CONDITION MONITORING OF POWER TRANSFORMERS USING FREQUENCY RESPONSE ANALYSIS R. P. TILAKERATNE This thesis was submitted to the Department of Electrical Engineering of the University of Moratuwa in partial fulfillment of the requirements for the Degree of Master of Engineering Supervised By: Prof. J. R. Lucas and Mr. K. B.M. 1. Perera Department of Electrical Engineering University of Moratuwa Sri Lanka 2004 82146 Abstract Power transformers are the most expensive and important equipment of a high voltage power system. Therefore it is essential to have a suitable and effective condition monitoring system to assess the conditions of power transformers well in advance to maintain the reliability of the power system by averting unexpected expensive failures. Conventional condition .monitoring techniques do not conclusively indicate mechanical conditions of the transformer such as winding movement, loss ·of clamping pressure, disc movements etc. which can take place during handling and transport, short circuit forces, faults in the power system network near the transformer or a high voltage stress which affects inductance or capacitance. This project thesis deals with Frequency Response' Analysis technique for condition monitoring of power transformers, which is suitable for detecting the deformation of transformer windings. A mathematical model for obtaining frequency response characteristics has been developed for 'a typical transformer, and the effect of variation of parameters on frequency response has been examined. Case studies based on frequency response tests on typical 31.5 MVA, 132/33 kV grid transformers have been presented. Case studies reveal that any mechanical deformation in the transformer winding is clearly reflected in the frequency response characteristics. The frequency response characteristics obtained in the field for different aged power transformers are found to be somewhat similar in nature to those obtained analytically using ladder network equivalent circuit. DECLARATIONS I certify that this thesis has not been previously presented in whole or part to any University or Institution for a higher degree. ~ R. P. Tilakeratne August 2004 ~~ Prof. J. R. Lucas Supervisor :/&}~ Mr. K. B. M. I. Perera Supervisor I ~ II ACKNOWLEDGEMENT I wish to express my appreciation and sincere thanks to the University of Moratuwa for providing me with the opportunity of following the Master's Degree Programme in Electrical Engineering and Professor J. R. Lucas of Department of Electrical Engineering and Mr. K. B. M. I. Perera, Managing Director of M/s. Lanka Power Promoters (Pvt) Limited, who guided and assisted me as Project Supervisors in selecting the topic and preparing the thesis report despite their load of work. Their advices and insight were immeasurable. I would extend my sincere gratitude to the Ceylon Electricit/Board, Mr. C. M. B. Ekanayake and Mr. Nilanga Abewickrama of University of Peradeniya and fellow engineers who helped me in taking relevant measurements on selected transformers in the Ceylon Electricity board. Whilst I regret my inability to specifically mention individuals, I am grateful to all the staff of the University of Moratuwa and my colleagues who were helpful me in numerous ways to make my endeavor a success. _,. Last, but not least, I thank my beloved wife Sunethra and children Lakrnal, Nissansala and Anuradhitha for their affection, appreciation, support and understanding towards me in achieving the aspiration. -- iv LIST OF FIGURES Fig. 6.1- Simplified ladder network of HV winding of the transformer Fig. 6.2 -Frequency response of HV winding at medium frequency Fig. 6.3 -Frequency response of HV winding at high frequency Fig. 6.4 - Frequency response of HV winding at specified frequency Fig. 6.5 - Effect of increase in inductance on frequency response of HV winding at high frequency Fig. 6.6- Effect of increase in inductance on frequency response of HV PAGE NO. 26 27 28 29 32 winding at specified frequency 33 Fig. 6.7- Effect of increase in series capacitance on frequency r~ponse of HV winding at high frequency 34 Fig. 6.8- Effect of increase in series capacitance on frequency response of HV winding at specified frequency 35 Fig. 6.9- Effect of increase in shunt capacitance on frequency response of HV winding at high frequency 36 Fig. 6.10- Effect of increase in shunt capacitance on frequency response of HV winding at specified frequency 37 Fig. 7. 1- Open and short circuit impedance of a distribution transformer 41 .. Fig. 7.2 - Phase comparison of 300 MY A auto transformer Fig. 7.3- Open circuit impedance with simulated faults Fig. 7.4- Comparison of low frequency response of failed converter 43 44 transformer with healthy unit of same design (LV delta winding) 46 Fig. 7.5 - Comparison of low frequency response of failed converter transformer with healthy unit of same design (HV winding of delta limb) Fig. 8.1 - MS 4630 A Network Analyzer ~ Fig. 9.1 -Frequency response of HV winding of Phase A of Pannipitiya and FRA of transformer model Fig. 9.2- Frequency response of HV winding of Phase A of Pannipitiya and FRA of transformer model at specified frequency v 47 49 56 57 Fig. A.l-The effect of breakdown voltage (BDV) of transformer oil on Loss Tangent (tan o) 66 Fig. B.l-Frequency response of open circuit impedance of HV winding of Phase A of Pannipitiya and Kiribathkumbura transformers for complete frequency range- Amplitude verses Frequency 71 Fig. B.2-Frequency response of open circuit impedance of HV winding of Phase A of Pannipitiya and Kiribathkumbura transformers for complete frequency range- Phase Angle verses Frequency Fig. B.3-Frequency response of open circuit impedance of HV winding of Phase A of Pannipitiya and Kiribathkumbura transf9rrners for low ;I frequency range- Amplitude verses Frequency Fig. B.4-Frequency response of open circuit impedance of HV winding of Phase A of Pannipitiya and Kiribathkumbura transformers for low 72 73 frequency range- Phase Angle verses Frequency 74 Fig. B.S-Frequency response of open circuit impedance of HV winding of Phase A of Pannipitiya and Kiribathkumbura transformers for medium frequency range- Amplitude verses Frequency 75 Fig. B.6-Frequency response of open circuit impedance of HV winding of Phase A of Pannipitiya and Kiribathkumbura transformers for medium ,. frequency range- Phase Angle verses Frequency 76 Fig. B.7-Frequency response of open circuit impedance of HV winding of Phase A of Pannipitiya and Kiribiithkumbura transformers for high frequency range- Amplitude verses Frequency 77 Fig. B.8-Frequency response of open circuit impedance of HV winding of Phase A of Pannipitiya and Kiribathkumbura transformers for high frequency range- Phase Angle verses Frequency 78 Fig. C.l-Frequency response of HV winding of open circuit impedance of --Phase A of Sapugaskanda transformer No.1 and Kiribathkumbura transformer for complete frequency range- Amplitude verses Frequency vi Fig. C.2- Frequency response of HV winding of open circuit impedance of Phase A of Sapugaskanda transformer No.1 and Kiribathkumbura transformer for complete frequency range- Phase Angle verses Frequency 82 Fig. D.l-Frequency response of HV winding of open circuit impedance of Phase A of Sapugaskanda transformer No.2 and Kiribathkumbura transformer for complete frequency range- Amplitude verses Frequency 85 Fig. D.2-Frequency response of HV winding of open circuit impedance of Phase A of Sapugaskanda transformer No.2 and Kjribathkumbura transformer for complete frequency range- Phase Angle verses Frequency 86 Fig. E.l- Frequency response of open circuit impedance of HV windings of Phases A, Band C of defective transformer at Sapugaskanda Grid Sub Station for complete frequency range- Amplitude verses Frequency 88 Fig. E.2-Frequency response of open circuit impedance of HV windings of Phases A, Band C of defective transformer at Sapugaskanda Grid Sub Station for medium-frequency range- Amplitude verses Frequency 89 Fig. E.3- Frequency response of open circuit impedance of LV winding terminals of RB, RY and YB of defective transformer at Sapugaskanda Grid Sub station for complete frequency range- Amplitude verses Frequency 90 Fig. E.4- Frequency response of open circuit impedance of LV winding terminals of RB, RY and YB of defective transformer at Sapugaskanda Grid Sub station for medium frequency ranse- Frequency 91 VII Fig. E.5- Frequency response of open circujt impedance of HV windings of Phases A, B and C of defective transformer at Sapugaskanda Grid Sub Station for complete frequency range after removing noises (Smoothed)- Amplitude verses Frequency Fig. F.l- Frequency response of open circuit impedance of HV windings of All three Phases of defective transformer at WimaJasurendra Power Station (WPS) and Phase A of Kiribathkumbura transformer for Complete frequency range- Amplitude verses Frequency Fig. F.2- Frequency response of open circuit impedance of HV windings of All three Phases of defectjve transformer at Wi~lasurendra Power Station (WPS) and Phase A of Kiribathkumbura transformer for 92 94 Complete frequency range - Phase Angle verses Frequency 95 .. ·~ viii LIST OF TABLES PAGE NO. Table 2.1- Generator and Transmission transformers in the CEB network in Sri Lanka 6 I .. ·~ ix CONTENTS FRONT PAGE DECLARATION ABSTRACT ACKNOWLEDGEMENT UST OF FIGURES UST OFT ABLES 1. INTRODUCTION 1.1 General 1.2 Objective 1.2.1 Methodology used to accomplish objectives 2. THE OPERATIONS AND MAINTENANCE OF POWER TRANSFORMERS 2.1 Power Transformers 2.2 Distribution of Power Transformers in the CEB 2.2.1 Loading Pattern 2.2.2 Condition Monitoring 2.3 Life Time 2.3.1 The Parameters Contribute to the Normal Operation 2.3.2 Deterioration of dielectric Strength of Oil 2.3.3 Ageing 2.3.4 Stresses 2.4 Operation and Maintenance 2.5 Criterion for Overhauling "' .. / 3. METHODS USED TO EVALUATE CONDITIONS OF POWER TRANSFORMERS 3.1 Continues and Automatic Monitoring Systems 3.2 Indirect Methods 3.2.1 Gas Chromatography 3.2.2 Sensing of Hydrogen 3.2.3 Condenser Bushing 3.2.4 Insulation Resistance 3.3 Loss Tangent and Capacitance 3.3.1 Single Frequency Method 3.3.2 Two - Frequency Method 3.3.3 Two -Temperature Method 3.3.4 Frequency Domain Method (over a wide frequency domain) X ii iii IV v IX 1 1 3 3 5 5 5 7 7 8 8 8 9 9 10 10 11 11 11 12 12 12 13 ·~ 13 14 14 14 15 4. BEHAVIOUR OF LOSS TANGENT (TAN b) IN FREQUENCY DOMAIN 4.1 Dielectric Response Measurements 4.1.1 Loss Tangent (Tan o) Measurements in Frequency Domain 4.2 The Response of Tan b with Breakdown Voltage of Transformer Oil 4.2. 1 Frequency Response Analysis (FRA) 5. THEORY OF CONDITION MONITORING: FREQUENCY RESPONSE ANALYSIS APPROACH 5.1 Introduction 5.2 Frequency Response Analysis 5.3 Frequency Response of Transformer Windings 5.3.1 Characteristics of Frequency Response 5.3.2 Low Frequency Response 5.3.3 Medium Frequency Response 5.3.4 High Frequency Response I 5.3.5 Effect of Variation of Parameters on the Frequency Response 5.4 Merits of FRA 5.5 Demerits of FRA 6. TRANSFORMER MODEL FOR FREQUENCY RESPONSE ANALYSIS (FRA) 6.1 Frequency Response of Transformers 6.2 Frequency Response of HV Winding of Transformer (Base Case) 6.2.1 High Frequency Response 6.2.2 Specified Frequency Respotfse 6.2.3 Medjum Frequency Response 6.3 Effect of Variation of Parameters on the Frequency Response 6.3.1 Effect of Increase in Inductances 6.3.2 Effect of Increase in Series Capacitances 6.3.3 Effect if Increase in Shunt Capacitances 7. RESEACH CARRIED OUT ON FRA IN DETECTING FAULTS ON POWER TRANSFORMERS 7.1 FRA as a Diagnostic Tool 7.2 Frequency Response Characteristics 7 .2.1 Short Circuit Impedance 7.2.2 Voltage Transfer Ratio 7.2.3 Ageing 7.3 Diagnosing Mechanical Faults 7.3. 1 Short Circuit Turns xi -- 16 16 16 16 17 18 18 19 20 21 21 22 22 22 23 24 25 25 27 30 30 31 31 32 34 36 38 38 38 39 39 39 40 42 7.3.2 Major Winding Movement 8. CASE STUDIES 8.1 Behavior of Frequency Response 8.2 Test Equipment 8.2.1 Test Method 8.2.2 Interpretation of Measured Frequency Response 8.3 Case Studies 8.3.1 Case I 8.3.2 Case IT 8.3.3 Case ill 8.4 Frequency Response of Defective Transformers 8.4.1 Case IV 8.4.2 Case V 9. CORRELATION OF FREQUENCY RESPONSE CHARACTERISTICS WITH TRANSFORMER MODEL 9.1 Comparison of Frequency Responses 9.2 Examination of Frequency Responses 9.3 Reasons for Deviations of Frequency Responses 9.4 Complete Transformer Model 9.4.1 Francisco de Lean and Adam Semlyen Model 9.4.2 Syed Mofizul Islam Model I 10. FINDINGS, IMPLEMENTATION, RECOMMENDATION AND CONCLUSION .. 10.1 Findings 10.1.1 Detecting Mechanical Faults .. 10.1.2 Development of Sensitive Transformer Model for FRA 10.2 Recommendations 10.3 Conclusion REFERENCES APPENDICES APPENDIX A- Breakdown Voltage of Transformer oi l on tan o APPENDIX B - Case Study I APPENDIX C - Case Study IT APPENDIX D - Case Study ill APPENDIX E - Case Study IV APPENDIX F - Case Study V xii 45 48 48 48 50 50 51 51 52 52 53 53 54 55 55 55 58 58 58 59 60 60 60 60 61 61 63 66 66 68 79 ~ 83 87 93