DYNAMIC OVERVOLTAGES DUE TO LOAD REJECTION IN POWER SYSTEMS by BABU RAM A Thesis submitted to the Indian Institute of Technology, Delhi for the award of the degree of DOCTOR OF PHILOSOPHY CENTRE OF ENERGY STUDIES INDIAN INSTITUTE OF TECHNOLOGY, DELHI MARCH, 1983
DEDICATED TO THE MEMORY OF MY FATHER SHRI BALAK RAM AND MY MOTHER SHRIMAT I RAMAVAT I DEVI
CERTIFICATE Certified that the thesis entitled, "Dynamic Overvoltages due to Load Rejection in Power Systems", which is being submitted by Shri Babu Ram in partial fulfilment for the award of the degree of Doctor of Philosophy in Centre of Energy Studies of the Indian Institute of Technology, Delhi is a record of the student's own work carried out by him under our supervision and guidance. The matter embodied in this thesis has not been submitted for the award of any other Degree or Diploma. Dr. R. Balasubramanian Assistant Professor Centre of Energy Studies Indian Institute of Technology New Delhi 110 016, India,S4L4,2tA6- e. Dr. S.C. Tripathy Professor
iii ACKNOWLEDGEMENTS The author wishes to express his gratitude and deep indebtedness to Professor Sarat C. Tripathy and Dr. R. Balasubramanian for their valuable guidance, right direction and attention at every stage of this work. The author is thankful to Dr. S.C. Kapoor, Deputy General Manager of B.H.E.L. for providing technical literature and data. Thanks are due to laboratory staff, in particular to Messrs Dalel Singh and V.K. Rana for their cooperation. The author is also thankful to many of his teachers and friends for their constant encouragement and help. The author gratefully acknowledges the cooperation and help given by his family members. The author is thankful to Mr. P.M.Padmanabhan Nambiar for typing the manuscript of the thesis.
iv The thesis is based on the following papers published by the author 3 1. Data Acquisition and Monitoring of Micro-Alternator, J.I.E.(India), Vol. 62, pp. 101-107, Dec. 1981. 2. Real-Time Torque Angle Monitoring of Micro-Alternator, J.I.E.(India), Ref: EM-697 (Accepted for Publication, 1982). 3. Microprocessor Based Governor for Steam Turbine, J.I.E.(India), Ref : CP-690 (Accepted for Publication, 1983). 4. Interfacing of Micro-Alternator with Micro-Computer for Real-Time Monitoring, (Communicated to IJEEE (England), 1983). 5. Temporary Overvoltages Due to Load Rejection on a Series Compensated Line., Proc. mr, tio1.130,!jett C, No.1, 'bp. 8-15, Jan. 1983. 6. A Study of Self-excitation on a Series Compensated Line and its Control (Communicated to IEEE Summer Meeting, 1983). 7. Control of Dynamic Overvoltages due to Load RejectiOn by Static Shunt Compensation (Communicated to International Journal of Electrical Power and Energy Systems, England, 1983).
ABSTRACT A method of analysis for computing dynamic overvoltages following load rejection is presented in this thesis. The method incorporates detailed dynamic models of synchronous machine, excitation system and transmission network. The problem is simulated on ICL 2960 computer system. The receiving end voltage envelopes following load rejection have been evaluated for a typical 400 kv system for different models of synchronous machines using the digital computer program. These studies reveal the need for detailed representation of machine and excitation control system as otherwise rate of decay of overvoltage response becomes considerably slower. This kind of study is very useful for choosing the voltage rating of the current limiting gap type surge arresters which have the capability to discharge repetitively a few times and reseal against dynamic overvoltages. Furthermore, the performance of excitation systems, viz. static excitation system and D.C. rotating system in the control of dynamic overvoltages due to load rejection is studied. Next, the effects of various parameters, such as line length, power factor of the load rejected and excitor amplifier gain on dynamic overvoltages are studied. The analysis is further extended to include series compensation on transmission lines. Receiving end voltage envelopes following
xi load rejection on a series compensated network are presented in this thesis for different degrees of series compensation, line loads and excitation systems. These results reveal the occurrence of self excitation in the particular power system considered when the compensation level reaches 70 percent. Hence those studies are useful for choosing safe series compensation level for a particular system. Furthermore, the performance of static compensator as a device for control of dynamic overvoltage is studied in this thesis. The regulator employed in the static compensator control scheme is a proportional plus integral type. The effect of control parameters such as gain and integration delay of P I regulator is investigated for a typical 400 kv system. The effect of shunt reactors on dynamic overvoltages both for series compensated and nonseries compensated networks is analysed in this thesis. The performance of joint series plus static shunt compensation is also studied in this thesis. These studies are useful for selecting kind and quantum of compensation required for a particular power System. The application of this analysis for choosing economic voltage rating of surge arrester which is to be installed in an EHV station is presented. A real time technique for on line monitoring of overvoltages following load rejection is also presented in this thesis. This
xii technique has been implemented on a microcomputer interfaced with a model synchronous generator which delivers power to a load through a model transmission lino. This technique may be used for monitoring overvoltages in a load rejection field test conducted on a real power system.
CONTENTS Page ABSTRACT LIST OF TABLES LIST OF FIGURES LIST OF SYMBOLS xiii xiv xviii CHAPTER I INTRODUCTION 1 1.1 Dynamic Overvoltages due to Load 3 Rejection 1.2 Literature Survey 4 1.3 Outline of the Thesis 5 CHAPTER II MATHEMATICAL MODELLING FOR LOAD REJECTION 8 OVERVOLTAGE STUDIES 2.1 2.2 Introduction, Objectives 8 9 2.3 Mathematical Modelling of the System 9 2.4 Synchronous Machine Modelling 11 2.5 Rotor Electrical Equations 15 2.6 Derivation of a.-p, Equivalent Circuits of Synchronous Machine 16 2.7 Rotor Mechanical Equations 19 2.8 Transmission Network Modelling 20 2.9 Modelling of Excitation System 25 2.9.1 Static Excitation System 26
vi CONTENTS (CONTD.) Page 2.10 Numerical Example 26 2.11 Solution Procedure 28 2.12 Computation of Initial Conditions 28 2.12.1 Rotor Electrical State 29 Variables 2.12.2 Network State Variables 31 2.12.3 Rotor Mechanical Equations 32 2.12.4 Rotating D.C. Excitation System 32 2.13 Discussion and Results 33 2.13.1 Effect of Excitation Controls 36 2.13.2 Effect of Line Length on 36 Dynamic Overvoltages 2.13.3 Effect of Load Power Factor 39 2.13.4 Effect of Amplifier Gain 39 2.14 Conclusion 41 CHAPTER III DYNAMLC UVERVOLTAGES DUE TO LOAD REJECTION ON 43 A SERIES COMPENSATED TRANSMISSION LINE 3.1 Introduction 43 3.2 Location of Series Capacitor in a 45 Transmission Network 3.3 Formulation of the Problem 46 3.4 Excitation System Models 51 3.5 Numerical Example 51 3.6 Method of Solution 52
vii CONTENTS (CONTD.) Page 3.7 Discussion of Results 52 3.7.1 Effect of Quantum of Series 52 Compensation 3.7.2 Ef'ect of Different Excitation 57 Systems 3.7.3 Effect of Change in Line Length 57 3.7.4 Effect of Change in Exciter 57 Amplifier Gain 3.8 Conclusion 60 CHAPTER IV CONTROL OF DYNAMIC OVERVOLTAGES BY STATIC 62 SHUNT COMPENSATION 4.1 Introduction 62 4.2 Control Strategy 63 4.3 Mathematical Modelling of the System 65 4.4 Model of Static Compensator 70 4.4.1 Measuring Devices 70 4.4.2 Regulator 70 4.4.3 Thyristor Firing System 72 4.5 Numerical Example 74 4.5.1 Static Shunt Compensator 74 4.6 Solution Procedure 74 4.7 Discussion and Results 74 4.7.1 Effect of Control Parameters on 75 Dynamic Overvoltages 4.7.2 Dynamic Performance of Static 78 Compensator 4.8 Conclusion 78
viii CONTENTS (CONTD.) Page CHAPTER V EFFECT OF SHUNT REACTORS ON DYNAMIC 81 OVERVOLTAGES 5.1 Introduction 81 5.2 Effect of Shunt Reactors on a Non 82 series Capacitor Compensated Line 5.3 Effect of Reactors on a Series 84 Compensated Line 5.4 Series Compensated Network with Static 93 Compensator 5.5 Formulation of the Problem 93 5.6 Model of Static Compensator 97 5.7 Method of Solution 97 5.8 Discussion and Results 98 5.9 Conclusion 98 CHAPTER VI SELECTION OF SURGE ARRESTER VOLTAGE RATING 102 6.1 Introduction 102 6.2 Choice of Voltage Rating of Surge 103 Arrester for a Sample System 6.3 Choosing Size of Shunt Reactor 105 6.4 Conclusion 105 CHAPTER VII REAL TIME MONITORING OF OVERVOLTAGES FOLLOWING 107 LOAD REJECTION 7.1 Introduction 107 7.2 Experimental Set Up 108 7.3 Data Acquisition System 110
ix CONTENTS (CONTD.) Page 7.4 Microcomputer System Organization 110 7.4.1 Central Processing Unit 112 7.4.2 Memory 112 7.4.3 Input/Output Devices 112 7.5 Real Time Programming Technique 112 7.5.1 Intermixing of PL/S with 112 Assembly Language 7.5.2 Data Acquisition Subroutine 114 7.5.3 Subroutine for Packing Real Time 121 Data 746 Test Results 123 7.7 Conclusion 123 CHAPTER VIII CONCLUSIONS AND SUMMARY 126 8.1 Conclusions 126 8.2 Summary 130 APPENDIX A ELEMENTS OF SYSTEM MATRICES 133 APPENDIX B ELEMENTS OF SYSTEM MATRICES FOR STUDIES ON 135 SERIES COMPENSATED LINES APPENDIX C ELEMENTS OF SYSTEM MATRICES FOR STUDIES ON 137 STATIC SHUNT COMPENSATED LINES APPENDIX 0 ELEMENTS OF SYSTEM MATRICES FOR STUDIES ON 139 LINES WITH COMBINED SERIES AND STATIC SHUNT COMPENSATION APPENDIX E 141 REFERENCES 142