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Electrical power systems quality second edition
Electrical Power Systems Quality, Second Edition CHAPTER 1: INTRODUCTION What is Power Quality? Power Quality -- Voltage Quality Why Are We Concerned About Power Quality? The Power Quality Evaluation Procedure Who Should Use This Book Overview of the Contents CHAPTER 2: TERMS AND DEFINITIONS Need for a Consistent Vocabulary General Classes of Power Quality Problems Transients Long-Duration Voltage Variations Short-Duration Voltage Variations Voltage Imbalance Waveform Distortion Voltage Fluctuation Power Frequency Variations Power Quality Terms Ambiguous Terms CBEMA and ITI Curves References CHAPTER 3: VOLTAGE SAGS AND INTERRUPTIONS Sources of Sags and Interruptions Estimating Voltage Sag Performance Fundamental Principles of Protection Solutions at the End-User Level Evaluating the Economics of Different Ride-Through Alternatives Motor-Starting Sags Utility System Fault-Clearing Issues References CHAPTER 4: TRANSIENT OVERVOLTAGES Sources of Transient Overvoltages Principles of Overvoltage Protection Devices for Overvoltage Protection Utility Capacitor-Switching Transients Utility System Lightning Protection Managing Ferroresonance Switching Transient Problems with Loads Computer Tools for Transients Analysis References CHAPTER 5: FUNDAMENTALS OF HARMONICS Harmonic Distortion Voltage versus Current Distortion Harmonics versus Transients Harmonic Indexes Harmonic Sources from Commercial Loads Harmonic Sources from Industrial Loads Locating Harmonic Sources System Response Characteristics Effects of Harmonic Distortion Interharmonics References Bibliography CHAPTER 6: APPLIED HARMONICS Harmonic Distortion Evaluations Principles for Controlling Harmonics Where to Control Harmonics Harmonic Studies Devices for Controlling Harmonic Distortion Harmonic Filter Design: A Case Study Case Studies Standards of Harmonics References Bibliography CHAPTER 7: LONG-DURATION VOLTAGE VARIATIONS Principles of Regulating the Voltage Devices for Voltage Regulation Utility Voltage Regulator Application Capacitors for Voltage Regulation End-User Capacitor Application Regulating Utility Voltage with Distributed Resources Flicker References Bibliography CHAPTER 8: POWER QUALITY BENCHMARKING Introduction Benchmarking Process RMS Voltage Variation Indices Harmonics Indices Power Quality Contracts Power Quality Insurance Power Quality State Estimation Including Power Quality in Distribution Planning References Bibliography CHAPTER 9: DISTRIBUTED GENERATION AND POWER QUALITY Resurgence of DG DG Technologies Interface to the Utility System Power Quality Issues Operating Conflicts DG on Distribution Networks Siting DGDistributed Generation Interconnection Standards Summary References Bibliography CHAPTER 10: WIRING AND GROUNDING Resources Definitions Reasons for Grounding Typical Wiring and Grounding Problems Solutions to Wiring and Grounding Problems Bibliography CHAPTER 11: POWER QUALITY MONITORING Monitoring Considerations Historical Perspective of Power Quality Measuring Instruments Power Quality Measurement Equipment Assessment of Power Quality Measurement Data Application of Intelligent Systems Power Quality Monitoring Standards References Index 1 Introduction Both electric utilities and end users of electric power are becoming increasingly concerned about the quality of electric power. The term power quality has become one of the most prolific buzzwords in the power industry since the late 1980s. It is an umbrella concept for a mul- titude of individual types of power system disturbances. The issues that fall under this umbrella are not necessarily new. What is new is that engineers are now attempting to deal with these issues using a system approach rather than handling them as individual problems. There are four major reasons for the increased concern: 1. Newer-generation load equipment, with microprocessor-based con- trols and power electronic devices, is more sensitive to power qual- ity variations than was equipment used in the past. 2. The increasing emphasis on overall power system efficiency has resulted in continued growth in the application of devices such as high-efficiency, adjustable-speed motor drives and shunt capacitors for power factor correction to reduce losses. This is resulting in increasing harmonic levels on power systems and has many people concerned about the future impact on system capabilities. 3. End users have an increased awareness of power quality issues. Utility customers are becoming better informed about such issues as interruptions, sags, and switching transients and are challenging the utilities to improve the quality of power delivered. 4. Many things are now interconnected in a network. Integrated processes mean that the failure of any component has much more important consequences. Chapter 1 Source: Electrical Power Systems Quality Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. The common thread running though all these reasons for increased concern about the quality of electric power is the continued push for increasing productivity for all utility customers. Manufacturers want faster, more productive, more efficient machinery. Utilities encourage this effort because it helps their customers become more profitable and also helps defer large investments in substations and generation by using more efficient load equipment. Interestingly, the equipment installed to increase the productivity is also often the equipment that suffers the most from common power disruptions. And the equipment is sometimes the source of additional power quality problems. When entire processes are automated, the efficient operation of machines and their controls becomes increasingly dependent on quality power. Since the first edition of this book was published, there have been some developments that have had an impact on power quality: 1. Throughout the world, many governments have revised their laws regulating electric utilities with the intent of achieving more cost-com- petitive sources of electric energy. Deregulation of utilities has compli- cated the power quality problem. In many geographic areas there is no longer tightly coordinated control of the power from generation through end-use load. While regulatory agencies can change the laws regarding the flow of money, the physical laws of power flow cannot be altered. In order to avoid deterioration of the quality of power supplied to customers, regulators are going to have to expand their thinking beyond traditional reliability indices and address the need for power quality reporting and incentives for the transmission and distribution companies. 2. There has been a substantial increase of interest in distributed generation (DG), that is, generation of power dispersed throughout the power system. There are a number of important power quality issues that must be addressed as part of the overall interconnection evalua- tion for DG. Therefore, we have added a chapter on DG. 3. The globalization of industry has heightened awareness of defi- ciencies in power quality around the world. Companies building facto- ries in new areas are suddenly faced with unanticipated problems with the electricity supply due to weaker systems or a different climate. There have been several efforts to benchmark power quality in one part of the world against other areas. 4. Indices have been developed to help benchmark the various aspects of power quality. Regulatory agencies have become involved in performance-based rate-making (PBR), which addresses a particular aspect, reliability, which is associated with interruptions. Some cus- tomers have established contracts with utilities for meeting a certain quality of power delivery. We have added a new chapter on this subject. 2 Chapter One Introduction Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 1.1 What Is Power Quality? There can be completely different definitions for power quality, depend- ing on one’s frame of reference. For example, a utility may define power quality as reliability and show statistics demonstrating that its system is 99.98 percent reliable. Criteria established by regulatory agencies are usually in this vein. A manufacturer of load equipment may define power quality as those characteristics of the power supply that enable the equipment to work properly. These characteristics can be very dif- ferent for different criteria. Power quality is ultimately a consumer-driven issue, and the end user’s point of reference takes precedence. Therefore, the following def- inition of a power quality problem is used in this book: Any power problem manifested in voltage, current, or frequency devia- tions that results in failure or misoperation of customer equipment. There are many misunderstandings regarding the causes of power quality problems. The charts in Fig. 1.1 show the results of one survey conducted by the Georgia Power Company in which both utility per- sonnel and customers were polled about what causes power quality problems. While surveys of other market sectors might indicate differ- ent splits between the categories, these charts clearly illustrate one common theme that arises repeatedly in such surveys: The utility’s and customer’s perspectives are often much different. While both tend to blame about two-thirds of the events on natural phenomena (e.g., light- ning), customers, much more frequently than utility personnel, think that the utility is at fault. When there is a power problem with a piece of equipment, end users may be quick to complain to the utility of an “outage” or “glitch” that has caused the problem. However, the utility records may indicate no abnor- mal events on the feed to the customer. We recently investigated a case where the end-use equipment was knocked off line 30 times in 9 months, but there were only five operations on the utility substation breaker. It must be realized that there are many events resulting in end-user prob- lems that never show up in the utility statistics. One example is capaci- tor switching, which is quite common and normal on the utility system, but can cause transient overvoltages that disrupt manufacturing machinery. Another example is a momentary fault elsewhere in the sys- tem that causes the voltage to sag briefly at the location of the customer in question. This might cause an adjustable-speed drive or a distributed generator to trip off, but the utility will have no indication that anything was amiss on the feeder unless it has a power quality monitor installed. In addition to real power quality problems, there are also perceived power quality problems that may actually be related to hardware, soft- Introduction 3 Introduction Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. ware, or control system malfunctions. Electronic components can degrade over time due to repeated transient voltages and eventually fail due to a relatively low magnitude event. Thus, it is sometimes dif- ficult to associate a failure with a specific cause. It is becoming more common that designers of control software for microprocessor-based equipment have an incomplete knowledge of how power systems oper- ate and do not anticipate all types of malfunction events. Thus, a device can misbehave because of a deficiency in the embedded software. This is particularly common with early versions of new computer-controlled 4 Chapter One Other 3% Other 0% Utility 17% Utility Perception Customer Perception Utility 1% Natural 60% Natural 66% Neighbor 8% Neighbor 8% Customer 12% Customer 25% Figure 1.1 Results of a survey on the causes of power quality problems. (Courtesy of Georgia Power Co.) Introduction Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. load equipment. One of the main objectives of this book is to educate utilities, end users, and equipment suppliers alike to reduce the fre- quency of malfunctions caused by software deficiencies. In response to this growing concern for power quality, electric utilities have programs that help them respond to customer concerns. The phi- losophy of these programs ranges from reactive, where the utility responds to customer complaints, to proactive, where the utility is involved in educating the customer and promoting services that can help develop solutions to power quality problems. The regulatory issues facing utilities may play an important role in how their programs are structured. Since power quality problems often involve interactions between the supply system and the customer facility and equipment, regulators should make sure that distribution companies have incen- tives to work with customers and help customers solve these problems. The economics involved in solving a power quality problem must also be included in the analysis. It is not always economical to eliminate power quality variations on the supply side. In many cases, the optimal solution to a problem may involve making a particular piece of sensi- tive equipment less sensitive to power quality variations. The level of power quality required is that level which will result in proper opera- tion of the equipment at a particular facility. Power quality, like quality in other goods and services, is difficult to quantify. There is no single accepted definition of quality power. There are standards for voltage and other technical criteria that may be mea- sured, but the ultimate measure of power quality is determined by the performance and productivity of end-user equipment. If the electric power is inadequate for those needs, then the “quality” is lacking. Perhaps nothing has been more symbolic of a mismatch in the power delivery system and consumer technology than the “blinking clock” phenomenon. Clock designers created the blinking display of a digital clock to warn of possible incorrect time after loss of power and inad- vertently created one of the first power quality monitors. It has made the homeowner aware that there are numerous minor disturbances occurring throughout the power delivery system that may have no ill effects other than to be detected by a clock. Many appliances now have a built-in clock, so the average household may have about a dozen clocks that must be reset when there is a brief interruption. Older-tech- nology motor-driven clocks would simply lose a few seconds during minor disturbances and then promptly come back into synchronism. 1.2 Power Quality ϭ Voltage Quality The common term for describing the subject of this book is power qual- ity; however, it is actually the quality of the voltage that is being Introduction 5 Introduction Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. addressed in most cases. Technically, in engineering terms, power is the rate of energy delivery and is proportional to the product of the volt- age and current. It would be difficult to define the quality of this quan- tity in any meaningful manner. The power supply system can only control the quality of the voltage; it has no control over the currents that particular loads might draw. Therefore, the standards in the power quality area are devoted to maintaining the supply voltage within certain limits. AC power systems are designed to operate at a sinusoidal voltage of a given frequency [typically 50 or 60 hertz (Hz)] and magnitude. Any significant deviation in the waveform magnitude, frequency, or purity is a potential power quality problem. Of course, there is always a close relationship between voltage and current in any practical power system. Although the generators may provide a near-perfect sine-wave voltage, the current passing through the impedance of the system can cause a variety of disturbances to the voltage. For example, 1. The current resulting from a short circuit causes the voltage to sag or disappear completely, as the case may be. 2. Currents from lightning strokes passing through the power system cause high-impulse voltages that frequently flash over insulation and lead to other phenomena, such as short circuits. 3. Distorted currents from harmonic-producing loads also distort the voltage as they pass through the system impedance. Thus a dis- torted voltage is presented to other end users. Therefore, while it is the voltage with which we are ultimately con- cerned, we must also address phenomena in the current to understand the basis of many power quality problems. 1.3 Why Are We Concerned about Power Quality? The ultimate reason that we are interested in power quality is eco- nomic value. There are economic impacts on utilities, their customers, and suppliers of load equipment. The quality of power can have a direct economic impact on many industrial consumers. There has recently been a great emphasis on revitalizing industry with more automation and more modern equip- ment. This usually means electronically controlled, energy-efficient equipment that is often much more sensitive to deviations in the sup- ply voltage than were its electromechanical predecessors. Thus, like the blinking clock in residences, industrial customers are now more 6 Chapter One Introduction Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. acutely aware of minor disturbances in the power system. There is big money associated with these disturbances. It is not uncommon for a single, commonplace, momentary utility breaker operation to result in a $10,000 loss to an average-sized industrial concern by shutting down a production line that requires 4 hours to restart. In the semiconductor manufacturing industry, the economic impacts associated with equip- ment sensitivity to momentary voltage sags resulted in the develop- ment of a whole new standard for equipment ride-through (SEMI Standard F-47, Specification for Semiconductor Process Equipment Voltage Sag Immunity). The electric utility is concerned about power quality issues as well. Meeting customer expectations and maintaining customer confidence are strong motivators. With today’s movement toward deregulation and competition between utilities, they are more important than ever. The loss of a disgruntled customer to a competing power supplier can have a very significant impact financially on a utility. Besides the obvious financial impacts on both utilities and industrial customers, there are numerous indirect and intangible costs associated with power quality problems. Residential customers typically do not suffer direct financial loss or the inability to earn income as a result of most power quality problems, but they can be a potent force when they perceive that the utility is providing poor service. Home computer usage has increased considerably in the last few years and more trans- actions are being done over the Internet. Users become more sensitive to interruptions when they are reliant on this technology. The sheer number of complaints require utilities to provide staffing to handle them. Also, public interest groups frequently intervene with public ser- vice commissions, requiring the utilities to expend financial resources on lawyers, consultants, studies, and the like to counter the interven- tion. While all this is certainly not the result of power quality problems, a reputation for providing poor quality service does not help matters. Load equipment suppliers generally find themselves in a very com- petitive market with most customers buying on lowest cost. Thus, there is a general disincentive to add features to the equipment to withstand common disturbances unless the customer specifies otherwise. Many manufacturers are also unaware of the types of disturbances that can occur on power systems. The primary responsibility for correcting inad- equacies in load equipment ultimately lies with the end user who must purchase and operate it. Specifications must include power perfor- mance criteria. Since many end users are also unaware of the pitfalls, one useful service that utilities can provide is dissemination of infor- mation on power quality and the requirements of load equipment to properly operate in the real world. For instance, the SEMI F-47 stan- dard previously referenced was developed through joint task forces Introduction 7 Introduction Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. . Electrical Power Systems Quality, Second Edition CHAPTER 1: INTRODUCTION What is Power Quality? Power Quality -- Voltage Quality Why Are. CHAPTER 8: POWER QUALITY BENCHMARKING Introduction Benchmarking Process RMS Voltage Variation Indices Harmonics Indices Power Quality Contracts Power Quality