DEVELOPMENT OF AN UNDER FREQUENCY 
LOAD SHEDDING ALGORITHM 
Nelum Shanthi Kalansur iya 
This thesis was submitted in partial fulfillment of the requirement for the 
degree of Master of Engineering 
Project Supervisors: Prof. J R. Lucas, Dr. H.J.C. Peiris 
University of Moratuwa 
78976 
Department of Electrical Engineering 
University of Moratuwa 
Sri Lanka. 
July 2003 
78976 
ABSTRACT 
In overload conditions caused by sudden outages in a Generator or a 
Transmission line in a Power System, loads have to be shed at selected 
feeders in the distribution system, to maintain system stability, namely 
frequency. 
Different load shedding schemes can result in quite different performance, in 
recovery of system frequency. 
In this research project, initially the existing load shedding scheme employed 
in the Sri Lankan Power System was studied. Improvements to the existing 
scheme using the rate of change of frequency (df/dt),is proposed. 
A typical network was modelled using MATLAB/Simulink software package 
and a Load Shedding Scheme was simulated with this model. Improved 
performance was observed when the combination of Frequency (f) and Rate 
of change of Frequency (df/dt)were employed in the Load Shedding Scheme. 
ACKNOWLEDGEMENT 
I wish to express my deepest gratitude to the Prof. J.R. Lucas and Dr. Jahan 
Peiris of Dept of Electrical Engineering, for the guidance and enthusiasm they 
have shown and the valuable advices offered as my Project Supervisors. 
My special appreciation goes to the course coordinators of , the Master of 
Engineering course, and other staff members of the depa_.9ment of Electrical 
Engineering, University of Moratuwa for the support given in making my 
project a success. 
I would like to pay my sincere thanks to Ceylon Electricity Board and all 
friends and colleagues who helped me in many ways and sharing their views 
with me during this approach. 
Finally, my special thanks goes to my husband for his inspiring support given 
·-
to me in following this Master of Engineering course. 
CONTENTS 
Page no. 
1. INTRODUCTION ............................................................................... 1 
1.1. GENERAL .................................................................. ........................ 1 
1.2. OBJECTIVE ....................................................................................... 2 
2. THEORY BEHIND SYSTEM OVERLOAD ....................................... .4 
2.1. INTRODUCTION ....................................................... ........................ 4 
2.2. SYSTEM FREQUENCY BEHAVIOUR. .............................................. 5 
2.2.1. Influence of load ...................................................................... 5 
2.2.2. Influence of Governors ............................ ...... : ......................... 6 
2.2.3. System Inertia ............................... ......... ;1: ............................. 6 
2.2.4. Load Reduction Factor ............................................................ 7 
2.3. RATE OF FREQUENCY DECLINE ................................ ... ................ 8 
3. BASIS OF LOAD SHEDDING ......................................................... 12 
3.1. REQUIREMENT .................................... ............. ............................. 12 
3.2. FORMULATING A LOAD SHEDDING SCHEME ........ .................... 14 
3.2.1. Maximum Anticipated Overload ............................................ 14 
3. 2. 2. Number of Load shedding Stages ......................................... 16 
3.2.3. Size of the Load Shed at each Stage .................................... 16 
3.2.4. Frequency Settings ........................ .... ................................... 17 
3.2.5. Time Delay ............................................................................ 18 
3.2.6. Location of Frequency Relays ............................................... 19 
4. CONCEPTS FOR DEVELOPMENT ................................................ 20 
4.1. EXISTING UNDER FREQUENCY LOAD SHEDDING SCHEME IN 
SRI LANKAN NETWORK ............. .. ................. .. .............................. 20 
4.1.1. Present Load Shedding Criteria ............................................ 20 
4.1.2. Deficiencies of the present Load Shedding Scheme ............. 23 
4.2. CONCEPTS FOR DEVELOPMENT ................ ... ........... .......... ........ 23 
4.2.1. Weaknesses in Static Load Shedding scheme ..................... 26 
4.3. DYNAMIC LOAD SHEDDING ........ ........ ........ ..................... .. ........... 27 
5. MODELLING APPROACH .............................................................. 30 
5.1. A TYPICAL NETWORK MODEL ...................... .... ........................... 30 
5.2. METHODOLOGY ............. ...................... ......................................... 34 
5.3. RESULTS AND ANALYSIS ............................................... .............. 37 
5. 3. 1. Results for a Generator Tripping from the Network ............... 37 
5.3.2. Network with Under Frequency Load Shedding Scheme ...... 38 
5. 3. 3. Network with f supervised dfldt Load Shedding Scheme . .... 39 
6. APPLICATION ON SRI LANKAN NETWORK ........................ 42 
6.1. MODELLING THE SRI LANKAN POWER NETWORK. .................. .42 
6.2. RESULTS OBTAINED W ITH SIMPLIFIED NETWORK. .................. 43 
6.2.1. Result 1 ................................................................................. 44 
6.2.2. Result 2 ............................................................... /:, .............. 44 
7. IMPLEMENTATION, DISCUSSION AND CONCLUSION ............... 46 
7.1. PRACTICAL IMPLEMENTATION .................................................... 46 
7.2. DISCUSSION .................................................................................. 47 
7.3. CONCLUSION ................................................................................. 48 
8. REFERENCES ................................................................................ 50 
9. APPENDICES ............................................................... , ................. 52 
9.1. APPENDIX 1; TABLE 4.1 .................................................................. 52 
9.2. APPENDIX 2; MATLAB/SIMULINK BLOCKS ........................................ 53 
9.3. APPENDIX 3; ACTUAL FREQUENCY VARIATIONS ................................ 64 
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