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.
- e.
,S4L4,2tA6
Dr. S.C. Tripathy
Dr. R. Balasubramanian
Professor
Assistant Professor
Centre of Energy Studies
Indian Institute of Technology
New Delhi 110 016, India
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 par-
ticular 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
xiii
LIST OF TABLES
LIST OF FIGURES
xiv
LIST OF SYMBOLS
xviii
CHAPTER I
CHAPTER II
INTRODUCTION
1
1.1
Dynamic Overvoltages due to Load
Rejection
3
1.2
Literature Survey
4
1.3
Outline of the Thesis
5
MATHEMATICAL MODELLING FOR LOAD REJECTION
OVERVOLTAGE STUDIES
8
2.1
Introduction ,
8
2.2
Objectives
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
Variables
29
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
CHAPTER III
33
2.13.1 Effect of Excitation Controls
36
2.13.2 Effect of Line Length on
Dynamic Overvoltages
36
2.13.3 Effect of Load Power Factor
39
2.13.4 Effect of Amplifier Gain
39
2.14 Conclusion
41
DYNAMLC UVERVOLTAGES DUE TO LOAD REJECTION ON
A SERIES COMPENSATED TRANSMISSION LINE
43
3.1 Introduction
43
3.2
Location of Series Capacitor in a
Transmission Network
45
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 Series52
Compensation
3.7.2 Ef'ect of Different Excitation57
Systems
3.7.3 Effect of Change in Line Length57
3.7.4 Effect of Change in Exciter57
Amplifier Gain
3.8 Conclusion
60
CHAPTER IVCONTROL OF DYNAMIC OVERVOLTAGES BY STATIC62
SHUNT COMPENSATION
4.1 Introduction
62
4.2 Control Strategy
63
4.3 Mathematical Modelling of the System65
4.4 Model of Static Compensator70
4.4.1 Measuring Devices70
4.4.2 Regulator
70
4.4.3 Thyristor Firing System72
4.5 Numerical Example
74
4.5.1 Static Shunt Compensator74
4.6 Solution Procedure
74
4.7 Discussion and Results 74
4.7.1 Effect of Control Parameters on75
Dynamic Overvoltages
4.7.2 Dynamic Performance of Static78
Compensator
4.8 Conclusion
78
viii
CONTENTS (CONTD.)
Page
CHAPTER VEFFECT OF SHUNT REACTORS ON DYNAMIC
81
OVERVOLTAGES
5.1 Introduction
81
5.2 Effect of Shunt Reactors on a Non—
series Capacitor Compensated Line
82
5.3 Effect of Reactors on a Series
Compensated Line
84
5.4 Series Compensated Network with Static
Compensator
93
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 VISELECTION OF SURGE ARRESTER VOLTAGE RATING
CHAPTER VII
102
6.1 Introduction
102
6.2 Choice of Voltage Rating of Surge
Arrester for a Sample System
103
6.3 Choosing Size of Shunt Reactor
105
6.4 Conclusion
105
REAL—TIME MONITORING OF OVERVOLTAGES FOLLOWING
LOAD REJECTION
107
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
Assembly Language
112
7.5.2 Data Acquisition Subroutine
114
7.5.3 Subroutine for Packing Real—Time
Data
121
746 Test Results
123
7.7 Conclusion
123
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
SERIES COMPENSATED LINES
135
APPENDIX C
ELEMENTS OF SYSTEM MATRICES FOR STUDIES ON
STATIC SHUNT COMPENSATED LINES
137
APPENDIX 0
ELEMENTS OF SYSTEM MATRICES FOR STUDIES ON
LINES WITH COMBINED SERIES AND STATIC SHUNT
COMPENSATION
139
CHAPTER VIII
APPENDIX E
141
REFERENCES
142