HARDWARE IMPLEMENTATION OF A 
POWER SYSTEM STABILIZER 
H.C.S. HETTIGODA 
A dissertation submitted in the partial fulfillment of the requirements for the degree of 
Master of Engineering 
NOVEMBER 2002 
DEPARTMENT OF ELECTRICAL ENGINEERING 
UNIVERSITY OF MORATUWA 
SRI LANKA 
754S1 
University of Vterattiwa 
78481 
Abstract 
The aim of this project is to implement a Power System Stabilizer (PSS) and incorporate 
it to a Generator to enhance its dynamic performance. When a generator is connected to a 
power system the rotor angle oscillations due to small load changes can be observed This 
is due to fact that once a generator is constructed the damping effect of it is fixed and it is 
unable to offer additional damping to damp down these rotor angle oscillations. The 
power system stabilizer (PSS) provides supplementary damping for low frequency rotor 
angle oscillations ranging from 0.1 Hz to 3 Hz which covers local mode, Inter area mode 
and inter unit mode power system oscillations. Here the PSS output is fed back to the 
excitation system summing point to damp down rotor angle oscillations. 
In general, small load changes in a power system cause rotor angle oscillations. In order 
to damp down these oscillations an electrical torque should be produced on the rotor 
shaft, in phase with the speed deviation. Power system stabilizer adds a closed loop 
auxiliary feedback signal to the reference voltage of Automatic voltage regulator (AVR) 
in proportion to either speed deviation, frequency deviation or power deviation. 
Normally the terminal voltage of a generator is adjusted by the AVR after computing the 
difference between reference voltage and feedback terminal voltage. In my project the 
PSS is designed to operate as follows. A voltage signal in proportion to the frequency 
deviation caused by small load changes is first produced. This voltage signal which 
represents the frequency deviation is then added to the reference voltage of AVR. This in 
turn causes an electrical torque to be added on the rotor shaft to enhance the small signal 
stability performance. 
The developed PSS consists of a High pass filter (HPF), a Zero crossing detector (ZCD), 
a Frequency to voltage conveitor (F/V), a Summer I and a Summer 2. The frequency 
deviation is filtered by using the HPF in order to block steady changes in frequency. The 
filtered signal is sent through the ZCD to maintain a constant amplitude signal to the F/V 
converter because it responds to amplitude of input signal. The output of F/V converter 
and the voltage corresponding to base load frequency which is injected separately using a 
Calibration instrument, are summed in summer 1 to obtain a voltage deviation 
corresponding to frequency deviation. The output of summer I and reference voltage of 
AVR are summed in summer^ and fed back to LM723, which is the main control chip of 
the AVR. 
In order to analyze the effect of PSS, following parameters were observed. 
1 Output of frequency to voltage convertor 
2 Generator Current 
3 Induction Motor Current 
Finally two MATLAB routines are used to extract actual low frequency oscillations from 
observed signals. 
1 A low pass filter is designed using ellip() and filter() functions and observed 
signals are filtered with a cutoff frequency of 3 Hz. The outputs of the filter 
clearly show the effect of PSS in enhancing the dynamic performance of the 
generator. 
2. Observed signals are analyzed using Fast Fourier transform technique in 
MATLAB in order to observe the low frequency components. 
Preface 
111 
Design of a power system stabilizer came into my mind after completion of power system 
stability module of MEng I PG Diploma course.At that time I had only a conceptual idea 
of power system stabilizer. I gathered more knowledge on Power System Stabilizers by 
searching the Internet. 
I am indebted to Prof. Rohan Lucas, Dr. Jahan Peiris and Dr. Nalin Wickramarachchi for 
their valuable encouragement and direction to implement this project. 
/ 
I sincerely thank Mr. A.K. Samarasinghe, Chief Engineer (Electronics & Instrumentation) 
and Mr. D.N. Navaratne, Electrical Engineer (Controls & Instrumentation) at Mahawelli 
Complex for providing me with the generator and Automatic Voltage Regulator (A VR) 
which were used during the project and for their comments on practical circuits, concepts, 
etc. 
And I wish to thank Mr. A.R.Nawamani, Electrical Engineer (Instrumentation & 
Development) at Victoria Power Station for his support given during the project. 
.,. 
Finally I thank my wife Nayana for her encouragement provided during the project. 
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List of Figures 
Figure 3.1 Generator and infinite bus system ................................................. 15 
Figure 4.1 Complete block diagram of the system ............................................ 19 
Figure 4.2 Complete circuit of the system ..................................................... 20 
Figure 4.3 Circuit diagram of high pass filter ................................................ 21 
Figure 4.4 Circuit diagram of zero crossing detector ....................................... 22 
Figure 4.5 Circuit diagram ofFN convertor .................................................. 22 
Figure 4.6 Circuit diagram of Summer 1 ....................................................... 23 
Figure 4.7 Circuit diagram of Summer 2 ............................... ~ ..................... 24 
Figure 5.1 Block diagram of apparatus of the system ....................................... 27 
Figure 6.1 FN convertor output (ac coupled) without PSS ................................ 29 
Figure 6.2 FN convertor output (ac coupled) with PSS .................................... 29 
Figure 6.3 Generator current without PSS .................................................... 30 
Figure 6.4 Generator current with PSS ........................................................ 30 
Figure 6.5 Induction motor current without PSS ............................................. 31 
Figure 6.6 Induction motor current with PSS ................................................... 31 
Figure 7.1 Percentage of oscillations in observed signals .................................. 32 
... 
Figure 7.21 LPF output of generator current without PSS ................................. 33 
Figure 7.22 LPF output of generator current with PSS ..................................... 33 
Figure 7.23 LPF output of induction motor current without PSS .......................... 34 
Figure 7.24 LPF output of induction motor current with PSS .............................. 34 
Figure 7.31 Fast fourier transform of generator current without PSS ........................... 35 
Figure 7.32 Fast fourier transform of generator current with PSS ................................ 35 
Figure 7.33 Fast fourier transform of induction motor current without PSS ................ 36 
Figure 7.34 Fast fourier transform of induction motor current with PSS ..................... 36 
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Figure 11.1 Output of zero crossing detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 
Figure 11.2 Output ofFN convertor .......................................................... 43 
List of Tables 
Table 7.1 Voltage deviation and percentage of oscillations 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0000 32 
Table 11.3 Characteristics ofFN convertor 0 0 00 0 0 0 0 0 0 0 00 0 00 0 00 0 00 0 0 0 0 0 0 0 0 0 0 00 0 00 0 00 0 00 0 0 0 0 0 0 0.44 
Table 11.4 Testing ofPSS 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.45 
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Abstract 
Preface 
List of figures 
List of tables 
Contents 
Chapter 1 Intr oduction 
Contents 
I I Background ... .. ...... .... ..................................................................... .. ... ... .... ........... . 
iii 
iv 
v 
vi 
1 
1.2 Thesis objective ....... ........... .............................. ............................. ··l· .. . . .. .. .. .. . . .... 2 
1.3 Thesis outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . .. .. .. .. . . . .. .. .. .. .. .. .. .. . .. . . . . . . . .. .. .. . . . . .. . . . . . .. . . . .. 2 
Chapter 2 Present and historical background of power system stability 5 
2 I Utility practice . . .. . . .. . . .. .. .... . . .. . . .. .. .... .. . . .. .. ... . .. . . .. .... .. .. .. . . .. . . .. . . .. .... .. .. . . .. . ... ...... .. .... . 5 
2 2 Experience with power system osc illat ions and summary of probable causes . . .. 9 
2 3 Methods of mitigating of power system osc illations . . ................. .. ... ... .. .......... .. ... 12 
Chapter 3 Th eor y 15 
Chapter 4 Design and Implem entation 19 
4 I Complete block diagram of the system. .................... > ............................. .... .... : ... 19 
4 2 Complete circuit of the system. ........... :~.' ........................................ ...... ...... ........... 20 
4 3 High pass filter .............................................. , ........................................... .... ... ..... 21 
4 4 Zero crossing detector ............................................................................ .. ... ..... .... 2 1 
4 5 Frequency to Voltage convertor (F/V) .. ...... .......................................................... 22 
4.6 Summer I . . . .. .. .. .. . . .. . . . . . . . . . . .. .. .. .. . . .. .. . . .. .. . . .. .. . . . .. . . . . .. . . .. . . ...... . . .. . ... . . .. . . .. . . .. ...... .. .. .... . 23 
4 7 Summer 2 .... .. .... .... .... .......... .. .... .. ........ ... ...... .. ........ .. .. .. .. .... .. .. .. .. .. ..... ... .. .. .. .. .... ... ... 24 
('hapter 5 Instrumentation and Testing procedure 26 
5.1 Instrumentation ... .. .. ...... ..... ...... ...... .. .... ...... ... ........ .......... ...... .. ...... .. .... .. .. .... .. .. .. .... 26 
52 Testing procedure .... .... ....... ................. .. ...... ......................... .. .. .. .... ... ..... ... :~ ...... 27 
Chapter 6 Obser vations 29 
6 I FN convettor output without PSS ... ... ................. ... .... .... .... . .. ... . .. ...... .............. ... 29 
6 2 FN convertor output with PSS .. .. .. .. . . .. .. .. .. .. .. . .. .. .. .. .. .. .... .... ... . .. .. ... . .... ... . .. .. .. .... .. . 29 
o 3 Generator current without PSS .............................................. ........................... 30 
Contents VII 
6.4 Generator current with PSS .... . . . . . . . . . .. ........ ...... .... ........ .......................... .... ..... .. 30 
6.5 Induction motor current without PSS...... .. . . . .... .......................... .. .. .. ... ... .... ........ 31 
6.6 Induction motor current with PSS ...................................................................... 31 
Chapter 7 Results 32 
7.1 Voltage deviation and percentage of oscillations ................................................. 32 
72 LPF MATLAB simulations with observations ..................................................... 33 
7.21 LPF output of generator current without PSS ............................................ 33 
7.22 LPF output of generator current with PSS ................................................. 33 
7.23 LPF output of induction motor current without PSS .. ... ... /. ....................... 34 
7.24 LPF output of induction motor current with PSS ........................ .. ............. 34 
7.3 Fourier analysis using fast fourier transform in MATLAB .. .. ......... .. ... ... ........... 35 
7.31 Fast fourier transform of generator current without PSS ............................ 35 
7.32 Fast fourier transform of generator current with PSS .................................. 35 
7.33 Fast fourier transform of induction motor current without PSS .................. 36 
7.34 Fast fourier transform of induction motor current with PSS ....................... 36 
Chapter 8 Discussion 37 
... 
Chapter 9 Conclusion 39 
Chapter 1 0 References 
~ 
41 
Chapter 11 Appendices 42 
11.1 Appendix 1 - Output of zero crossing detector . . . . . . . . . . . . . . . .. . . .. . . . .. .. . . . . .. . ... . . .. . . . . . . 42 
11.2 Appendix2- Output ofFN convertor ............................. .. ... ... .. ... ..................... 43 
11.3 Appendix3 - Characteristics ofFN convertor ....................... .... .. .. ... .. ... .. .. .. .. .. 44 
11.4 Appendix4 - Testing ofPSS .............................................................................. 45 
11.5 Appendix5- Matlab routine to simulate the low pass filter (LPF) ................... 46 
11.6 Appendix6- Matlab routine to simulate the fast fourier transform ................. 47 
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