MODIFICATIONS IN TRANSFORMER DESIGN TO REDUCE TEMPERATURE RISE

CHONG SlEW WEI

A thesis submitted in fulfillment of the requirement for the degree of

Master of Engineering

Faculty of Engineering UNIVERSITI MALAYSIA SARA W AK

2008

To My wife, Florence;

my daugbter, Estber;

and

my son, Elvis

Acknowledgements

I have pleasure in acknowledging the advice and assistance given to me in my research

and preparation of this thesis. In particular, grateful thanks are offered to my supervisor,

Dr. Mohammad Shahril Osman, and the co-supervisor, Mr. Martin Anyi, for their

invaluable guidance and comments. To the engineers in the Research and Development

Department of Efacec Energia of Portugal, Mr. Anacleto Cardoso, Mr. Inacio Ribeiro,

Mr. Constantino Silva and Mr. Fernando Travessa, I would like to express my sincere

gratitude for their suggestions and ideas. Last but not least, I wish to thank the staff of

Sesco-Efacec Sdn Bhd for their help in constructing and testing ofthe prototype unit.

ii

Abstract

~n ideal transfonner is one which has no lOsses) f'bis would entail having purely

inductive coils wound on a loss-less core, impossible to realize in practic~) [rransfonner

losses are dissipated as heat, leading to higher operating temperatures which cause the

transfonner insulating materials to deteriorate fasteJ Iff there is a breakdown of the J '

insulation system, transfonner will fail) (This research identifies causes of the heat

generation from the windings and magnetic core of a transfonner in operation, and

methods of reducing this from the aspects of design, materials used and workmanship

during manufacturing of transfonne~ ~t also covers how this heat can be removed

efficiently through proper arrangement of cooling ducts, selection of good transfonner

oil as the agent for heat removal, and optimal designs of transfonner tank and fin walls

for efficient heat dissipation to the surrounding medium. A transfonner prototype was

manufactured based on the most optimal design in tenns of the heat generation and

dissipation. The main amendments to the existing design include the use of conductors

of bigger sizes in the primary and secondary windings; use of a larger magnetic core

with silicon steel of higher grade; better wiring connections and handling of core

laminations, repositioning and creation of additional cooling ducts, as well as increased

cooling fin depth. Tests perfonned on the prototype unit and simulation results verify

that the heat losses and temperature rise have been reduced significantly. Thus, the

modifications made to the new design, with the ultimate aim of achieving lower

transfonner heat losses and temperature rise, have proven to be effective. The lower

temperatures will slow down the rate of deterioration of transfonner insulating

iii

materials, such as oil and kraft insulating paper. Therefore, transformer with the new

design is anticipated to have a longer life span.

iv

Abstrak

.,..

Transformer yang unggul tidak mempunyai kehilangan. Ini memerlukan lilitan gelung

yang hanya induktif ke atas suatu teras yang tidak ada kehilangan, dan adalah mustahil

pada praktiknya. Kehilangan transformer dalam bentuk tenaga haba seterusnya akan

meningkatkan suhu transformer dan menyebabkan kadar kemerosotan bahan penebat

dalam transformer yang lebih cepat. Transformer akan rosak seandainya sistem

penebatnya roboh. Penyelidikan ini mengenalpasti sebab-sebab penjanaan haba dan

lilitan and teras magnet. Cara-cara untuk mengurangkannya juga dikaji dari segi

rekabentuk, bahan mentah yang digunakan and kemahiran kerja semasa pengilangan

transformer. Laporan ini juga meliputi sudut keberkesanan penyebaran haba dari lilitan

transformer dengan penyusunan saluran penyejukan yang sesuai, pemilihan minyak

transformer yang baik sebagai pengantar bagi penyingkiran haba, dan rekabentuk tangki

transformer dan dinding sirip yang optimum bagi penyebaran haba yang berkesan ke

persekitaran. Seterusnya satu prototaip dihasilkan berdasarkan rekabentuk yang paling

optimum dari segi penjanaan dan penyebaran haba. Antara perubahan utama yang

dibuat termasuklah penggunaan konduktor yang lebih besar di Iilitan utama and lilitan

sekunder, penggunaan teras magnet yang lebih besar dan dengan gred yang lebih baik,

pengambilan tindakan berjaga-jaga semasa penyambungan wayar dan pengendalian

lapisan-Iapisan teras, pengalihan kedudukan dan penyelitan saluran penyejutan

tambahan, dan juga penggunaan sirip penyejutan di tangki transformer yang lebih lebar.

Ujian yang dijalankan ke atas unit prototaip ini and keputusan simulasi mengesahkan

bahawa kehilangan and peningkatan haba dapat dikurangkan dengan banyaknya.

Pembaikan yang dibuat ke atas rekabentuk baru, dengan tujuan muktamad untuk

v

menghasilkan transformer yang kurang kehilangan and peningkatan haba, disahkan

berkesan. Peningkatan haba yang kurang bermakna kadar kemerosotan bagi bahan­

bahan mentah transformer seperti minyak dan kertas penebat kraf akan dikurangkan.

Dengan itu, dijangkakan bahawa transformer dengan rekabentuk baru ini akan

mempunyai hayat yang lebih panjang.

vi

Table of Contents

Chapter 1

Chapter 2

Chapter 3

Chapter 4

ChapterS

Introduction and Thesis Overview L 1 Introduction and research problem ................................. 1.2 Research objective .................................................. . 3 1.3 Organization ofthe thesis ........................................... 3

Literature Review 2.1 Introduction ........................................................... 5 2.2 Transfonner oil ....................................................... 5 2.3 Transfonner insulating paper ...................................... . 9 2.4 Causes ofheat generation ......................................... .. 9

2.4.1 Transfonner I2R loss ...................................... 10 2.4.2 Transfonner stray losses .................... ...... ........ 12 2.4.3 Transfonner hysteresis loss........... ................... 14 2.4.4 Transfonner eddy current loss ........................... 16

2.5 Summary ............................................................. 19

Methods to Reduce Heat Generated 3.1 Introduction ............................................................ 20 3.2 Methods of reducing the t2R loss ................................... 20 3.3 Methods of reducing the stray losses .............................. 21 3.4 Methods of reducing the hysteresis loss .............. ............. 22 3.5 Methods of reducing the eddy current loss .................... .... 23 3.6 Summary............................................................... 23

Methods to Dissipate Heat Efficiently 4.1 Introduction......................................... ................... 25 4.2 Changes made on winding for lower thermal gradient.......... 26

4.2.1 Software simulation ofthennal gradient........ ........ 26 4.2.2 Calculation ofthennal gradient in transfonner

winding .................................................. 36 4.3 Changes made on tank for better heat dissipation to ambient ... 37

4.3.1 Software computation oftransfonner top oil temperature rise ......................................... 39

4.3.2 Calculation of transformer top oil temperature rise ..•. 41 4.4 Summary................ ............................................... 42

Results 5.1 Introduction ........................................................ .... 44 5.2 Result of the transfonner heat losses test .......................... 44 5.3 Result of transfonner temperature rise test....... . .. . .. . .. . ..... 46 5.4 Summary ............................. ................................ 52

vii

Chapter 6 Discussion 6.1 Comparison of test results .......... ........................... ...... 53 6.2 Factors that contribute to lower transformer heat losses.... . ... 54 6.3 Factors that contribute to lower transformer oil and windings

temperature rises ................................................. 54

Chapter 7 Conclusion and Future Works 7.1 Conclusion .... ................... ........................ ........ ...... 57 7.2 Future works .......................................................... 58

References ........................................................................................ 59

Appendix A Basic Operation and Construction of a Transformer A.l Basic operation ofa transformer ................................. ... 62 A.2 Construction ofa transformer ....................................... 63

Appendix B Transformer with Existing Design and Its Heat Losses and Temperature Rise

8.1 Existing design of transformer . .......... .......................... 74 8.2 Iron and copper losses of transformer with existing design .... 75 8.3 Temperature rise of transformer with existing design ........ ... 76

Appendix C Calculations of Transformer Heat Losses C.1 Calculation of the copper loss .......................... ... ......... 79 C.2 Calculation of the iron loss.............. .. ............... ....... ... 83

Appendix D Simulation Outcomes of Other Designs D.l Simulations of thermal gradient for other designs ..... ... ....... 87

Appendix E Calculation ofThermal Gradient and Top Oil Temperature Rise E.1 Theory ofcalculation of thermal gradient........................ 99 E.2 Calculation ofthermal gradient in the transformer windings

with new design .................................................. 103 E.3 Calculation oftransformer top oil temperature rise ..... ........ 108

Appendix F Testing of Transformer Heat Losses and Temperature Rise F.l Test method and circuit for iron loss. ................ ............. 114 F.2 Test method and circuit for copper loss .. ............... ...... .... 115 F.3 Test method and circuit for transformer temperature rise .. ..... 116

Appendix G Costing of Transformer G.l Costing oftransformer .................................. ............. 122

viii

List orFigures

Figure 1.1

Figure 1.2 Figure 2.] Figure 2.2 Figure 2.3 Figure 2.4

Figure 2.5 Figure 2.6 Figure 2.7

Figure 2.8

Figure 3.1

Figure 4.1

Figure 4.2 Figure 4.3 Figure 4.4 Figure 4.5 Figure 4.6 Figure 4.7 Figure 4.8 Figure 4.9 Figure 4.10 Figure 5.1 Figure 6.1

Figure A.I Figure A.2 Figure A.3 Figure A.4 Figure A.5 Figure A.6 Figure A.7 Figure A.8 FigureA.9 Figure A.]O Figure A.II Figure A.12 Figure A.I3

: Damaged transformer winding due to perforation ofoverheated copper foil ............................................................ . 2

: Severe burning of kraft insulating paper due to high temperature .. . 3 : Water content in transformer oil versus temperature ................. . 5 : Breakdown voltage versus water content ................................ 6 : Heat transfer coefficient versus viscosity .............................. . 7 : Graph of stray losses (in percentage ofI2R loss) vs. secondary

current .................................................................. 13 : Specific loss of silicon steel vs. flux density ......................... .. 15 : Stack of many thin laminations ......................................... . 17 : Burrs of silicon steel sheet due to poor cutting ........................ . 18

: Unwanted gaps left between the core and the yoke ..................... 19

: Grouping of leads for reduction of stray losses ....................... . 22 : Arrows showing dissipation of heat from winding to the

surrounding ............................................................ 25 : A sample ofgeometry modeling ....................................... .. 27 : A sample ofmesh generation ............................................ . 28 : Top view of the transformer winding ............................................ . 31 : Outcome ofthe simulation for block A ofthe secondary winding .. 32 : Outcome of the simulation for block B of the secondary winding .. 33 : Outcome ofthe simulation for block A of the primary winding ..... 34 : Outcome ofthe simulation for block B of the primary winding .. .. 35 : Outline drawing ofa transformer tank ................................. . 38 : Graph showing top oil temperature rise versus heat generated ...... . 40 : Cooling down curves ofthe primary and secondary windings ...... . 51 : Three portions of secondary winding with insertion ofduct at 1/3

position ................................................................. 56 : Diagram ofan ideal transformer .......................................... 62 : Silicon steel coil ........................................................... . 63 : Stacking of silicon steel sheet to form magnetic core ............... .. 63 : Partially completed magnetic core ...................................... . 64 : Manufacturing of secondary winding .................................... 65 : Cooling strips are inserted between layers ofcopper foil ............ . 65 : Side view of a secondary winding ...................................... . 66 : Top view ofa secondary winding ...................................... .. 66 : Manufacturing ofprimary winding ....................................... 67 : Cooling strips are inserted between layers ofthe primary winding .. 68 : Completed primary and secondary windings .......................... . 68 : Assembling ofwindings on to the magnetic core ..................... . 69 : Completed active part ofa transformer .................................. 70

ix

Figure A.14 Figure A.15 Figure A.16 Figure 8.1 Figure C.1 Figure C.2

Figures C.3

Figure 0.1 Figure 0.2 Figure 0.3 Figure 0.4 Figure 0.5 Figure 0.6 Figure 0.7 Figure 0.8 Figure 0.9 Figure 0.10 Figure 0.11 Figure 0.12 Figure 0.13 Figure 0.14 Figure 0.15 Figure E.1 Figure E.2 Figure E.3 Figure E.4 Figure E.5 Figure E.6 Figure E.7 Figure F.l Figure F.2 Figure F.3 Figure F.4

: Assembling of transformer active part into the tank ................. .. 70 : Transformer active part immersed in the oil .......................... .. 71 : Outlook ofa transformer ................................................... 72 : Top view of the existing transformer winding .......................... 74 : Top view ofthe winding of 1000kVA llkV10.433kV transformer .. 79 : Top view of the magnetic core ofl000kVA l1kV/0.433kV

transformer............................................................ .. 84 : Side view ofthe magnetic core of 1000kVA IlkV/0.433kV

transformer ........................................................... . 84 : Top view of the winding (trial J) ......................................... 87 : Outcome ofthe simulation for the secondary winding ............... . 88 : Outcome ofthe simulation for the primary winding .................. . 88 : Top view ofthe winding (trial 2) ....................................... .. 89 : Outcome ofthe simulation for the secondary winding (Block A) .. . 90 : Outcome ofthe simulation for the secondary winding (Block B) .. . 90 : Outcome of the simulation for the primary winding (Block A) ..... . 91 : Outcome of the simulation for the primary winding (Block B) ..... . 91 : Top view ofthe winding (Trial 3) ..................................... .. 93 : Outcome ofthe simulation for the secondary winding (Block A) .. . 94 : Outcome ofthe simulation for the secondary winding (Block B) .. . 94 : Outcome ofthe simulation for the secondary winding (Block C) .. . 95 : Outcome of the simulation for the primary winding (Block A) ., ... . 95 : Outcome ofthe simulation for the primary winding (Block B) ..... . 96 : Outcome ofthe simulation for the primary winding (Block C) .... .. 96 : Decomposition ofpartial temperature drop in winding .............. . 99 : Conversion of surface area from round wire to rectangular wire .. .. 102 : Cross section ofthe primary winding .................................... 103 : Detail ofthe secondary winding .......................................... 104 : Detail of the primary winding ............................................. 106 : Top view ofa transformer tank ......................................... .. 109 : Evacuation coefficient vs. fm depth .................................... .. 110 'J

3: Connection for iron loss test ............................................ .. 114 . 2: Connection for copper loss test ........................................... 115

: Test circuit for measuring winding resistance (line in bold) .......... 117 : Test circuit for temperature rise test ...................................... 118

x

Table 2.1 Table 2.2 Table 4.1 Table 4.2 Table 4.3 Table 4.4 Table 4.5

/~4.6

~ able 4.7 , Table 4.8

Table 4.9

Table 5.1 Table 5.2

Table 5.3 Table 5.4 Table 5.5 Table 5.6

Table 5.7

Table 5.8

Table 6.1

Table B.l Table 8.2 Table B.3

Table D.I Table D.2 Table D.3 Table G.l

TableG.2

List of Tables

: Viscosity ofoil at 40°C ..................................................... 8 : Resistivities of various conductive materials ........................... 11 : Thermal conductivities of copper and kraft insulating paper .......... 29 : Outcome of simulations for various sections ofthe windings ........ 36 : Outcome ofcalculations ofthermal gradient ........................... 36 : Dimensions ofactive part and transformer tank ........................ 37 : Properties ofair, coolant, materials for tank and fin ................... 39 : Top oil temperature rise for five options ofdifferent fin depths ...... 41 : Dimensions ofoptimal tank and fin design ............................... 41 : Summary of the outcomes of simulation and calculation of thermal

gradient ................................................................ 42 : Summary ofthe outcomes of software computation and calculation

of top oil temperature rise ........................................... 43 : Result of the iron loss test ................................................. 44 : Result ofthe I2R loss test .................................................. 45

: Result ofthe measurement of PI ....................................................... 45 : Results ofthe heat losses test and the calculated values ............... 46 : Result for determination of reference values RI and Tl ............... 46 : Readings taken during total heat losses and rated current

injections .............................................................. 47 : Results of primary winding resistance measurement after test

power supply was shut down ....................................... 49 : Results ofsecondary winding resistance measurement after test ~ ,

power supply was shut down ....................................... 50 j

: Test results for transformers with the existing design and new ~

design .................................................................. 53 'f

: Dimensions of the existing transformer tank ........................... 75 (• , '~

: Iron loss and copper loss tests results for 5 units of transformer .... 75 , , ,: Top oil and winding temperature rise test results for 5 units of ~ .. . ?(

transformer ................................................. '" ........ 76 .1 j~: Summary ofthe simulation outcomes for trial 1 ........................ 89 3~..~: Summary ofthe simulation outcomes for trial 2 ........................ 92 ::It

: Summary ofthe simulation outcomes for trial 3 ........................ 97 '';:14 ~ : Material cost for transformer with the existing design ................ 122 z "

: Material cost for transformer with the new design ..................... 122

xi

r

Chapter 1 Introduction and Thesis Overview

1.1 Intrpduction and research problem

In- this rapid developing world, the use of electricity is in great demand. For

transmission and distribution networks to transfer large amounts of electricity over a

long distance with minimal losses and least cost, different voltage levels are required in

various parts of the networks. Transformers enable these changes in voltage to be

carried out efficiently.

The process oftransforming electrical power from the primary winding to the secondary

winding of a transformer incurs some losses. These losses, namely the copper loss and

iron loss, amount to the electrical energy which is converted to heat energy, causing

temperature rise in the transformer. High operating temperatures adversely affect the

properties of the insulating materials of a transformer such as transformer oil and kraft

insulating paper.

Transformer oil is used as insulation between the windings and the tank. Life span of

the oil can be monitored through regular sampling and analysis using standard

guidelines. Oil can be retreated or replaced when necessary. However, the kraft

insulating paper, which is used as insulation between layers of the winding, cannot be

retreated or replaced. Any failure in the kraft insulating paper is irreversible.

Thermal degradation of the kraft insulating paper will take place in transformers at

normal operating temperature of 60 to 90°C [1]. This degradation is accelerated at

1

higher temperatures. In the temperature range of 80°C to 140°C, every increment of

approximately 6°C doubles the rate ofageing [1].

Degraded kraft insulating paper may have acceptable electrical properties, but its

mechanical properties might be sufficiently weakened. It will no longer be able to

withstand the mechanical vibrations or stresses associated with transformer operation.

The paper can become brittle and break away from the transformer windings. This will

cause an internal electrical short between the layers of winding and overheat the

winding. Figure 1.1 and Figure 1.2 show a damaged transformer - the result of an

overheated transformer winding where kraft insulating paper and winding conductors

are perforated.

Figure 1.1 : Damaged transformer winding due to perforation ofoverheated copper foil

2

Figure 1.2 : Severe burning of kraft insulating paper due to high temperature

1.2 Research objective

This research will put-emphasis on two aspects of enhancements made to a transformer

in order to reduce its temperature rise. Firstly, how to reduce generation of heat, and

secondly, how heat can be dissipated efficiently from the transformer windings to the

surrounding medium. Various designs are considered, and a transformer prototype is

manufactured based on the most optimized design. Transformer heat losses and

temperature rise tests are then conducted on the prototype unit for the purpose of

verification. The ultimate aim is to reduce the temperature rise of a transformer in order

to prolong its life span.

1.3 Organization of the thesis

In Chapter 2, relevant studies related to the temperature rise of a transformer are

reviewed. These are, for example, the suitable types of oil and paper to be used in the

3

--.f'-----------------------------.....-.~.--.

transfonner as insulators and the causes of generation of heat when a transfonner is in

operation.

Chapter 3 elaborates on how improvements can be made to the existing design to reduce

the heat losses from aspects of the design, materials used and workmanship during

manufacturing of transfonner.

Chapter 4 describes a simulation done using finite element analysis software on various

winding designs. This is to identifY the parts in the design of transfonner windings

where heat can be dissipated efficiently from the windings in order to achieve a low

thennal gradient. Software is also used to calculate the sufficient number ofcooling fins

attached to the transfonner tank needed for good dissipation ofheat to the ambient.

Chapter 5 details the results of transfonner heat losses and temperature rise tests

perfonned on a prototype of transfonner manufactured based on the most optimized

design in tenns of heat generation and dissipation. In Chapter 6, the test results of old

and new designs are compared, and the factors that contribute to the improvements are

described. Chapter 7 is the conclusion and proposed future works, which may include

studies on the effect of different types of loading duty on the operating temperature of a

transfonner.

4

Chapter 1 Literature Review

2.1 Introduction

The chapter outlines relevant studies related to the temperature rise of a transformer.

These are, for example. the type of transformer oil used (such as mineral oil. coconut oil

and sunflower oil), the suitability of kraft insulating paper as insulator, and also the

causes of generation of heat when a transformer is in operation.

2.2 Transformer oil

Transformer oil serves both as an insulating material and the agent for heat removal. In

general, when the temperature increases, the solubility of water (from the cellulose of

the insulating paper) in transformer oil will also increase [2]. In other words,

transformers operating at higher temperatures will have higher water content in the oil.

This is shown in Figure 2.1.

Water content ppm

200

175

150

125

100

75

50

25

O+--------T--------~------~------~------~

o 10 20 30 40 50 Temperat~re 'C

Figure 2.1 : Water content in transformer oil versus temperature [2]

5

[

I

The result of the electrical breakdown voltage test done on oil with different water

contents is shown in Figure 2.2. The test was done based on the International

Electrotechnical Commission IEC60296 Standard [3] and it shows that the electrical

breakdown voltage oftransformer oil decreases when the water content is higher.

go.-~----------------------------------------~

70

60

Figure 2.2: Breakdown voltage versus water content [3]

:' 'j

\ .\

Therefore, it is important to fill transformers with an effective coolant (transformer oil)

which can improve heat transfer and lower the transformer temperature rise. Based on

the experiments done by Nynas .Naphthenics Research and Development Department,

the major factor that determines the cooling ability oftransformer oil is its viscosity [4].

The graph shown in Figure 2.3 was obtained experimentally. with a cooling strip

channel of 2m long, 30mm wide; assuming that oil velocity is O.5m1s at 60°C [4]. It

shows that the flow characteristic of transformer oil depends on its viscosity. The heat

transfer coefficient reduces at higher viscosity.

6

Heat Transfer Coefficient vs Transformer Oil Viscosity

400

350 ­$2' ~ -- 300 ­ - turbulent~ ... fbw = 250~ - laminar ~ fbw ~ 0 200 U,., ~.., =I

150

=,., ~ 100 - ............... -----­... ~ =: 50 -

0 0 5 10 15 20 25 30

Figure 2.3 : Heat transfer coefficient versus viscosity [4]

With viscosity of Ilmm2/s and above, laminar flow is expected. This means there is an

even layer of oil along the boundary between the windings and the oil. With viscosity

below Ilmm2/s, there will be turbulent flow of the oil. In turbulent flow, this layer is

disturbed and the oil is constantly mixed, so that new parts of oil continuously come

into contact with the windings. This provides a much more effective cooling, with

higher heat transfer coefficient compared to laminar flow. The existing standard for

transformer oils (lEC60296 Standard) [3] states that the viscosity should be at most

Il.Omm2/s at 40°C Nynas Nytro IOGBX transformer oil has viscosity of 9.0mm2/s at

40°C, and as such, is deemed suitable for use in transformer.

7

----------------....--~-

There have been works done on coconut oil for use in tropical countries where there is

an abundance ofenvironmentally friendly coconut oil available. Purified coconut oil has

a relatively good breakdown voltage of about 20kV (for an electrode gap of2.5mm) at

room temperature [5]. However, as cooling of transformer windings is mainly through

circulation of oil, it is important to use transformer oil with a low viscosity to facilitate

good convection. The viscosity of coconut oil at 40°C is 29 mm2/s [6], which is much

higher than the value specified in the existing standard for transformer oils. Therefore,

coconut oil is not very suitable to be used as a cooling medium in a transformer.

Sunflower oil is of vegetable origin and is obtained from the fatty kernels of sunflowers.

It has been used as transformer oil in several countries. The main disadvantage of

sunflower oil is that its viscosity is about 50mm2/s at 40°C [7]. Furthermore the cost of

sunflower oil is very high compared to mineral oil [7].

Table 2.1 summarizes the viscosities ofNynas Nytro IOGBX, coconut oil and sunflower

oil, compared with the requirement based on IEC60296 Standard.

Table 2.1 : Viscosity ofoil at 40°C

Requirement based on IEC60296 Standard viscosity :'511.0 mml/s Nynas Nytro lOGBX viscosity = 9.0 mm"is

Coconut oil viscosity = 29.0 mm"/s sunflower oil viscosity = 50.0 mmJls

8

T 2.3 Transfonner insulating paper

Paper is used between layers ofcopper foil and copper wire in transfonner windings as

an insulating material. Good electrical insulators, by nature, tend to be good thennal

insulators as well, which is undesirable. What is required is a system having maximum

electrical insulation and minimal thennal insulation characteristics.

Thennal conductivity of 'nonnal' paper is quite low, in the order of about 0.05W /(m.K)

[8]. There are several types of insulating paper with higher thennal conductivity and

higher breakdown voltage. and these are more suitable for use as an insulating material

in transfonner. Kraft insulating paper from August Krempel of Gennany, for instance,

has both a high breakdown voltage of more than IOkV/mm, and a good thennal

conductivity ofabout 0.2W /(m.K)[9].

Kraft insulating paper and transfonner oil are good insulators. Their insulating

properties are more effective when both are used together. This is exemplified in the

observed synergism of paper impregnated with oil: the dielectric strengths of oil and

paper on their own are 40 and 12 kV per mm respectively; however their dielectric

strength in combination is 64 kV per mm, which is a significant improvement [10].

2.4 Causes of heat generation

Generally, transfonner heat losses can be categorized into the Copper Loss and the Iron

Loss. The copper loss comprises the 12R loss and stray losses, whereas iron loss

comprises the hysteresis loss and eddy current loss in the magnetic core lamination.

9

f These losses amount to the quantity of electrical energy which is converted into heat

energy, causing the operating temperature of a transformer to increase.

2.4.1 Transformer I2R loss

As its name implies, the 12R loss is equal to the square of the phase current multiplied

by the resistance of the winding. It is the amount of heat generated due to resistive

heating of the conductor when current flows through.

where

Ip: phase current

R: resistance ofthe winding

(2.1)

Phase current, Ip. is determined by the rated power, SR and the line voltage. V L.

For a Delta-connected circuit,

(2.2)

1 :1

<

For a Star-connected circuit,

(2.3)

Resistance ofwinding, R =pll A

where

p : resistivity of the conductor

I : length of the conductor

(2.4)

lO

r'

A : cross-sectional area of the conductor

Table 2.2 shows the resistivities of various materials in ohm-meter [11].

Table 2.2 : Resistivities ofvarious conductive materials [11]

Material Resistivity (in obm-meter) at 20Ge Aluminium 2.65 x 10""

Brass 6.00-8.00 x 10.6

Copper 1.72 x 10.... Iron 9.80 x 10-lS

Mercury 95.80 x 10-11

Nickle 7.80 x lO'lS Platinum 9.00-15.50 x 1O-l!

Silver 1.64 x tOolS Tungsten 5.50 x 10'lS

Materials with high resistivities generate more heat when current passes through them.

The resistivity of copper is low and is therefore a good conductor. Silver has a lower

resistivity. but in view of its cost, it is too expensive to be used as electrical conductor.

A superconductor is a material that can conduct electricity. or transport electrons from

one atom to another, with no resistance. Unfortunately, most materials must be in an

extremely low energy state (cooled to a very low temperature) in order to become

superconductive. Research is underway to develop compounds that become

superconductive at higher temperatures. American Superconductor Corporation and

China's Institute of Electrical Engineering have successfully demonstrated a

transformer prototype, utilizing a high temperature superconductor in year 2005 [12].

However, at this stage, superconductors are not commercially available.

11

2.4.2 TransfQrmer stray losses

Stray losses can be subdivided into two key components - stray loss in winding and

stray loss in other components. These are the losses due to stray electromagnetic flux in

the winding, core clamps, tank walls and so on.

Various formulae have been put forward from time to time to calculate stray losses, but

there are too many factors such as shunt and inter-winding capacitance, stray

inductance, magnetic losses, winding resistance, an so on [13] which must be

considered in the calculation. It is more practical and common to express the stray

losses as a percentage of the }2R loss rather than to attempt to calculate it by means of

formulae [14]. Figure 2.4 shows the graph of stray losses (in percentage of the eR loss)

at different values of secondary current [13], obtained empirically.

'.,

, .

12