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DESIGN AND CONSTRUCTION OF HIGH VOLTAGE AC TESTING UNIT

A THESIS SUBMITTED TO THE GRADUATE SCHOOL OF NATURAL AND APPLIED SCIENCES OF THE MIDDLE EAST TECHNICAL UNIVERSITY

BY AYDEMİR ARISOY

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN THE DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

NOVEMBER 2000

Approval of the Graduate School of Natural and Applied Sciences.

-----------------------------Prof.Dr.TayfurÖZTURK Director I cirtify that this thesis satisfies all the requirements as a thesis for the degree of Master of Science.

------------------------------Prof.Dr.Fatih CANATAN Head of the Department This is to certify that we have read this thesis and that in our opinion it is fully adequate,in scope and quality ,as a thesis for the degree of Master of Science.

-------------------------------Prof.Dr.Mirzahan HIZAL Supervisor

Examining Committee Members ……………………………………..

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ABSTRACT

DESIGN AND CONSTRUCTION OF HIGH VOLTAGE AC TESTING UNIT

ARISOY,Aydemir M.S.,Department of Electrical and Electronic Engineering Supervisor :Prof. Dr. Mirzahan HIZAL September 2000,

pages

In this thesis,a complete High Voltage Alternating Current Testing Unit (HVAC TU) is designed and constructed to perform high voltage AC tests on electrical apparatus and material in accordance with IEC 60,and IEC 270 test standarts. Transient voltage distribution in the test transformer windings subjected to standart impulse waveform is investigated. Also an electrical field analysis of the transformer is realized by ANSYS 5.5.1 software program using FEM (Finite Element Method). Key Words:Test Transformer, insulation,impulse voltage,electrical field.

ÖZ

YÜKSEK GERİLİM ALTERNATİF AKIM TEST UNİTESİNİN TASARLANMASI VE İMALATI

ARISOY,Aydemir Yüksek Lisans,Elektrik ve Elektronik Mühendisliği Bölümü Tez Yöneticisi:Prof.Dr.Mirzahan HIZAL Eylül 2000, sayfa Bu tezde, IEC 60 ve IEC 270 deneme stadartlarına uygun olarak yüksek

gerilim

alternatif

akım

denemeleri için

elektrik

cihaz

ve

malzemelerine uygulanabilen bir Yüksek Gerilim Alternatif Akım Test Sistemi (YGAA TS) tasarlandı ve imalatı yapıldı. Geçici gerilimlerin test transformatörü sargılarındaki dağılımı standart darbe gerilimi dalga şekli test transformatörü sargılarına uygulanarak araştırıldı. Ayrıca, test transformatörünün elektrik alan analizi

Sonlu

Elemanlar Yöntemini (SEY) kullanan yazılım programı ANSYS 5.5.1 ile gerçekleştirildi.

Anahtar Sözcükler: Test Transformatörü,izolasyon,darbe gerilimi,elektrik alan.

To My Family… Z.Funda , Y.Kıvanç,N.Aslı

ACKNOWLEDGEMENTS

I would like to convey my sincere thanks and regards to Prof.Dr.Mirzahan HIZAL for his invaluable support , advice, guidance and endless patience throughout this thesis work. I would like to extend my thanks to Turkish Air Force for providing education opportunity and METU Electrical and Electronics Engineering Department for the facilities provided. I also express my appreciation to research assistant M.Efe ÖZBEK whose kind and patient helping hand has greatly guided this work. I also want to thank my wife Z.Funda for her support and patience during my late studies.Meanwhile I don’t have to forget my son,Y.Kıvanç, and my baby daughter , N.Aslı, special thanks to them.

TABLE OF CONTENTS

ABSTACT ………………………………………………………….

iii

ÖZ …………………………………………………………. ……….

iv

ACKNOWLEDGEMENTS …………………………………………

vi

TABLE OF CONTENTS ……………………………………………

vii

LIST OF TABLES …………………………………………………..

xii

LIST OF FIGURES ………………………………………………...

xiii

LIST OF SYMBOLS ………………………………………………..

xvi

CHAPTER 1. INTRODUCTION 1.1 Scope and Objective………………………………….

1

2.1 Outline of the Study……………………………………

5

2. AC HIGH VOLTAGE GENERATION AND TEST TECHNOLOGY 2.1 The Brief History of High Voltage Engineering…….

7

2.2 Classification of Dielectric Stress …………………

8

2.3 Introduction to High Voltage Testing ………………

9

2.3.1 Basic Testing Techniques

…………………

10

2.3.2 Testing Voltages ……………………………...

10

2.3.3 Testing with Power Frequency Voltages …..

11

2.3.4 Testing with Lightning Impulse Voltages ……

11

2.3.5 Testing with Switching Impulses …………….

12

2.4 Generation of High Alternating Voltages …………

12

2.4.1 Single Step-Up Testing Transformers ….…..

13

2.4.2 Cascaded Transformers …………...……….

15

2.4.3 Series Resonant Circuits ……………...……

17

2.5 Voltage Control of the Test Transformer………….

18

2.6 One-Line Diagram for HVAC Test System ……….

21

3. DESIGN OF THE HVAC TEST UNIT (HVAC TU) 3.1 Design Requirements and Specifications …………

22

3.2 Design of the Test Transformer …………………….

22

3.2.1 Design of The Core Cross-Section……………

23

3.2.2 Winding Calculation of the Test Transformer

23

3.2.3 Internal Insulation Design of HV Test Transformer ……………………………………

25

3.2.4 High Voltage Winding Design…………………

27

3.2.5 Low Voltage Winding Design …………………

30

3.2.6 Winding Dimensions in Core Section…………

30

3.2.7 Main Insulation of Test Transformer .………..

32

3.2.8 End Insulation ………………………………….

38

3.2.9 Inner Connections………………………………

40

3.3 External Insulation of Test Unit………….…………..

46

3.3.1The Bushings of HVAC TU……………………..

46

3.3.2 Air End Clearance Calculation of the HVAC TU Bushings ……………………….

50

3.3.3 Oil End Clearance of HVAC TU Bushings……

51

3.4 Cooling and Vacuum Treatment of Oil Test Transformers 3.4.1 Cooling……………………………………………

52

3.4.2 Vacuum Treatment of Oil Transformers………

53

3.5 Design of the Control and Measurement Unit………

54

3.5.1 Inrush Current Phenomena……………………

55

3.5.2 Solving Methods of Inrush Current Problem….

59

3.5.3 Inrush Current Calculations…………………….

59

4. ELECTRIC FIELD AND IMPULSE VOLTAGE ANALYSIS OF HV TEST TRANSFORMER WINDINS 4.1 Introduction……………………………………………

61

4.2 Voltages Appearing Across The Transformer Terminals 4.2.1 The service voltage…………………………….

62

4.2.2 Over-voltages…………………………………..

62

4.2.2.1 Internal over-voltages …………………

62

4.2.2.2 External over-voltages ……………….

64

4.3 Voltages Inside The Transformers …………………

66

4.3.1 Standing Waves and Travelling Waves……….

66

4.3.2 Determination of Voltage Distribution Within the Winding………………………………….…

70

4.3.2.1 Initial Voltage Distribution …………..

70

4.3.2.2 Final Voltage Distribution …………….

72

4.3.2.3 Voltage Oscillations ………………….

73

4.4 Mathematical model for the test transformer windings……………………………………………….

74

4.5 Impulse Voltage Characteristics of Test Transformer Windings………………………………..

75

4.5.1 Calculation of Capacitances………………….

75

4.5.2 Self Inductances……………………………….

83

4.6 Analysis of the Calculation and Experimental Results…………………………………………………

84

4.6.1 Verification of the Calculation and Measurement Results…………………………

85

4.6.2 Transient Voltage Distribution in HV Winding Due to Breakdown of the Test Object….……..

87

4.7 Electric Field Analysis of the Test Transformer……

91

4.7.1 Introduction……………………………………..

91

4.7.2 The Mathematical Basis of Electrical Field Problems…………………………………………

92

4.7.3 Electric Field Analysis by Using ANSYS 5.5 ……………………………………..

94

4.7.4 Electric Field Analysis Results………………..

95

5. RESULTS AND DISCUSSIONS 5.1 Lightning Impulse Voltage Distribution in Test Transformer Winding………………….……………

100

5.2 Transient Voltage Distribution, Due to the Breakdown of the Test Object, in Test Transformer winding…….…………………………..

107

5.3 Electric Field Analysis of Test Transformer……….

109

5.4 Reliability Tests of Test Transformer………………

111

6. CONCLUSIONS ………………………………………..

115

REFERENCES……………………………………………………..

118

APPENDICES A. DEFINITIONS IN HIGH VOLTAGE TEST TECHNIQUE …………………………………………..

120

B.1 POLLUTION LEVELS, EQUIVALENT AMBIENT SEVERIETIES AND MINIMUM SPECIFIC CREEPAGE DISTANCE……………………………….

124

B.2 FACTORS INHOMOGINITY …………………………

125

C. TECHNICAL SPECIFICATIONS………………………

127

D. DESIGN DRAWING OF TEST TRANSFORMER …..

131

E. CONTROL AND MEASUREMENT UNIT CIRCUIT DIAGRAMS …………………………………

132

F. CALCULATION RESULTS OF SERIES CAPACITANCE OF THE HV WINDING ……………..

136

G. INSULATING OIL PROPERTIES IN VARIOUS STANDARTS …………………………………………..

139

H. CONSTRUCTION STAGES AND PICTURES………………………………………………..

140

LIST OF TABLES TABLE 3.1 Rated withstand voltages for transformer windings with highest voltage for equipment Um