Thesis Project: Power Quality Analysis at Murdoch University eng470: Engineering Honours Thesis
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Thesis Project: Power Quality Analysis at Murdoch University ENG470: Engineering Honours Thesis School of Engineering and Information Technology Murdoch University Written By: Abdullah Abdullah Academic Supervisor: Dr. Ali Arefi Unit Coordinators: 1. Prof. Parisa Bahri 2. Dr. Gareth Lee A thesis submitted to the School of Engineering and Information Technology, Murdoch University to fulfil the requirements for the degree of: H1264: Bachelor of Engineering Honours [BE(Hons)] 1. Electrical Power Engineering 2. Renewable Energy Engineering Date: 02/07/2018 Author’s Declaration I, Abdullah Abdullah, do hereby declare that this thesis is an original work and in its entirety is a product of my own efforts and commitment to the research of power quality analysis at Murdoch University. This report signifies the final component of the mandatory requirements for the achievement of a Bachelor’s Degree in Engineering (Honours) majoring in Electrical Power and Renewable Energy Engineering. To the best of my ability, I have ensured that all relevant sources which have contributed to the ideas within this thesis be it directly or indirectly, have been credited and suitably acknowledged. Name: . . . . . . . . . . . . . . . . . . . . . . . . . . Date: . . . . . . . . . . . . . . . . . . . . . . . . . . . Signature: . . . . . . . . . . . . . . . . . . . . . . . Power Quality Analysis at Murdoch University iii Abstract Introduction of new electronic technologies that are more sensitive to disturbances in the power network has made it necessary to monitor the quality of power supplied. This project presents a study of the power quality at Murdoch University, and aims at identifying the causes of poor power quality and provide the solutions to these power quality problems. The main objective of this project is to analyse the power quality of the electricity system of Murdoch University in order to identify the causes of poor power quality and provide solutions to improve the power quality. Furthermore, for all the specific goals that had been set before the commencement of this project have all been accomplished. The most common power quality issues in most electrical networks globally include voltage sags, voltage swells, voltage transients, and frequency harmonics. Other power quality issues that can be experienced include voltage flicker, voltage unbalance, inter-harmonics, brownouts, direct current, short-term and long-term power interruption. A discussion of all these issues has been conducted in this report with their impacts on equipment in the network identified to be wear and tear, overheating, tripping of circuit breakers, and improper functioning of equipment. The compensating devices that can be used for improving power quality such as Active and Passive filters, Distribution Static Compensator, Dynamic Voltage Restorer, and Unified Power Quality Conditioner have also been analysed in this report. The direct and indirect costs of power quality have been reviewed in this project with the finding that 50 % of the global turnover of the electricity sector equating to 500 billion Euros is lost annually due to poor power quality. IEEE 519, AS/NZS 61000.3.2.2003, ANSI C84.1, IEEE 1459, EN 50160, and IEC 61000 are some of the power quality standards that have been discussed in Power Quality Analysis at Murdoch University iv this project. Based on the data collected from Murdoch University, particularly from 1.330, in substation 12, South Street Building 330, chancellery. Subsequently, the power quality problems experienced at the institution include voltage unbalance of 0.295 or 0.3486 % based on NEMA and IEC formula respectively, a 42.26 % current unbalance, distortions in current angle, power factor, and harmonic distortion in the range of 1.8 to 2.8 %. Power Quality Analysis at Murdoch University v Acknowledgements This report could not have been conceived without the wise guidance and caring support of those around me. It is here that I would like to express my deepest gratitude for their unwavering and much needed support throughout the course of my education at Murdoch University. First of all, I would like to thank my supervisor Dr. Ali Arefi for his help in this project, through his constant guidance and by sharing his expertise in Electrical Power Systems. Not only has it been a pleasure to work with Dr. Ali Arefi, but his experience in Electrical Engineering was invaluable to me as to improve as an Electrical Power Engineer. Moreover, his outstanding work ethic, remarkable integrity, and the care he showed to me as his student encouraged me to do my best in developing this project. It has been an honour Dr. Ali Arefi. Secondly, I would like to extend my gratitude and love to my family and friends, who have consistently pushed and supported me throughout my education. This would not have been possible without them. Furthermore, I would like to thank the Government of Kuwait and specifically the Ministry of Higher Education for believing in me and for sponsoring me with this scholarship. Last but certainly not least, I would like to thank all of the academic staff at Murdoch University. In particular Prof. Parisa Bahri, Dr. Gareth Lee, Dr. Martina Calais, A/Prof. Graeme Cole, Dr. David Parlevliet, Dr. Gregory Crebbin, Dr. Jonathan Whale, Dr. Farhad Shahnia, Dr. Xiangpeng Gao, Dr. Manickam Minakshi, Dr. Linh Vu, and Dr. GM Shafiullah. Power Quality Analysis at Murdoch University vi Table of Contents Author’s Declaration .................................................................................................................... Abstract ..................................................................................................................................... iii Acknowledgements .................................................................................................................... v List of Figures ............................................................................................................................. x List of Tables ............................................................................................................................ xii List of Abbreviations and Acronyms ....................................................................................... xiii Chapter 1: Introduction ............................................................................................................. 1 1.1 Background Information about Power Quality ........................................................... 1 1.2 Aims and Objectives .................................................................................................... 3 1.3 Significance of the Project ........................................................................................... 3 1.3.1 Significance to Power Consumers ............................................................................. 3 1.3.2 Significance to the Electric Utility ............................................................................. 4 1.4 Thesis Project Outline ...................................................................................................... 5 Chapter 2: Literature Review ..................................................................................................... 6 2.1 Introduction into Power................................................................................................... 6 2.2 Power Quality Problems ................................................................................................ 10 2.2.1 Voltage Sags ............................................................................................................ 11 2.2.2 Voltage Swells ......................................................................................................... 12 Power Quality Analysis at Murdoch University vii 2.2.3 Voltage Flicker ......................................................................................................... 13 2.2.4 Voltage Unbalance .................................................................................................. 15 2.2.5 Voltage Transients .................................................................................................. 16 2.2.6 Voltage Notching .................................................................................................... 17 2.2.7 Brownouts ............................................................................................................... 19 2.2.8 Short Interruptions of Power .................................................................................. 19 2.2.9 Long Interruptions of Power ................................................................................... 19 2.2.10 Direct Current ....................................................................................................... 20 2.2.11 Electrical Noise ...................................................................................................... 21 2.2.12 Harmonic Distortion.............................................................................................. 21 2.2.13 Interharmonics ...................................................................................................... 23 2.2.14 Electromagnetic Interference (EMI) ..................................................................... 24 2.3 Power Quality Standards ............................................................................................... 25 2.3.1 IEEE 519 Standard ................................................................................................... 25 2.3.2 IEEE 1459 Standard ................................................................................................. 25 2.3.3 ANSI C84.1 Standard ............................................................................................... 25 2.3.4 EN 50160 Standard ................................................................................................. 26 2.3.5 IEC 61000 Standard ................................................................................................. 26 2.3.6 AS/NZS 61000.3.2:2003 Standard ........................................................................... 27 2.4 Power Quality Monitoring ............................................................................................. 28 Power Quality Analysis at Murdoch University viii 2.5 Using Compensating Devices to Improve Power Quality Disturbances ........................ 29 2.5.1 Active and Passive Filters ........................................................................................ 29 2.5.2 Distribution Static Compensator (D-STATCOM) ..................................................... 32 2.5.3 Dynamic Voltage Restorer (DVR) ............................................................................ 33 2.5.4 Unified Power Quality Conditioner (UPQC) ............................................................ 34 2.6 Power Quality Case Studies ........................................................................................... 35 2.7 Costs of Power Quality Problems .................................................................................. 38 Chapter 3: Methodology .......................................................................................................... 40 Chapter 4: Results and Discussion of Data Analysis ................................................................ 42 4.1 Voltage Unbalance ......................................................................................................... 42 4.2 Phase Distortion ............................................................................................................. 46 4.3 Harmonic Distortion ....................................................................................................... 47 4.4 Power Factor .................................................................................................................. 48 Chapter 5: Conclusion and Future Work ................................................................................. 57 5.1 Future Work Opportunities for the Power Quality Analysis at Murdoch University .... 59 Reference List ........................................................................................................................... 61 Appendix .................................................................................................................................. 68 Appendix Part A: EM133 Meter Manual .............................................................................. 68 Appendix Part B: Calculation of Apparent Power and Power Factor .................................. 70 Appendix Part C: Calculation of Voltage Unbalance ............................................................ 71 Power Quality Analysis at Murdoch University ix Appendix Part D: Risk Assessment ....................................................................................... 72 Power Quality Analysis at Murdoch University x List of Figures Figure 1: Impedance Triangle [36] ............................................................................................. 7 Figure 2: Energy Consumption by Sectors in European Union Countries [12] .......................... 9 Figure 3: Most Common Power Quality Problems [16] ........................................................... 11 Figure 4: Waveform Illustrating Voltage Sags [19] .................................................................. 12 Figure 5: Waveform Showing Voltage Swells [19] ................................................................... 13 Figure 6: Waveform Indicating Voltage Flicker [23] ................................................................ 14 Figure 7: Voltage Unbalance between Three Phases [25] ....................................................... 15 Figure 8: Relationship between Voltage Unbalance and Increase in Temperature [24] ........ 16 Figure 9: Types of Transients [19] ............................................................................................ 17 Figure 10: Voltage Notching [27] ............................................................................................. 18 Figure 11: Direct Current Offset [29] ....................................................................................... 20 Figure 12: Electrical Noise [19] ................................................................................................ 21 Figure 13: Harmonic Distortion [34] ........................................................................................ 23 Figure 14: Passive Low Pass Filter [1] ...................................................................................... 30 Figure 15: High Pass Filter [1] .................................................................................................. 30 Figure 16: Ideal D-STATCOM [20] ............................................................................................ 33 Figure 17: Schematic of a DVR [20].......................................................................................... 33 Figure 18: UPQC Schematic [20] .............................................................................................. 35 Figure 19: System Operation Snapshot [1] .............................................................................. 37 Figure 20: Voltage versus Time for Three Voltage Lines ......................................................... 42 Figure 21: Voltage versus Time Including Average Voltage of the Three Phases ................... 43 Power Quality Analysis at Murdoch University xi Figure 22: Percent Voltage Unbalance for all Three Phases against Time of the Day ............ 44 Figure 23: Current Magnitude versus Time ............................................................................. 45 Figure 24: Percent Current Unbalance for all Three Phases against Time of the Day ............ 45 Figure 25: Voltage Angles for the Three Phases Plotted against Time .................................... 46 Figure 26: Current Angles for Three Phases Plotted against Time .......................................... 47 Figure 27: THD for all the Phases ............................................................................................. 48 Figure 28: Power Factor for all Phases .................................................................................... 49 Figure 29: Real Power for all Phases ........................................................................................ 50 Figure 30: Reactive Power for all Phases ................................................................................. 50 Figure 31: Apparent Power for all Phases ............................................................................... 51 Figure 32: Current Demanded by the Load in each Phase ...................................................... 52 Figure 33: Phase to Phase and Phase to Neutral Voltages at Varying Power Factors............. 53 Figure 34: Unbalance Voltage Factor ....................................................................................... 55 Figure 35: EM133 Meter Manual [65] ..................................................................................... 69 Power Quality Analysis at Murdoch University xii List of Tables Table 1: Power Quality Standards ............................................................................................ 27 Table 2: Comparison between Active and Passive filter [43] .................................................. 32 Table 3: Power Data for the Phase A at 12:00:30 am .............................................................. 70 Table 4: Voltage Unbalance Using IEC 61000 Standard........................................................... 71 Power Quality Analysis at Murdoch University xiii List of Abbreviations and Acronyms A Ampere a Delay Angle AC Alternating Current ANSI American National Standard Institute AS/NZS Australian/New Zealand Standard ASDs Adjustable Speed Drives C Capacitor DC Direct Current D-STATCOM Distribution Static Compensator DVR Dynamic Voltage Restorer E DC Theoretical Direct Voltage EMI Electromagnetic Interference EN European Standard EPRI Electrical Power Research Institute E X Direct Voltage Drop f b Fundamental Frequency of the System GPO General Purpose Outlet Hz Hertz I DC Direct Current in Per Unit on a Converter Base IEC Electromagnetic Compatibility IEEE Institute of Electrical and Electronic Engineers Power Quality Analysis at Murdoch University xiv I s Current Source IZ Current and Impedance kHz Kilohertz kVA Kilo Volt Ampere kVA Kilovolt-Ampere kVAr Kilo Volt Ampere Reactive kW Kilowatt L Inductor NEMA National Electrical Manufacturers Association NPL National Power Laboratory P Real Power PCC Point of Common Coupling PQ Power Quality PQM Power Quality Monitoring PV Photovoltaic Q Reactive Power R Resistor RFI Frequency Interference RMS Root Mean Square S Apparent Power THD Total Harmonic Distortion u Commutation Angle UBF Unbalance Factor Power Quality Analysis at Murdoch University xv UPQC Unified Power Quality Conditioner UPS Uninterruptible Power Supply V Volt V av Average Voltage V frms RMS Voltage of Fundamental Frequency V nrms RMS Voltage of the nth Harmonic V s Voltage Source VSC Voltage Source Converter VSD Variable Speed Drive VSIs Voltage Source Inverters V t Terminal Voltage X s System Reactance in Per Unit on a Converter Base X t Converter Transformer Reactance in Per Unit on a Converter Base Power Quality Analysis at Murdoch University xvi Power Quality Analysis at Murdoch University 1 Chapter 1: Introduction 1.1 Background Information about Power Quality According to Institute of Electrical and Electronic Engineers (IEEE), power quality is defined as the idea of powering and grounding electronic equipment in such a way that it is fit for the functioning of that equipment and well matched with the wiring system of the premises and other connected equipment [1]. Grounding refers to a conducting connection through which electrical equipment have a connection to earth [2]. Power quality can also be defined as the ability of a grid or electrical network to supply clean stable power that is always available when required by a consumer. The cleanliness of the power is in terms of the wave shape, which should be pure sinusoidal, noise-free, and within the allowable tolerances for voltage and frequency [3]. The cost and reliability of any electrical system significantly depend on the supplied and consumed power quality of the system. The demand for clean and reliable power in the world has increased in the last several decades due to the introduction of new electronic technologies [4]. There has been tremendous growth in the use of power electronics in almost every type of electronic device making them more sensitive to the present disturbances in the network. In most cases, there has not been an improvement in the quality of designs for electronic devices to accompany its growth [5]. This has resulted in the distortion of the distribution network. Therefore, the electronic devices may malfunction during operation under the distorted distribution networks. For this reason, power quality monitoring and analysis need to be conducted to predict the occurrence of a disturbance, its causes, and to Power Quality Analysis at Murdoch University 2 develop a strategy in order to minimize the disturbance with the aim of protecting the devices connected in the network. Power quality monitoring (PQM) involves the collection, analysis, and interpretation of raw power measured data into information that is useful. This process consists of measuring the currents and voltages of the supply over a specified period and studying their waveforms. Wiring, grounding, and equipment connections can be inspected throughout power quality monitoring [6]. PQM is vital to detect potential and present power quality disturbances, which may damage equipment or decrease their lifespans. Instruments that use monitoring power quality include: 1) Power quality metre and analyser used for measuring, recording and storing signal frequency, harmonics, real and apparent power, phase rotation, current, and voltage. 2) Oscilloscope for measuring and displaying harmonics, current, and voltage. 3) Circuit monitor for measuring voltage sag, swell, flicker, and harmonics. 4) Flicker meter for voltage flicker measurement. 5) Disturbance analyser for voltage transients, power outages, and voltage sags measurement. 6) The in-plant power monitor used for monitoring voltage profile, voltage swells, sags, flicker, and harmonics in the power system [6]. This project has attempted to perform a power quality analysis at Murdoch University. The focus of this thesis is to identify the causes of poor power quality at the institution, to quantify its impacts on equipment and operations and to provide a solution for the improvement of the quality of power. An extensive study of various literature sources has been conducted to get background information about power quality, monitoring techniques, disturbances, Power Quality Analysis at Murdoch University 3 impacts, and mitigation strategies. Data has then been collected from the university and analysed to determine the power quality issues affecting Murdoch University, and solutions are introduced based on the literature reviewed. Afterward, the data is further analysed to improve the power quality at the institution. 1.2 Aims and Objectives The major objective of this project is to analyse the power quality of the electricity system of Murdoch University to identify the causes of poor quality and provide possible solutions in order to improve the power quality. This can be achieved through the following specific objectives: 1. To identify power quality issues. 2. To investigate causes, effects, and proper ways of improving poor power quality. 3. To study the IEEE STD 519 standard and others power quality standards. 4. To obtain power data from Murdoch University and using it afterward for quality analysis against the standards. 5. To calculate the power quality parameters based on collected data. 1.3 Significance of the Project Monitoring, analysis, and mitigation of (PQ) are factors vital to various sections of the economy as well as several industries. Subsequently, the significance of this project can be divided into two main parts: to consumers and the electric utility. 1.3.1 Significance to Power Consumers Through the collection and analysis of power quality data that has been conducted in this research, the occurrence of recurrent power quality disturbances can be accurately predicted. Power Quality Analysis at Murdoch University 4 This would be significant to consumers as accurate prediction and mitigation of the disturbances would result in decreased malfunction of electronic devices connected to the distribution network. This project is also substantial to consumers, as it would help to decrease the costs associated with repairs or replacement of malfunctioned electronic devices in the distribution network. By providing a solution to the power quality disturbances, the likelihood of occurrence of these disturbances and their impacts would be minimized significantly. Thus, there would be fewer malfunctioned consumer devices. The significance of this project to industrial consumers is in the form of experiencing lower interruptions during production due to power quality issues. The effect of this is increased production and reduced losses due to equipment damage. Moreover, this reduces the costs based on idle personnel during interruptions. 1.3.2 Significance to the Electric Utility Through this study, the primary power quality disturbance in a distribution network has been identified. The important source of the disturbance either generation, distribution, transmission, or consumer that can also be determined. These issues make this research essential to the electric utility operators, as it would help them understand their network better. Power Quality Analysis at Murdoch University 5 1.4 Thesis Project Outline This dissertation is divided into five chapters which consist of: Chapter 1 contains the background information on power quality, a brief description of the research problem and the research focus. Project objectives and significance of the research are also found in this chapter. Chapter 2 provides the present literature, the various ideas as well as issues in power quality analysis. Specifically, it will consider the implementation of compensating devices to mitigate the effects of power quality disturbances. In addition, the financial repercussions of power quality problems, along with a multitude of power quality standards will be examined. Finally, this chapter explores some power quality case studies. Chapter 3 describes the methodology of the project. Here the techniques employed in the research and execution of the project is presented and discussed. Chapter 4 discusses the data analysis and calculations done to determine the numerous of power quality problems at the university. Then the results are presented and reviewed to validate that indeed the power quality problems have been identified correctly. Chapter 5 will conclude the project and provide some future work opportunities for the power quality analysis at Murdoch University. Power Quality Analysis at Murdoch University 6 Chapter 2: Literature Review 2.1 Introduction into Power An electric power system is made up of three main parts: a generation system, which is the source of power; a delivery system made up of the transmission and distribution networks and the load that can be transport (traction/railroad), residential, commercial, industrial, and governmental consumers [7]. Electric power defined as the rate of converting electric energy [8] can be generated using conventional methods such as hydroelectric power generation, thermal generating plants, synchronous machinery, and distributed machinery. Moreover, electric power can be generated using nonconventional methods such as photovoltaic (PV) modules, wind generation, tidal generation, and advanced technologies like fuel cells [9]. In direct current (DC) systems, power is given as the product of current and voltage [8] thus: Power (Watts) = Voltage (V) × Current (A) (1) P = V × I In alternating current (AC) systems, instantaneous power is given as the product of the instantaneous current, and instantaneous voltage expressed mathematically as [10]: p (t) = v(t)i(t) = V m I m 2 [cos ϕ + cos(2ωt − ϕ)] (2) Where: ϕ is the phase angle. The resistor, inductor, and capacitor define the total impedance of an AC circuit, which affects the current and voltage waveforms and hence their phase difference. In AC circuits, the impedance of the circuit is defined as the ratio of the current and voltage phasors created by Power Quality Analysis at Murdoch University 7 a circuit component. Both reactance and resistance make up the total impedance (Z) that restricts the flow of current around the circuit. The following Figure 1 shows an impedance triangle, which is used to demonstrate the geometric relationship between reactance, resistance, and impedance [36] From the impedance triangle, it is clear that the algebraic sum of the reactive and resistive ohmic values is not equal to the impedance, as a 90 0 phase difference exists between a pure reactance and a pure resistance. As a result, the impedance is a vector sum of the reactance and the resistance and has both of phase angle (𝜃) and of magnitude (Z). When the frequency of the AC supply changes, the reactance (X) changes but the resistance remains constant, thus there will be a change in shape of the impedance triangle. The following relationships hold between resistance, reactance, and impedance [37]. Z 2 = R 2 + jX 2 (3) cos θ = R Z (4) In AC systems, the average power consumed in a circuit is referred to as real power (also referred to as active power or true power) and is dependent on the power factor (cosine of the phase angle). The power supplying stored energy in reactive elements is known as the Figure 1: Impedance Triangle [36] Power Quality Analysis at Murdoch University 8 reactive power. As opposed to active power and reactive power, which reduces the power in the circuit in order to create and reduce both the inductive magnetic fields and capacitive electrostatic fields thereby decreasing the supply of true power to the load. The power stored by a capacitors electrostatic field tries to control the voltage while the power stored in the inductors magnetic field tries to control the current. This result in the generation of reactive power by capacitors and consumption of the reactive power by inductors. Apparent power is defined as the product of RMS values of current and voltage across a circuit [10]. These mathematically are expressed as shown using the following equations 5-7 [11]: Real Power: P = |V̅||I̅| cos ϕ (5) Reactive Power: Q = |V̅||I̅| sin ϕ (6) Apparent Power = |S̅| = |V̅||I̅| = √P 2 + Q 2 (7) Power factor defined as the ratio of the active power (P) to the apparent power (S) is an essential part of an AC circuit [38]. Power factor defines the phase angle between the voltage and current waveforms and is expressed mathematically as [38]: Power Factor = P S = VI cos θ VI = cos θ (8) In a purely resistive circuit, there is zero phase difference between the real and the apparent power as the voltage and current waveforms are in phase. Accordingly, the power factor will be unity. Power Factor = cos(0) = 1 (9) Unity power factor means that the actual power consumed is equal to the apparent power consumed. In a purely reactive circuit, the voltage waveform and the current waveform are 90 degrees out of phase. Thus the power factor will be: Power Quality Analysis at Murdoch University 9 Power Factor = cos(90) = 0 (10) This means that the wattage consumed is zero, but there is a voltage and current supplying the reactive load. Lagging power factor refers to an inductive circuit where the current lags the voltage while a leading power factor refers to a capacitive circuit where the current leads the voltage. Improving the power factor by decreasing the reactive power component is therefore important to have efficient use of the power by the load. Energy consumption is divided into four major sectors as illustrated in Figure 2 below for European Union countries. From Figure 2, it is indicated that industry accounts for the highest consumption of electrical power at 36 % followed by households at 31 %, commercial sectors at 30 %, and finally transport at 3 % [12]. Figure 2: Energy Consumption by Sectors in European Union Countries [12] Power Quality Analysis at Murdoch University 10 2.2 Power Quality Problems Power quality issues are caused mainly by the use of sensitive electronic loads such as digital controllers and computers. Some of these devices have problems due to either their proximity to other electrical equipment or due to disturbances in the powerline since higher power loads generate greater disturbances. Examples of devices that create high power loads: electric variable speed drives, welders, and arc furnaces [1]. Impacts of poor power quality include accelerated tear and wear, overheating, improper function of electrical and electronic equipment, tripping of circuit breakers, and this sometimes results in hazardous conditions [13]. A survey by experts in power quality show that about half of all power quality problems are associated with ground current, ground loops, neutral to ground voltages, ground bonds, grounding, and other ground related issues [14]. Essential power parameters that affect the quality of power delivered include voltage sags, voltage swells, voltage flicker, voltage unbalance, voltage transients, voltage notching, brownouts, short-term and long-term power interruptions, direct current, electrical noise, harmonic distortion, inter-harmonics, and electromagnetic interference. According to research conducted by EPRI, power quality problems arising from customers premises account for 70 % of all disturbances, while the network side accounts for 30 % [15]. From these parameters, the most common power quality problems among American consumers were identified as voltage sags, voltage swells, voltage transients caused by capacitor switching, and harmonics among other related issues [16][17]. Figure 3 demonstrates the most common power quality problems, and this is discussed below. Power Quality Analysis at Murdoch University 11 2.2.1 Voltage Sags Voltage sags, which are also known as voltage dips, define a decrease of the voltage supply level by 10 -90 % of the normal voltage, over a short duration of time (half a cycle to 1 minute) [18]. Voltage sags are usually caused by distribution and transmission system faults. Moreover, faults in installations at the consumer premises, energization of the transformer and large motors starts, may result in voltage sags [19]. The following Figure 4 represents a voltage sag. 48% 22% 15% 6% 5% 2% 1% 1% Download 1.28 Mb. Do'stlaringiz bilan baham: 
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