MODIFICATIONS IN TRANSFORMER DESIGN TO REDUCE 

TEMPERATURE RISE 

CHONG SlEW WEI
A thesis submitted 

in fulfillment of t...
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 oft...
Abstract
~n ideal transfonner is one which has no lOsses) f'bis would entail having purely
inductive coils wound on a loss...
materials, such as oil and kraft insulating paper. Therefore, transformer with the new
design is anticipated to have a lon...
Abstrak
.,..

Transformer yang unggul tidak mempunyai kehilangan. Ini memerlukan lilitan gelung
yang hanya induktif ke ata...
menghasilkan transformer yang kurang kehilangan and peningkatan haba, disahkan
berkesan. Peningkatan haba yang kurang berm...
Table of Contents
Chapter 1 

Chapter 2 

Chapter 3 

Chapter 4 

ChapterS
Introduction and Thesis Overview 

L1 Introduct...
Chapter 6 Discussion
6.1 Comparison oftest results .......... ........................... ...... 53 

6.2 Factors that con...
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...
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...
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
Tab...
r
Chapter 1 Introduction and Thesis Overview
1.1 Intrpduction and research problem
In- this rapid developing world, the us...
higher temperatures. In the temperature range of 80°C to 140°C, every increment of
approximately 6°C doubles the rate ofag...
Figure 1.2 : Severe burning of kraft insulating paper due to high temperature
1.2 Research objective
This research will pu...
--.f'-----------------------------.....-.~.--.
transfonner as insulators and the causes of generation of heat when a trans...
Chapter 1 Literature Review
2.1 Introduction 

The chapter outlines relevant studies related to the temperature rise of a ...
The result of the electrical breakdown voltage test done on oil with different water
contents is shown in Figure 2.2. The ...
Heat Transfer Coefficient vs Transformer Oil Viscosity
400
350 ­$2'
~
-- 300 ­ - turbulent
~... fbw
=250~
- laminar
~ fbw
...
----------------....--~-
There have been works done on coconut oil for use in tropical countries where there is
an abundan...
T 

2.3 Transfonner insulating paper
Paper is used between layers ofcopper foil and copper wire in transfonner windings as...
f These losses amount to the quantity of electrical energy which is converted into heat
energy, causing the operating temp...
r' 

A : cross-sectional area ofthe conductor
Table 2.2 shows the resistivities ofvarious materials in ohm-meter [11].
Tab...
2.4.2 TransfQrmer stray losses
Stray losses can be subdivided into two key components - stray loss in winding and
stray lo...
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  1. 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. 2. To My wife, Florence; my daugbter, Estber; and my son, Elvis
  3. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 11
  24. 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. '., , . 12

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