Transient Overvoltages and Distance Protection: Problems and Solutions

Transcription

1 Transient Overvoltages and Distance Protection: Problems and Solutions A thesis submitted in fulfillment of the requirements for the degree of Master of Engineering by Research Leonardo Torelli B.Eng. (Hons) School of Electrical and Computer Engineering RMIT University November /138

2 Table of Contents The Author...5 Acknowledgement...6 Declaration...7 Abbreviations...8 Executive Summary Introduction Neutral Earth Resistor and Transient Overvoltages Introduction Method of Earthing Review of the Existing Body of Knowledge Design of the Experiment The Software Package The Model The experimental design System Configuration Simulation Characteristics Model Components Simulation Results Simulation 1 : Unloaded Line - Fault at the End of the Line Simulation 2 : Unloaded Line - Fault at the Beginning of the Line Simulation 3 : Capacitor Bank. Fault at the End of the Line Simulation 4 : Capacitor Bank. Fault at the Beginning of the Line Simulation 5 : Light inductive Load. Fault at the End of the Line Simulation 6 : Energized 66/22kV Transformer Fault at the 66kV Bus Overvoltages and Traveling Waves Impact of Time of Fault with the Overvoltage Overvoltage Response on Phase a,b and c Overvoltage Harmonic Response Conclusion CVT and Transient Overvoltages Overview /138

3 3.2 The CVT Model Simulation Settings Simulation Results CVT Conclusion Distance Protection Scheme Introduction MHO Characteristics Theory Transient Overvoltages and Distance Protection: Solutions Introduction Polarisation Techniques Polarising Techniques CVT Techniques Setting Advice Conclusion Research Findings Areas for Further Investigation References Appendix A th International Conference on Developments in Power System Protection, March 2010, Manchester, UK -Abstract Appendix B Overvoltage Study PSCAD Data Appendix C Overvoltage study Steady State Voltage Data Appendix D Overvoltage Study- PSCAD Model Appendix E Voltage and Current as function of SIR /138

4 Appendix F CVT Study PSCAD Data /138

5 The Author My name is Leonardo Torelli. I have been working for fourteen years in power industry. I have worked in Italy in Utilities and in Australia in consulting firms. I am currently working at Hydro Tasmania Consulting and involved in Protection and Power System Analysis projects. It is not a surprise that the topic of this research goes across these two areas of electrical engineering. The aim of this study was, indeed, proving how important is the knowledge of power system analysis for Protection Engineers. As result of this study, I presented a paper at the 10th International Conference on Developments in Power System Protection, March 2010, Manchester, UK This research has given me the opportunity to expand my skills, challenging my knowledge and improved my analytical skills. I am committed to use this experience at RMIT University as starting point for further studies. 5/138

6 Acknowledgement I thank Dr Selva Moorthy for his contribution, support and expert advice during this study. I have appreciated his sincere interest in my professional development and support of my personal life. Dr Moorthy s feedback helped me to stay in line with my schedule and achieve the completion of this research. I would like to thank my former and existing colleagues at Hydro Tasmania Consulting for their encouragement during this journey. Last, I would like to thank my three beautiful children, my family and friends for their warm and enthusiastic appreciation of my work.. 6/138

7 Declaration I certify that except where due acknowledgement has been made, the work is that of the author alone; the work has not been submitted previously, in whole or in part, to qualify for any other academic award; the content of the thesis is the result of work which has been carried out since the official commencement date of the approved research program; and, any editorial work, paid or unpaid, carried out by a third party is acknowledged.. Leonardo Torelli Date 7/138

8 Abbreviations The following terms are used in this thesis: HVDC - High Voltage Direct Current MHO relay - Distance relay with circular characteristic which passes through the origin of the R-X plane NER - Neutral Earth Resistor NEX - Neutral Earth Reactance PSCAD-EMTDC - Power system software package SIR- Source Impedance Ratio TRV- Transient Recovery Voltage 8/138

9 9/138

10 Executive Summary This thesis investigates the relationship between transient overvoltages that follow a fault and the functionality of the digital distance protection. The study is conducted in a 66kV subtransmission network with a neutral earth resistor as method to earth the power system. The results of this study show that fault current limitation and consequent reduction of the thermal stress and aging of the equipment are successfully supported by modern digital distance relays. This thesis begins by performing an extensive power system analysis on ground fault transient overvoltages. The study is conducted by using the PSCAD-EMTC software package. The results of this analysis are discussed to determine the impact of a disturbed voltage signal fed to the distance relay, with particular attention to the polarisation and CVT transient mitigation techniques as implemented by digital distance relays. Finally, this research provides recommendation for the design of the protection settings of the distance relay. This thesis supports the implementation of digital distance protection relays on short lines, lines with ineffective grounding system and weak impedance source as a valuable alternative solution to the differential protection scheme. 10/138

11 1 Introduction Power systems are designed and operated to supply electrical energy to the customers in a safe, reliable and economical way. Faults represent one of the main challenges for the power system. Among all the types of faults, phase to ground faults represent the majority of the events, with records between 80 to 90 % of all faults. Ground faults are generated from insulation breakdown, atmospheric conditions and accidental contacts of birds or branches of trees with power lines. Therefore, these faults are transitory in nature. During this post fault initial period, the voltage change dramatically from a pre fault steady state value to a post fault steady state value. In addition, the voltage signal is disturbed by higher and lower frequency components. Therefore, transient overvoltages could affect the accuracy of the protection scheme. At the same time, the protection scheme should be able to operate in a fast manner to reduce the risk to personnel, equipment damage, and system stability. It is evident that achieving a short operating time and high accuracy of the protection scheme represents a major challenge for protection relay manufacturers and protection engineers. This research investigated the performance of modern digital distance relay during the transient overvoltages period which follows a ground fault. Initially, this study focused on power system analysis to have a clear understanding of the transient overvoltage phenomena. The overvoltage study was conducted using PSCAD-EMTC software package. The investigation was performed by using the experimental method that involves the scientific manipulation of the variables involved in the process and the systematic study of the behavior of the system. The study was conducted on a radial system using a neutral earth resistor, NER, as a method to earth the 66 kv network. However, the theory and the results 11/138

12 obtained are largely applicable to higher transmission voltages and in power systems earthed via a neutral earth reactor, NEX. Following the transient overvoltages study, the research focused on the performance of five modern digital distance relays. In summary, the project objectives are as follow: Determine the characteristics of transient overvoltages generated by a phase to ground fault. This work should highlight the main factors that influence the voltage disturbances Analyze the characteristics of modern digital distance relays to determine if the transient overvoltages could affect the accuracy ad operating time of the protection relay Establish a method to predict the performance of the distance protection relay following a phase to ground fault Elaborate a list of setting advice that could enhance the application of the distance protection relay in relation to the transient overvoltages disturbance This study aimed to validate the existing body of knowledge, highlight new insight and views and determine new findings. The research is organised in six Sections: Section 2 -NER and Overvoltage Section 3 - NER and Capacitor Voltage Transformers Section 4- NER and Distance Protection Section 5- Distance Relay Applications 12/138

13 In the last part of this study, Section 6, research conclusions and recommendations for areas of further investigations are also provided. 13/138

14 2 Neutral Earth Resistor and Transient Overvoltages 2.1 Introduction This Section investigated the influence of the neutral earth resistor on transient overvoltages. The study focused on a 66kV radial overhead subtransmission system. The expected deliverables of this section are: Documenting the transient overvoltage phenomena under a series of system configurations and a set of neutral earth resistor sizes using PSCAD-EMTC transient software package Determining whether the neutral earth resistor has particular and/or peculiar effect on the overvoltage that may be considered in design and planning of the power system. The research only focused on the impact of the resistor under ground faults. Therefore this work did not pursue the study of overvoltages during normal switching condition and lighting phenomena. The method and the technologies to control the overvoltage and the impact of the size of the resistor to the overall insulation coordination were also excluded from this study. 14/138

15 2.2 Method of Earthing The earthing of the power system involves the implementation of an electrical connection between the neutral point and the ground. Connection of the neutral to the earth can be done in several methods: Solidly earthing Neutral earth resistor, NER Neutral earth reactance, NEX Compensated earthing Ungrounded The type and size of the earthing system will affect the following system parameters [1-4]: Fault current Personnel safety System Protection Steady state overvoltage during ground faults Transient overvoltage during ground faults Thermal stress on the equipment Effect on communication circuit Harmonic current on neutral connection System stability 15/138

16 System availability Operating procedure Equipment selection Cost While the power system is operating in normal condition, the method of earthing is not relevant. The method and design of the system earthing becomes critical during a ground fault. In this condition the fault current magnitude and fault duration will affect critical elements of the power system. For a 66 kv subtransmission network, system earthing has developed in different ways in different parts of the world[2]. Generally, English speaking country such as Australia, England and America prefers solidly earthed or low impedance earthing. North European Countries, Italy and France have instead a good experience with compensated earthing using the Petersen Coil at the Substation. High Impedance grounding or isolated earthing is instead limited to a relatively smaller part of systems. It is also noted that the value of the impedance is not consistent among utilities. Moreover, some utilities use a resistor and others use a reactor. For example, In Australia, SPAusnet, Ergon and Energex use a reactor. Jemena and many industrial consumers often prefer a resistor. 2.3 Review of the Existing Body of Knowledge Overvoltages are a common disturbance in power systems. The increase of the system voltage can create malfunction, damage of the equipment and consequently disruption of the service[5]. 16/138

17 Transient overvoltages are produced by sudden changes in the electrical system as caused by lightning, faults or switching [6]. The method of earthing system does not have any effect on the system during normal operation [4]. However, during the transient period the method of earthing has an impact on the system response and magnitude of the transient overvoltage. Power demand, power generated and system configuration vary continuously with time. Therefore the power system always changes from one steady state to another steady state. In this very short periods current and voltages may reach dangerous values for the equipment and the insulation of the system. Overvoltages are often classified according to their duration in two groups [7, 8]: Temporary Overvoltage above 200 ms Transient Overvoltage below 200 ms Alternatively, overvoltages can be classified according to their origin: External overvoltage caused by atmospheric phenomena as lighting and electrostatic charges Internal overvoltage generated within the power systems This type of disturbances are also classified according to the frequency of the two major overvoltage components [9]: Power frequency overvoltage Natural frequency overvoltage of a short duration superimposed on the power frequency overvoltage The sum of the power and natural frequency overvoltage is the voltage recorded in the field. This voltage is commonly defined as a transient overvoltage. 17/138

18 The overvoltage component with a power frequency does not theoretically have any decaying component and it often termed as steady state overvoltage. Instead, the short duration component usually decays in less than 100 ms. Above all; the magnitude of the overall overvoltage is of greatest interest for the impact on the planning, design and financial cost of the power system. A fault in the power system can be represented and analysed as the closing of a switch in the electrical system. This change in the power system develops a new redistribution of stored energy in the system [6]. Phase to ground faults represent the majority of faults in the power system. These types of faults are produced by atmospheric condition, mechanical breakdown of the insulation, objects such as birds or branches or trees in contact with the overhead line and poles or structures. Recent statistics conducted by protection relay manufacturers found that these types of faults represent between 80 and 90 % of the faults in a given power system [3, 4, 10]. A large majority of these faults involving overhead lines also have transient characteristics and, therefore, self extinguish in a short period of time. When a phase to ground fault occurs, an overvoltage can be measured in the system during the fault itself or after clearing [10]. The maximum transient overvoltage is obtained by adding the peak of the temporary overvoltage at power frequency overvoltage at 50Hz sinusoidal pattern to the transient overvoltage which usually has a higher frequency. This relation is based on the worst case scenario with the assumptions that the two components will have maximum peak values at the same time. This is plausible because of the different time frames. Power frequency peak lasts long in the context of high frequency oscillations. The magnitude of the power frequency overvoltage is strictly related to the characteristic parameters of the system. This overvoltage can be calculated using the symmetrical components theory. Considering some system simplification as necessary, this engineering technique produces accurate results. The fundamental frequency voltage, which is also referred to the steady state overvoltage, is absent 18/138

19 in a solidly earthed system. In an ungrounded system the overvoltage reaches the maximum value of 3= In this latter case, the phase to ground voltage is equal to the phase to phase voltage. Application of the neutral earth resistor as a method of grounding increases the fundamental frequency overvoltage from none to a maximum of for the insulated system. Between these two boundaries of the overvoltage measured in the system, the institution of Electrical and Electronic Engineers established an arbitrary limit of the overvoltage measured, which is named effective grounding [11]. The effective grounding term indicates that, during a ground fault, the voltage on the healthy phases will not exceed 80 % of the maximum line to line voltage. The term effective grounding is commonly used in the power industry as a point of reference for the design of the earthing system and sizing of the neutral impedance. Therefore a power system can be effectively grounded or ineffectively grounded. In other words, in a 66kV subtransmission system, the phase to ground voltage during fault condition will be: 19/138

20 V Nominal Vph-ph=66kV Vph-g = 66kV/ 3=38.1 kv Solidly Grounded System Vph-g fault= 66/ 3 = 38.1 kv Effective grounding System Vph-g fault= 66 * 0.8= 52.8 kv Ungrounded System Vph-g fault= 66kV The main advantage of having an effective grounded system is limiting the overvoltage magnitude which has a direct impact on the cost of the system. For instance, the required insulation medium can be reduced with direct benefits to the cost of the electrical equipment. This advantage is a key factor for the design of subtransmission and transmission power systems. The effective grounding system is nowadays considered the best compromise between reducing the phase to ground fault and keeping the overvoltages at a reasonable level. Therefore the effective grounding is the preferred solution for the 20/138

21 higher voltages in countries where the power system is grounded via an impedance[4]. How to achieve an effective grounded system? IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems states the required system conditions: The system must be grounded through a sufficient low impedance such that for all system conditions the ratio of zero sequence reactance to positive sequence reactance is positive and equal or less than ( X0/X1 3) and the ratio of zero sequence resistance to positive-sequence reactance is positive and less than 1( R0/X1<1) These system conditions, if satisfied, maintain the power frequency overvoltage within 80 % of the nominal voltage (ground reference) Instead, to limit the natural frequency overvoltage the system conditions are: XC0 R0.Total zero sequence resistance less than one third total zero 3 sequence reactance. This condition requires that the current flowing through the neutral earth resistor shall be at least 3 times the charging current per phase of the system[4] Non compliance with this system condition could lead to dangerous faults with subsequent restrikes and overvoltages, usually named as arcing faults R0 2X 0 Zero sequence impedance system resistance at least twice the value of zero sequence reactance. This conditions shows that grounding the power system through a resistance contains transient overvoltages 21/138

22 X 0 20, which is inherently met in resistance grounding and with delta X1 star power transformers [12], unless the power system is earthed via a large reactor From the system conditions stated above, it can be derived that the size of neutral earth resistor or neutral earth reactor is not critical by itself for the magnitude of the overvoltage. The relation with other system parameters determines the magnitude of the overvoltage. The above conditions also highlights that unplanned increase of the size of the resistor will increase the value of the overvoltage due to the fault clearing restrike. Besides, the system conditions show that the reactance of the overall zero sequence network shall be inductive and not capacitive. These system conditions aim to limit transient overvoltages and, at the same time, reduce the phase to ground fault current between 10 and 25 % of the steady state fault current [9]. The lower level is dictated by the minimum current requirement for relay operation. Nowadays with modern digital relays, this requirement could be reconsidered. The 25 % upper level takes into consideration the resistor cost in comparison to the reactive option. Plots of power frequency overvoltage following faults as a function of different system parameters or system parameters or ratios of system parameters have been published and are available in the existing literature[9]. Existing studies on overvoltage and impact of the neutral earth resistor on the measured overvoltages are based on field testing, software simulation and theoretical analysis. First researches on transient overvoltages during faults were conducted before the Second World War. Overvoltages were analytically determined and tested using 22/138

23 miniature models[9]. Maximum peak voltages were recorded mainly between 3 and 4 pu with 20 % above 4 pu. The researchers also considered current chopping produced by the circuit breakers and restrike phenomena known as transient recovery voltage, usually abbreviated as TRV. Overvoltage calculations and tests using miniature system have also been conducted by other researchers[6, 8]. These papers provide a complete explanation of the maximum overvoltage in relation to system parameters and formed the basis for future IEEE standards. Measurements of overvoltages following a fault were performed in the distribution network in the 10kV Croatian distribution network[13]. The system was earthed using the NER. Maximum overvoltage factors are below 2 pu and minimum variations are recorded for different values of the resistor. A transient software program was used to validate the results. It is possible to determine the maximum transient overvoltage by applying the RLC circuit analysis and the use of symmetrical components. [14]. Generalized plots which took into consideration the damping effect of the resistor were also produced. An overall indication of the effect of the resistor is defined by: R0 2X 0 circuit is oscillatory R0 2X 0 critical damped R0 2X 0 over damped Recent studies usually apply the traveling waves theory. In line with this theory, modern transient software packages also avoid the use of lumped elements [15]. Lumped elements representation is adequate for steady state analysis but give inaccurate results in transient studies. In the latter studies, where travel time of the electromagnetic waves and the energy exchange between capacitance and inductance must be taken into consideration, traveling waves theory is the most 23/138

24 accurate method for the computation of the overvoltages along the transmission line. Fault generated traveling waves propagates not only on the overhead line to line. but also line to earth with different propagation parameters [16]. A phase to ground fault can be represented as disturbance that generates current and voltage traveling waves on a subtransmission line. At each point of discontinuity the traveling waves will be refracted and reflected according to the surge impedance characteristic of the line [17].It is possible to represent the large number of forward and backwards waves and determine the maximum overvoltage generated as a function of time by using the Bewley Lattice diagram. However, there are also some other considerations which must be dealt with which include mutual coupling with other conductors and wave shape distortion experienced along the transmission line. It is important to point out that overvoltage studies and traveling wave application mainly focus on switching and lightning transients [16]. However, overvoltages produced by switching episode can be used as a reference for this type of study. To conclude, what overvoltage magnitude can be developed under a ground fault? From the existing literature it can be inferred generally that the maximum overvoltages following a phase to ground fault are below 3 pu [18-20]. Maximum overvoltage can instead reach 4.1 pu with trapped charges on the line as in the case of switching a capacitor bank connected on the line [19]. 24/138

25 2.4 Design of the Experiment The Software Package Usually, software packages are used to facilitate transient overvoltage studies. In fact, these investigations involve complex and time consuming computation that can be achieved with sufficient accuracy with software packages. PSCAD-EMTDC is a state of the art software package developed using the wave propagation theory first published by DR Hernmann W Dommel in the transaction of IEEE Power Apparatus and Systems in based on this paper another two transient analysis software packages well known in the industry, EMTP and, later, the ATP software package [21] were developed. PSCAD is a very powerful and flexible graphical interface that uses the EMTDC simulation program. The first version of PSCAD was produced by Denis Woodford to model and simulate the high voltage DC system in Canada. The software is now developed and maintained by the Manitoba HVDSC Research Centre and is continually upgraded to incorporate the latest validated research. The software allows one to model an electrical circuit, run a simulation, analyze the results and manage the data information in an efficient way. The software package is provided with an equipment library, but also allows the creation of new models and building a personal equipment library. It is used by consulting engineers, equipment manufacturers and laboratories for planning and design of power systems. Typical projects involve transient stability, dynamic stability, relay coordination, transformer saturation, insulation coordination studies, HVDC studies, harmonic studies, power electronics studies and optimisation of controller parameters. Further information is located on the website 25/138

26 All PSCAD software editions must be licensed except for the student license. The student license version, which was used for this research, contains a limitation of 15 electric nodes. However, considering the purpose of this study, PSCAD was still found suitable. PSCAD-EMTDC was selected for its performance, the graphical interface and the technical support provided by Application Engineers at Manitoba HVDC Research Centre The Model The system modeled is a basic radial subtransmission system. The model contains a 220 kv source, one 220/66 kv transformer and one overhead line. Although power systems are more complex and contain more components, the overhead line and the power transformer represent the backbone of the system and have the major influence in the transient overvoltage. It also contains the part of the system that is affected by the fault current. This system reduction is in line with previous studies [22]. This type of configuration also allows keeping at minimum the required number of nodes and the use of PSCAD student license version. The measured overvoltage can be represented as the superposition of the temporary overvoltage at power frequency and the natural overvoltage component at higher frequency. Power Frequency V + Natural Frequency V = Measured Overvoltage PSCAD software was used to measured the absolute value of the instantaneous peak of the three phase to ground voltages. Measurement was conducted at the fault location using the maximum/minimum recorder function offered by PSCAD. The maximum value of the three phases was selected for the analysis. 26/138

27 Steady state overvoltage for a given value of the neutral earth resistor was calculated by applying symmetrical components theory. Factor K2 and K0 were used to determine fault currents and steady state overvoltages[4]. K2 Z2/Z1 K0 Z0/Z1 Where: Z1= Positive system impedance Z2= Negative system impedance Z0= Zero system impedance The factor K0 gives us a prompt understanding of the power frequency overvoltage on the healthy phases during a ground fault. The factor K2 is assumed to be 1. If the fault occurs close to the generator the factor K2 may increase to 1.4 [4]. Knowing the steady state overvoltage and the total overvoltage, the transient component can be calculated with: Natural Frequency V = Measured Overvoltage - Power Frequency V The experimental design The most relevant factors that influence the overvoltage magnitude during phase to ground faults are as follow: 27/138

28 Method of earthing Type of fault System configuration Number of earthing points and location in respect to the fault location Electrical parameters of the equipment Size of the neutral earth resistor Location of the fault Time of the fault applied Type of subtransmission network as underground cables and overhead lines have different electrical characteristics Type of overhead line Length of the line Fault impedance Each variable was analyzed in order to determine the most critical and relevant factors to be used in this research. In the end, the study was based on the following five factors: System configuration Type of faults Size of the neutral earth resistor 28/138

29 Location of the fault Time of the fault System Configuration Four system configurations were selected. These configurations were considered the most likely configurations to produce the highest overvoltages during a ground fault [5, 14]: RL #1 #2 T 66kV Subtransmission Line + Resis Figure 2.1: Overhead line with no load at the end of the line model RL #1 #2 T 66kV Subtransmission Line + Resis 30 [MVAR] Figure 2.2: Overhead line with capacitor bank at the end of the line 29/138

30 RL #1 #2 T 66kV Subtransmission Line + Resis 2.0 [MVAR] Figure 2.3: Overhead line with light inductive load at the end of the line RL #1 #2 #1 #2 + Resis Figure 2.4: 66/22kV transformer connected directly to the secondary side of the 220/66kV transformer Type of faults Phase to ground fault and phase to phase to ground fault. Neutral earth resistor The sizes of the resistor selected cover the entire spectrum between a solidly earthed system (no resistor installed) and ungrounded system (infinite resistor installed). 15 resistor sizes were selected. In particular, the data was selected between K0=0 to K0=10 as the research focused on low resistance grounding. System response was assumed to be linear between adjacent samples. Location of the fault 30/138

31 Faults were simulated at the beginning and at the end of line. Time of fault Considering the symmetry of the alternating voltage cycle, the samples were selected only on one half cycle. As one semi cycle at 50Hz takes 10 ms, 21 sample spaced 0.5 ms apart were selected. Line length Line lengths were set to 100, 10 and 0.2 km. These line lengths are based on the minimum, maximum and typical line length in the Victorian Network. Typical subtransmission lines vary between km in the urban area around Melbourne and increase to km in the country area. In Victoria the maximum line length is about 80 Km. The line length of 100 km reflects the need to investigate the system in the worst case scenario Simulation Characteristics In this study no randomization factor is applied as the chance of biasing the results in a software simulation is considered negligible. Therefore, simulation studies were conducted in a logical order to maximize the efficiency of the work. The simulations were conducted using PSCAD multiple run function. This software tool allows completion of complex studies in an efficient manner by manipulating variables sequentially and automatically. PSCAD Multiple Run tool can run and modulate up to six variables. 31/138

32 Maximum instantaneous voltage peak was recorded for each run and on each phase. Only the maximum overvoltage among the three phases was selected for the maximum overvoltage record. Duration of the run time was set to 0.6 s with fault start at 0.4 s plus selected time delay. The end time of the fault was 0.1 s with clearing fault at the first zero crossing intersection of the fault current. According to the Nyquist_Shannon Sampling Theorem, to avoid aliasing and incorrect representation of the selected variable, sampling rate should be at least double of the variable frequency to be represented. PSCAD recommends that 50µs for the time step of the process and computation would be sufficient. However, time step resolution was reduced to 20 µs to capture transient components up to 25 khz [22]. Sampling rate=20 µs Frequency rate= 1/T=10 6 /20= 50kHz Model Components Source The source represents the power system which is upstream the 220/66 kv power transformer in the substation. The voltage source selected from the PSCAD library enabled to specify the positive, negative and zero-sequence impedance. The impedance value is selected to produce a three fault current of 30 ka on the 220 kv bus at the substation. Positive impedance is equal to negative impedance. Zero impedance was set to be equal to the positive sequence impedance to allow for a phase to ground fault level equal to the three phase fault level. The X/R ratio was set to /138

33 The control system for the output voltage was set using the Fixed Control function offered by PSCAD software package. Through several trial and error experiments this type of control produced a model behavior very similar to an infinite bus. This setting enhances the robustness of the source model in respect to disturbances on a lower voltage level. It is assumed that the impact of the overvoltage on the 66 kv radial line is limited on the higher transmission network. In summary: Source Type = Three Phase Voltage-Source Model 3 MVA = 100 MVA Vph- Vph = 220 kv Frequency = 50 Hz Time constant = 0.06 s Z1=Z2=Z0= 4.23 Ohm Z1 angle= deg Voltage Fixed Control Es = 220 kv Ph = 0.0 deg Frequency = 50 Hz Transformer The two winding 220/66 kv power transformer connects the transmission system to the subtransmission system. 33/138

34 Often substation have more than one transformer operating in parallel with the bus tie on the 220 kv closed and on the 66 kv bus open or closed according to system requirement, the fault level in the system and the rating of the equipment. To simplify the study the model represents only one transformer. Winding connection of the transformer is delta on the HV side and star on the LV side. Tap changer operation is disabled The transformer is represented in PSCAD according to the classical approach of the transformer theory. Magnetising current is represented as well as core saturation placed on the HV side. In PSCAD the HV side is the closest winding to the core of the transformer. Hysteresis losses in the transformer core are included. Copper losses are disabled to reduce the number of nodes in the system. Considering that a typical X/R for a power transformer of 100 MVA and 220/66 kv is above 30, this simplification was considered acceptable. TRANSFORMER DATA: 3 Phase 2 Winding Transformer MVA Rating: MVA Freq= 50 Hz, V ph-ph primary= 220 kv Vph-ph secondary= 66kV No Load Losses= 0.01 pu 34/138

35 Copper Losses=0.0 pu. Positive Sequence Reactance= 0.15 pu. Saturation enabled simulated on primary winding Air core reactance= 0.3 pu In Rush Decay Time Constant= 0.03 pu. Knee Point Voltage: 1.25 pu Time to release Flux Clipping=0.1s Magnetising Current= 0.02 pu. Overhead line In electromagnetic simulation studies there are two main methods to represent transmission lines[21]:. Simplified studies generally rely on the PI Section model Distributed transmission line, which is most suited for transient line response modeling using a digital computer. This method was used for this study PSCAD operates applying the traveling wave theory but also implementing mutual coupling between conductors and wave shape attenuation produced by the transmission line. Transmission lines under PSCAD transient software packages are modeled using one of the three traveling wave models: Bergeron Model Frequency Dependent Mode Model 35/138

36 Frequency Dependent Phase Model The Frequency Dependent Phase Model was selected for this study as is the most accurate model. This model is a distributed RLC traveling wave model, which incorporates the frequency dependence of all parameters in the model. This model is recommended by the PSCAD User s Guide for this type of study. The overhead line parameters are based on a typical 66 kv line as represented in Victoria. Conductor Data 400 mm 2 aluminum Conductor Resistance= Ohm/km Conductor Geometric Mean Radius m Capacitor bank The capacitor bank is implemented in Simulation 3 and 4. The capacitor bank is connected at the end of the line to produce maximum peak value of the system response during the transient period. The capacitance value was set under the assumption that the load on the overhead line is 60 MVA with a power factor of Load Current = S/ ( 3 * V ) = 60 *106 / 3 * )= A P = S * PF = MW If PF is Power angle á= 45 degrees 36/138

37 P=Q = MVAR The selected size for the MVAR is set to correct the power factor above 0.95 Q/P= tan ( cos -1 PF ) = tan ( cos )= Q final= P * = MVAR Q capacitor bank = Q load- Q final = = MVAR The capacitor bank rating is set to 30 MVAR and connected in star to the network. The star point of the capacitor bank is ungrounded Light Inductive Load The rating of the inductive load is 2 MVAR and a resistive load of 0.2 MW. The load is star connected and is ungrounded. The load size is designed a very light 66 kv subtransmission load or a substation at the end of the line with no load and supplying only the substation auxiliary services. 66/22kV Transformer Simulation 6 is configured to simulate the 220/66 kv transformer that feeds a 66/22 kv transformer within the substation. The 66/22 kv transformer rating is 30 MVA. For simplicity the internal parameters of the transformers are set as the 220/66kV transformer. 37/138

38 TRANSFORMER DATA: 3 Phase 2 Winding Transformer MVA Rating: 30 MVA Freq= 50.0 Hz, V ph-ph primary=66 kv Vph-ph secondary= 22 kv No Load Losses= 0.1 pu Copper Losses=0.0 pu. Positive Sequence Reactance= 0.15 pu. Saturation enabled simulated on primary wounding Air core reactance= 0.3 pu In Rush Decay Time Constant= 0.03 pu. Knee Point Voltage: 1.25 pu Time to release Flux Clipping=0.1 s Phase to Ground Fault PSCAD fault logic enables to specify the type of fault, time of fault and fault duration. 38/138

39 Phase to ground and phase to phase to ground the fault were simulated. Current interruption was simulated at 0 current crossing to avoid any overvoltage related to magnetic field being trapped in the circuit. This type of overvoltage is usually referred as current chopping [3]. Fault resistance is set to zero to evaluate the impact of the neutral earth resistor by itself. In summary: Fault Type = Phase to ground and phase to phase to ground Fault start =0.4 s to fault s Fault length =0.1 s Fault ON resistance = 0.01 ohm Fault OFF resistance = 10 6 ohm Current Chopping limit= 0 ka 39/138

40 2.5 Simulation Results Simulation results are organized according to the six system configuration and related simulations. Simulation results data is attached in Appendix B. Each simulation result Section contains two graphs. The first plot summarizes the main objective of this investigation which is the maximum overvoltage as a function of the reduced fault current. The fault current was calculated by applying the symmetrical components theory as illustrated in Appendix B. The second plot highlights the specific weight of the transitory overvoltage component in respect to the steady state overvoltage component. Power frequency overvoltage was subtracted from the measured overvoltage only if the recorded maximum overvoltage took place during the fault and not at fault clearing. After fault clearing, the power frequency overvoltage can be considered absent as the three phases are now connected to a symmetrical voltage source. Additional investigations were also performed on the following topics: Overvoltages and traveling waves Influence of the fault time Overvoltage response on healthy and faulty phases Frequency spectrum analysis 40/138

41 2.5.1 Simulation 1 : Unloaded Line - Fault at the End of the Line TABLE 2-1 SIMULATION 1 RESULTS Line Length= 100km Max Measured Iph-g with V from NER/ Iph-g V Meas / Power Natural NER (Ohm) PSCAD (kv) solidly earthed K0 Nominal Voltage Frequency V (pu) Frequency V (pu) Line Length=10km /138

42 Max Measured Iph-g with V from NER/ Iph-g V Meas / Power Natural NER (Ohm) PSCAD (kv) solidly earthed K0 Nominal Voltage Frequency V (pu) Frequency V (pu) Line Length=0.2km Configuration 1- Fault at the of the line- No load- Measured Overvoltage 2.5 Overvoltage ( pu) L=100km L = 10km L= 0.2km Ifault/I ph-g solidly earthed (pu) Figure 2.5: Configuration 1 Measured overvoltage result 42/138

43 The maximum overvoltage reaches 2.5 times the value of the normal phase to earth voltage. Maximum overvoltage below 3.0 pu is in line with existing literature[6, 20]. The maximum overvoltage was recorded with the longest subtransmission line which is 100km. This characteristic is common with previous studies[23] and tests with a miniature circuit. It is also in line with all the other simulations of this study. The characteristic is due to the larger line capacitance and inductance which are proportional to the length of the line. As the capacitance increases, the energy stored in the electric field increases producing more severe transient phenomena upon a sudden change in the power system. The simulation results show that the maximum recorded overvoltage plots have a consistent pattern across the neutral earth resistor spectrum. The increase of the resistor size slightly increases the measured overvoltage due to the highest contribution of the power frequency overvoltage. A high maximum overvoltage is recorded once the resistor size is increased to reduce the phase to ground fault below 0.1 pu. The high value of transient overvoltage is due to the effect of transient phenomena after fault clearing, also called transient restrike, as is also shown in Figure 2.5. As we increase the size of the resistor and the fault current decreases, the maximum overvoltage is recorded during the fault. The restrike is due to the excessive increase of the neutral earth resistor. In other words the ratio R0/Xco becomes well above the recommended limit of 1/3[9]. This factor was found to be more relevant of the positive damping provided by the resistor. As per Reference [14], the increase of the resistor should damp the transient overvoltage as R0>2ZO Please also note that the shift of the peak voltage from the fault clearing time to the fault period takes place first in the shortest line system configuration, then with line length of 10km and last to the longest subtransmission line. This behavior can be 43/138

44 explained using the factor K0=Z0/Z1. The implementation of the resistor will increase Z0. As the neutral impedance has a bigger impact on the factor K0, in the shortest line the expected steady state overvoltage has a bigger increase The results show that the maximum overvoltages recorded in the simulation with a line length of 10km are similar to the results obtained from the simulation with a line length of 0.2km. This characteristic is recorded throughout the study. We can explain this characteristic by referring to the overvoltage plots as function of system parameters of IEEE standards[18]. The different line length does not have a particular impact on the Xco/X1. For both line lengths, the ratio is larger than 100. Beyond this value the expected overvoltage is considered constant.[6]. Xsource= pu X Tx= 0.15 pu X line= Ohm/km C= 9.4 nf/km Line length 10km Xc = 1/(2Ð f C )/ 10 = 1/( 2* Ð 50 * )/ 10 = Ohm X1= Xsource+ Xtx + X line = ( 10km) = pu Z base = 484 Ohm Xc in pu = Xc value/ Z base= 33879/484= 70 pu Ratio Xco/X1= 70/ = Line length 0.2km 44/138

45 Xc = 1/(2Ð f C )/0.2 = 1/( 2* Ð 50 * )/ 0.2 = Ohm X1= Xsource+ Xtx + X line( 0.2km) = = pu Z base = 484 Ohm Xc in pu = Xc value/ Z base= 636/484= 3500 pu Ratio Xco/X1= 3500/ = Line length 100 km Xc = 1/(2Ð f C )/0.2 = 1/( 2* Ð 50 * )/ 100 = Ohm X1= Xsource+ Xtx + X line( 100km) = = pu Z base = 484 Ohm Xc in pu = Xc value/ Z base= /484= 7 pu Ratio Xco/X1= 7/ = 43.6 The Xco/X1 ratio for line length of 100km is also proved to be below 100. Therefore, the line will produce a higher overvoltage. 45/138

46 Configuration 1- Fault at the of the line- No load- Transient Overvoltage Contribution L=100km L = 10km L= 0.2km Overvoltage ( pu) Ifault/I ph-g solidly earthed (pu) Figure 2.6: Configuration 1 Transient overvoltage component Figure 2.6 shows the contribution of the transient overvoltage to the maximum recorded overvoltage. 46/138

47 2.5.2 Simulation 2 : Unloaded Line - Fault at the Beginning of the Line TABLE 2-2 SIMULATION 2 RESULTS Line Length= 100km Max Measured Iph-g with V from NER/ Iph-g V Meas / Power Natural NER (Ohm) PSCAD (kv) solidly earthed K0 Nominal Voltage Frequency V (pu) Frequency V (pu) Line Length=10km /138

48 Max Measured Iph-g with V from NER/ Iph-g V Meas / Power Natural NER (Ohm) PSCAD (kv) solidly earthed K0 Nominal Voltage Frequency V (pu) Frequency V (pu) Line Length=0.2km Configuration 2- Fault at the bus- No load- Measured Overvoltage 2.5 Overvoltage ( pu) L=100km L = 10km L= 0.2km Ifault/I ph-g solidly earthed (pu) Figure 2.7- Configuration 2 Measured overvoltage result 48/138

49 Configuration 2- Fault at the bus- No load- Transient Overvoltage Contribution 2.5 Overvoltage ( pu) L=100km L = 10km L= 0.2km Ifault/I ph-g solidly earthed (pu) Figure 2.8: Configuration 2 Transient overvoltage component This simulation has the same system configuration of the first study except the location of the fault. The results however show that different transmission line lengths do not produce any particular effect on the maximum overvoltage except to a slightly lower level of the recorded overvoltage. Lower results of the maximum overvoltages are due to different effect produced by the traveling waves that propagates along the subtransmission lines and different impact of the system conditions. 49/138

50 2.5.3 Simulation 3 : Capacitor Bank. Fault at the End of the Line TABLE 2-3 SIMULATION 3 RESULTS Line Length= 100km Max Measured Iph-g with V from NER/ Iph-g V Meas / Power Natural NER (Ohm) PSCAD (kv) solidly earthed K0 Nominal Voltage Frequency V (pu) Frequency V (pu) Line Length=10km /138

51 Max Measured Iph-g with V from NER/ Iph-g V Meas / Power Natural NER (Ohm) PSCAD (kv) solidly earthed K0 Nominal Voltage Frequency V (pu) Frequency V (pu) Line Length=0.2km Configuration 3- Fault at the end of the line- Cap Bank connected at the end of the line- Measured Overvoltage 12.0 Overvoltage ( pu) L=100km L = 10km L= 0.2km Ifault/I ph-g solidly earthed (pu) Figure 2.9: Configuration 3 Measured overvoltage result 51/138

52 Configuration 3- Fault at the end of the line- Cap Bank connected at the end of the line- Transient Overvoltage Contribution L=100km L = 10km L= 0.2km Overvoltage ( pu) Ifault/I ph-g solidly earthed (pu) Figure 2.10: Configuration 3 Transient overvoltage component The dangerous effects of capacitor switching are well known in the engineering field [23]. The results obtained with this simulation show that, following a fault in the network with no or small loads and a capacitor bank connected to the grid represent a danger to the insulation of the power system. The overvoltages measured in this simulation are the highest of the entire study reported here. The results are in line with previous studies [5, 14] except for results obtained with line length of 100 km. This is due to the combination of the following system parameters: Length of the line NER size XC0 These parameters affect the R0 transient overvoltages condition. If this 3 condition is not met, dangerous fault restrikes are generated [9]. Is it worth noting that field experience of arcing ground fault above 7 pu are quite rare as the insulation of the power system usually breaks down between 6 to 7 pu [9] 52/138

53 Line length impact is shown in Figure 2.9 and Instead, to review the impact of the NER size further simulations were performed. Four snapshots with a NER of 10, 100, 1000 and Ohm were taken as shown in Figures 2.11 to Voltage (kv) Vs Figure 2.11: Configuration 3 Transient overvoltage Snapshot with NER of 10 Ohms /138

54 Figure 2.14: Configuration 3 Transient overvoltage Snapshot with NER of Ohms The graphs show that the increase of the resistor will increase the transient overvoltages. The dangerous transient overvoltages are also due to the conservative approach in selecting the size of the capacitor bank. A further simulation is conducted to validate this theory. A smaller capacitor bank of 15 MVA was used for the analysis. Configuration 3- Line Length 100km- Comparison between 30MVAR and 15MVAR Capacitor Bank Results- Measured Overvoltage 12.0 Overvoltage ( pu) MVAR 30MVAR Ifault/I ph-g solidly earthed (pu) Figure 2.15: Configuration 3 30 and 15 MVAR Cap bank overvoltage comparison 54/138

55 The system response with a smaller capacitor gives a result which is acceptable and in line with previous studies [5, 19]. Maximum overvoltage with the insulated system is 4.1 pu. 55/138

56 2.5.4 Simulation 4 : Capacitor Bank. Fault at the Beginning of the Line TABLE 2-4 SIMULATION 4 RESULTS Line Length= 100km Max Measured Iph-g with V from NER/ Iph-g V Meas / Power Natural NER (Ohm) PSCAD (kv) solidly earthed K0 Nominal Voltage Frequency V (pu) Frequency V (pu) Line Length=10km /138

57 Max Measured Iph-g with V from NER/ Iph-g V Meas / Power Natural NER (Ohm) PSCAD (kv) solidly earthed K0 Nominal Voltage Frequency V (pu) Frequency V (pu) Line Length=0.2km Configuration 4- Fault at the bus- Cap Bank at the end of the line- Measured Overvoltage Overvoltage ( pu) L=100km L = 10km L= 0.2km Ifault/I ph-g solidly earthed (pu) Figure 2.16: Configuration 4 Measured overvoltage result 57/138

58 Configuration 4- Fault at the bus- Cap Bank at the end of the line- Transient Overvoltage Contribution L=100km L = 10km L= 0.2km Overvoltage ( pu) Ifault/I ph-g solidly earthed (pu) Figure 2.17: Configuration 4 Transient overvoltage component This simulation uses the same system configuration of Simulation 3 except for the location of the fault. Measured overvoltage shows that these results are similar to the results obtained from Simulation 3. Maximum overvoltages are recorded with the transmission line of 100km and after fault clearing. As discussed in Simulation 2, location of the fault in proximity of the neutral earth resistor decreases the value of the maximum overvoltages. 58/138

59 2.5.5 Simulation 5 : Light inductive Load. Fault at the End of the Line. TABLE 2-5 SIMULATION 5 RESULTS Line Length= 100km Max Measured Iph-g with V from NER/ Iph-g V Meas / Power Natural NER (Ohm) PSCAD (kv) solidly earthed K0 Nominal Voltage Frequency V (pu) Frequenc y V (pu) Line Length=10km /138

60 Max Measured Iph-g with V from NER/ Iph-g V Meas / Power Natural NER (Ohm) PSCAD (kv) solidly earthed K0 Nominal Voltage Frequency V (pu) Frequenc y V (pu) Line Length=0.2km Configuration 5- Fault at the of end the line- Inductive Load connected at the end of the line- Measured Overvoltage Overvoltage ( pu) L=100km L = 10km L= 0.2km Ifault/I ph-g solidly earthed (pu) Figure 2.18: Configuration 1 Measured overvoltage result 60/138

61 Configuration 5- Fault at the end of the line- Inductive Load connected at the end of the line- Transient Overvoltage Contribution L=100km L = 10km L= 0.2km Overvoltage ( pu) Ifault/I ph-g solidly earthed (pu) Figure 2.19: Configuration 5 Transient overvoltage component Maximum overvoltages recorded are similar to the results from Simulation 1. It is likely that this inductive load was not large enough to produce any relevant impact on the overvoltages. Therefore, not further analysis was conducted using the results of this system configuration. 61/138

62 2.5.6 Simulation 6 : Energized 66/22kV Transformer Fault at the 66kV Bus. TABLE 2-6 SIMULATION 61 RESULTS Max Measured Iph-g with V from NER/ Iph-g V Meas / Power Natural NER (Ohm) PSCAD (kv) solidly earthed K0 Nominal Voltage Frequency V (pu) Frequenc y V (pu) Configuration 6- Fault at the bus- Tx 30 MVA delta/star at the bus- Measured Overvoltage Overvoltage ( pu) Ifault/I ph-g solidly earthed (pu) Figure 2.20: Configuration 6 Measured overvoltage result 62/138

63 Configuration 6- Fault at the bus- Tx 30 MVA at the bus- Transient Overvoltage Contribution Overvoltage ( pu) Ifault/I ph-g solidly earthed (pu) Figure 2.21: Configuration 6- Transient overvoltage result Overvoltages recorded in this study are relatively lower than in the other simulations. Maximum overvoltage reaches a maximum of 1.8pu as phase to ground fault decreases below 0.6 pu. Please note that the transient overvoltage component disappears as the phase to ground fault decreases below pu. Therefore, for high resistor values the overvoltages are only produced by the power frequency voltage. No further analysis was conducted using the data from this simulation Overvoltages and Traveling Waves This section documents the propagation of the voltage traveling waves along the transmission line. A series of snapshots are presented to describe the behavior of the overvoltages using the data from Simulation 1 No load at the end of the lineand with the line length of 100 km 63/138

64 Is Bus Current,Bus Voltage : Graphs Current (ka) Voltage (kv) Vs Time Figure 2.22: Configuration 1 - Maximum Overvoltage - Phase to Ground Fault NER 0 Ohm Snapshots were recorded at fault inception and fault clearing time. The graph also shows that the transient overvoltage at higher frequency is superimposed on a lower frequency overvoltage. The overvoltage has a period of 0.7 ms which is equivalent to a frequency of 1400 Hz. The high frequency component decays in approximately 10ms. Traveling waves are triggered at the same time on both healthy phases and travel on the conductors. The steps pattern of the transient overvoltage is the effect of the traveling waves that reflect back and forward between the two ends of the line. 64/138

65 Is Bus Current,Bus Voltage : Graphs Current (ka) Voltage (kv) Vs Time Figure 2.23: Configuration 1- Snapshot of after clearing overvoltage- Phase to Ground Fault NER 0 Ohm Transient overvoltage superimposed on the power frequency voltage has a frequency of 590 Hz. Maximum overvoltage appears on the phase which has the highest voltage at the clearing time. The overvoltage is produced by the energy stored in the electric field of the capacitance that will discharge at fault clearing time. 65/138

66 Is Bus Current,Bus Voltage : Graphs Current (ka) Vs 100 Voltage (kv) Time Figure 2.24: Configuration 1- Snapshot of Maximum Overvoltage- Phase to Phase to Ground Fault NER 0 Ohm In the phase to phase ground fault the maximum overvoltage is recorded at the fault clearing time. Frequency of the superimposed transient overvoltage is 590 Hz as in the phase to ground faults. The overvoltages on each phase have the same frequency. This transient overvoltage decays after 30ms. 66/138

67 Current (ka) Voltage (kv) Is Vs Bus Current,Bus Voltage : Graphs Time Figure 2.25: Configuration 1- Snapshot of Maximum Overvoltage- Phase to Ground Fault NER 50 Ohm Is Bus Current,Bus Voltage : Graphs Current (ka) Vs Time Voltage (kv) Figure 2.26: Configuration 1- Snapshot of Maximum Overvoltage- Phase to Phase to Ground Fault NER 50 Ohm 67/138

68 As the NER approaches 50 Ohm, the pattern of the plots remains similar to plots with NER equal to 0 Ohm. Overvoltage frequencies are 590 Hz in all three phases Is Bus Current,Bus Voltage : Graphs Current (ka) Voltage (kv) Vs Time Figure 2.27: Configuration 1- Snapshot of Maximum Overvoltage- Phase to Ground Fault NER 1000 Ohm The large size of the resistor now makes the phase to ground fault as the most prominent overvoltage. Traveling waves are visible in the current and voltage plots Impact of Time of Fault with the Overvoltage This investigation highlights the relation between the time of fault and the amplitude of the measured overvoltage. Plots are divided into phase to ground fault and phase to phase to ground fault. 68/138

69 Simulation 1- Measured Overvoltage Ph-g fault- All Data Plot Overvoltage Solidly Earthed System Insulated System L=100km L=10km L=0.2km Sample Figure 2.28: Configuration 1- All Data Plot 2.5 Simulation 2- Measured Overvoltage Ph-g Fault- All Data Plot 2 Overvoltage Solidly Earthed System Insulated System L=100km L=10km L=0.2km Sample Figure 2.29: Configuration 2 All Data Plot 12 Simulation 3- Measured Overvoltage Ph-g Fault- All Data Plot 10 Overvoltage 8 6 Solidly Earthed System Insulated System L=100km L=10km L=0.2km Sample Figure 2.30: Configuration 3 All data plot 69/138

70 Simulation 4- Measured Overvoltage Ph-g Fault- All Data Plot Overvoltage 8 6 Solidly Earthed System Insulated System L=100km L=10km L=0.2km Sample Figure 2.31: Configuration 4- All data plot 3 Simulation 1- Measured Overvoltage Ph-Ph-G Fault- All Data Plot Overvoltage(pu) Solidly Earthed System Insulated System Series1 Series2 Series Simulation Figure 2.32: Configuration 1- All data plot 2.5 Simulation 2- Measured Overvoltage Ph-Ph-G Fault- All Data Plot 2 Overvoltage (pu) Solidly Earthed System Insulated System L=100km L=10km L=0.2km Simulation Figure 2.33: Configuration 2 All data plot 70/138

71 5 Simulation 3- Measured Overvoltage Ph-Ph-G Fault- All Data Plot Overvoltage (pu) Solidly Earthed System Insulated System L=100km L=10km L=0.2km Simulation Figure 2.34: Configuration 3- All data plot 8 Simulation 4- Measured Overvoltage Ph-Ph-G Fault- All Data Plot 7 6 Overvoltage (pu) Solidly Earthed System Insulated System L=100km L=10km L=0.2km Simulation Figure 2.35: Configuration 3 All data plot These graphs are produced by using the maximum overvoltage recorded on each simulation. It is reminded that 11 simulations were performed for each NER size configuration. Besides, 1 ms additional delay time was applied on each simulation. Points were plotted sequentially, starting on the simulation with a NER of 0 Ohm to the maximum value of the NER, which corresponds to the insulated system. In these eight plots, Figure 2.28 to Figure 2.35, it is possible to recognize the 15 peaks related to the 15 resistor sizes used in the simulations. 71/138

72 Phase to ground faults for transmission line of 10 and 0.1km showed a smoother pattern with lower peak values. Only in Simulation 2 there is an irregularity in the maximum overvoltage. Overall, the maximum overvoltage in the phase to ground fault is therefore less dependent on the time of fault. Phase to ground fault maximum overvoltages show a similar pattern across the three different subtransmission line lengths. Maximum overvoltages appear only for a brief period of time and at the same time in all the three line lengths. The 8 plots also show similar results for simulations conducted with subtransmission line of 10km and 0.1km. These graphs confirm that time of fault is relevant in determining the maximum overvoltage during phase to phase to ground faults Overvoltage Response on Phase a,b and c The section investigates the relationship between faults and different overvoltage responses on each phase. This study was conducted only for subtransmission line of 100km in the simulation 1, 2, 3 and 4. Graphs are divided in phase to ground faults and phase to phase to ground faults. On PSCAD software package phase to ground fault takes place on phase a and phase to phase to ground fault takes place on phases a and b. 72/138

73 Simulation 1-Measured Overvoltage Phase Comparison-Phase to Ground Fault- L=100km Overvoltage(pu) 1.5 Solidly Earthed Insulated System Phase a Phase b Phase c 1 System Simulation Figure 2.36: Configuration 1 Phase comparison Simulation 2-Measured Overvoltage Phase Comparison-Phase to Ground Fault- L=100km Insulated Overvoltage(pu) Solidly Earthed System Phase a Phase b Phase c System Simulation Figure 2.37: Configuration 2 Phase comparison Simulation 3-Measured Overvoltage Phase Comparison-Phase to Ground Fault- L=100km Insulated System Overvoltage (pu) 6 Solidly Earthed Phase a Phase b Phase c 4 System Simulation Figure 2.38: Configuration 3 Phase comparison 73/138

74 Simulation 4-Measured Overvoltage Phase Comparison-Phase to Ground Fault- L=100km Insulated Overvoltage (pu) 6 System Phase a Phase b Phase c 4 2 Solidly Earthed System Simulation Figure 2.39: Configuration 4 Phase comparison Simulation 1-Measured Overvoltage Phase Comparison-Phase to Phase to Ground Fault- L=100km Overvoltage(pu) 1.5 Solidly Earthed Insulated System Phase a Phase b Phase c 1 System Simulation Figure 2.40: Configuration 1 Phase comparison Simulation 2-Measured Overvoltage Phase Comparison-Phase to Phase to Ground Fault- L=100km Overvoltage(pu) Solidly Earthed Insulated System Phase a Phase b Phase c System Simulation Figure 2.41: Configuration 2 Phase comparison 74/138

75 Simulation 3-Measured Overvoltage Phase Comparison-Phase to Phase to Ground Fault- L=100km Overvoltage(pu) Insulated System Phase a Phase b Phase c 1.5 Solidly Earthed System Simulation Figure 2.42: Configuration 3 Phase comparison Simulation 4-Measured Overvoltage Phase Comparison-Phase to Phase to Ground Fault- L=100km Insulated System Overvoltage(pu) Solidly Earthed System Phase a Phase b Phase c Simulation Figure 2.43: Configuration 4 Phase comparison This investigation shows that maximum overvoltage can be recorded in the healthy phases as well as in the faulty phases following fault clearing. In this case the impact of the power frequency overvoltage is null. This characteristic is particularly defined on phase to phase to ground faults at clearing time and for both types of faults for Simulation 3 and 4, where the capacitor bank is connected. In each phase the overvoltage pattern is same for all resistor values. Constant overvoltages are broken by regular small bumps at the time where the voltage on the related phases have the highest magnitude at fault clearing or fault inception. 75/138

76 As noted in Section 2.4.9, the plots for the phase to phase to ground faults show that the time of fault is a key element to determine in which phase the maximum overvoltage is recorded Overvoltage Harmonic Response A further analysis was conducted on the frequency spectrum of the transient overvoltage. The study was done using the Fast Fourier Transform (FFT) function, which determine the harmonic magnitude and phase of the input signal as a function of time[21]. Base frequency was set to 12.5 Hz to detect noise elements lower than power frequency voltage. Here below few snapshots of the healthy and faulty phases for a phase to ground fault following a fault are provided. Snapshots were captured after 10 ms fault injection. Referring to the graphs left hand side spectrum refers to 12.5 Hz component and right hand side spectrum refers to HZ noise element. The number in the plot frame refers to the voltage magnitude in the first frequency band, 12.5 Hz Ea 0.0 [1] Figure 2.44: Configuration 1 Harmonic analysis NER 0 ohms after 10ms 76/138

77 60.0 Ea 0.0 [1] Figure 2.45: Configuration 1 Harmonic analysis NER 5 ohms after 10ms 60.0 Ea 0.0 [1] Figure 2.46: Configuration 1 Harmonic analysis NER 50 ohms after 10ms From the graphs we can observe that voltage noise appears at frequencies higher and lower values in respect of the power frequency voltage The major components are in the band close to 50 Hz. Components with lower magnitude, less than 3 % of nominal voltage, are visible on high frequency up to 300 Hz. The harmonics were recorded on the healthy phase. The graphs show that the noise is not limited to odd harmonics. Therefore, filtering this natural frequency component could be problematic and slow down the information process of the digital relays. 77/138

78 2.6 Conclusion 1. Results obtained from this study are in line with the existing literature and previous studies in this field. Maximum overvoltages are below 2.5 pu for a ground fault in an unloaded subtransmission line and below 4 pu in an unloaded line with a capacitor bank connected at the end of the line 2. Maximum overvoltage is influenced by the power system parameters. The size of the neutral earth resistor itself does not give a clear indication of the system response and expected maximum overvoltage during a ground fault 3. Increasing the size of the neutral earth resistor controls the maximum overvoltage on unloaded line as the resistor has a positive impact in limiting the natural frequency overvoltage until the fault current is equal to 10 % of the fault current with a solidly earthed system 4. The limitation of the transitory overvoltage component is strictly connected to the increase of the ratio R0/X0 until the R0<1/3 Xc0 factor and subsequent fault clearing restrike takes place. Therefore, to design a high impedance grounding, a transient recovery voltage study, TRV, should be conducted to safely determine the magnitude of the resistor 5. The positive effect of the neutral earth resistor is negligible if a capacitor bank is connected to an unloaded line. In this event, further increase of the resistor could generate dangerous overvoltages 6. Phase to ground faults generally produce higher overvoltage than the overvoltage response for phase to phase to ground faults for high value of neutral earth resistor are in the higher end of the of the value considered 7. There is a clear connection between line length and maximum overvoltage. Line length increases transient overvoltages. Requirement for system 78/138

79 analysis and related transient studies should be carefully considered for long subtransmission lines as is usually done for transmission lines 8. There is also a consistent similarity of results for line length of 10km and 0.2km. This characteristic should be taken into consideration to optimize the work during planning and design activities for short and medium length subtransmission lines 9. Transient overvoltages appear in the faulty and healthy phases. There is a relation between time of fault and location where the maximum overvoltages takes place 10. There is not a strong relation between the time of fault and the magnitude of the maximum overvoltage during phase to ground faults. Relation between the magnitude of the maximum overvoltage and the time of fault is only relevant on the phase to phase to ground faults 11. Considering the overvoltage component at higher frequency, this study shows that beyond the debate of effective or non effective grounding, design of the neutral earth resistors could safely aim to limit the ground fault current to 0.1pu of the fault current in a solidly earthed system without compromising the insulation level of the system 79/138

80 3 CVT and Transient Overvoltages 3.1 Overview Capacitive voltage transformer, CVT, represents the most common method of voltage source for the impedance protection relays installed for transmission and subtransmission power systems. Until few years ago the use of the CVT was limited to extra high voltage system. In recent years the use of the CVT has become more popular even for application with nominal voltage down to 33kV (as applied by Energex and Ergon in Queensland) due the lower capital cost compared to the traditional voltage transformer, VT. CVTs also offer the advantage to couple power line carrier to inject teleprotection signals between substations. During a fault, ideally, the low voltage source provided by the CVT should be an exact copy of the primary voltage. Unfortunately this is not the reality. CVT contains a large number of stored energy components which must charge and discharge during voltage changes [24-29]. The transfer of energy between capacitances and inductances within the CVT requires some time to take place. Therefore, the CVT introduces some transients that affect the magnitude and shape of the voltage signal. CVT creates some problems to the distance relays especially during faults that severely depress the voltage at the relay location. Faults located at the beginning of the protected line can bring the voltage down to few percents of the nominal voltage and this could affect accuracy and operating time of the relay. This section aims to investigate the CVT transient response during phase to ground fault and the impact that this disturbance can have on the distance relay. 80/138

81 3.2 The CVT Model Introduction The main component of the CVT are the capacitive voltage divider, a tuning reactor, a voltage transformer, a ferroresonance suppression circuit and the burden made by the relay. The CVT model can be further simplified where the equivalent circuit is referred to the intermediate voltage: The CVT model has been design according to this References [24, 25, 27] Capacitive Voltage Divider The capacitive voltage divider steps down the voltage from 66 to kv. A lower voltage is required to reduce the insulation of the subsequent voltage transformation and, therefore, to the overall cost of the CVT. The design of the capacitors is one of the main challenges faced by manufacturers. In fact, the higher the size of the capacitance, the lower is the magnitude of the transient overvoltage during sudden changes in the power system [24]. At the same time, CVT with a high capacitor size are more expensive. CVTs are often classified as normal C, high C and extra high C according to the value of the stack capacitance [25]. Typically the value of the capacitors is in the range of 100nF [24, 27]. A Normal C capacitor with a total value of 100 nf was selected. Tuning reactor 81/138

82 The tuning reactor is required to cancel out the value of the capacitive divider at the system frequency [24]. Therefore, the reactor prevents the phase shift between the voltage signal at the transmission line and the voltage fed to the relay. The size of the reactor is tuned for the system frequency of 50 Hz. In this study the reactor is 111 H. Voltage transformer The voltage transformer reduces the voltage from kv to a voltage suitable for the relay which is usually 100 or 110 V. Copper and iron losses were taken into consideration by adding in the CVT model a resistance of 2000 Ohm. Ferroresonance Suppression Circuit The ferroresonance suppression circuit is installed in the low voltage side of the voltage transformer to prevent dangerous overvoltages due to the saturation of the iron core of the step down transformer. Two types of ferroresonance suppression circuits are normally used by CVT manufacturers [24, 25, 27]: Active ferroresonance suppression circuit Passive ferroresonance suppression circuit The active ferroresonance suppression circuit consists of a parallel tuning circuit in series with a damping resistor. The LC parallel circuit behaves as an open circuit at the nominal power frequency to prevent shunt current flowing through the circuit. During transient condition the tuning circuit will decrease the impedance to the value of the damping resistor. The resistor will attenuate the energy of the transient surge [24] 82/138

83 The passive ferroresonance suppression circuit has a resistor, a saturable inductor and air gap loading resistor. In normal condition the voltage is not high enough to flash the air gap. During transient conditions, the air gap will flash over through the resistor and will attenuate the transient energy. Existing literature and previous study demonstrated that the passive ferroresonance circuit has better transient performance than the active device [24, 25, 27, 28, 31, 32]. However, as the study aims to test the CVT under the worst case scenario, the active suppression circuit was implemented. Device Data R= Ohms C=29 nf L=111 H CVT Burden The burden connected at the end of the circuit represents the static or numerical digital relays. Modern relays have a small burden. Typically the burden is below 5 VA. The value of the burden is 1000 Ohms 83/138

84 3.3 Simulation Settings Fault time System faults can happen at any time. Considering the two voltage magnitude end range, the fault could take place at 0 voltage or at maximum voltage. Fault at zero voltage are less common than faults at the maximum voltage. In fact, this type of fault could only happen during lightning or close of the circuit breaker on fault condition. Faults at the maximum voltage usually take place when insulation failure occurs and are statistically more likely to happen. Simulations were conducted with fault at zero crossing of the primary voltage. At this time the energy store in the capacitor is at the maximum and this condition will generate the worst voltage transient conditions. The sudden change in the system will release the energy stored in the active elements of the CVT creating the worst transient overvoltage condition. These voltage oscillations are composed of lower and higher frequency components. Network and Source Settings 66 kv subtransmission line model is as per the model used in Section 1 (Overvoltage). The line model was set to 2 km and 10 km. Short lines were selected to increase the source impedance ratio, SIR, and testing the performance of the CVT with a higher SIR. It is reminded that SIR is the ration between the source impedance and the line impedance at the relay point. 84/138

85 The source impedance was set to determine a starting SIR of 1 and 10 without the implementation of the NER. Then, the neutral earth resistor was increased to obtain the desired SIR. To conclude, four network models were investigated: 10km line length- SIR of 1( No NER), 5, 10, 20, 30 and 60 10km line length- SIR of 10( No NER), 20, 30 and 60 2 km line length- SIR of 1( No NER), 5, 10, 20, 30 and 60 10km line length- SIR of 10( No NER), 20, 30 and 60 85/138

86 3.4 Simulation Results Simulations results are provided in Appendix D and summarised in the graphs here below: Transient Overvoltage- 10km line- Initial SIR 1 Overvoltage ( pu) SIR 1 SIR 5 SIR 10 SIR 20 SIR 30 SIR 60 Time (ms) Figure 3.1: Peak overvoltage after fault- Initial SIR 1- Line length 10 km Transient Overvoltage- 10km line- Initial SIR 10 Overvoltage ( pu) SIR 10 SIR 20 SIR 30 SIR Time (ms) Figure 3.2: Peak overvoltage after fault- Initial SIR 10- Line length 10 km 86/138

87 Transient Overvoltage- 2km line- Initial SIR 1 Overvoltage ( pu) Time (ms) SIR 1 SIR 5 SIR 10 SIR 20 SIR 30 SIR 60 Figure 3.3: Peak overvoltage after fault- Initial SIR 1- Line length 2 km Transient Overvoltage- 2km line- Initial SIR 1 Overvoltage ( pu) Time (ms) SIR 10 SIR 20 SIR 30 SIR 60 Figure 3.4: Peak overvoltage after fault Initial SIR 10 - Line length 2 km Some snapshots of the Voltage are shown in Figure 3.5 to 3.8. Er is the voltage at the CVT output. Ea is the voltage at the CVT input. These snapshots were captured during the simulation with line length of 10 km and increasing SIR from 1 to 60. Please note the transient time constant and phase shift. 87/138

88 Figure 3.5: Peak overvoltage - Line length 10 km SIR 1 Figure 3.6: Peak overvoltage - Line length 10 km SIR 10 Figure 3.7: Peak overvoltage - Line length 10 km SIR 30 88/138

89 Voltage (kv) Er Ea Figure 3.8: Peak overvoltage - Line length 10 km SIR 60 From this study, the following observations are derived: 1. The application of the resistor increases the SIR ratio. The increase of the SIR ratio decreases the voltage at the relay point during a fault and increases the magnitude and phase shift of the transient overvoltage in respect to the steady state overvoltage 2. Transient overvoltage decay between 2 and 4 cycles after the fault event. The overvoltage decays with a longer time with high SIR ratio. This phenomena is a consequence of the voltage gap between the pre fault and post fault steady state. Transient components are also highlighted from the overvoltage snapshots 3. The results are consistent throughout the 4 simulations. The results are also in line with previous studies [24, 25, 27]. 4. The four simulations produce similar results at the same SIR ratio as shown in Figure 3.9 to These results confirm that grounding the system via the resistor do not have a peculiar impact on the CVT performance. The critical element for the transient results is the SIR. It influences more than the length of the transmission line or the source impedance. A high SIR will generate a significant voltage depression at the CVT location. 89/138

90 Overvoltage Comparison- SIR Overvoltage (pu) ime Simulation 1 Simulation 2 Simulation 3 Simulation Time (ms) Figure 3.9: Overvoltage comparison - Line length 10 km SIR 10 Overvoltage Comparison- SIR Overvoltage (pu) Simulation 1 Simulation 2 Simulation 3 Simulation Time (ms) Figure 3.10: Overvoltage comparison - Line length 10 km SIR 20 90/138

91 Overvoltage Comparison- SIR 30 Overvoltage (pu) Simulation 1 Simulation 2 Simulation 3 Simulation Time (ms) Figure 3.11: Overvoltage comparison - Line length 10 km SIR 30 Overvoltage Comparison- SIR Overvoltage (pu) Simulation 1 Simulation 2 Simulation 3 Simulation Time (ms) Figure 3.12: Overvoltage comparison - Line length 10 km SIR The size of the capacitance has an impact on the transient overvoltage produced by the CVT [24, 27]. Three snapshots shown with a higher Capacitance of 200, 400nF of 1000 nf were recorded as shown on Figure 3.13 to /138

92 Figure 3.13: Overvoltage comparison - Line length 10 km SIR 10-Capacitance of 100 nf Figure 3-14:Overvoltage comparison - Line length 10 km SIR 10-Capacitance of 200 nf Figure 3.15:- Overvoltage comparison - Line length 10 km SIR 10-Capacitance of 400 nf 92/138

93 Figure 3.16: Overvoltage comparison - Line length 10 km SIR 10-Capacitance of 1000 nf The graphs show that the fist peak decreases from 8.12 kv, with a capacitance of 100 nf to 4.64 kv with a capacitance value of 1000 nf. Decay time is also reduced. These results confirm that CVT manufacturers need to consider the size of the capacitance which needs to be traded off with the higher cost of large capacitors. 6. To verify the CVT transient performance in relation to the power factor of the relay burden, three different simulation with SIR ratio of 10 and burden power factor of 1, 0.7 and 0.1 were performed. Voltage (kv) Er Ea Figure 3.17: Overvoltage comparison - Line length 10 km SIR 10-PF 1 93/138

94 Voltage (kv) Er Ea Figure 3.18: Overvoltage comparison - Line length 10 km SIR 10-PF 0.7 Figure 3.19: Overvoltage comparison - Line length 10 km SIR 10-PF 0.1 The results show that the CVT transient characteristic is affected by the inductive burden. Therefore, it is customary to keep the voltage input relay burden as close as possible to unity power factor. Therefore, digital relays with their resistive burden perform better than the old electromechanical relays with the inductive coil. 7. CVT transients are also affected by the magnitude of the connected burden[24]. A simulation has been conducted to determine the response of the CVT with a SIR 30 and a burden of Ohm 94/138

95 Er Ea Voltage (kv) Figure 3.20: Overvoltage comparison - Line length 10 km SIR 10-Burden Figure 3.21: Overvoltage comparison Additional simulation- Line length 10 km SIR 10- Burden The benefit of the additional damping resistor is confirmed as per existing literature [25, 27]. 8. The post fault voltage is the result of the superpositions of several frequency components. To simplify the analysis we can assume that four main frequency components are the major elements: Power frequency component High frequency decay component Low frequency decay component 95/138