequency change of 100 resulted in

increased change of IC by 105 to 133, 43 to 50, and 60 to 62 under TG, LG, and HG

configurations. The corresponding changes with IR are 7 to 20, 2 to 5, and 2.3 to12.5.

The effect of voltage is significant at the lowest tested frequency. With TG

configuration, more polarisation (IC) and leakage (IR) behaviour are observed across

low and high voltage winding insulation.

In old transformer T3 also, monotonically increasing current response is seen with

increase in frequency. The rate of change in IC with frequency follows the pattern of

T1. It is less than the resistive current component in the low frequency range. A

frequency change of 100 resulted in increased change of IC by 15.8 to 22, 15 to 25,

and 14.5 to 50 under TG, LG, and HG configurations. The corresponding changes

with IR are 1.89 to 2.2, 1.43 to 2 and 2.22 to 5. The effect of voltage in increasing the

current is seen in the entire frequency range. With HG and TG configurations, more

polarisation current (IC) trend is seen while increased leakage current (IR) behaviour is

seen with HG configuration.

Good new transformer, T2 resulted in increase IC current ratio in the range of 43 to

133 with increase in frequency. Similar range of variation is observed with T1. Bad


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transformer, T3 resulted in IC current ratio in the range of 14.5 to 50 with increase in

frequency.

Good new transformer, T2 resulted in increase in IR current ratio in the range of 2 to

20 with increase in frequency. The corresponding range of variation with old

transformer T1 is 1.88 to 7.5. Bad transformer, T3 results are like T1 with IR current

ratio in the range of 1.43 to 5 with increase in frequency.

Following that logic in T2, the insulation between low voltage and high voltage

winding is in good condition.

6.6 Trend of real and imaginary admittance variation with frequency

Figures 5.4 to 5.6 present the variation of admittance with frequency and voltage. The

real (AR) and imaginary (AC) admittances in general increase monotonically with

increase in frequency and voltage.

In old transformer T1, the rate of increase in AC with frequency is slightly more than

the rate of increase in AR with frequency. A frequency change of 100 resulted in

increased change of AC by 87.5 to100, 50 to 100, and 47 to 75 under TG, LG, and HG

configurations. The corresponding changes with IR are 2 to 2.1, 5.14 to 7.7 and 6 to 8.

The increase in voltage resulted in more admittance for all the cases in the entire

frequency range. More polarisation admittance (AC) is recorded in TG and HG

configurations. More leakage current response is observed in HG configuration.

In new transformer T2, the rate of increase in AC with frequency is more than the rate

of increase in AR with frequency. A frequency change of 100 resulted in increased

change of AC by 133 to 167, 56 to 67, and 17.5 to 20 under TG, LG, and HG

configurations. The corresponding changes with IR are 5.8 to 23.3, 4.3 to 5 and 2.8 to

10. The increase in voltage resulted in more admittance for all the cases in the entire

frequency range. More polarisation admittance (AC) is recorded in TG configuration.

More leakage current response is observed in HG configuration.


93

In old transformer T3, the rate of increase in AC with frequency is more than the rate

of increase in AR with frequency. A frequency change of 100 resulted in increased

change of AC by 16.7 to 20, 13.3 to 30, and 15 to 40 under TG, LG, and HG

configurations. The corresponding changes with IR are 1.1 to 2.14, 2.57 to 2.67 and

2.2 to 5. The increase in voltage resulted in more admittance for all the cases in the

entire frequency range. More polarisation admittance (AC) is recorded in TG and HG

configurations. More leakage current response is observed in HG configuration.

Good new transformer, T2 resulted an increase in AC current ratio in the range of 17.5

to 167 with increase in frequency. Similar range of variation is observed with T1. Bad

transformer, T3 resulted in AC current ratio in the range of 13.3 to 40 with increase in

frequency.

Good new transformer, T2 resulted an increase in AR current ratio in the range of 2.8

to 23.3 with increase in frequency. The corresponding range of variation with old

transformer T1 is 2 to 8. Results on bad transformer, T3 are like T1 with AR current

ratio lying in the range of 1.1 to 5 with increase in frequency.

Following that logic in T2, the insulation between low voltage and high voltage

winding is in good condition.

6.7 Trend of tanδ variation with frequency

Figures 5.7 to 5.9 show the variation of sensitive parameter tanδ with frequency and

voltage. The loss factor, tanδ decreases in all the cases with the increase in frequency

and perturbation voltage level.

In old transformer T1, for a frequency change of 100, the observed changes are 16 to

24, 8 to 11 and 8.5 to 11.8 under TG, LG, and HG configurations. The loss factor was

maximum under HG configuration indicating the weak low voltage winding

insulation with respect to tank.

In new transformer T2, the estimated loss factor is minimum. For a frequency change

of 100, the observed changes are 6.88 to 14, 11 to 12 and 4.35 to 5.5 under TG, LG,


94

and HG configurations. The loss factor was maximum under LG and HG

configuration.

In old transformer T3, maximum loss factor is observed. For a frequency change of

100, the observed changes are 8.7 to14, 8 to17 and 5.8 to 13.3 under TG, LG, and HG

configurations. The loss factor was maximum under HG configuration indicating the

weak low voltage winding insulation with respect to tank.

New transformer T2 had minimum loss factors and the variation with frequency and

voltage is minimum.

6.8 Effect of voltage on tanδ and admittance

Figure 5.10 shows the percentage variation of loss factor with voltage. With new

transformer, the change was maximum at the highest tested frequency. Increasing

trend of change is observed with increase in frequency.

Fig. 5.11 shows the percentage variation of real admittance with voltage. With new

transformer, more changes are observed. While Fig. 5.12 shows the variation of

imaginary admittance with voltage. The change is comparatively less on new

transformer.

6.9 Summary

This chapter discusses about the observed results under this research program. Section

6.1 presents in detail the developed new industrial relaxation instrumentation and the

test program. Section 6.2 analyses and discusses the results on response currents with

reference to aging indicators. Sections 6.3 and 6.4 discuss the trend of current and

phase shift to identify the aging tendency. Section 6.5 discusses the trend of resistive

and capacitive components of current with three transformers. While section 6.6

discusses on the estimated real and imaginary admittances. The section 6.7 analyses

the sensitive parameter of aging, tan δ variation with aging. The last section 6.8

discusses on the effect of perturbation voltage level on tanδ and admittance variation

on three transformers.


95

In simple terms, the study suggests that the aged insulation can be identified with

different analyses discussed in chapters 5 and 6. It can be quantified to relate with the

traditional aging rate measurements by fitting the trend of current, phase, admittance

and tanδ parameters with equations. The future field measurements with a number of

transformers may push the direction of this promising relaxation measurement to

identify aging tendency, aging level and aging rate.


96

CHAPTER 7

CONCLUSIONS AND SCOPE OF FUTURE WORK

This chapter summarizes the findings of this investigation and presents the possible

future work which may be undertaken further.

The main objectives of the research as stated in Chapter 1 are:

1. To develop and evaluate the specifications of a portable relaxation

instrumentation for industrial use.

2. To test the instrument and collect relaxation responses on known three aged

oil-filled transformers.

3. To analyse the data and identify aging conditional indicators.

4. Study the variation of aging indicators by varying the frequency range,

perturbation voltage level and terminals of test.

To the best of my abilities, the research tasks have been completed. The task will be

complete if the software and hardware is tested at any substation site transformer

away from controlled and protected QUT environment.

7.1 Conclusion

Objective 1:

The developed lab view software (i) to generate the digital control waveform, (ii) to

convert that data to suitable analog level signal to amplify, (iii) to store the

proportional high voltage polarisation and response relaxation current signal. The

developed software can run with lab view software platform. With the converted

*.exe module, it can be loaded to any computer – bench-top or lab top to interface

with the instrument. The communication is done through USB cable. The instrument

is calibrated with a known resistive load and the conversion factors are provided in

Table 4.1. It is found that HV amplifier generated phase shift error above 1.5Hz in

three current ranges.


97

In the appendix, the developed *.exe software to generate sine wave, triangular wave

and square wave is loaded in CD. The corresponding source programs *.vi are also

included. In this project, the testing is done with sine wave only. Most of the

measurements are carried only in the middle range of the incorporated 3 amplifier

ranges. The instrument is able to provide a maximum of 1 mA at a maximum voltage

level of ± 200 Vdc. In the conducted experiments, a frequency range from 15 mHz to

1.5Hz is selected. The limitation on that range is due to modulated noise current level

from the transformers in the lowest frequency level of 15 mHz. The software and

hardware are tested in lap-top as well as bench-top. It is also tested by other fellow

research students.

It is to be tested for use of substation site transformer.

Objective 2:

The instrument is tested with three known ‗SWER‘ oil filled transformers of different

aged levels. T2 is a new transformer. T1 is 30 years old transformer as per name plate

details. This transformer T1 is a reconditioned one after decommissioning after 25

years of service. The transformer oil was replaced with fresh oil and the utility

donated to QUT. T3 is another used transformer but it was used in a continuous mode

in utility and at QUT for pollution testing work. All the transformers had metallic

terminals from the low voltage winding, high voltage winding and tank. All the

shorting links were removed for the measurements and the tank was mounted on an

insulated platform for carrying the required tests. The available terminals were

shorted to ground before and after the relaxation measurements.

Three terminals insulation measurement is carried out to identify the degradation

between (i) low voltage winding and tank by grounding the high voltage winding, (ii)

high voltage winding and tank by grounding the low voltage winding,(iii) low voltage

winding and high voltage winding by grounding the tank. This way, the insulation

status of the windings can be evaluated in a better way. The relaxation sine wave

measurements in the selected frequency range were carried on a transformer with

three different configurations.


98

From the theory, it is learnt that the insulation will have two components of current. It

was planned to investigate the role of applied polarisation voltage level on the

relaxation current components. Using the instrument, it is found that the transformers

can be tested in a frequency range from 15 mHz to 1.5 Hz without much modulated

noise. Five set frequencies: 15 mHz, 30 mHz, 150 mHz, 300 mHz, and 1.5 Hz are

used for the test to get a frequency spectrum. Each set of measurement in that

frequency range took about 5 to 6 minutes. It is found that the initial perturbation

generated some distortion in the periodic response and the third consecutive cycle is

used for analysis.

The number of sampled points for each cycle was 1000 points for any frequency

range. A minimum current level of about 30 nA can be recorded with this unit and

the maximum perturbation voltage can be ± 200 V. Reproducible results are obtained

with the unit. Teflon coated shielded cables are used to connect the instrument to the

test transformer.

Objective 3:

The data are stored as *.txt files. Using the developed Matlab routines, the data are

analysed. Identification of aging condition indicators is made by the trend of data with

frequency, perturbation voltage level and terminals of test. All the relaxation current

responses are found to retain the sine wave shape and they are found to have leading

phase shift with reference to applied perturbation sine wave voltage.

Initial analysis is made on variation of current magnitude and phase shift with

frequency, perturbation voltage level and three test terminals configurations.

Then the derived analysis is made.

The variation of real/in phase /resistive current component and imaginary/90º phase

shifted/capacitive component with frequency, perturbation voltage level and three

test configurations are studied with three transformers. The loss factor or tanδ is

computed as a ratio of resistive current component to capacitive current component.


99

After that, admittance with real and imaginary admittance components is analysed.

The effect of perturbation voltage on tanδ and admittance responses is analysed. It is

found that the current and phase shift changed with increase in voltage. The

measurements are done at 141V, 176V and 195V. By taking 176V reading as

reference, the upward and downward voltage variation effects are analysed.

The ageing indicators tanδ, admittance and its changes with voltage in the lowest test

frequency can be effectively used as ageing predictors. At this stage, a correct

relationship could not be established as the method is not tested on many transformers

and also, the degree of polymerisation data to quantify aging using traditional method

is not available.

Objective 4:

It is found that the response relaxation current increases with increase in frequency

and perturbation voltage level. Chapter 6 provides the trend in some quantitative way.

It is found that T2 is in very good condition followed by T1 and then last more aged

as T3.

7.2 Scope of future work

The scope of this condition monitoring work appears to be enormous.

(i) This technique has to be validated in the substation test site oil filled

transformers of different rating and some valid data base to relate with

aging must be developed along with other condition indicators.

(ii) The instrumentation must be tested and improved for ‗On-line

measurement‘, and in that square wave or single pulse perturbation modes

may be of great use.

(iii) It seems that more curve fitting and extensive time and frequency domain

analysis can be made to identify the most sensitive conditional parameters

with aging.


100

(iv) This method may be extended to other electrical power apparatus for

ageing identification.

(v) ‗Effect of operation Temperature‘ and ‗Wave shape of perturbation

signal‘ to identify aging on transformers will be good area for the applied

research.


101

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APPENDIX A

1 Lab view software:

The one objective of this thesis is to develop the lab view software program for

(i) To generate the digital control waveform.

(ii) To convert that data to suitable analog level signal to amplify.

(iii) To store the proportional high voltage polarisation and response relaxation

current signal.

Three transformers are tested using this lab view software. This software is used to

collect the relaxation data for each of the transformer. The lab view software is used

under three different voltages ± 141V, ± 176V, and ± 195V and the each voltage is

operated under five frequency range of 15MHz to 1.5Hz. The each transformer is

tested under three conditions as shown in the chapter 3 in table 3.3. The relaxation

data is collected using this lab view software and plotted in the chapter 4 in fig 4.2 to

4.10. The graphical software code is shown in Fig.3.1.

2 Mat lab Program for Results:

In the chapter 4, the results are shown.

(i) The relaxation current response, from the data collected using lab view

software; the results are extracted using the Mat lab software. The M-file

for the fig 4.11, 4.12 and 4.13. The same M-file is used for the figures

4.11, 4.12 and 4.13. Just the values, magnitude and transformers are

changed in the program.

clear; F=[0.015 0.03 0.15 0.3 1.5]; TG=[[0.02+0.005 0.025+0.01 0.075+0.055 0.13+0.115

0.6+0.59]*3.9542*10-6]; LG=[[0.01+0.005 0.01+0.01 0.035+0.03 0.06+0.055 0.25+0.25]*3.9542*10-

6]; HG=[[0.02+0.02 0.03+0.02 0.08+0.07 0.14+0.13 0.61+0.61]*3.9542*10-6]; plot(F,TG,F,LG,F,HG); grid; legend('TG','LG','HG'); xlabel('Frequency in Hz'); ylabel('Response Sine Peak Current in A'); title('T1 141V');


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(ii) The relaxation leading phase shift, from the data collected using lab view

software; the results are extracted using the Mat lab software. The M-file

for the fig 4.14, 4.15 and 4.16. The same M-file is used for the figures

4.14, 4.15 and 4.16. Just the values, phase and transformers are changed in

the program.

clear; F=[0.015 0.03 0.15 0.3 1.5]; TG=[32.4 57.6 75.6 77.4 66.6+21.49]; LG=[27 50.4 60.12 68.4 61.2+21.49]; HG=[23.4 43.2 51.12 64.8 57.96+21.49]; plot(F,TG,F,LG,F,HG); grid; legend('TG','LG','HG'); xlabel('Frequency in Hz'); ylabel('Leading phase shift in degrees'); title('T1 141V');

(iii) Program to find Phase and Admittance

clear; k=3.1236e-6 load a20_01.txt; a1=a20_01(:,1); a2=a20_01(:,2); a4=a20_01(:,4);%4.5 to 0.5 i=1:3002; V1=max(a4); V2=min(a4); V3=(V1-V2)*0.5; V4=a4-V3-V2; plot(V4); V5=V4./(max(V4)); V6=V5*195; plot(V6);%***********voltage I1=a2; I2=I1*k; plot(a1,I2); V=V6(1250:length(V6)-10); I=I2(1250:length(V6)-10); T=a1(1250:length(V6)-10); save ND20_01 T V I; clear; load ND20_01;%T V I; plot(T,V); MV=max(V); MI=max(I); Adm=MI/MV %**********Phase NV=V./max(V);


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NI=I./max(I); t=1:length(NV); plot(t,NV,t,NI); grid;


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APPENDIX B

The analysis is done using the equations 1, 2, 3 and 4 in chapter 5, to find the

resistive current, capacitive current, resistive admittance, capacitive admittance

and loss factor. The mat lab program is done to show the plots plotted in the

chapter 5.

(i) Relaxation IR and IC current response for the fig 5.1, 5.2 and 5.3:

clear; F=[0.015 0.03 0.15 0.3 1.5]; TGIC=[1.79E-9 2.49E-9 2E-9 1.93E-9 2.02E-9]; LGIC=[2.33e-9 2.27e-9 1.04e-9 9.17e-10 8.57e-10]; HGIC=[3.42e-9 3.21e-9 1.94e-9 3.9e-9 2.03e-9]; TGIR=[0.113E11 0.1E11 0.621E10 0.370E10 0.168E10]; LGIR=[0.704E10 0.840E10 0.535E10 0.439E10 0.29E10]; HGIR=[0.408E10 0.465E10 0.204E10 0.869E9 0.84E9]; plot(F,TGIR,F,TGIC,F,LGIR,F,LGIC,F,HGIR,F,HGIC); legend('TGIR','TGIC','LGIR','LGIC','HGIR','HGIC'); xlabel('Frequency in Hz'); ylabel('Response peak IR and IC in A'); title('T1 141V');

(ii) Real and imaginary admittance response for the fig 5.4, 5.5 and 5.6:

clear; F=[0.015 0.03 0.15 0.3 1.5]; TGAR=[2.46E-9 3.97E-9 3.03E-9 3E-9 3.12E-9]; LGAR=[1.7e-9 1.71e-9 1.52e-9 1.39e-9 1.33e-9]; HGAR=[4.13e-9 4.17e-9 3.18e-9 3.28e-9 3.15e-9]; TGAC=[3.45E-8 2.27E-8 7.74E-9 6.58E-9 6.02E-9]; LGAC=[1.72E-8 1.08E-8 3.31E-9 3.2E-9 2.84E-9]; HGAC=[7.19E-8 4.21E-8 1.31E-8 1.05E-8 7.89E-9]; plot(F,TGAR,F,TGAC,F,LGAR,F,LGAC,F,HGAR,F,HGAC); legend('TGAR','TGAC','LGAR','LGAC','HGAR','HGAC'); xlabel('Frequency in Hz'); ylabel('Response AR and AC in Mhos'); title('T1 141V');


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(iii) Variation of loss factor for the fig 5.7, 5.8 and 5.9:

clear; F=[0.015 0.03 0.15 0.3 1.5]; TG=[1.57 0.63 0.26 0.22 0.03]; LG=[1.97 0.83 0.57 0.39 0.13]; HG=[2.31 1.06 0.8 0.47 0.19]; plot(F,TG,F,LG,F,HG); legend('TG','LG','HG'); xlabel('Frequency in Hz'); ylabel('Loss factor'); title('T1 141V');

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