This thesis examines modifications to transformer design to reduce temperature rise. The thesis identifies causes of heat generation in transformer windings and cores. It also discusses methods to reduce heat generated and dissipate heat efficiently. A prototype transformer is designed and tested based on the optimal design. Test results show the modifications successfully reduced heat losses and temperature rise compared to the existing design, which will slow insulation deterioration and increase transformer lifespan.

1. MODIFICATIONS IN TRANSFORMER DESIGN TO REDUCE
TEMPERATURE RISE
CHONG SlEW WEI
A thesis submitted
in fulfillment of the requirement for the degree of
Master ofEngineering
Faculty ofEngineering
UNIVERSITI MALAYSIA SARAWAK
2008

2. To My wife, Florence;
my daugbter, Estber;
and
my son, Elvis

3. Acknowledgements
I have pleasure in acknowledging the advice and assistance given to me in my research
and preparation ofthis 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

4. 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 ofadditional 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

5. materials, such as oil and kraft insulating paper. Therefore, transformer with the new
design is anticipated to have a longer life span.
iv

6. 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

7. 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

8. Table of Contents
Chapter 1
Chapter 2
Chapter 3
Chapter 4
ChapterS
Introduction and Thesis Overview
L1 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 ofreducing the t2R loss ................................... 20
3.3 Methods ofreducing the stray losses .............................. 21
3.4 Methods ofreducing the hysteresis loss .............. ............. 22
3.5 Methods ofreducing 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 oftransformer top oil temperature rise ..•. 41
4.4 Summary................ ............................................... 42
Results
5.1 Introduction ............................................................ 44
5.2 Result ofthe transfonner heat losses test .......................... 44
5.3 Result oftransfonner temperature rise test....... ............... 46
5.4 Summary ............................. ................................ 52
vii

9. Chapter 6 Discussion
6.1 Comparison oftest 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 oftransformer ........... .......................... 74
8.2 Iron and copper losses oftransformer with existing design .... 75
8.3 Temperature rise oftransformer with existing design ........ ... 76
Appendix C Calculations ofTransformer Heat Losses
C.1 Calculation ofthe copper loss .......................... ... ......... 79
C.2 Calculation ofthe iron loss.............. .. ............... .......... 83
Appendix D Simulation Outcomes of Other Designs
D.l Simulations ofthermal gradient for other designs ..... ... ....... 87
Appendix E Calculation ofThermal Gradient and Top Oil Temperature Rise
E.1 Theory ofcalculation ofthermal 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 ofTransformer 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 ofTransformer
G.l Costing oftransformer .................................. ............. 122
viii

10. 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 ofstray losses (in percentage ofI2
R loss) vs. secondary
current .................................................................. 13
: Specific loss ofsilicon steel vs. flux density ........................... 15
: Stack of many thin laminations ......................................... . 17
: Burrs ofsilicon steel sheet due to poor cutting......................... 18
: Unwanted gaps left between the core and the yoke..................... 19
: Grouping of leads for reduction ofstray losses ....................... . 22
: Arrows showing dissipation ofheat from winding to the
surrounding ............................................................ 25
: A sample ofgeometry modeling ......................................... 27
: A sample ofmesh generation............................................. 28
: Top view ofthe transformer winding ............................................. 31
: Outcome ofthe simulation for block A ofthe secondary winding .. 32
: Outcome ofthe simulation for block B ofthe secondary winding .. 33
: Outcome ofthe simulation for block A ofthe primary winding ..... 34
: Outcome ofthe simulation for block B ofthe 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 ofsecondary 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 ofsecondary 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

11. 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 oftransformer active part into the tank ................... 70
: Transformer active part immersed in the oil ............................ 71
: Outlook ofa transformer ................................................... 72
: Top view ofthe existing transformer winding .......................... 74
: Top view ofthe winding of 1000kVA llkV10.433kV transformer .. 79
: Top view ofthe magnetic core ofl000kVA l1kV/0.433kV
transformer.............................................................. 84
: Side view ofthe magnetic core of 1000kVA IlkV/0.433kV
transformer ............................................................ 84
: Top view ofthe 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 ofthe simulation for the primary winding (Block A) ...... 91
: Outcome ofthe 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 ofthe 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 ofsurface area from round wire to rectangular wire .... 102
: Cross section ofthe primary winding .................................... 103
: Detail ofthe secondary winding .......................................... 104
: Detail ofthe 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

12. 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 ofTables
: Viscosity ofoil at 40°C ..................................................... 8
: Resistivities ofvarious conductive materials ........................... 11
: Thermal conductivities ofcopper and kraft insulating paper .......... 29
: Outcome ofsimulations 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 ofthe outcomes of simulation and calculation ofthermal
gradient ................................................................ 42
: Summary ofthe outcomes ofsoftware computation and calculation
oftop oil temperature rise ........................................... 43
: Result ofthe iron loss test ................................................. 44
: Result ofthe I2R loss test .................................................. 45
: Result ofthe measurement ofPI ....................................................... 45
: Results ofthe heat losses test and the calculated values ............... 46
: Result for determination ofreference values RI and Tl ............... 46
: Readings taken during total heat losses and rated current
injections .............................................................. 47
: Results ofprimary 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 ofthe existing transformer tank ........................... 75 (
•
,'~
: Iron loss and copper loss tests results for 5 units oftransformer .... 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

13. 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

14. 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

15. 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

16. --.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 oftransfonner.
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 ofdifferent types of loading duty on the operating temperature ofa
transfonner.
4

17. 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 oftransformer oil used (such as mineral oil. coconut oil
and sunflower oil), the suitability of kraft insulating paper as insulator, and also the
causes ofgeneration ofheat 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

18. 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

19. 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

20. ----------------....--~-
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 ofvegetable origin and is obtained from the fatty kernels ofsunflowers.
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

21. 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 ofheat 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

22. f These losses amount to the quantity of electrical energy which is converted into heat
energy, causing the operating temperature ofa 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 ofthe 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. VL.
For a Delta-connected circuit,
(2.2)
1:1
<
For a Star-connected circuit,
(2.3)
Resistance ofwinding, R =pll A
where
p : resistivity ofthe conductor
I : length ofthe conductor
(2.4)
lO

23. r'
A : cross-sectional area ofthe conductor
Table 2.2 shows the resistivities ofvarious materials in ohm-meter [11].
Table 2.2 : Resistivities ofvarious conductive materials [11]
Material Resistivity (in obm-meter) at 20G
e
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.
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24. 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 ofstray losses (in percentage of the eR loss)
at different values ofsecondary current [13], obtained empirically.
'.,
, .
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