STUDY ON THE NON LINEAR CHARACTERISTIC OF POWER TRANSFORMER AND THEIR EFFECT FERRORESONANCE SALIZAWATI BT HJ. SHAMSUDDIN

Transcription

1 STUDY ON THE NON LINEAR CHARACTERISTIC OF POWER TRANSFORMER AND THEIR EFFECT FERRORESONANCE SALIZAWATI 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

2 To my beloved mother and father

3 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.

4 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.

5 ABSTRAK Feroresonan berlaku ke atas sistem kuasa elektrik yang boleh menyebabkan kerosakan pada sistem tersebut. Contohnya kejadian pemanasan lebihan pada pengubah voltan ataupun voltan lampau pada pengubah kuasa. Kajian ini melibatkan kerja simulasi bagi pelbagai keadaan yang boleh berlakunya feroresonan pada pencawang elektrik EHV. Program ATP-EMTP merupakan satu program simulasi yang berkeupayaan untuk menghasilkan pelbagai model komponen sistem kuasa dan seterusnya melakukan simulasi ke atas sistem. Kajian simulasi ini melaporkan kesan sifat tak lelurus keluli pengubah kuasa terhadap feroresanan dan kaedah untuk mengelakkan berlakunya feroresonan. Justeru itu, kerosakan dan kerugian komponen dapat dihindarkan.

6 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 ii iii iv v vi vii xi xii xv xvi 1 INTRODUCTION Introduction Objectives Scope of Work Project Flow Chart Organisation of Thesis 4 2 LITERATURE REVIEW Introduction Case Study by Zia Emin and Yu Kwong Tong: 5 Ferroresonance Experience in UK: Simulation and Measurement

7 2.2.1 Objective Voltage Transformer Comparison Field Measurement and Simulation 7 Result Single Phase Traction Supply Transformer Comparison Field Measurement and Simulation 9 Result Conclusion Case Study by YK Tong: NGC Experience on Ferroresonance 10 In Power Transformer and Voltage on HV Transmission System Introduction Power Transformers Voltage Transformer Measurement to Predict or Prevent Ferroresonance Case Study by David A.N. Jacobson: Example of 12 Ferroresonance in High Voltage Power System Objective Wound Potential Transformer-Circuit Breaker 12 Grading Capacitor Description of Disturbance Simulation Result Mitigation Options Transformer Circuit Breaker Grading Capacitor Description of Disturbance Simulation Result Mitigation Options Open Delta Potential Transformer Description of Disturbance Simulation Result Mitigation Options Conclusion Summary 17

8 3 FERRORESONANCE Basic of Ferroresonance Main Characteristic Sensitivity to System Parameter Value Sensitivity to Initial Condition Classification of Ferroresonance Mode Fundamental Mode Sub harmonic Mode Quasi Periodic Mode Chaotic Mode Power System Ferroresonance Symptoms of Ferroresonance Audible Noise Overheating Arrestor and Surge Protector Failure Flicker Cable Switching 30 4 METHDOLOGY System Modeling ATP-EMTP Simulation Selected model and Validation Resistor and Capacitor Model Overhead Transmission Lines Transformer Model Nonlinear and frequency Dependent Parameter Modelling of Iron Core Modelling of Eddy Currents Effects 37 5 SIMULATION: 400kV DOUBLE CIRCUIT 38 CONFIGURATION 5.1 Introduction Simulation Procedures Circuit Description Simulation Model 40

9 5.4.1 Typical Overhead Line Spacing for 400kV BCTRAN Transformer Model Non-linear Inductance Resistor and Capacitor Model Result of Simulation for 400kV Double Circuit Simulation by Changing the Magnetization Characteristic Simulation Model Simulation Result Simulation Result for Curve Simulation Result for Curve Simulation Result for Curve Simulation Result for Curve Mitigation Technique Simulation Model Simulation Result 64 6 CONCLUSION AND RECOMMENDATION Conclusion Recommendation 71 REFERENCES 72

10 LIST OF TABLES TABLE NO. TITLE PAGE 5.1 Transformer Characteristic Transformer Short Circuit Factory Data Transformer Magnetizing Characteristic Simulation Result for 400kV Double Circuit Configuration Transformer Magnetizing Characteristic Curve Transformer Magnetizing Characteristic Curve Transformer Magnetizing Characteristic Curve Transformer Magnetizing Characteristic Curve The Effect of Using the Saturation -Curve The Effect of Using the Saturation -Curve The Effect of Using the Saturation -Curve The Effect of Using the Saturation -Curve The Effect of Adding Resistor on Secondary Side 65

11 LIST OF FIGURES FIGURE NO. TITLE PAGE 1.1 Project Flow Chart 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 Component 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 23 Phenomenon 3.5 Sensitivity Initial Condition Diagrams Illustrating the Fundamental Mode 25 of Ferroresonance

12 3.7 Diagrams Illustrating the Subharmonic Mode of 26 Ferroresonance 3.8 Diagrams Illustrating the Quasi Periodic Mode of 26 Ferroresonance 3.9 Diagrams Illustrating the Chaotic Mode of Ferroresonance A Single Line Diagram of the Brinsworth/Thorpe Marsh 40 Circuit 5.2 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 - R Phase The Output Current Waveform at TR1 Terminal- Y Phase The Output Current Waveform at TR1 Terminal- B Phase Variation of Magnetization Curve The Saturation Curve The Saturation Curve The Saturation Curve The Saturation Curve 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 61

13 5.25 The Output Current Waveform at TR1 Terminal R Phase Simulated Power Transformer Circuit by Adding Loading 63 Resistor at Secondary Side 5.27 The Output Voltage Waveform by Adding R=1k Ohm 65 Resistance R Phase 5.28 The Output Voltage Waveform by Adding R=1k Ohm 66 Resistance Y Phase 5.29 The Output Voltage Waveform by Adding R=1k Ohm 66 Resistance B Phase 5.30 The Output Current Waveform by Adding R=1k Ohm 66 Resistance R Phase 5.31 The Output Current Waveform by Adding R=1k Ohm 67 Resistance Y Phase 5.32 The Output Current Waveform by Adding R=1k Ohm 67 Resistance B Phase 5.33 The Output Voltage Waveform by Adding R= 30k Ohm 67 Resistance R Phase 5.34 The Output Voltage Waveform by Adding R= 30k Ohm 68 Resistance Y Phase 5.35 The Output Voltage Waveform by Adding R= 30k Ohm 68 Resistance B Phase 5.36 The Output Current Waveform by Adding R= 30k Ohm 68 Resistance R Phase 5.37 The Output Current Waveform by Adding R= 30k Ohm 69 Resistance Y Phase 5.38 The Output Current Waveform by Adding R= 30k Ohm 69 Resistance B Phase

14 LIST OF ABBREVIATIONS DC - Direct Current AC - Alternating Current ATP - Alternative Transient Program EMTP - Electro Magnetic Transient Program PT - Potential Transformer GB - Grading Bank HV - High Voltage LV - Low Voltage

15 LIST OF SYMBOLS X L - Inductance reactance X C - Inductance capacitance C - Capacitance L - Inductance C - Capacitive R - Resistance V L - Load Voltage E - Voltage source f - Frequency w - Frequency Z - Impedance

16 CHAPTER 1 INTRODUCTION 1.1 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. With a poor power quality, an electrical device (or load) may malfunction, fail prematurely or not operate at all. There are many ways in which electric power can be of poor quality and many more causes of such poor power quality. As a general statement, any deviation from normal of a voltage source (either DC or AC) categorized as a power quality issue. Power quality issues can be highspeed events such as voltage impulses / transients, high frequency noise, wave shape faults, voltage swells and sags and total power loss. Power quality issues will affect each type of electrical equipment differently. By analyzing the electrical power and evaluating the equipment or load, we can determine if a power quality problem exists.

17 Power quality problems manifest themselves in variations in the voltage has been obtained. This variation can be in the form of transients due to switching or lightning strikes, sags or swells in the amplitude of the voltage, a complete interruption in the supply, or harmonic distortion caused by non-linear loads in the system which may likely lead to the occurrence of ferroresonance. 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, features, components and arrangements of the substation power system. An alternative Transient Program- 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. 1.3 Scope of Work The main scope of this project is to simulate the various conditions of ferroresonance, which include: i. To prove or otherwise that ferroresonance can occur at 400kV double circuit substation; ii. To identify the effect of magnetization characteristic of power transformer on ferroresonance;

18 iii. To identify method to minimize the impacts of ferroresonance on power transformers. 1.4 Project Flow Chart To solve the problem, one has to first study the problem and come up with the process flow chart, which will guide the simulator throughout the project. 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 1.1. 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

19 Simulation and Analysis Recommendation and Conclusion Figure 1.1: Project Flow Chart 1.5 Organisation of Thesis Chapter 2 illustrates previous work done related on ferroresonance phenomenon in voltage transformer and power transformer. Besides, it is also includes some techniques for avoiding or mitigating ferroresonance. Chapter 3 describes the basics of ferroresonance, characteristics and types of ferroresonance. Chapter 4 describes the methods that are used for the simulation. Therefore, all information related the simulation is explained in detail with the operation. Chapter 5 presents the circuits that were used in the simulation and explains how the simulations techniques are implemented. Lastly, chapter 6 describes the conclusion and recommendation that is related to the project done.

20 CHAPTER 2 LITERATURE REVIEW 2.1 Introduction This chapter reviews the previous related work done on ferroresonance. Three case studies describe ferroresonance phenomena on voltage transformers and power transformers in detail. 2.2 Case Study by Zia Emin and Yu Kwong Tong: Ferroresonance Experience in UK: Simulations and Measurements [1] Objective The objective of this paper is model and analysis ferroresonance on the system by using program ATP-EMTP. This is to compares the result with system measurement to identify the crucial factor in ferroresonance. The case studies related to ferroresonance in wound voltage and single-phase supply transformers.

21 2.2.2 Voltage Transformer Throughout voltage transformer ferroresonance oscillation happens among the nonlinear inductance and capacitance that remaining connected to the voltage transformer. In this case study, energy attached to the nonlinear inductance of voltage transformer by the open circuit grading capacitance to maintain resonance. Figure 2.1 shows the single line diagram of voltage transformer. Ferroresonance can happen upon opening of disconnector 3 with circuit breaker open and either disconnector 1 or 2 closed. Alternatively it can also occur upon closure of both disconnector 1 or 2 with circuit breaker and disconnector 3 open. Figure 2.1: Single Line Diagram of Voltage Transformer The system arrangement in Figure 2.1 can be reduced to equivalent circuit in Figure 2.2. Figure 2.2: Reduced equivalent ferroresonant circuit

22 E is the rms supply phase voltage, C series is the circuit breaker grading capacitance and C shunt is the total phase-to-earth capacitance of the arrangement. The resistor R represents a voltage transformer core loss that has been found to be an important factor in the initiation of ferroresonance Comparison Field Measurement and Simulation Result The measured and simulated voltage transformer at secondary voltages has been compared. The measured was obtained after opening the disconnector 3 in Figure 2.1. As it observed that the voltage transformer haven t get into ferroresonance. Although the simulation does not match the measured test waveform exactly, its general appearance is a very close fit. 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. 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. The shapes of measured and simulated waveforms in Figure 2.3 and 2.4 are a good match. Figure 2.3: Measurement Output Voltage

23 Figure 2.4: Digital Simulation Output Voltage Single Phase Traction Supply Transformer Figure 2.3 shows the single line diagram of a traction supply transformer arrangement. The single phase transformer is fed from two phases of the three-phase system. Ferroresonance happens upon opening of circuit breakers 1 and 2 to deenergize the line and the transformer. Because energy is coupled to the de-energized network from the adjacent live parallel circuit through the inter-circuit coupling capacitance, C series in the equivalent ferroresonant circuit. The equivalent C shunt in the ferroresonance circuit is the phase-to-earth capacitance of the line and transformer winding and bushing capacitance. The single line diagram in figure 2.5 can reduced to equivalent circuit in Figure 2.2. Figure 2.5: Single Line Diagram of Traction Supply Transformer

24 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 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.

25 2.3 Case Study by YK Tong: NGC Experience on Ferro resonance in Power Transformer and Voltage Transformer on HV Transmission Systems [2] 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 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 Voltage Transformer Ferroresonance happen upon de-energization of the wound voltage transformer, an oscillation occurs between the voltage transformer inductance and

26 the capacitance-to-earth of any system that remaining connected to the voltage transformer 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.

27 2.4 Case Study by David A.N. Jacobson: Example of Ferroresonance in High Voltage Power System [3] 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 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 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.

28 Figure 2.8: Single line diagram of wound power transformer circuit Simulation Result The simulation result substantiation that ferroresonance was occurred at A2 bus. 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 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 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.

29 2.4.3 Transformer Circuit Breker Grading Capacitor Description of Disturbance Breaker failed to latch while energize the induction motor at Dorsey Converter Station. Ferroresonance over voltage occurred on B2 bus, where noise level from SST1 that higher than normal. Figure 2.9 shows a single diagram of main circuit component and Figure 2.10 shows a station service transformer. Figure 2.9: A single diagram of main circuit component Figure 2.10: A single diagram of station service transformer Simulation result Figure 2.11 exhibits simulation result of fundamental frequency mode of ferroresonance due to high distortion and over voltage near 1.5p.u.

30 Figure 2.11: The output voltage waveform of bus voltage Mitigation Option The mitigation option of this case is being connected 200 ohm loading resistors at secondary service transformer SST1 and SST Open Delta Potential Transformer Ferroresonance can happen during energization of the unloaded step down transformer and interruption of a single line- to-ground fault on the low side of the transformer. Three modes of ferroresonance can be observed in an open delta potential transformer. Low values of capacitance may cause third harmonic mode. Medium values of capacitance may cause fundamental model of feroresonance and higher values of capacitance could cause subharmonic modes of ferroresonance.

31 Description of Disturbance Figure 2.12 shows the single diagram of open delta potential transformer. The purpose of grounding bank is to provide zero sequence current during single line-to-ground for ground fault protection relay to operate. Generally, the open delta potential transformer as a backup to ground bank to taken out for service for maintenance. Figure 2.12: Typical station layout Simulation result Figure 2.14(a) shows the unbalanced fundamental mode of ferroresonance. Figure 2.14(b) shows the voltage output by connected loading resistor 83 ohm at secondary side and Figure 2.14(c) explains the voltage output waveform with connected grading bank. Fig. 2.14: Example of output voltages, a) no mitigation, b) 83 ohm resistor connected and c) Ground bank connected.

32 Mitigation Options There are several possible solutions to prevent ferroresonance: i. Replacing potential transformer with capacitive coupled voltage transformer. ii. Install potential transformer that are rated for line-to-line system voltage. iii. Install damping resistor in the broken delta Conclusion Three examples of circuit arrangements that can practice ferroresonance have been presented. The impact of ferroresonance can vary from relay or control disoperation to damage the equipment. 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. Open delta voltage transformer ferroresonance can be prevented by adding loading resistor across the open delta or close grounding transformer banks. 2.5 Summary The first case study, it has been proven that ferroresonance in voltage transformers and power transformers occurs due to nonlinear and dynamics in the system. The case study analyses the performance of ferroresonance in the system by comparing measurement data and the ATP-EMTP simulation results. The study shows the measured and simulated voltages agree very well.

33 The second case study describes the ferroresonance phenomena when connecting power transformers with overhead lines, or when connecting voltage transformers with isolated sections of busbars. 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, 162/3Hz and 10Hz. The mitigation methods of ferroresonance are by disconnecting the power transformer from the feeder, and for the case voltage transformers, 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. Common elements in a ferroresonance circuit are a nonlinear saturable inductor, capacitors, and a voltage source. The circuit also has a low-loss characteristic. Prevention methods are by loading resistors and replacement of the voltage transformer with a capacitor transformer.

34 CHAPTER 3 FERRORESONANCE 3.1 Introduction This chapter discusses the theories, characteristics, types and symptoms of ferroresonance. 3.2 Basics of Ferroresonance [4] Ferroresonance does not occur regularly and it is very hard to analyze. Any response to a sudden change in the system may jump out of a steady state into ferroresonance condition. Therefore, the basics of ferroresonance must be identifying. The simple RLC circuit in Figure 3.1 can be used as an aid in explaining resonance and hence ferroresonance.

35 Figure 3.1: Resonance in RLC Circuit This linear circuit is resonating when at some given source of frequency; the inductive reactance (X L ) and capacitive reactance (X C ) cancel each other out. These impedance values be forecast and they change with the 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), where w is the frequency of the source. X C = 1/ jwc (1) X L = jwl (2) I = V / (R + X L - X C ) *V / R (3) The current in equation (3), depends on the resistance (R). If this resistance is small, then the current can become very large in the RLC circuit. The size of current during resonance is equation (3).

36 If the inductor in Figure (2) replaced by an iron cored non-linear inductor, the exact values of voltage and current cannot be predicted as in a linear model. Equation (3) will not indicate the size of current produced. The inductance becomes nonlinear due to saturation of flux in the iron core. The understanding that ferromagnetic material saturates is very important. Ferromagnetic material has a property of causing an increase to the magnetic flux density, and therefore magnetic induction. Figure 3.2: Magnetization Curve As the current is increased, 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. This has been identified as the saturation point. Figure 3.2 illustrates the relationship between magnetic flux density and current. As the current increases in a ferromagnetic coil past the saturation point, the inductance of the coil changes very quickly. This allows the current to take on very unsafe high values. These high currents make damage on the transformer. Most transformers have cores made from ferromagnetic material. This is why ferroresonance is a concern for transformer operation. When ferroresonance happens, it can be identify by certain distinct characteristics. In ferroresonance, because the steel core is driven into saturation so an audible noise occurs. As the core goes into a high flux density, magnetostriction forces cause movement in the core laminations. This sound is different from the normal hum of a transformer. Ferroresonance can cause high

37 over-voltages and currents. This can cause electrical damage to both the primary and secondary circuits of a transformer. The heating cause by over-currents may cause permanent damage to transformer insulation. Eventually, the transformer could fail completely. 3.3 Main Characteristic [ 4 ] Figure 3.3 is a basic series ferroresonance circuit and the curve obtained from of the circuit shown in figure 3.5 and 3.6. These curves, illustrate the following characteristics of ferroresonance: i. Sensitivity to system parameter values, jump phenomenon ii. Sensitivity to initial condition Figure 3.3: Basic series ferroresonance circuit parameter Sensitivity to system parameter values, jump phenomenon Figure 3.4 illustrates the peak voltage V L at the terminals of the nonlinear inductance as a function peak amplitude E of the sinusoidal voltage source.

38 Figure 3.4: Sensitivity to system parameters and the jump phenomenon By gradually building up peak amplitude E from zero, the curve shows that there are three different types of behaviour according to the value or E, as well as the jump phenomenon: i. For E = E 1, the solution (M 1n ) is unique, and this corresponds to the normal state (in the linear ), ii. For E = E 2, there are three solutions (M 2n, M 2i, M 2f ), two of which are stable (M 2n and M 2f ). M 2n corresponds to the normal state, whereas M 2f corresponds to the ferroresonant state. The dotted part of the curve corresponds to unstable states. iii. For E = E 2, the voltage V L suddenly moves from the point M 2 to the point M 2 (the jump phenomenon). The point M 2 is known as a limit point, iv. For E = E 3, only the ferroresonance state (M 3f ) is possible. v. When the value of E decreases from E 3, the solution suddenly moves from the point M 1 (second limit point) to the point M 1. vi. 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.

39 3.3.2 Sensitivity to initial conditions Whether M 2n or M 2f obtained depends on the initial conditions. Figure 3.5 illustrates the trajectories of the transient of pairs (Ф, V C ) as a function of time for different initial conditions (M 01 and M 02 ). Curve C describes a boundary. If the initial conditions (residual flux, voltage at capacitor terminals) are on one side of the boundary, the solution converges to M 2n. If the initial conditions are on the other side, the solution converges to M 2f. As the point M 2i belongs to the boundary, the steady state effectively reached around this point is extremely sensitive to the initial conditions. Figure 3.5: Sensitivity initial condition

40 3.4 Classification of Ferroresonance Modes [4] The type of ferroresonance can identify by the spectrum of the current and voltage signals. The characteristics of each type of ferroresonance listed below [4]: i. Fundamental mode ii. Subharmonic mode iii. Quasi periodic mode iv. Chaotic mode Fundamental mode Figure 3.6 shows the diagrams to explain fundamental mode. The voltages and currents are periodic with a period T equal to the system period, and can contain a varying rate of harmonics. The signal spectrum is a discontinuous spectrum made up of the fundamental f 0 of the power system and of its harmonics (2 f0, 3 f0...). The stroboscopic image reduced to a point far removed from the point representing the normal state. Figure 3.6: Diagrams illustrating the fundamental mode of ferroresonance

41 3.4.2 Subharmonic mode Figure 3.7 shows the diagrams to explain the sub harmonic modes. The signals are periodic with a period nt which is a multiple of the source period. This state known as sub harmonic n or harmonic 1/n. Sub harmonic ferroresonant states are normally of odd order. The spectrum presents a fundamental equal to f 0 /n (where f 0 is the source frequency and n is an integer) and its harmonics (frequency f 0 is thus part of the spectrum) Figure 3.7: Diagrams illustrating the sub harmonic mode of ferroresonance Quasi-periodic mode Figure 3.8 shows the diagram for the quasi-periodic mode. This mode is not periodic. The spectrum is a discontinuous spectrum whose frequencies are expressed in the form: nf 1 +mf 2 (where n and m are integers and f 1 /f 2 an irrational real number).

42 Figure 3.8: Diagrams illustrating the quasi-periodic mode of ferroresonance Chaotic mode Figure 3.9 shows the explanation for the chaotic mode. The corresponding spectrum continuous, i.e. it not cancelled for any frequency. The stroboscopic image made up of completely separate points occupying an area in plane v, i known as the strange attractor. Figure 3.9: Diagrams illustrating the chaotic mode of ferroresonance Therefore, it can conclude that the ferroresonance is a complex phenomenon in which there are several steady states for a given circuit, the appearance of these states is highly sensitive to system parameter values and the appearance of these states is highly sensitive to initial conditions.

43 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. The modes most commonly encountered are the fundamental and subharmonic ones. Abnormal rates of harmonics, over voltages/ currents, either as stable oscillation or as transients caused by ferroresonance, often represent a risk for electrical equipment. Steady state ferroresonance sustained by the energy supplied by the power system. 3.5 Power System Ferroresonance Ferroresonance in a power system can result in any of the following, alone or in combination [5]: i. high sustained overvoltages, both phase to phase and phase to ground, ii. high sustained overcurrents, iii. high sustained levels of distortion to the current and voltage waveforms, iv. transformer heating and excessively loud noise, v. electrical equipment damage (thermal or due to insulation breakdown) vi. apparent mis-operation of protective devices, There are four elements for ferroresonance to occur [5]: i. A sinusoidal voltage source -A power system generator will do quite nicely. ii. Ferromagnetic inductances -These can be power transformers or voltage transformers. iii. Capacitance -This can come from installed power system capacitors, the capacitance to ground of transmission lines, the large capacitance of

44 underground cable, or the capacitance to ground of an ungrounded system. iv. Low resistance - This can be lightly loaded power system equipment, low short circuit power source, or low circuit losses. 3.6 Symptoms of Ferroresonance There are several modes of ferroresonance with varying physical and electrical displays. Some have very high voltages and currents while others have voltages close to normal. In this section, it will reveal the symptoms of ferroresonance Audible Noise Audible noise occurs when the steel core driven into saturation. Therefore the core goes into a high flux density, it make due to the magnetostriction of the steel and to the actual movement of the core laminations. Ferroresonance happen when the noise is louder than the normal hum of transformer 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. The effect is bubbling of the paint on the top of the tank. Ferroresonance happen, when it has

45 continued extended to reason overheating of some of the larger internal connections. This may cause damage insulation structures Arrester and Surge Protector Failure The arrester failures related to heating of the arrester block. Normally, the people determine an open fused cut out and just replace the fuse. In the meantime, 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. Under-oil arresters are less vulnerable to the problem because it can dissipate the heat due to the ferroresonance current more quickly. Nowadays surge protectors are common in computers, office equipment and factory machines Flicker Utility customers often face with the problem of a wavering voltage magnitude. For example, the light bulbs will flicker between very bright and dim. In addition, some electronic appliances are reportedly very susceptible to the voltages that result from some types of ferroresonance, the alleged failure mode is unknown Cable Switching The transformers themselves can usually withstand the over voltages without failing. However, it not accepted for stress repeatedly because the forces often shake things loose inside and scrape insulation structures. The cable also in little risk unless its insulation stress reduced by aging or physical harm.

46 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. It because the currents could be much higher than expected and the peak voltages could be high enough to cause reigniting of the arc. Therefore, it may be difficult to clear arcs when pulling cable elbows if ferroresonance in progress. The solution of ferroresonance during cable switching is to always drag the elbows and energize the unit at the primary terminals. These will no external cable capacitance to cause ferroresonance. Only a little internal capacitance and the losses of the transformers are mostly enough to prevent resonance with the small capacitance.

47 CHAPTER 4 METHODOLOGY 4.1 System Modelling The main emphasis is to identify the models of substation components to use in the ferroresonance studies. For each component, the importance of the model parameters will describe and typical values shall be providing. Afterwards, the models will be simulated using ATP-EMTP program and the output is to be analyzed, in particular, whether ferroresonance has occurred or otherwise. 4.2 ATP-EMTP Simulation The ATP is the PC version of the Electromagnetic Transients Program (EMTP). The EMTP is primarily a simulation program of the electric power industry. 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

48 system representation. Generally, this simulation software can use in design of an electrical system or in detecting or predicting an operating problem of a power system. ATP is a universal program system for digital simulation of transient phenomena of electromagnetic as well as electromechanical nature. With this digital program, 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. To represent the electrical response of the transmission system, electrical model of the transmission system apparatus have to be select and validated to gain high accuracy result. 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. In ATP-EMTP simulation, the input and output that usually observed are current, voltage, power and energy. A complete set of representation of a transmission system are made of models for all of the power system components.

49 4.4 Resistor and Capacitor Model There are various types of resistors and capacitors available in the ATP library. This can vary from linear to non-linear and capacitor braches. However, for this work only adopted linear branch capacitor and resistor and focusing on the nonlinear inductance. 4.5 Overhead Transmission Lines [6] Overhead transmission lines model depends on the line length and the highest frequency to be simulated. This simulation, the overhead transmission lines are double circuit 400kV. The length of the overhead line circuit is approximately 37km. It represented by multi-phase models considering the distributed nature of the line parameters due to the range of frequencies involved, phase conductors, shield wires are explicitly modelled between towers, and only a few spans are considered. The line parameters being clarify by line constants, using the tower structure geometry and conductor data as input. 4.6 Transformer models [7] Transformer model can be classified into three types. The first type is matrix representation (BCTRAN model). The second type is saturable transformer component model to multi-phase transformer. Both type of model can be implemented in the ATP-EMTP. The third type is topology-based models [7].

50 Matrix representation (BCTRAN model) will be used to represent single and three phase, two and three winding transformers by considering the number of phases, the number of windings, the type of core ( single phase core, triplex and three phase shell type) and test frequency. The parameter of open circuit and short circuit data can be obtained from factory test data. In addition, clarify the ratings of the line voltage, rated power and type of coupling. Besides that, identify the winding connection such as an autotransformer, wye and delta. If the connection is an auto transformer or wye, the rated voltage is automatically divided by 3 to get the winding voltage. 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 (4) Z*L T = ZL T (5) Z*H T = ZL H.VH. VL + ZH T.VH + ZL T.VL (6) ( VH VL) 2 ( VH VL) ( VH VL) where ZL H, ZL T, ZH T are the impedance (%) values and Z*H L, Z*L T and Z*H T are vales that written to the BCTRAN file. BCTRAN transformer of ATPDraw, 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 transient calculations can represent as;

51 [di/ dt] = [L] -1 [v] [L] -1 [R] [i] (8) For simulation of saturable cores, excitation may be omit from the matrix description and attached externally at the model terminals in the form of non-linear elements Nonlinear and Frequency-Dependant Parameters [7] Some transformer parameters are non-linear and/or frequency dependent due to three key effects: saturation, hysteresis and eddy currents. Saturation and hysteresis are included in the representation of the iron core and introduce distortion in waveforms. Excitation losses are cause by hysteresis and eddy current effects, although in modern transformers they are mostly due to eddy current Modeling of Iron Cores Iron core behaviour is represent by a relationship between the magnetic flux density B and the magnetic field intensity H. Hysteresis loops usually have a negligible influence on the magnitude of the magnetizing current, although hysteresis losses and the residual flux can have a major influence on some transients, such as inrush currents. Magnetic saturation of an iron core is representing by the hysteretic curve, the B H relationship that would be obtain if there were no hysteresis effect in the material. The saturation characteristic can be modelled by a piecewise linear inductance with two slopes, since increasing the number of slopes does not considerably improve the accuracy. In ferroresonance, the detailed of the saturation characteristic is required. The specification of such inductor requires a curve relating the flux linkage, to the current, i. The information usually available is the rms voltage as a function of the rms current.

52 Modelling of Eddy Current Effects Eddy current effects, occur at the same time in a loaded transformer that result in a non-uniform distribution of current in the conductors, 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. These circuits must be of infinite order exactly to reproduce the impedance at all frequencies. However, a computationally efficient circuit can be deriving by fitting only at certain pre-established frequencies. A series model of order equal or less than 2 is adequate for low-frequency transients. A change in the magnetic field induces also eddy currents in the iron. Because of this, the flux density will be lower than that given by the normal magnetization curve. As frequency changes, flux distribution in the iron core lamination also changes. For high frequencies, the flux is confine to a thin layer close to the lamination surface, whose thickness de-creases as the frequency increases. This indicates that inductances representing iron path magnetization and resistances representing eddy current losses are frequency dependent. 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. Inductive components of these models represent the magnetizing reactances and have to make non-linear to account for the hysteresis and saturation effects. Since the high frequency components do not contribute appreciably to the flux in the transformer core, it can be assume that only low frequency components are responsible for driving the core into saturation. It can be justifiable to represent as non-linear only the first section of the model, so for low frequency transients an equivalent circuit with order equal or less than two may suffice.

53 CHAPTER 5 SIMULATION OF 400KV DOUBLE CIRCUIT CONFIGURATION 5.1 Introduction In this work, a 400kV/275kV double circuit extra high Voltage substation was chosen for the simulation study. The purpose of the simulation is to determine conditions in which ferroresonance can occurs. If a ferroresonance does occur, then how best can be the ferroresonance be mitigated or its effect minimised. Some of the components data are not available. Hence, in this simulation work, typical data or estimates were used for each case. 5.2 Simulation Procedures In order for the simulation work to be successfully carried out, the following procedures were adopted: (i) Determinations of all parameters such as the overhead transmission line, power transformer saturation characteristics, the power transformer and

54 shunt capacitance, etcetera. Estimate been made in the cases where actual physical values cannot be obtained. (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. (iv) Determine whether ferroresonance occurs or otherwise under the given circuit parameters. (v) Determine the mitigation techniques to prevent ferroresonance from occurring. The first part of the process was to gather information on the system. This involved finding circuit diagrams, information on transformers, capacitors etcetera and finally the nature of the load magnetization characteristic and type of nonlinear inductance. 5.3 Circuit description The 400kV double circuits used in this simulation were taken from Brinsworth / Thorpe Marsh (UK transmission network) design parameter [9]. The purpose is to exhibit of ferroresonance to occurring on the system. The length of the parallel overhead line circuit is approximately 37 km and the feeder has a 1000MVA 400/275/13kV power transformer. Figure 5.1 shows single diagram of the Brinsworth /Thorpe Marsh circuit arrangement.

55 X303 3 x X103 3 X420 x TR1 T10 0 x x Thorpe Marsh 400kV Overhead line 37km TR2 Brinsworth 275kV Figure 5.1: A single line diagram of the Brinsworth/Thorpe Marsh circuit Following is the circuit equipment condition: i. Before switching (Transformer energized) - All disconnector and circuit breaker were close. ii. After switching (Transformer de-energized) -Disconnector X303, circuit breaker X420 and T10 were open. -Disconnector X103 was close. 5.4 Simulation Model The system arrangement in Figure 5.1 can be reduce to equivalent circuit in Figure 5.2. The main components of the network are; i. Typical overhead line spacing for a 400kV double circuit; ii. BCTRAN transformer matrix mode ; iii. Non-linear inductance; iv. Resistor and capacitor.

56 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

57 5.4.1 Typical overhead line spacing for a 400kV double circuit In this simulation, the J.Marti model is used. The model is dependent on frequency with constant transformation matrix. The geometrical and material data for overhead line conductors are specify as below [6]; Phase no Phase number. 0=ground wire (eliminated) RESIS: Conductor resistance at DC (with skin effect) or at Freq. Init. (no skin effect) REACT: The frequency independent reactance for one unit spacing (meter/foot). Only available with no skin effect. Rout: Outer radius (cm or inch) of one conductor Rin: Inner radius of one conductor. Only available with skin effect. Horiz: Horizontal distance (m or foot) from the center of bundle to a user selectable reference line. VTower: Vertical bundle height at tower (m or foot). VMid: Vertical bundle height at mid-span (m or foot). The height h= 2/3* VMid + 1/#*VTower is used in the calculations. If Auto bundling checked: Separ: Distance between conductors in a bundle (cm or inch) Alpha: Angular position of one of the conductors in a bundle, measured counter-clockwise from the horizontal line.

58 NB: Number of conductors in a bundle. Figure 5.3 shows the ATP draw input window for the transmission line/cable. A cable length of 37 km was used. Figure 5.4 shows the line configuration. (a) : Selection of system type (Line/Cable) (b) Specification of conductor data Figure 5.3: Line/Cable dialog box. (a) Selection of type (Line/Cable), standard data (grounding and frequency) and model data (type of model and frequency)

59 (b) Specification of conductor data. Figure 5.4: Line configuration BCTRAN transformer model In this simulation, BCTRAN transformer matrix modelling represent three phase and three winding transformer. The transformer characteristics available from test report of 1000MVA transformer shown in Table 5.1. Table 5.1: Transformer Characteristic Rating 1000MVA Type 400/275/13 kv (auto) Core Construction Five Limb Core Vector Yy0 Bolt main No Bolt Yoke No Vector% Ratio 4 & 5/Y/M 60/60/100

60 The transformer has been model in the BCTRAN component of ATP Draw which use an admittance matrix representation of the form [9]; [ I ] = [ Y ]* [ V ] (9) and in transient calculations can be represented as [ di/dt ] = [ L ] -1 [ V ] [ L ] -1 * [ R ][ I ] (10) The elements of the matrix are deriving from open circuit and short circuit test that made in the factory. Table 5.2 shows the data of short test factory. Table 5.2: Transformer short circuit factory data Impedance Power (MVA) Loss (kw) HV-LV HV-TV (60) ( ) LV-TV (60) (1792.5) The data used in this simulation model include impedances and losses are rate at 1000 MVA (400/275/13kV) [9]. Figure 5.5 shows the ATPDraw input window consist of the number of phases, the number of windings, the type of core and test frequency. The BCTRAN data is based on the test report of a 1000 MVA (400/275/13 kv) transformer shown in Table 5.1 and Table 5.2. It used autotransformer-winding connection for the primary and secondary winding (HV- LV). Therefore, the impedance calculated as shown in equation (4), (5), and (6).

61 (a) Open Circuit Data (b) Short Circuit Data Figure 5.5: BCTRAN dialog box of data according to Table 5.1 and Table 5.2 (a) Open Circuit Data (b) Short Circuit Data. Saturation effect has considered by attaching the non-linear inductances. Furthermore, the average no load loss at rated voltage and frequency was 74.4kW and average magnetizing current was 0.012% at 1000MVA base. In this simulation, the zero sequence data was not available because the model has been set equal to the positive data.