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TECHTORIAL Electric Power QualityThe strong connection with power electronics |
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| M.M. Morcos and J.C. Gomez | ||
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Early power equipment was designed to withstand disturbances such as lightning, short circuits, and sudden overloads without extra expenditure. Current power electronics (PE) prices would be much higher if the equipment was designed with the same robustness. Pollution has been introduced into power systems by nonlinear loads such as transformers and saturated coils; however, perturbation rate has never reached the present levels. Due to its nonlinear characteristics and fast switching, PE create most of the pollution issues. This article shows how PE and electric power quality (PQ) are related in almost every aspect of commercial, domestic, and industrial application. OverviewPower quality is a young subject that was introduced in the early 1980s. A high level of misunderstanding regarding the quality of power still exists to the extent that the European Community standards and American standards use different nomenclature, definitions, and acceptable disturbance rates. Disturbances distinctly affect the electric power utilities, customers, sensitive equipment (SE) manufacturers, and government regulation agencies. A new concept of "disturbance flow" can be described as when one source may pollute the system without being affected itself, whereas another that does not cause any disturbances could be seriously affected by that pollution. The solution should be the result of a joint agreement among all parties involved.The frequency of occurrence of disturbances is of real importance in regard to the effect of transient system perturbations on the sensitive equipment but not to the pollution created by PE, which is a deterministic process. Perturbations and pollution levels are highly changeable with time of day, day of the week, season, and environmental conditions. Another important concept is the vulnerable zone, inside which PQ problems can be detected, perhaps more than 100 miles from the disturbance source location. Equipment Using Power ElectronicsThe following devices represent some of the types of many SE using power electronics.
How Is a PQ Problem Detected?The problem could have several symptoms, depending on the type of issue involved. Some of these are very obvious and others more vague, such as:
Are PQ Issues a Consequence of PE?Disturbances due to lightning, short circuits, and sudden overloads have always existed in electric power systems from the very beginning. Early power equipment was designed to withstand these phenomena without high cost; however, PE prices today would be much higher if the equipment was designed with the same robustness.Pollution was introduced into power systems by nonlinear loads such as transformers and saturated coils; however, perturbation level has never reached the present magnitudes. Most of the pollution issues are created due to the nonlinear characteristics and fast switching of PE. Approximately 10% to 20% of today's energy is processed by PE; the percentage is estimated to reach 50% to 60% by the year 2010, due mainly to the fast growth of PE capability. The trend should accelerate even more with the development of devices based on SiC technology. Figure 1 shows the increasing trend of the devices' voltage ratings. A race is currently taking place between increasing PE pollution and sensitivity, on the one hand, and the new PE-based corrective devices, which have the ability to attenuate the issues created by PE, on the other hand. Why Is the Concept Becoming Stronger and Stronger?Power quality issues are becoming stronger because SE will be more sensitive for market competition reasons, equipment will continue polluting the system more and more due to cost increase caused by the built-in compensation, and sometimes for lack of enforced regulations. Also, more sensitive and pollution-prone equipment will be used, with extended applications not only to industry but practically everywhere. Efficiency and cost are considered today almost at the same level.Other reasons include deregulation exacerbating the situation due to the fierce competition between utilities and regulations or standards, measuring devices, and analysis techniques becoming very powerful and sophisticated as well as the high cost involved in any PQ issue due to industrial or commercial load shut-down losses. These factors will always exist until a consensual solution is reached. Power QualityThe PQ issue is defined as "any occurrence manifested in voltage, current, or frequency deviations that results in damage, upset, failure, or misoperation of end-use equipment." Almost all PQ issues are closely related with PE.Power ElectronicsThe invention of solid-state semiconductor materials in 1950 has allowed the creation of a large family of conduction-controlled devices. Semiconductor devices have gradual and on/off conduction properties that have added to the possibility of high-frequency utilization based on its high commutation speed. The devices have allowed an unimaginable new set of applications. The devices family started from the simple single-phase, half-wave, noncontrolled rectifiers using diodes [Figure 2(a)]. Voltage and current waveforms are shown in Figure 2(b).The highly nonsinusoidal current, shown in Figure 2(b), would produce a nonsinusoidal voltage drop on the system impedance, depriving other customers—who are connected to the same power source—of sinusoidal supply. Besides, current and voltage distortion depend on the connected load, which can be widely variable. The extension of the same phenomenon toward more complex and high-power devices, used in pulse-width- modulated (PWM) adjustable speed drives and other applications, explains the voltage distortion generated in the present power systems. Another fundamental aspect is that PE are normally designed to work in nonpolluted power systems, so they would suffer from malfunctions when the supply voltage is not pure sinusoidal. Power Quality/Power Electronics InteractionPQ Problems Generated by PE EquipmentPE are the most important cause of harmonics, interharmonics, notches, and neutral currents. The types of pollution generated by PE, the equipment responsible for the pollution, and the affected equipment (e.g., harmonics generated by battery chargers that affect power transformers) are as follows:
Figure 3 shows current waveforms of a PWM-type ASD with and without inductor (choke), where high levels of generated harmonic distortion are noticeable. The current THD values were 131% and 45%, respectively. PQ Problems Suffered by PE EquipmentOther issues that can seriously affect PE include:
Figure 5 shows a phase-angle jump of approximately 45° (delay) due to voltage sag to 80% and with duration of 55 ms, caused by a short circuit at transmission level. Equipment presents different levels of sensitivity to PQ issues, depending on the type of both the equipment and the disturbance. Several surveys have been carried out to determine the relationship between perturbation and damage; results are not always comparable due to discrepancies in the definition of perturbation characteristics. A typical equipment sensitivity/perturbation relationship is shown in Figure 6(a). Furthermore, the effect on the PQ of electric power systems, due to the presence of PE, depends on the type of PE utilized. Figure 6(b) shows typical effects of the most common PE applications. ASDs are extremely sensitive to PQ problems, and much research has been carried out on this subject. The severity of disturbances is closely related with the probability of their occurrence, as shown in Figure 7 for voltage perturbations. In the case of voltage sags and swells, the recommendations given by the Information Technology Industry Council (ITIC) in graphical format as voltage/time characteristics would be used. The graph has two borderlines that enclose an area where voltage perturbations do not cause SE dropout. The graph shows the CBEMA-ITIC borderlines and the typical number of perturbations as a yearly average percentage for some specific areas on the graph. The number of locations in a given power system having harmonic pollution, and the corresponding THD values, is shown by the distribution histogram in Figure 8. The maximum acceptable values of harmonic contamination are specified in IEEE Standard 519. PQ Problems Solved by PE EquipmentThe solution to PQ problems can be achieved in one or more of the following ways:
The second measure is by far the most popular, and the choice of mitigation equipment depends on the characteristics of the protected device. Usually, the customer buys mitigation equipment when the available PQ is below what is required by the SE. Auxiliary equipment has the general name of power conditioners; they are mainly characterized by the amount of stored energy or stand-alone supply time; most are the consequence of PE development. The most commonly used devices are ferroresonant transformers, uninterruptible power supply (UPS), superconducting magnetic energy storage (SMES) systems, and power compensators. The most commonly used devices are discussed below. Ferroresonant transformers were initially proposed more than 60 years ago, based on a three-winding transformer, having one winding in parallel with a capacitor. The consequence of this connection is that the magnetic flux through the iron core is independent of supply voltage; hence, the output voltage is not a function of the supply voltage. A modern version of this device uses power electronic converters to keep the load current at unity power factor, optimizing the device behavior. A UPS, under normal operation, takes the power from the supply, rectifies the ac voltage to dc, and inverts it back to ac with the same frequency and rms value. At the dc link, a suitable battery is connected, which receives the energy from the supply maintaining its full charge. During a voltage sag or interruption, the battery maintains the voltage at the dc bus for some time, depending on the amount of the energy stored. This equipment is widespread due to its low cost and versatility. However, it has two main drawbacks: the double conversion losses and the need of frequent maintenance, besides harmonic generation. One of the common enhancements is the addition of a static transfer switch (stand-by UPS) that connects the load to the UPS in less than 4 ms, reducing the power loss. Its main application is in business offices for PC backup powering. In an SMES system the energy is stored in a superconducting coil, and when its high-speed controller senses a decrease in voltage, an isolation switch opens that separates the critical loads from the utility system. During this time period, the load is supplied with the energy stored in the coil through an inverter circuit. Normally, the energy is enough for full load work during 2 seconds. Commercial applications are reported for stored energy up to 2.4 MJ and power ratings up to 4 MVA, using liquid helium as cooling medium. There are some studies on the application of high-temperature superconductors, with prototypes in an early stage. Power compensators are specifically designed for the main producer of voltage fluctuation, the arc furnace, and they are listed below:
Custom Power and Premium Power ParksDue to the general characteristics of the devices mentioned above, in addition to the 70% to 80% of outages that originate on the utility distribution system, a new methodology called custom power was presented in the last decade. Custom power means that power conditioner equipment should have the following characteristics:
Such drawbacks lead to the fact that average users would be eager to hand over the problem to somebody else (the power utility in this case). Custom power is possible today due to PE development, e.g., insulated gate bipolar transistor (IGBT) and integrated gate commutated thyristor (IGCT). Several devices are readily available, which are self-contained, built in a standard container, and trailer-mounted (can be installed and later moved to a different place, according to service requirements). Custom power involves a series of devices:
The primary function of the solid-state circuit breaker is to interrupt fault currents quickly enough to prevent them from affecting service on adjacent feeders, using SCR and gate turn-off (GTO) technologies. Rated voltage is 13.8 kV, reacting in 1/4 cycle. The solid-state transfer switch is used to switch industrial and commercial facilities from one feeder to another, using two SCR thyristor switches connected back-to-back. The rated voltage and current are 15 kV and 600 A, respectively, and the total switching time is about 1/2 cycle. The distribution static compensator serves as the protection of the distribution system from power pollution caused by disturbing customer loads. Normally, it is placed between the feeder and the fluctuating load, connected in parallel with them. It consists of an IGBT-based dc-ac power inverter connected through a coupling transformer. In this way, it is able to exchange only reactive power with the line; it would also supply real power if a storage energy system is added. Rated voltage and power are 25 kV and 2 MVAR, respectively. The dynamic voltage restorer is connected in series with the primary distribution feeder providing power to a sensitive load, compensating for momentary voltage sags, swells, transients, and harmonics, by exchanging real and reactive power with the line. It consists of an IGBT-based dc–ac power inverter, connected in series with distribution line through a set of three single-phase injection transformers. A rechargeable energy storage system (capacitor-based) can be connected to the dc terminals to provide additional power to ride through deep sags. Rated voltage and power are 22 kV and 6 MVA, respectively; response time is less than 3 ms, and correction time is up to 5 seconds. Various forms of energy storage have been proposed, such as SMES, which stores the electrical energy in a superconducting coil, and the battery energy storage system (BESS), which uses a large battery bank to store the energy. The EPRI-sponsored transportable BESS is a new development; its characteristics are 1–2 MW, one-hour discharge capability, and ramp rate of less than 4 ms. Furthermore, intensive research is devoted to the supercapacitor (also called electrochemical capacitor), which is a multiple-layer capacitor having energy densities comparable to batteries, without the lifetime and maintenance problems of the latter.
For Further ReadingIEEE Recommended Practice for Evaluating Electric Power System Compatibility with Electronic Process Equipment, IEEE Standard 1346, 1998.T. Moore, "Beyond silicon: Advanced power electronics," EPRI J., vol. 22, no. 6, pp. 30–36, 1997. IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, IEEE Standard 519, 1992. IEEE Recommended Practice for Powering and Grounding Sensitive Electronic Equipment, IEEE Standard 1100, 1992. G.T. Heydt, Electric Power Quality. Scottsdale, AZ: Starts in a Circle Publications, 1991. M.M. Morcos and J.C. Gomez, "Power electronics and power quality (Tutorial)," presented at European Power Electronics Conf., Graz, Austria, 2001. M. McGranaghan, "Advances in power quality monitoring using the Internet," Power Quality Assurance, vol. 11, no. 2, pp. 42–45, 2000. M. McGranaghan, "Using revenue meters in a power quality information system," Power Quality Assurance, vol. 10, no. 8, pp. 22–26, 1999. N.G. Hingorani, "Introducing custom power," IEEE Spectrum, pp. 41–48, June 1995. A. Arora, K. Chan, A. Kara, T. Jauch, and E. Wirth, "Innovative system solution for power quality enhancement," ABB Rev., no. 3, pp. 4–12, 1998. J. Duran-Gomez, P. Enjeti, and A. von Jouanne, "An approach to achieve ride-through of an adjustable-speed drive with flyback converter modules powered by super capacitors," IEEE Trans. Ind. Applicat., vol. 38, pp. 514-522, Feb. 2002. Medhat M. Morcos is professor of electrical and computer engineering and Distinguished Teaching Scholar at Kansas State University, Manhattan. He received the B.S. and M.S. degrees in electrical engineering from Cairo University, Egypt, in 1966 and 1978, respectively, and the Ph.D. degree in electrical engineering from the University of Waterloo, Ontario, Canada, in 1984. His current research includes artificial intelligence applications in power quality and power systems protection, power electronics, insulation systems, and high-voltage engineering. Dr. Morcos is a member of the editorial boards of Electric Power Components and Systems and IEEE Power Engineering Letters. He is also a member of the America Society for Engineering Education, Eta Kappa Nu, Sigma Xi, Tau Beti Pi, and Phi Kappa Phi. J. Carlos Gomez received the B.S. degree from National University of Cuyo, Mendoza, Argentina, in 1974, and the Ph.D. degree in electrical engineering from Sheffield Hallam University, Sheffield, U.K., in 1994. Currently, he is director of the Electric Power System Protection Institution at the National University of Rio Cuarto (NURC), Argentina, where he has been since 1980. He is also professor of electrical engineering at NURC and was with the National University of Cuyo for four years. His research interests include power quality and distribution protection. Dr. Gomez is a member of the IEEE Power Engineering Society. |
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