STUDY ON THE NON LINEAR CHARACTERISTIC OF POWER TRANSFORMER AND THEIR EFFECT FERRORESONANCESALIZAWATI BT HJ. SHAMSUDDIN A project report submitted in partial fulfilment of the requirement for the award of the degree of Master of Engineering (Electrical Engineering) Faculty of Electrical Engineering Universiti Teknologi Malaysia May 2008 To my beloved mother and father ACKNOWLEDGEMENTS Particularly thanks to God for the blessing that gives me a patience and courage in finishing my project report. Firstly, I would like to take this opportunity to thank Prof. Madya Dr. Zulkurnain Bin Abdul Malek, my thesis supervisor for his unfailing enthusiasm and encouragement. I also would like to thank him for all his valuable advice and assistance, suggestions and comments in completing this project report. I also would like to express many thanks to my parents, family and friends for their understanding, consistent commitment and moral support in order for me to write this project report. ABSTRACT Ferroresonance can occur in electrical power system and consequently can cause damage such as due to voltage transformer overheating or power transformer overvoltages. This study involves simulation work to simulate various conditions under which ferroresonance can occur in typical extra high voltage substations. The ATP-EMTP simulation program was used to model various power system components and simulate the ferroresonance phenomena. The effects of the non-linear characteristics of power transformers are also studied. Methods to prevent the ferroresonance conditions from occurring and hence avoiding equipment damages and losses were also proposed based on the simulation work. kerosakan dan kerugian komponen dapat dihindarkan. . Kajian simulasi ini melaporkan kesan sifat tak lelurus keluli pengubah kuasa terhadap feroresanan dan kaedah untuk mengelakkan berlakunya feroresonan.ABSTRAK Feroresonan berlaku ke atas sistem kuasa elektrik yang boleh menyebabkan kerosakan pada sistem tersebut. Program ATP-EMTP satu program simulasi yang berkeupayaan untuk menghasilkan pelbagai model komponen sistem kuasa dan seterusnya melakukan simulasi ke atas sistem. Justeru itu. Kajian ini melibatkan kerja simulasi merupakan bagi pelbagai keadaan yang boleh berlakunya feroresonan pada pencawang elektrik EHV. Contohnya kejadian pemanasan lebihan pada pengubah voltan ataupun voltan lampau pada pengubah kuasa. TABLE OF CONTENS CHAPTER TITLE PAGE DECLARATION DEDICATION ACKNOWLEDGEMENT ABSTRACT ABSTRAK TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES LIST OF ABBREVIATIONS LIST OF SYMBOLS 1 INTRODUCTION 1.2 Objectives 1.1 Introduction 2.2 Case Study by Zia Emin and Yu Kwong Tong: Ferroresonance Experience in UK: Simulation and Measurement 5 5 .5 Organisation of Thesis 2 LITERATURE REVIEW ii iii iv v vi vii xi xii xv xvi 1 1 2 2 3 4 5 2.1 Introduction 1.4 Project Flow Chart 1.3 Scope of Work 1. 2.4 Summary 5 6 7 8 9 9 2. Jacobson: Example of Ferroresonance in High Voltage Power System 2.3 Mitigation Options 2.3 Case Study by YK Tong: NGC Experience on Ferroresonance 10 10 10 11 11 12 12 12 12 13 13 14 14 14 15 15 16 16 17 17 17 .4.4.4.1 Description of Disturbance 2.4.3.4.2 Simulation Result 2.3.1 Comparison Field Measurement and Simulation Result 2.1 Objective 2.4.4 Case Study by David A.2.2.2.4 Conclusion In Power Transformer and Voltage on HV Transmission System 2.3.4.3 Transformer – Circuit Breaker Grading Capacitor 2.5 Conclusion 2.N.3 Mitigation Options 2.4.2 Voltage Transformer 2.2.2 Power Transformers 2.3 Single Phase Traction Supply Transformer 2.2.4.4.2.3.4.2 Simulation Result 2.2 Wound Potential Transformer-Circuit Breaker Grading Capacitor 2.4 Open Delta Potential Transformer 2.4.1 Objective 2.1 Description of Disturbance 2.2.2.3 Voltage Transformer 2.2.3.3.3.1 Comparison Field Measurement and Simulation Result 2.1 Description of Disturbance 2.3.2 Simulation Result 2.4 Measurement to Predict or Prevent Ferroresonance 2.4.4.4.2.4.1 Introduction 2.3 Mitigation Options 2.4. 5 4.2 Modelling of Eddy Currents Effects 5 SIMULATION: 400kV DOUBLE CIRCUIT CONFIGURATION 5.3 4.5 Cable Switching 4 METHDOLOGY 4.5 Symptoms of Ferroresonance 3.3.2 4.3 Quasi Periodic Mode 3.4 Flicker 3.5.3.6.1 Audible Noise 3.2 5.4 Introduction Simulation Procedures Circuit Description Simulation Model 19 22 22 24 25 25 25 26 26 28 28 29 29 29 30 30 32 32 32 33 34 34 34 36 36 37 38 38 38 39 40 .2 Sub harmonic Mode 3.4 Chaotic Mode 3.3 Classification of Ferroresonance Mode 3.3.2 Overheating 3.1 Sensitivity to System Parameter Value 3.5.1 Modelling of Iron Core 4.4 Power System Ferroresonance 3.1.2.1 Fundamental Mode 3.1 5.6 System Modeling ATP-EMTP Simulation Selected model and Validation Resistor and Capacitor Model Overhead Transmission Lines Transformer Model 4.3 5.3 FERRORESONANCE 19 3.1.5.6.1 Basic of Ferroresonance 3.1 Nonlinear and frequency Dependent Parameter 4.5.5.6.3.1 4.2 Main Characteristic 3.2 Sensitivity to Initial Condition 3.3 Arrestor and Surge Protector Failure 3.4 4.2. 2 Simulation Result 6 CONCLUSION AND RECOMMENDATION 6.2.6.1 6.6.4.6.7.3 Non-linear Inductance 5.2 REFERENCES Conclusion Recommendation 42 44 47 48 48 51 51 56 56 58 59 60 62 64 64 70 70 71 72 .1 Simulation Model 5.2 Simulation Result for Curve 2 5.6 Result of Simulation for 400kV Double Circuit Simulation by Changing the Magnetization Characteristic 5.3 Simulation Result for Curve 3 5.2.6.4 Simulation Result for Curve 4 5.2.4.2.6.5 5.4.1 Typical Overhead Line Spacing for 400kV 5.7 Mitigation Technique 5.1 Simulation Model 5.6.2 BCTRAN Transformer Model 5.4.7.2 Simulation Result 5.1 Simulation Result for Curve 1 5.4 Resistor and Capacitor Model 5.5. 10 5.8 5. 5.7 5.9 5.6 5.12 5.3 5.5 5.11 5.4 5.1 5.13 TITLE Transformer Characteristic Transformer Short Circuit Factory Data Transformer Magnetizing Characteristic Simulation Result for 400kV Double Circuit Configuration Transformer Magnetizing Characteristic – Curve 1 Transformer Magnetizing Characteristic – Curve 2 Transformer Magnetizing Characteristic – Curve 3 Transformer Magnetizing Characteristic – Curve 4 The Effect of Using the Saturation -Curve 1 The Effect of Using the Saturation -Curve 2 The Effect of Using the Saturation -Curve 3 The Effect of Using the Saturation -Curve 4 The Effect of Adding Resistor on Secondary Side PAGE 44 45 47 49 52 53 53 54 57 58 59 61 65 .LIST OF TABLES TABLE NO.2 5. 8 2.1 2.7 2. 1.10 2.4 3.3 2.3 3.5 2.13 3.1 2.6 2.2 2.9 2.2 3.11 2.5 3.LIST OF FIGURES FIGURE NO.6 Project Flow Chart TITLE PAGE 4 6 6 7 8 8 9 9 13 14 14 15 16 16 20 21 22 23 24 25 Component Single Line Diagram of Voltage Transformer Reduced Equivalent Ferroresonance Circuit Measurement Output Voltage Digital Simulation Output Voltage Single Line Diagram of Traction Supply Transformer Measurement Output Volatge Digital Simulation Output Voltage Single Line Diagram of Wound Power Transformer Circuit A Single Diagram of Main Circuit A Single Diagram of Station Service Transformer The Output Voltage Waveform of Bus Voltage Typical Station Example of Output Voltage Resonance in RLC Circuit Magnetization Curve Basic Series Ferroresonance Circuit Parameter Sensitivity to the System Parameter and Jump Phenomenon Sensitivity Initial Condition Diagrams Illustrating the Fundamental Mode of Ferroresonance .1 3.12 2.4 2. 3.7 3.17 5.23 5.5 5.18 5.3 5.9 5.13 5.9 5.2 5.8 5.24 Diagrams Illustrating the Subharmonic Mode of Ferroresonance Diagrams Illustrating the Quasi Periodic Mode of Ferroresonance Diagrams Illustrating the Chaotic Mode of Ferroresonance A Single Line Diagram of the Brinsworth/Thorpe Marsh Circuit Equivalent Circuit of Power Transformer Line/Cable Dialog Box Line Configuration BCTRAN Dialog Box The Saturation Curve for Nonlinear Inductor The Output Voltage Waveform at TR1 Terminal–R Phase The Output Voltage Waveform at TR1 Terminal–Y Phase The Output Voltage Waveform at TR1 Terminal– B Phase The Output Current Waveform at TR1 Terminal .14 5.Y Phase The Output Current Waveform at TR1 Terminal.12 5.21 5.B Phase Variation of Magnetization Curve The Saturation Curve 1 The Saturation Curve 2 The Saturation Curve 3 The Saturation Curve 4 The Output Voltage Waveform at TR1 Terminal–R Phase The Output Current Waveform at TR1 Terminal –R Phase The Output Voltage Waveform at TR1 Terminal –R Phase The Output Current Waveform at TR1 Terminal–R Phase The Output Voltage Waveform at TR1 Terminal –R Phase The Output Current Waveform at TR1 Terminal –R Phase The Output Voltage Waveform at TR1 Terminal – R Phase 26 26 27 40 41 43 44 46 48 49 49 50 50 50 51 52 54 55 55 56 57 57 58 59 60 60 61 .6 5.4 5.8 3.11 5.7 5.20 5.R Phase The Output Current Waveform at TR1 Terminal.1 5.15 5.22 5.19 5.16 5.10 5. 25 5.31 5.35 5.28 5.32 5.33 5.26 The Output Current Waveform at TR1 Terminal –R Phase Simulated Power Transformer Circuit by Adding Loading Resistor at Secondary Side 61 63 5.38 The Output Voltage Waveform by Adding R=1k Ohm Resistance – R Phase The Output Voltage Waveform by Adding R=1k Ohm Resistance – Y Phase The Output Voltage Waveform by Adding R=1k Ohm Resistance – B Phase The Output Current Waveform by Adding R=1k Ohm Resistance – R Phase The Output Current Waveform by Adding R=1k Ohm Resistance – Y Phase The Output Current Waveform by Adding R=1k Ohm Resistance – B Phase The Output Voltage Waveform by Adding R= 30k Ohm Resistance – R Phase The Output Voltage Waveform by Adding R= 30k Ohm Resistance – Y Phase The Output Voltage Waveform by Adding R= 30k Ohm Resistance – B Phase The Output Current Waveform by Adding R= 30k Ohm Resistance – R Phase The Output Current Waveform by Adding R= 30k Ohm Resistance – Y Phase The Output Current Waveform by Adding R= 30k Ohm Resistance – B Phase 65 66 66 66 67 67 67 68 68 68 69 69 .34 5.30 5.29 5.27 5.37 5.36 5.5. LIST OF ABBREVIATIONS DC AC ATP EMTP PT GB HV LV - Direct Current Alternating Current Alternative Transient Program Electro Magnetic Transient Program Potential Transformer Grading Bank High Voltage Low Voltage . LIST OF SYMBOLS XL XC C L C R VL E f w Z - Inductance reactance Inductance capacitance Capacitance Inductance Capacitive Resistance Load Voltage Voltage source Frequency Frequency Impedance . wave shape faults. By analyzing the electrical power and evaluating the equipment or load. we can determine if a power quality problem exists. .CHAPTER 1 INTRODUCTION 1. Power quality issues will affect each type of electrical equipment differently. As a general statement. Power quality issues can be highspeed events such as voltage impulses / transients. There are many ways in which electric power can be of poor quality and many more causes of such poor power quality. fail prematurely or not operate at all. high frequency noise. voltage swells and sags and total power loss.1 Introduction A power quality is a term used to describe electrical power that motivates an electrical load and the load’s ability to function properly with that electric power. an electrical device (or load) may malfunction. With a poor power quality. any deviation from normal of a voltage source (either DC or AC) categorized as a power quality issue. Power quality problems manifest themselves in variations in the voltage has been obtained. features. sags or swells in the amplitude of the voltage. To prove or otherwise that ferroresonance can occur at 400kV double circuit substation. 1. This variation can be in the form of transients due to switching or lightning strikes.3 Scope of Work The main scope of this project is to simulate the various conditions of ferroresonance. components and arrangements of the substation power system. . or harmonic distortion caused by non-linear loads in the system which may likely lead to the occurrence of ferroresonance. ii. An alternative Transient Program. a complete interruption in the supply. 1.2 Objective The main objectives of this project is to simulate the ferroresonance event on extra high voltage substation power transformer based on parameters. To identify the effect of magnetization characteristic of power transformer on ferroresonance.Electromagnetic Transient Program (ATP-EMTP) will be used to carry out the simulation in order to study the phenomenon and therefore to determine methods to minimize or reduce the risk of ferroresonance to power transformers. which include: i. Review previous work done on Ferroresonance Review Ferroresonance simulation done in power system Analyze the Ferroresonance work done Modelling the ferroresonance circuit arrangement in ATP/EMTP . It is also to give the simulator the basic overview of the system and what is required before simulations be completed. The flow chart of this project is as shown in Figure 1.4 Project Flow Chart To solve the problem.iii. To identify method to minimize the impacts of ferroresonance on power transformers.1. one has to first study the problem and come up with the process flow chart. which will guide the simulator throughout the project. 1. chapter 6 describes the conclusion and recommendation that is related to the project done. Besides. Therefore. it is also includes some techniques for avoiding or mitigating ferroresonance.Simulation and Analysis Recommendation and Conclusion Figure 1.1: Project Flow Chart 1. Chapter 3 describes the basics of ferroresonance. Chapter 4 describes the methods that are used for the simulation. . Chapter 5 presents the circuits that were used in the simulation and explains how the simulations techniques are implemented.5 Organisation of Thesis Chapter 2 illustrates previous work done related on ferroresonance phenomenon in voltage transformer and power transformer. characteristics and types of ferroresonance. Lastly. all information related the simulation is explained in detail with the operation. Three case studies describe ferroresonance phenomena on voltage transformers and power transformers in detail.1 Objective The objective of this paper is model and analysis ferroresonance on the system by using program ATP-EMTP. 2.2.CHAPTER 2 LITERATURE REVIEW 2. This is to compares the result with system measurement to identify the crucial factor in ferroresonance.2 Case Study by Zia Emin and Yu Kwong Tong: Ferroresonance Experience in UK: Simulations and Measurements [1] 2.1 Introduction This chapter reviews the previous related work done on ferroresonance. The case studies related to ferroresonance in wound voltage and single-phase supply transformers. . Alternatively it can also occur upon closure of both disconnector 1 or 2 with circuit breaker and disconnector 3 open.2 Voltage Transformer Throughout voltage transformer ferroresonance oscillation happens among the nonlinear inductance and capacitance that remaining connected to the voltage transformer.2. Figure 2.2: Reduced equivalent ferroresonant circuit .2. Figure 2. In this case study.1 shows the single line diagram of voltage transformer.2. Figure 2. energy attached to the nonlinear inductance of voltage transformer by the open circuit grading capacitance to maintain resonance.1: Single Line Diagram of Voltage Transformer The system arrangement in Figure 2. Ferroresonance can happen upon opening of disconnector 3 with circuit breaker open and either disconnector 1 or 2 closed.1 can be reduced to equivalent circuit in Figure 2. 2. Figure 2.3 and 2. When the same system is tested for a second time by changing the parameter the response is totally different from earlier and it become into sub-harmonic ferroresonance. As it observed that the voltage transformer haven’t get into ferroresonance.1 Comparison Field Measurement and Simulation Result The measured and simulated voltage transformer at secondary voltages has been compared. The resistor R represents a voltage transformer core loss that has been found to be an important factor in the initiation of ferroresonance.4 are a good match. Cseries is the circuit breaker grading capacitance and Cshunt is the total phase-to-earth capacitance of the arrangement.3: Measurement Output Voltage . It can be said that the ATP model used in this case study is capable of predicting feasible ferroresonance modes as long as the parameters of the system are known or can be estimated.E is the rms supply phase voltage. its general appearance is a very close fit.2. Although the simulation does not match the measured test waveform exactly. 2.1. The measured was obtained after opening the disconnector 3 in Figure 2. The shapes of measured and simulated waveforms in Figure 2. C series in the equivalent ferroresonant circuit. Figure 2. Because energy is coupled to the de-energized network from the adjacent live parallel circuit through the inter-circuit coupling capacitance.5: Single Line Diagram of Traction Supply Transformer . Ferroresonance happens upon opening of circuit breakers 1 and 2 to deenergize the line and the transformer.Figure 2. The single line diagram in figure 2.4: Digital Simulation Output Voltage 2. The equivalent C shunt in the ferroresonance circuit is the phase-to-earth capacitance of the line and transformer winding and bushing capacitance.2.3 Single Phase Traction Supply Transformer Figure 2.3 shows the single line diagram of a traction supply transformer arrangement.2. The single phase transformer is fed from two phases of the three-phase system.5 can reduced to equivalent circuit in Figure 2. 2.2.3.1 Comparison Field Measurement and Simulation Result Figure 2.6 and 2.7 shows the simulation result of field measurement and digital simulation of a traction supply transformer. Obviously, the two phases generate the transformer in prolong fundamental frequency mode ferroresonance. Figure 2.6: Measurement Output Voltage Figure 2.7: Digital Simulation Output Voltage 2.2.4 Conclusion It can be concluded that the ATP model used in this case study is capable of predicting feasible ferroresonance modes as long as the parameters of the system are known or can be estimated. 2.3 Case Study by YK Tong: NGC Experience on Ferro resonance in Power Transformer and Voltage Transformer on HV Transmission Systems [2] 2.3.1 Introduction Ferroresonance or nonlinear resonance can happen when power transformer connected to the overhead lines and voltage transformer connected to the isolated of busbars. Hence, energy coupled through the capacitance of the parallel line or open circuit breaker grading capacitance to maintain the resonance. In linear resonance condition, the current and voltage dependent on frequency but ferroresonance dependent on frequency, transformer magnetic flux and point on wave of the initiating switching event. 2.3.2 Power Transformers On power transformers circuit, ferroresonance can happen between the overhead line and the transformer magnetising inductance. Subsequently the transformer feeder de-energisation the transformer determined into saturation due to the discharge of the capacitance -to-earth on the isolated network and a non-linear oscillation happens between the reactive components. Ferroresonance can happen at 50Hz fundamental frequency or subharmonic frequencies at 331/2 Hz, 162/3 Hz and 10Hz. 2.3.3 Voltage Transformer Ferroresonance happen upon de-energization of the wound voltage transformer, an oscillation occurs between the voltage transformer inductance and the capacitance-to-earth of any system that remaining connected to the voltage transformer. 2.3.4 Measure to Predict or Prevent Ferroresonance Power transformer can be avoid by the disconnection either the transformer from feeder or parallel circuit excitation source that can provides the supply power for sustained ferroresonance. Other practices are built-in the surge arrestors at the power transformer that the circuit may be re–energised from the main system and then re-energised again. A ferroresonance damping device at secondary side is likelihood for controlling voltage transformer ferroresonance. Voltage transformer can prevent by adding damping device such as resistor at the secondary side. The resistance load burden will insert as soon as the ferroresonance detected. Presently, ferroresonance solution can acquire by using ATP simulation. The program is very sensitive to circuit parameters and the magnetising characteristic of the power and voltage transformer. 2.4 Case Study by David A.N. Jacobson: Example of Ferroresonance in High Voltage Power System [3] 2.4.1 Objective This paper represent three practical example of ferroresonance in a high voltage power system (33kV or greater) and method of mitigating among ATP simulation. The first example discusses a wound power transformer-circuit breaker grading capacitor and the second discusses a transformer-circuit breaker grading capacitor. The final case studies investigate open delta power transformer. 2.4.2 Wound Potential Transformer – Circuit Breaker Grading Capacitor Circuit breaker used as an advanced interrupting .In high voltage applications, multiple interrupting chamber connected in series are required to interrupt the current and withstand the high recovery voltage. Grading capacitor fitted in parallel with the each break to achieve an equal voltage distribution. 2.4.2.1 Description of Disturbance Figure 2.8 demonstrates the single line diagram of wound power transformer circuit at Dorsey s HVdc converter station 230kV. The station consists of four bus sections on which the converter valves and transmission lines are terminated. Bus A2 was removed for replacement breakers and to perform disconnects maintenance and trip testing. Afterwards, potential transformer (V13F) failed. The switching procedure de-energized bus and power transformer connected to energize B2 through grading capacitor. Normally connected to bus A2, but had been disconnected. The ferroresonance effects failure to the power transformer. 2.2. The damping resistor needs to dissipate energy faster than the system can supply energy in order for the ferroresonant state to disintegrate into ferroresonance 60 Hz operating mode.4.2 Simulation Result The simulation result substantiation that ferroresonance was occurred at A2 bus.Figure 2.2.8: Single line diagram of wound power transformer circuit 2.4.3 Mitigation Options Resistors were connected on the power transformer secondary side of Dorsey 230kV substation. . It explains that connected 200 ohm to the secondary of the system was established to prevent ferroresonance. The voltage output waveform of phase A and B experienced indiscriminate jumps between sub harmonic and normal oscillations but phase C did not practices ferroresonance. Figure 2. Figure 2.3 Transformer – Circuit Breker Grading Capacitor 2.9: A single diagram of main circuit component Figure 2.2 Simulation result Figure 2.4. where noise level from SST1 that higher than normal. Ferroresonance over voltage occurred on B2 bus.u.10 shows a station service transformer.5p.4. .3.1 Description of Disturbance Breaker failed to latch while energize the induction motor at Dorsey Converter Station.3.9 shows a single diagram of main circuit component and Figure 2.10: A single diagram of station service transformer 2.2.11 exhibits simulation result of fundamental frequency mode of ferroresonance due to high distortion and over voltage near 1.4. 2. . Low values of capacitance may cause third harmonic mode.4.4 Mitigation Option The mitigation option of this case is being connected 200 ohm loading resistors at secondary service transformer SST1 and SST2. Three modes of ferroresonance can be observed in an open –delta potential transformer. Medium values of capacitance may cause fundamental model of feroresonance and higher values of capacitance could cause subharmonic modes of ferroresonance.4 Open Delta Potential Transformer Ferroresonance can happen during energization of the unloaded step down transformer and interruption of a single line.4.Figure 2.11: The output voltage waveform of bus voltage 2.to-ground fault on the low side of the transformer.3. Generally.4. a) no mitigation.1 Description of Disturbance Figure 2. 2.4.12 shows the single diagram of open –delta potential transformer.14: Example of output voltages. The purpose of grounding bank is to provide zero sequence current during single line-to-ground for ground fault protection relay to operate. Figure 2.14(c) explains the voltage output waveform with connected grading bank. b) 83 ohm resistor connected and c) Ground bank connected. .4. Fig.2 Simulation result Figure 2. the open – delta potential transformer as a backup to ground bank to taken out for service for maintenance.4.14(b) shows the voltage output by connected loading resistor 83 ohm at secondary side and Figure 2. Figure 2.14(a) shows the unbalanced fundamental mode of ferroresonance.12: Typical station layout 2.2. Open delta voltage transformer ferroresonance can be prevented by adding loading resistor across the open delta or close grounding transformer banks.5 Summary The first case study.4. Therefore mitigation strategies can be designed before equipment is put into service to avoid ferroresonance by adding loading resistors or replacement of the voltage transformer with capacitor voltage transformer.3 Mitigation Options There are several possible solutions to prevent ferroresonance: i.5 Conclusion Three examples of circuit arrangements that can practice ferroresonance have been presented.4. Install potential transformer that are rated for line-to-line system voltage. The case study analyses the performance of ferroresonance in the system by comparing measurement data and the ATP-EMTP simulation results. 2. ii. iii. . The impact of ferroresonance can vary from relay or control disoperation to damage the equipment.2. it has been proven that ferroresonance in voltage transformers and power transformers occurs due to nonlinear and dynamics in the system. Install damping resistor in the broken – delta. Replacing potential transformer with capacitive coupled voltage transformer.4. 2. The study shows the measured and simulated voltages agree very well. 162/3Hz and 10Hz. Prevention methods are by loading resistors and replacement of the voltage transformer with a capacitor transformer. . The circuit also has a low-loss characteristic. The mitigation methods of ferroresonance are by disconnecting the power transformer from the feeder. and a voltage source.The second case study describes the ferroresonance phenomena when connecting power transformers with overhead lines. Common elements in a ferroresonance circuit are a nonlinear saturable inductor. or when connecting voltage transformers with isolated sections of busbars. capacitors. by adding damping devices at the secondary side of the voltage transformer. The third case study reports the practical examples of ferroresonance in a high voltage system. and for the case voltage transformers. The energy in the capacitance of the parallel lines or open circuit breaker grading capacitance is responsible for the ferroresonance occurrence at 50Hz fundamental frequency or sub-harmonics frequencies at 331/2Hz. 1 can be used as an aid in explaining resonance and hence ferroresonance. characteristics. Therefore. the basics of ferroresonance must be identifying.2 Basics of Ferroresonance [4] Ferroresonance does not occur regularly and it is very hard to analyze. types and symptoms of ferroresonance. The simple RLC circuit in Figure 3.1 Introduction This chapter discusses the theories. 3. Any response to a sudden change in the system may jump out of a steady state into ferroresonance condition. .CHAPTER 3 FERRORESONANCE 3. If this resistance is small. the inductive reactance (XL) and capacitive reactance (XC) cancel each other out. These impedance values be forecast and they change with the frequency. depends on the resistance (R).Figure 3. where w is the frequency of the source.XC ) *V / R (1) (2) (3) The current in equation (3). XC = 1/ jwC XL = jwL I = V / (R + XL . The size of current during resonance is equation (3).1: Resonance in RLC Circuit This linear circuit is resonating when at some given source of frequency. . Capacitance (C) will always have a capacitive reactance as in equation (1) and inductance (L) will always have an inductive reactance as in equation (1). then the current can become very large in the RLC circuit. This has been identified as the saturation point. This is why ferroresonance is a concern for transformer operation. the magnetic flux density until a certain point where the slope no longer linear and an increase in current lead to smaller and smaller increases in magnetic flux density. The understanding that ferromagnetic material saturates is very important.2 illustrates the relationship between magnetic flux density and current. and therefore magnetic induction. the inductance of the coil changes very quickly. As the core goes into a high flux density. This sound is different from the normal hum of a transformer. These high currents make damage on the transformer. When ferroresonance happens. Figure 3. As the current increases in a ferromagnetic coil past the saturation point. Ferroresonance can cause high . Equation (3) will not indicate the size of current produced. it can be identify by certain distinct characteristics. because the steel core is driven into saturation so an audible noise occurs. Ferromagnetic material has a property of causing an increase to the magnetic flux density.If the inductor in Figure (2) replaced by an iron cored non-linear inductor. The inductance becomes nonlinear due to saturation of flux in the iron core. Most transformers have cores made from ferromagnetic material. This allows the current to take on very unsafe high values. Figure 3. the exact values of voltage and current cannot be predicted as in a linear model. In ferroresonance.2: Magnetization Curve As the current is increased. magnetostriction forces cause movement in the core laminations. These curves.5 and 3.3. illustrate the following characteristics of ferroresonance: i. jump phenomenon Sensitivity to initial condition Figure 3. This can cause electrical damage to both the primary and secondary circuits of a transformer.over-voltages and currents.3 Main Characteristic [ 4 ] Figure 3. .3 is a basic series ferroresonance circuit and the curve obtained from of the circuit shown in figure 3. Eventually. 3. ii.6.3: Basic series ferroresonance circuit parameter 3. The heating cause by over-currents may cause permanent damage to transformer insulation.1 Sensitivity to system parameter values. jump phenomenon Figure 3. Sensitivity to system parameter values. the transformer could fail completely.4 illustrates the peak voltage VL at the terminals of the nonlinear inductance as a function peak amplitude E of the sinusoidal voltage source. For E = E’2. A small variation in the value of a system parameter or a transient can cause a sudden jump between two very different stable steady states. iv. When the value of E decreases from E3. the solution suddenly moves from the point M1 (second limit point) to the point M’1. v. only the ferroresonance state (M3f) is possible.4: Sensitivity to system parameters and the jump phenomenon By gradually building up peak amplitude E from zero. M2n corresponds to the normal state. the voltage VL suddenly moves from the point M2 to the point M’2 (the jump phenomenon). ii. The point M2 is known as a limit point. and this corresponds to the normal state (in the linear ). For E = E3. For E = E1. M2i. as well as the jump phenomenon: i. M2f). iii. . The dotted part of the curve corresponds to unstable states. there are three solutions (M2n. the solution (M1n) is unique. whereas M2f corresponds to the ferroresonant state. vi. two of which are stable (M2n and M2f). For E = E2. the curve shows that there are three different types of behaviour according to the value or E.Figure 3. the solution converges to M2f. If the initial conditions are on the other side. If the initial conditions (residual flux. voltage at capacitor terminals) are on one side of the boundary.5 illustrates the trajectories of the transient of pairs (Ф.2 Sensitivity to initial conditions Whether M2n or M2f obtained depends on the initial conditions. Figure 3.3.5: Sensitivity initial condition . Curve C describes a boundary. the steady state effectively reached around this point is extremely sensitive to the initial conditions. VC) as a function of time for different initial conditions (M01 and M02). Figure 3.3. the solution converges to M2n. As the point M2i belongs to the boundary. The signal spectrum is a discontinuous spectrum made up of the fundamental f0 of the power system and of its harmonics (2f0. The voltages and currents are periodic with a period T equal to the system period.. The characteristics of each type of ferroresonance listed below [4]: i.3.6: Diagrams illustrating the fundamental mode of ferroresonance . and can contain a varying rate of harmonics.4. iv..4 Classification of Ferroresonance Modes [4] The type of ferroresonance can identify by the spectrum of the current and voltage signals. 3f0.6 shows the diagrams to explain fundamental mode. ii. iii. Fundamental mode Subharmonic mode Quasi –periodic mode Chaotic mode 3. The stroboscopic image reduced to a point far removed from the point representing the normal state.).1 Fundamental mode Figure 3. Figure 3. 7 shows the diagrams to explain the sub harmonic modes. The spectrum presents a fundamental equal to f0/n (where f0 is the source frequency and n is an integer) and its harmonics (frequency f0 is thus part of the spectrum) Figure 3.4. This state known as sub harmonic n or harmonic 1/n. The signals are periodic with a period nT which is a multiple of the source period.4.3.3 Quasi-periodic mode Figure 3. . Sub harmonic ferroresonant states are normally of odd order.8 shows the diagram for the quasi-periodic mode.2 Subharmonic mode Figure 3.7: Diagrams illustrating the sub harmonic mode of ferroresonance 3. This mode is not periodic. The spectrum is a discontinuous spectrum whose frequencies are expressed in the form: nf1+mf2 (where n and m are integers and f1/f2 an irrational real number). 4 Chaotic mode Figure 3. the appearance of these states is highly sensitive to system parameter values and the appearance of these states is highly sensitive to initial conditions. it not cancelled for any frequency.8: Diagrams illustrating the quasi-periodic mode of ferroresonance 3. i. The corresponding spectrum continuous. i known as the strange attractor.9: Diagrams illustrating the chaotic mode of ferroresonance Therefore.Figure 3. Figure 3.4.e. it can conclude that the ferroresonance is a complex phenomenon in which there are several steady states for a given circuit. The stroboscopic image made up of completely separate points occupying an area in plane v. .9 shows the explanation for the chaotic mode. transformer heating and excessively loud noise. iii. iii. Abnormal rates of harmonics. high sustained overvoltages. ii. A sinusoidal voltage source -A power system generator will do quite nicely. often represent a risk for electrical equipment. the large capacitance of . high sustained levels of distortion to the current and voltage waveforms. The modes most commonly encountered are the fundamental and subharmonic ones. iv. Steady state ferroresonance sustained by the energy supplied by the power system. alone or in combination [5]: i. Ferromagnetic inductances -These can be power transformers or voltage transformers. both phase to phase and phase to ground. v. over voltages/ currents. either as stable oscillation or as transients caused by ferroresonance. There are four elements for ferroresonance to occur [5]: i. 3.5 Power System Ferroresonance Ferroresonance in a power system can result in any of the following. ii. Capacitance -This can come from installed power system capacitors. high sustained overcurrents. apparent mis-operation of protective devices. electrical equipment damage (thermal or due to insulation breakdown) vi. the capacitance to ground of transmission lines.Small variations in the value of a system parameter or a transient may cause a sudden jump between two very different steady states and initiate one of the four ferroresonance types. Some have very high voltages and currents while others have voltages close to normal. Therefore the core goes into a high flux density.6. or low circuit losses. when it has . iv.6 Symptoms of Ferroresonance There are several modes of ferroresonance with varying physical and electrical displays. or the capacitance to ground of an ungrounded system.This can be lightly loaded power system equipment. 3. low short circuit power source. Ferroresonance happen when the noise is louder than the normal hum of transformer. it make due to the magnetostriction of the steel and to the actual movement of the core laminations. Low resistance . Ferroresonance happen.1 Audible Noise Audible noise occurs when the steel core driven into saturation. it will reveal the symptoms of ferroresonance.underground cable. The effect is bubbling of the paint on the top of the tank.6. 3. In this section. 3.2 Overheating Another symptom of the high magnetic field is due to stray flux heating in parts of the transformer where magnetic flux come across into the tank wall and other metallic parts because of the core saturated repeatedly. 6. the light bulbs will flicker between very bright and dim. This may cause damage insulation structures. the arrester becomes very hot on the phase and goes into thermal runaway upon restoration of full power to that phase. The failure is often from the arrester housing.4 Flicker Utility customers often face with the problem of a wavering voltage magnitude. it not accepted for stress repeatedly because the forces often shake things loose inside and scrape insulation structures. 3. In addition. In the meantime.5 Cable Switching The transformers themselves can usually withstand the over voltages without failing. 3. office equipment and factory machines. For example. the people determine an open fused cut out and just replace the fuse. 3.3 Arrester and Surge Protector Failure The arrester failures related to heating of the arrester block.6.6. Nowadays surge protectors are common in computers. Under-oil arresters are less vulnerable to the problem because it can dissipate the heat due to the ferroresonance current more quickly. The cable also in little risk unless its insulation stress reduced by aging or physical harm.continued extended to reason overheating of some of the larger internal connections. some electronic appliances are reportedly very susceptible to the voltages that result from some types of ferroresonance. Normally. . However. the alleged failure mode is unknown. it may be difficult to clear arcs when pulling cable elbows if ferroresonance in progress. It because the currents could be much higher than expected and the peak voltages could be high enough to cause reigniting of the arc. The solution of ferroresonance during cable switching is to always drag the elbows and energize the unit at the primary terminals. Only a little internal capacitance and the losses of the transformers are mostly enough to prevent resonance with the small capacitance. These will no external cable capacitance to cause ferroresonance. .Some utilities will not execute the cable switching involving three-phase pad mount transformers without first verifying that there is substantial load on the transformers. Therefore. 1 System Modelling The main emphasis is to identify the models of substation components to use in the ferroresonance studies. It a computer simulation program specially designed to study transient phenomenon in the power system. It contains a large variety of detailed power equipment models or builds in setups that simplify the tedious work of creating a . whether ferroresonance has occurred or otherwise. The EMTP is primarily a simulation program of the electric power industry. in particular. 4. the models will be simulated using ATP-EMTP program and the output is to be analyzed. the importance of the model parameters will describe and typical values shall be providing.2 ATP-EMTP Simulation The ATP is the PC version of the Electromagnetic Transients Program (EMTP).CHAPTER 4 METHODOLOGY 4. Afterwards. For each component. electrical model of the transmission system apparatus have to be select and validated to gain high accuracy result. voltage.system representation. A complete set of representation of a transmission system are made of models for all of the power system components. In ATP-EMTP simulation. complex networks and control systems of arbitrary structure can simulate. ATP-EMTP used in this simulation process of observing the electrical response of the transmission system. power and energy. . this simulation software can use in design of an electrical system or in detecting or predicting an operating problem of a power system. 4.3 Selected Model and Validation Models are circuit or mathematical or electrical representation of physical equipment so that its characteristic determined by means of an output when applied with certain input. ATP is a universal program system for digital simulation of transient phenomena of electromagnetic as well as electromechanical nature. Generally. To represent the electrical response of the transmission system. With this digital program. the input and output that usually observed are current. . It represented by multi-phase models considering the distributed nature of the line parameters due to the range of frequencies involved. Both type of model can be implemented in the ATP-EMTP.4. for this work only adopted linear branch capacitor and resistor and focusing on the nonlinear inductance. The second type is saturable transformer component model to multi-phase transformer. However. The length of the overhead line circuit is approximately 37km. The line parameters being clarify by line constants. shield wires are explicitly modelled between towers. 4. the overhead transmission lines are double circuit 400kV. The first type is matrix representation (BCTRAN model). This simulation.4 Resistor and Capacitor Model There are various types of resistors and capacitors available in the ATP library. 4. and only a few spans are considered. phase conductors. using the tower structure geometry and conductor data as input. This can vary from linear to non-linear and capacitor braches.6 Transformer models [7] Transformer model can be classified into three types.5 Overhead Transmission Lines [6] Overhead transmission lines model depends on the line length and the highest frequency to be simulated. The third type is topology-based models [7]. triplex and three phase shell type) and test frequency. ZH‐T are the impedance (%) values and Z*H‐L . ZL‐T . Z*L‐T and Z*H‐T are vales that written to the BCTRAN file. two and three winding transformers by considering the number of phases.VH. the rated voltage is automatically divided by √3 to get the winding voltage.VH + ZL‐T . Besides that. which use an admittance matrix representation of the form: [v] = [R] [i] + [L] [di / dt] (7) where [R] and j ù [L] are the real and the imaginary part of the branch impedance matrix. In addition. If autotransformer selected for primary and secondary winding (HV‐LV) the impedance are recalculate as shown below [8]: Z*H‐L = ZL‐H ( VH/ VH – VL)2 Z*L‐T = ZL‐T ( VH – VL)2 ( VH – VL) ( VH – VL) (4) (5) Z*H‐T = ZL‐H . the number of windings.VL (6) where ZL‐H. In transient calculations can represent as. If the connection is an auto transformer or wye. identify the winding connection such as an autotransformer. VL + ZH‐T . . rated power and type of coupling. wye and delta. the type of core ( single phase core.Matrix representation (BCTRAN model) will be used to represent single and three‐phase. The parameter of open circuit and short circuit data can be obtained from factory test data. BCTRAN transformer of ATPDraw. clarify the ratings of the line‐voltage. 6. hysteresis and eddy currents.[di/ dt] = [L]-1 [v] – [L] -1 [R] [i] (8) For simulation of saturable cores. Saturation and hysteresis are included in the representation of the iron core and introduce distortion in waveforms. The information usually available is the rms voltage as a function of the rms current. the B–H relationship that would be obtain if there were no hysteresis effect in the material. Magnetic saturation of an iron core is representing by the hysteretic curve. The saturation characteristic can be modelled by a piecewise linear inductance with two slopes. although in modern transformers they are mostly due to eddy current. In ferroresonance. such as inrush currents.6.1. The specification of such inductor requires a curve relating the flux linkage. 4. although hysteresis losses and the residual flux can have a major influence on some transients. since increasing the number of slopes does not considerably improve the accuracy.1 Nonlinear and Frequency-Dependant Parameters [7] Some transformer parameters are non-linear and/or frequency dependent due to three key effects: saturation. 4. the detailed of the saturation characteristic is required. i.1 Modeling of Iron Cores Iron core behaviour is represent by a relationship between the magnetic flux density B and the magnetic field intensity H. . to the current. Hysteresis loops usually have a negligible influence on the magnitude of the magnetizing current. Excitation losses are cause by hysteresis and eddy current effects. excitation may be omit from the matrix description and attached externally at the model terminals in the form of non-linear elements. A change in the magnetic field induces also eddy currents in the iron. Efficient models intended for simulation of frequency dependent magnetizing inductances have been derive by synthesizing Cauer equivalent circuits to match the equivalent impedance of either a single lamination or a coil wound around a laminated iron core limb. . the flux is confine to a thin layer close to the lamination surface. and manifest themselves as an increase in the effective resistance and winding losses with respect to those for direct current eddy current effects in transformer windings can be model by Foster equivalent circuits. It can be justifiable to represent as non-linear only the first section of the model. Since the high frequency components do not contribute appreciably to the flux in the transformer core. a computationally efficient circuit can be deriving by fitting only at certain pre-established frequencies. the flux density will be lower than that given by the normal magnetization curve. For high frequencies. These circuits must be of infinite order exactly to reproduce the impedance at all frequencies.1. A series model of order equal or less than 2 is adequate for low-frequency transients. flux distribution in the iron core lamination also changes. occur at the same time in a loaded transformer that result in a non-uniform distribution of current in the conductors. This indicates that inductances representing iron path magnetization and resistances representing eddy current losses are frequency dependent. whose thickness de-creases as the frequency increases. As frequency changes. Inductive components of these models represent the magnetizing reactances and have to make non-linear to account for the hysteresis and saturation effects.6. However. Because of this. so for low frequency transients an equivalent circuit with order equal or less than two may suffice.4.2 Modelling of Eddy Current Effects Eddy current effects. it can be assume that only low frequency components are responsible for driving the core into saturation. 1 Introduction In this work.CHAPTER 5 SIMULATION OF 400KV DOUBLE CIRCUIT CONFIGURATION 5. The purpose of the simulation is to determine conditions in which ferroresonance can occurs. typical data or estimates were used for each case.2 Simulation Procedures In order for the simulation work to be successfully carried out. Hence. 5. Some of the components’ data are not available. in this simulation work. If a ferroresonance does occur. a 400kV/275kV double circuit extra high Voltage substation was chosen for the simulation study. power transformer saturation characteristics. the following procedures were adopted: (i) Determinations of all parameters such as the overhead transmission line. then how best can be the ferroresonance be mitigated or its effect minimised. the power transformer and . capacitors etcetera and finally the nature of the load – magnetization characteristic and type of nonlinear inductance.1 shows single diagram of the Brinsworth /Thorpe Marsh circuit arrangement. (v) Determine the mitigation techniques to prevent ferroresonance from occurring.3 Circuit description The 400kV double circuits used in this simulation were taken from Brinsworth / Thorpe Marsh (UK transmission network) design parameter [9]. The length of the parallel overhead line circuit is approximately 37 km and the feeder has a 1000MVA 400/275/13kV power transformer. . This involved finding circuit diagrams. (ii) Develop an equivalent circuit to represent the actual power transformer and its interconnections based on actual station. (iii) Analysis of voltage and current output from an equivalent circuit. The first part of the process was to gather information on the system. (iv) Determine whether ferroresonance occurs or otherwise under the given circuit parameters. The purpose is to exhibit of ferroresonance to occurring on the system. information on transformers. Estimate been made in the cases where actual physical values cannot be obtained. etcetera. 5. Figure 5.shunt capacitance. BCTRAN transformer matrix mode . Non-linear inductance. Resistor and capacitor.X303 3x X103 TR1 3 T10 0x x Overhead line 37km X420 x Thorpe Marsh 400kV TR2 Brinsworth 275kV Figure 5. 5.1 can be reduce to equivalent circuit in Figure 5. i. . iii. iv. ii. After switching (Transformer de-energized) -Disconnector X303. ii. -Disconnector X103 was close.2. Before switching (Transformer energized) . Typical overhead line spacing for a 400kV double circuit.All disconnector and circuit breaker were close.1: A single line diagram of the Brinsworth/Thorpe Marsh circuit Following is the circuit equipment condition: i.4 Simulation Model The system arrangement in Figure 5. circuit breaker X420 and T10 were open. The main components of the network are. Transformer nonlinear characteristic Series and Shunt Capacitances of Windings Disconnector Circuit Breaker Cable 170 m Circuit Breaker Figure 5.2: Equivalent circuit of power transformer . (no skin effect) ♦ REACT: The frequency independent reactance for one unit spacing (meter/foot). the J. Only available with no skin effect. Only available with skin effect. ♦ Phase no ♦ RESIS: Phase number. If Auto bundling checked: ♦ Separ: ♦ Alpha: Distance between conductors in a bundle (cm or inch) Angular position of one of the conductors in a bundle. Vertical bundle height at mid-span (m or foot). Init.1 Typical overhead line spacing for a 400kV double circuit In this simulation. The height h= 2/3* VMid + 1/#*VTower is used in the calculations. Horizontal distance (m or foot) from the center of bundle to a user selectable reference line. ♦ Rout: ♦ Rin: ♦ Horiz: Outer radius (cm or inch) of one conductor Inner radius of one conductor. ♦ VTower: ♦ VMid: Vertical bundle height at tower (m or foot). 0=ground wire (eliminated) Conductor resistance at DC (with skin effect) or at Freq. measured counter-clockwise from the horizontal line.5.4. The model is dependent on frequency with constant transformation matrix. The geometrical and material data for overhead line conductors are specify as below [6].Marti model is used. . (a) : Selection of system type (Line/Cable) (b) Specification of conductor data Figure 5.3: Line/Cable dialog box.3 shows the ATP draw input window for the transmission line/cable. standard data (grounding and frequency) and model data (type of model and frequency) . A cable length of 37 km was used.4 shows the line configuration. Figure 5. (a) Selection of type (Line/Cable).♦ NB: Number of conductors in a bundle. Figure 5. 4.4: Line configuration 5. Figure 5.1.(b) Specification of conductor data. Table 5.2 BCTRAN transformer model In this simulation. The transformer characteristics available from test report of 1000MVA transformer shown in Table 5. BCTRAN transformer matrix modelling represent three phase and three winding transformer.1: Transformer Characteristic Rating Type Core Construction Vector Bolt main Bolt Yoke Vector% Ratio 4 & 5/Y/M 1000MVA 400/275/13 kV (auto) Five Limb Core Yy0 No No 60/60/100 . 5 Power (MVA) 1000 1000(60) 1000(60) Loss (kW) 1764 28677 (1720. Table 5. Table 5. the number of windings. the impedance calculated as shown in equation (4). (5). The BCTRAN data is based on the test report of a 1000 MVA (400/275/13 kV) transformer shown in Table 5. Therefore. the type of core and test frequency.62) 29875 (1792.The transformer has been model in the BCTRAN component of ATP Draw which use an admittance matrix representation of the form [9].8 117.5) The data used in this simulation model include impedances and losses are rate at 1000 MVA (400/275/13kV) [9]. It used autotransformer-winding connection for the primary and secondary winding (HVLV). . Figure 5.2 shows the data of short test factory.2: Transformer short circuit factory data Impedance HV-LV HV-TV LV-TV 15. and (6). [ I ] = [ Y ]* [ V ] and in transient calculations can be represented as [ di/dt ] = [ L ]-1 [ V ] – [ L ]-1 * [ R ][ I ] (10) (9) The elements of the matrix are deriving from open circuit and short circuit test that made in the factory.5 shows the ATPDraw input window consist of the number of phases.2.2 91.1 and Table 5. . Saturation effect has considered by attaching the non-linear inductances. In this simulation.2 (a) Open Circuit Data (b) Short Circuit Data. the average no load loss at rated voltage and frequency was 74.1 and Table 5. the zero sequence data was not available because the model has been set equal to the positive data.5: BCTRAN dialog box of data according to Table 5. Furthermore.012% at 1000MVA base.4kW and average magnetizing current was 0.(a) Open Circuit Data (b) Short Circuit Data Figure 5. 37 35. Table 5. Subroutine SATURATION has designed to do the conversion with some simplifying assumptions.5.09 5531.18 7.85 8.27 58. and low compared slope in the unsaturated region.77 52.3 Non-linear inductance The saturation effect has considered by attaching the non-linear inductance (Type-98) characteristic externally in form of a non-linear inductive element branch.92 66.04 Flux Linkage (Wb-Turn) 48.02 65.02 55. .4.35 19.48 222.40 78.6 shows the magnetic core characteristics curve that been represented by peak flux linkages versus peak currents characteristic by using an internal SATURA – routine.72 Figure 5.3 shows the transformer magnetization curve based on manufacturer’s data. Non-linear can be modelled as a two slope piecewise linear inductances with accuracy.52 61. which is almost linear.77 65.3: Transformer magnetizing characteristic Current (A) 7. The slope in the saturated region above the knee reflects the air core inductance. [9] Table 5. it has to observed and recorded before and after the open switching operation (after 0.5 Result of simulation for 400kV double circuit configuration The sinusoidal AC supply peak voltage was 326.6: The saturation curve for nonlinear inductor 5. Upon disconnected TR1.25 . 5. X420 and T10 disconnected after 0. for this work only adopted linear branch capacitor and resistor and focusing on the nonlinear inductance.4 Resistor and Capacitor Model There are various types of resistors and capacitors available in the ATP library.Figure 5. X103 is closed and circuit breaker X303.4. This can vary from linear to non-linear and capacitor braches. In order to get the output voltage. However.6 kV with the 50 Hz frequency.25 second. 4 shows the simulation results for 400kV double circuit configuration.7: The output voltage waveform at TR1 terminal. Table 5.R phase Figure 5.4: Simulation Result for 400kV double circuit configuration Peak Voltage at Transformer Phase Before Red Yellow Blue 181 183 180 (kV) After 313 308 307 Before 4 4 3 Peak Current at Transformer (A) After 254 243 246 Figure 5.Y Phase .8: The output voltage waveform at TR1 terminal . Table 5.12 show the output voltage and current waveform for red. Figure 5.second). yellow. and blue phase.7 to 5. R Phase Figure 5.11: The output current waveform at TR1 terminal –Y Phase .Figure 5.10: The output current waveform at TR1 terminal.9: The output voltage waveform at TR1 terminal -B Phase Figure 5. 13 shows the variation of magnetization curve. Once the data collected.1 Simulation model This simulation is to investigate the effect of changing the saturation curve of the nonlinear inductance.Figure 5. 5.6 Simulation by changing the magnetization characteristic of transformer 5. Figure 5. It obtains by varying the curve 1.6.12: The output current waveform at TR1 terminal-B Phase It can be concluded that the fundamental mode ferroresonance waveform have been produced by simulation at specific time.25 second. . it will gather into the simulation. After 0. the power transformer voltage supposes to be zero but some how the voltage or current still maintain. 13: Variation of magnetization curve Table 5.85 8.02 65.77 52.04 Flux Linkage (Wb-Turn) 48.92 66.09 5531.6 to 5.48 222.40 78.Fluxlinked Wb-T 3 2 1 4 Current (A) Figure 5.35 19.72 . Table 5.5: Transformer magnetizing characteristic-curve 1 Current (A) 7.8 show the value of transformer magnetizing characteristics curve 1 to curve 4.52 61.37 35.02 55.77 65.27 58.18 7. 00 Flux Linkage (Wb-Turn) 48.35 19.02 55.77 52.40 78.18 7.48 222.35 19.73 .48 222.92 68.09 4200.02 65.77 52.02 55.09 3000.52 61.27 58.7: Transformer magnetizing characteristic-curve 3 Current (A) 7.85 8.02 65.72 Table 5.6: Transformer magnetizing characteristic-curve 2 Current (A) 7.27 58.00 Flux Linkage (Wb-Turn) 48.Table 5.85 8.77 65.40 78.92 75.37 35.77 65.52 61.37 35.18 7. 00 5531.77 52.89 Figure 5.00 90.14: The saturation curve 1 .Table 5.02 55.04 Flux Linkage (Wb-Turn) 48.00 20.77 61.52 60.09 61.00 50.27 58. Figure 5.14 to 5.8: Transformer magnetizing characteristic-curve 4 Current (A) 7.18 10.17 shows the saturation curve plotted using peak flux linkages as a function of peak currents for nonlinear inductor Type 98.00 30. 15: The saturation curve 2 Figure 5.16: The saturation curve 3 .Figure 5. 5.2.6. .9 shows the effect of using the saturation curve 1. Upon disconnected TR1.6 kV. The output voltage and current waveform of the red phase shown in Figure 5.18 and 5.25 second.17: The saturation curve 4 5. In this simulation.2 Simulation result The simulation was carried out using different magnetization saturation curves. The sinusoidal AC supply peak voltage is 326.Figure 5.1 Simulation result for curve 1 Table 5. X420 and T10 disconnected after 0. X103 is closed and circuit breaker X303. four saturation curves have been presented.19 but the ferroresonance still occurs.6. 9: The effect of using the saturation curve 1 Peak Voltage at Curve Transformer (kV) Before R Y B 181 183 180 After 313 308 307 Before 4 4 3 Peak Current at Transformer (A) After 254 243 246 Yes Yes Yes Ferroresonance occurrence Figure 5.Table 5.18: The output voltage waveform at TR1 terminal– R phase Figure 5.19: The output current waveform at TR1 terminal– R Phase . 54 Peak Current at Transformer (A) After 225 204 203 Yes Yes Yes Ferroresonance occurrence Figure 5. The output voltage and current waveform of the Red Phase had shown in Figure 5. Table 5.20 and 5.6.10 show the effect of using the saturation curve 2.21 but the ferroresonance still occurs.5.10: The effect of using the magnetization curve 2 Peak Voltage at Curve Transformer (kV) Before R Y B 181 182 178 After 208 253 297 Before 2.48 3.2.34 2.2 Simulation result for curve 2 Table 5.20: The output voltage waveform at TR1 terminal.R Phase . 21: The output current waveform at TR1 terminal– R Phase 5.23 but the ferroresonance still occurs.3 Simulation result for curve 3 Table 5.6.34 0. Table 5.Figure 5.22 and 5.11 shows the effect of using the saturation curve 3. The output voltage and current waveform of the Red Phase shown in Figures 5.74 125 Yes Yes Yes Ferroresonance occurrence .013 Peak Current at Transformer (A) After 188 51.11: The effect of using the magnetization curve3 Peak Voltage at Phase Transformer (kV) Before R Y B 181 183 178 After 294 173 296 Before 4 3.2. 22: The output voltage waveform at TR1 terminal– R Phase Figure 5. .4 Simulation result for curve 4 Table 5.12 show the effect of using the saturation curve 4. Figure 5.23 and 5.Figure 5.25 show the voltage output is reduced after 0.2.25 second but the current output shows that ferroresonance still occurs.23: The output current waveform at TR1 terminal – R Phase 5.6. 12: The effect of using magnetization curve 4 Peak Voltage at Phase Transformer (kV) Before R Y B 181 182 178 After 60 10 62 Before 6.7 8.24: The output voltage waveform at TR1 terminal – R Phase Figure 5.Table 5.1 0.24: The output current waveform at TR1 terminal–R Phase .11 Peak Current at Transformer (A) After 165 135 156 Yes Yes Yes Ferroresonance occurrence Figure 5. It can conclude that the fundamental mode ferroresonance waveforms have presented.7 Mitigation Techniques From the previous studies. In this simulation. there are a number mitigation method(s) been used in order to dampen the occurrences of ferroresonance in the power system. The above results show that changes in the magnetization curves of the transformer do not actually prevent ferroresonance from occurring. This is due to the already highly non-linear characteristic of the iron. . mitigation techniques were analysed based on the previous studies by adding resistor at secondary side of the power transformer. 5. A Figure 5.26: Simulated power transformer circuit by adding loading resistor at secondary side (A) . 13 shows the effect of adding the resistor on secondary side for the case of red phase.1 Simulation model Figure 5.5.7.27 to 5. 5. Figure 5. X420 and T10 disconnected after 0. Table 5.38 show the output waveform when a 30k Ω loading resistance was used.25 second). Figure 5. Upon disconnection of TR1.7.2 Simulation results It suggested that loading resistances values from 10 Ω until 30k Ω could be use to prevent ferroresonance.33 to 5. X103 is closed and circuit breakers X303.25 second. In order to get the output voltage it must be observe and recorded before and after switching operation (after 0.32 show the output waveform when a 1kΩ loading resistance was used.26 shows the simulated power transformer circuit by adding resistor at secondary side. . 7 0 241 282 No No No No No No No No No No Yes Yes Ferroresonance occurence Figure 5.9 1.2k 2k 4k 6k 10k 20k 30k 143 174 177 124 125 125 125 125 125 125 124 125 Peak Voltage at Transformer (kV) After 0 0 0 0 0 0 5.7 11 160 224 Before 438 345 838 117 59 49 29 15 10 6 2. R Phase .1 1 7.8 0.Table 5:13: The effect by adding resistor on secondary side .27: The output voltage waveform by adding R=1k ohm resistance.R Phase Resistor Value (Ω ) Before 10 50 100 500 1k 1.1 0.97 Peak Current at Transformer (A) After 0 0 0 0 0 0 0. B Phase Figure 5.Figure 5.28: The output voltage waveform by adding R=1kohm resistance .29: The output voltage waveform by adding R=1k ohm resistance.30: The output current waveform by adding R=1k ohm resistance. R Phase .Y Phase Figure 5. Y Phase Figure 5.33: The output voltage waveform by adding R= 30k Ohm resistance. B Phase Figure 5. R Phase .Figure 5.32: The output current waveform by adding R=1k Ohm resistance.31: The output current waveform by adding R=1k Ohm resistance. Figure 5. Y Phase Figure 5. R Phase .36: The output current waveform by adding R= 30k Ohm resistance. B Phase Figure 5.34: The output voltage waveform by adding R= 30k Ohm resistance.35: The output voltage waveform by adding R= 30k Ohm resistance. 38: The output current waveform by adding R= 30k Ohm resistance. it fails to prevent ferroresonance from occurring. . Y Phase Figure 5. If the higher resistances are used (more than 10k Ω). B Phase It can be concluded that the fundamental mode ferroresonance waveform have been produced by simulation at specific time.Figure 5. This method is shown to able to absorb energy during ferroresonace conditions and therefore damp out the phenomenon. The suggested loading resistance is ranging from 10 Ω to 10k Ω. Loading resistance acts as damping elements that can prevent ferroresonance from occurring.37: The output current waveform by adding R= 30k Ohm resistance. the parallel circuit should be switch off. It seen that the variation of the magnetization curve does not affect the ferroresonance.CHAPTER 6 CONCLUSION AND RECOMMENDATIONS 6.1 Conclusion The simulation of ferroresonance in 400kV double circuit substation has been presented in this work. This is due the highly non-linear characteristic of the iron. Both instrument and power transformers can be subject to ferroresonance. An actual magnetization curves may have to be obtaining from either the manufacturer or a field test to achieve the reliable ATP simulation. The effects of changing the magnetization characteristics of the power transformer were also presented. It is established when one side of the double circuit transmission line connected to the transformer is de-energized but remain energized because of coupling from the parallel circuit. To avoid ferroresonance. Failure to detect a ferroresonance condition can lead to overheating the transformer. . Carefully evaluate the location and value of resistors added to counteract ferroresonance. when the resistor is added between neutral and ground it will reduce the fault current.A number of mitigation methods have been proposed in order to dampen the occurrences of ferroresonance in power system. the proper value for the resistive load may be connected either in the secondary or the neutral to ground of the transformer. Then. . Nevertheless. It depend on the transformer connection of the suspected circuit.2 Recommendations It is suggested to determine the values of R. The R. L and C elements in the system will give inductive and capacitive reactances in series or parallel with the source of the voltage need to be identified. Therefore the value of resistance must select to the specific ferroresonance damping requirements. if not interfering with circuit fault protection system. The appropriate values of resistance to eliminate ferroresonance have been presented in this work. 6. identify the required resistance value to eliminate ferroresonance. the actual magnetization curves have to be obtained from either the manufacturer or field test. In addition. L and C for the suspected circuit that will support ferroresonance occur. Morched. K. Lee. A. Mork. Zia Emin. A. Mork. 8. A. C. A.Paper 006. M. Ferroresonance Experience in UK: Simulations and Measurements IPST '01 . Ferroresonance Destroy Transformers (1191). Peter Daay.J. Mark Osborne and Paul Jarman. Z.5. Bruce A. Examples of Ferroresonance in a High Voltage Power System Member.2 2. S. A. Martinez. Wang.Rio de Janeiro. Hans.. Electrical Electrical Engineering Tutorials: Ferroresonance. New Features in ATPDraw Version 3. Classification and Characteristic 20 March. Martinez-Velasco. Parniani. Brazil. Shirmohammadi. Charalambos Charalambous. International Conference on Power Systens Transients. A. R. Yu Kwong Tong.F. Chaudhary. IEE 3. B. J. Transformer Modeling for Low Frequency Transient.Introduction.The State of the Art IPST 2003.(2001). K. ATPDRAW version 3. Jacobson. Hassan.K. S. I.(2000) Modeling and Analysis Guidelines for Slow . Laszlo Pikler.List of References 1. B. June 2428. Hokaidan. M. (2002) 6. Woodford. Bruce A. A.5 for Windows 9x/NT/2000/XP Users' Manual (2002) 7. Jie Li. Juan A.(2007)Validation of a Power Transformer Model for Ferroresonance with System Tests on a 400 kV Circuit. D. J. W.R Iravani. IPST 10. 2007 5. David A.D. N. 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