ANALYSING OF VOLTAGE SURGES IN GRID SUBSTATION AUXILIARY SUPPLY By J D K G Hemaratne 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 in Electrical Engineering Department of Electrical Engineering Faculty of Engineering University of Moratuwa, Sri Larka January 2004 7 3 5 9 ; Abstract In Sri Lankan National Electricity Grid, severe voltage fluctuations have been noticed in the Grid Substation auxiliary supply. The auxiliary supply is quantitatively 20CkVA, but very important to keep in good health because it provides power to all substation equipment installed in GSSs, such as battery charger units and power supply modules for various types of protective monitoring and measuring instruments. Due to these fluctuations, not only the protective equipment, but also the small power equipment such as computers, Air conditioners and substation lights are also had frequent failures. It is observed all these failures to the equipment and substation lights have been experiencedjust after the tr pping of feeders due to an earth fault. The objective of the research is to analyze the problem of voltage fluctuations in the auxiliary supply in Grid Substations and find the possible remedies. The studies presented in this report are mainly focused on the Earthing Transformer, which provides the power to the Auxiliary equipments. The earthing transformers in bulk of the GSSs are a Zig-Zag wound transformer with an low voltage auxiliary winding. Unfortunately, there are no models for Zig-Zag transformer readily available for simulation of the problem using computer software. Therefore the purpose of simulation of the problem in this report is mainly based on a Prototype transformer, which was wound specifically for the purpose. The possible cause to the voltage surges experienced in the auxiliary supply at an earth fault is due to the extra flux in the grounding transformer limbs and the leakage flux of primary windings directly influencing the auxiliary winding, resulting an extra voltage induced in the auxiliary winding. The arrangement of the winding in each limb has made a huge impact on the problem. The integration of two functions has created a negative impact on the performance of the transformer. As a solution, firstly, this report suggest to split the two functions integrated in the grounding transformer i e use separate transformer for grounding the 33kV side of the GSS and an other separate transformer for auxiliary and control supply for the GSS. Secondly, re-arrange the windings in such a way that the no resultant flux induces due to the fault current injected into the neutral of the grounding transformer DECLARATION I hereby declare that this submission is my own work and that, to the best of my knowledge and behalf, it contains no material previously published or written by another person nor material, which to substantial extent, has been accepted for the award of any other academic qualification of an university or institute of higher learning except wKere acknowledgment is made in the text. t\t:J .,.. - ~y 4.,..-..._ J D K G Hemaratne PGIEF115/99 January 2004 -~~ Dr. J.P. Karunadasa Project Supervisor January 2004 .... .... Acknowledgments I am most grateful and privileged to have Dr. J P Karunadasa, Senior Lecturer, University of Moratuwa and Eng. Y R L Jayawikrama, Electrical Engineer, Ceylon Electricity Board as supervisors. Their kind and encouraging assistance with regard to the problem formulation, construction and comments on the results is greatly appreciated. I am deeply indebted to all the lecturers, colleagues and friends, too numerous to mention here, who have contributed so generously in various stages of this project over few months. Special mention should make to the engjteers namely Kusum Shanthi and Bernal Jayawardana of CEB, and my friends Thusitha Eranjeewa and Thushara Priyadarshana of CECB. Many thanks are owed especially to the Head and the Staff of the Department of Electrical Engineering University of Moratuwa, colleagues of the batch of the M.Eng!PG Diploma Course in Electrical Engineering, technical staff of the machine laboratory of the Department of Electrical Engineering University of Moratuwa and non executive staff of the computer section of CECB for assistance in reviewing, complying and editing this report. ... Finally this work is dedicated to mx beloved parents and to my wife who have always been with me in every hurdle I cleared. JDKG Hemaratne January 2004 . _ Contents ACRONYMS .................................................................................................... 3 1 INTRODUCTION ...................................................................................... 4 1.1 General .............................................................................................................. 4 1.2 The Problem ..................................................................................................... 4 1.3 Damages ............................................................................................................ 4 1.4 GSS- Introduction ........................................................................................... 5 I 1.5 GSSs where the problem is experienced ........................................................ 7 1.6 GSS where the problem is not experienced ................................................... 9 2 PROBLEM OBSERVATION ................................................................... 1 0 2.1 Observation of Voltage & Current profiles ................................................. I 0 2.2 Observation of Instantaneous Waveforms .................................................. ! 0 2.3 Analysis of observed voltage profiles and waveforms ................................ 18 .... 3 ANALYSIS OF THE PROBLEM ............................................................. 21 3.1 General action of Zig-Zag (earthing) Transformer .................................... 21 3.2 Core - flux under earth fault conditions ....................................................... 26 3.3 Simulated flux phase voltages under fault condition .................................. 33 3.3.1 Simulation using prototype transformer .................................................. 35 3.4 Experiment results ......................................................................................... 41 3.4 .1 Experiment setup 1 results .... .... .. ...... .. ....... .................. ... .... .... ..... .. ......... .41 3.4.1 Experiment setup 2 results .. ....................... .... ...... .................................... 41 3.4.2 Experiment setup 3 results ................ .. ...... .... ........................................... 43 ...... 3.5 Analysis of Experiment Results .................................................................... 43 4 SUGGESTED TESTS FOR EARTHING AND AUXILIARY TRANSFORMERS ......................................................................................... 46 4.1 Existing Tests .................................................................................................. 46 4.1.1 Routine Tests ... ....... : .. .............................................................................. 46 4.1.2 Type tests ......... .... .. ......................................................... ..... .... ..... ........... 46 1 4.1.3 Special Tests ... .. .... ............................. .......... ... .... ..................................... 47 4.2 Suggested Tests ............................................................................................... 47 4.2.1 Type tests ................................ ... .... .... .. ............... .. ..... .......... ... .... ..... .... ... . 47 4.2.2 Special Tests ................. ... .... .......... ................. .... ........ .... ........ ......... ........ 47 5 CONCLUSION & RECOMMENDATION ................................................ 51 5.1 Splitting of Grounding Transformer ........................................................... 52 5.2 Re-arrangement of Windings in the Grounding Transformer .................. 52 5.3 Proposals for Earthing and Auxiliary Transformer tests .......................... 52 I .... ·Mo. 2 ACRONYMS CEB - Ceylon Electricity Board MW - Mega Watt kV - killo Volt HV - High Voltage LV - Low Voltage GSS - Grid Substation AVR - Automatic Voltage Regulator/ AC - Air Conditioner rms - Root Mean Square .. ·~ 3