wind turbines [3]. The use of power ... The inverter is a basic component of the UPS system that ... âA New Analog Controller for Three-Phase Voltage Gen-.
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Guest Editorial
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NINTERRUPTIBLE power supply (UPS) systems are becoming important due to the increase in critical loads, such as telecommunication systems, computer sets, and hospital equipment. In the last few years, an increasing number of publications about UPS system research have appeared, and at the same time, different kinds of industrial UPS units have been introduced in the market place. Furthermore, the development of novel energy storage systems, power electronic topologies, fast electrical devices, high-performance digital processors, and other technological advances have yielded new opportunities for UPS systems [1], [2]. New electrical energy concepts such as distributed generation and microgrids require storage energy systems to be able to manage the energy near the consumption points. In this sense, distributed UPS systems are becoming important in integrating variable renewable energy, such as photovoltaic or wind turbines [3]. The use of power electronics lets us control the parts that comprise a UPS system, thereby improving power quality and reliability. Nowadays, the UPS scenario is very wide since there is not only a broad range of power rating but also different kinds of storage energy systems. In addition, digital signal processors are making real control techniques that could not be implemented in the past, allowing new power system configurations. The UPS system field is a multidisciplinary area that encompasses power stage topologies, control techniques, technological storage solutions, and complex power systems, among others. In the light of worldwide interest among engineers, manufacturers, researchers, and users of UPS systems, this Special Section on Uninterruptible Power Supply Systems is provided. Researchers have been writing articles that cover a wide spectrum of UPS systems. The 21 high-quality papers approved for publication in this Special Section are organized according to the following six topics: • control of parallel UPS systems; • UPS inverter control; • double-conversion UPS; • line-interactive UPS; • battery management; • dynamic UPS. These topics and the papers included are explained in the following sections of this Guest Editorial. I. C ONTROL OF P ARALLEL UPS With the increasing interest in distributed UPS systems, parallel operation is a suitable application to increase reliability and expandability. The control of this kind of system is complex and requires the synthesis of multiple loops. This section con-
Digital Object Identifier 10.1109/TIE.2008.924159
tains one review article and three active current-sharing control proposals for single-phase parallel UPS systems. • “Control of Distributed Uninterruptible Power Supply Systems,” by Guerrero et al., reviews the control schemes for the parallel operation of UPS inverters, including active load sharing and droop control techniques. • “Distributed Control for UPS Modules in Parallel Operation With RMS Voltage Regulation,” by He and Xing, proposes a control scheme for parallel UPS, which includes a current load-sharing loop and a root-mean-square voltage regulation loop. • “Robust Model-Following Control of Parallel UPS SinglePhase Inverters,” by Pascual et al., develops a centralized current sharing controller by using the robust linear control theory and the model-following approach based on lowpass reference models. • “Model Predictive Control of Parallel-Connected Inverters for Uninterruptible Power Supplies,” by Low and Cao, develops a model of the state space of a paralleled UPS system, and model-predictive control is proposed. Voltage tracking and current sharing are formulated through a weighted cost function.
II. UPS I NVERTER C ONTROL The inverter is a basic component of the UPS system that is responsible for the power quality supplied to the critical loads. Consequently, the control design of UPS inverters is an important topic to take into account in high-performance UPS [2]. The authors, through the following four papers, have investigated analog and digital controllers for three-phase and single-phase UPS inverters. • “A New Analog Controller for Three-Phase Voltage Generation Inverter,” by Li et al., presents a simple low-cost analog control that avoids the use of coordinate transformations. The one-cycle control loop used effectively eliminates the dc-link disturbances. • “Analysis of Control-Delay Reduction for the Improvement of UPS Voltage-Loop Bandwidth,” by Mattavelli et al., studies the reduction of the control delay time. By shifting the output voltage sampling, they achieve increasing voltage-loop bandwidth without challenging the stability. • “A Direct PWM Technique for a Single-Phase Full-Bridge Inverter Through Controlled Capacitor Charging,” by Dalapati and Chakraborty, proposes a capacitor-chargingbased control technique applied to a single-phase UPS inverter. The controller allows zero-current switching operation and good output voltage total harmonic distortion. • “Comparison of Three Single-Phase PLL Algorithms for UPS Applications,” by Santos Filho et al., analyzes and
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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 55, NO. 7, JULY 2008
tests three single-phase phase-locked-loop structures suitable for synchronizing the output voltage of the UPS inverter with the utility grid. Their performances are compared in terms of computational cost, accuracy, and transient response. III. D OUBLE -C ONVERSION UPS Double-conversion UPS, which is often called online UPS, is the highest performance type of UPS, it provides total independence between the input and output voltage amplitude and frequency; thus, high output voltage quality can be obtained, although the price to pay is in terms of cost and efficiency [4], [5]. The importance of this kind of UPS system is depicted through the following seven papers—one about control issues and six that include single- and three-phase UPS topologies. • “Practical Control Implementation of a Three- to SinglePhase Online UPS,” by Kim et al., presents an overall control algorithm for all the power stage parts of a threeto single-phase UPS. The rectifier, the charger/discharger, and the inverter controllers are implemented in one single digital signal processor. • “High-Performance Transformerless Online UPS,” by Park et al., proposes a single-phase four-leg online UPS topology. By using the battery charger/discharger, the use of any transformer is avoided; the efficiency is improved; and the size, weight, and cost are reduced. • “Two-Stage Uninterruptible Power Supply With High Power Factor,” by Vázquez et al., presents a single-phase two-stage UPS topology, including an isolated inverter and a multifunctional converter, which provides the energy and acts like a battery charger. • “A Novel AC UPS With High Power Factor and Fast Dynamic Response,” by Rodriguez et al., proposes another single-phase double-conversion UPS topology, which includes an isolated PFC rectifier, a battery charger, and a boost inverter. • “A Nonisolated Single-Phase UPS Topology With 110-V/ 220-V Input–Output Voltage Ratings,” by Branco et al., presents a transformerless UPS topology, which includes a common neutral-point connection, a three-level rectifier, and a half-bridge inverter. • “A UPS With 110-V/220-V Input Voltage and High-Frequency Transformer Isolation,” by TorricoBascopé et al., proposes an isolated double-conversion UPS that uses a high-frequency transformer employing a 110/220-V full-bridge chopper in the input stage, a boost converter, and a full-bridge inverter. • “Single-Phase Uninterruptible Power Supply Based on Z-Source Inverter,” by Zhou et al., introduces a new UPS inverter topology that avoids the use of an additional boost conversion, being suitable even for low-voltage battery levels. IV. L INE -I NTERACTIVE UPS The line-interactive UPS configuration is considered as midway between the online and offline configurations. It consists
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of a single bidirectional converter that connects the batteries to the load. The efficiency is higher than that of online UPS, and the cost is normally lower although it cannot achieve frequency regulation or real isolation between the grid and the loads [4], [5]. Two practical papers are included in this section. • “Simplified Voltage-Sag Filler for Line-Interactive Uninterruptible Power Supplies,” by Arias et al., proposes a very simple system to eliminate voltage notches at the load. The controller directly modulates a capacitor voltage turning on and off a switch. This leads to a sinusoidal voltage being applied to the load. • “Line-Interactive UPS Using a Fuel Cell as the Primary Source,” by Tao et al., presents a three-port single-phase UPS topology for fuel cells that use supercapacitors. It includes a high-frequency transformer to isolate all the parts and a supercapacitor set to improve the dynamic response. V. B ATTERY M ANAGEMENT Although fuel cells can be an interesting future alternative, typical UPS systems use chemical batteries to store energy [1]. The control and management of the battery chargers of these systems are presented through the following two papers. • “Design of a Reflex-Based Bidirectional Converter With the Energy Recovery Function,” by Chen et al., proposes a bidirectional converter for online UPS applications. The topology can provide a proper current profile to charge the battery, obtaining high battery charging efficiency and prolonging the battery life cycle. • “High Efficiency and Low Stress ZVT–PWM DC-to-DC Converter for Battery Charger,” by Chuang and Ke, presents a topology for the battery charger, which consists of a zero-voltage-transition soft-switching buck converter. The approach is suitable for managing the charge of leadacid batteries. VI. D YNAMIC UPS Dynamic UPS systems, which are also named rotary UPS, have been around for a long time and have large power ratings. Normally, these UPS systems consist of a motor–generator set with heavy flywheels and engines. In addition, the combination of dynamic UPS systems with power electronic converters results in hybrid systems, which are more attractive in terms of system efficiency [6]. This section includes two papers about these kinds of dynamic UPS systems. • “Control of High-Speed Solid-Rotor Synchronous Reluctance Motor/Generator for Flywheel-Based Uninterruptible Power Supplies,” by Park et al., proposes feedforward control for a hybrid UPS consisting of a synchronous reluctance motor/generator and a flywheel. The control takes into account the flux-model machine, providing stability and good regulation. • “DFIG-Based Power Generation System With UPS Function for Variable-Speed Applications,” by Iwanski and Koczara, studies the output voltage control of a doubly fed induction generator working stand-alone in variable-speed
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operation for a UPS. The controller is also able to synchronize the UPS with the grid and to detect possible mains outages. ACKNOWLEDGMENT The Guest Editors would like to thank the IEEE Industrial Electronics Society, authors, reviewers, current TIE Editorin-Chief Prof. B. Wilamowski, past TIE Editor-in-Chief Prof. M. Kazmierkowski, TIE Administrator S. McLain, and past TIE Administrator K. M. Brzozowska for their constant help and support.
R EFERENCES [1] A. King and W. Knight, Uninterruptible Power Supplies and Standby Power Systems. New York: McGraw-Hill, 2003. [2] S. B. Bekiarov and A. Emadi, “Uninterruptible power supplies: Classification, operation, dynamics, and control,” in Proc. IEEE APEC, 2002, pp. 597–604. [3] J. M. Guerrero, L. García de Vicuña, and J. Uceda, “Uninterruptible power supply provide protection,” IEEE Ind. Electron. Mag., vol. 1, no. 1, pp. 28– 38, Spring 2007. [4] W. Sölter, “A new international UPS classification by IEC 62040-3,” in Proc. IEEE Telecommun. Energy Conf., 2002, pp. 541–545. [5] S. Karve, “Three of a kind,” IEE Rev., vol. 46, no. 2, pp. 27–31, Mar. 2000. [6] A. Kusko and S. Fairfax, “Survey of rotary uninterruptible power supplies,” in Proc. IEEE Telecommun. Energy Conf., Boston, MA, Oct. 1996, pp. 416–419.
JOSEP M. GUERRERO, Guest Editor Department of Automatic Control Universitat Politècnica de Catalunya 08036 Barcelona, Spain JAVIER UCEDA, Guest Editor División de Ingeniería Electrónica Universidad Politecnica de Madrid 28006 Madrid, Spain
Josep M. Guerrero (S’01–M’04–SM’08) received the B.S. degree in telecommunications engineering, the M.S. degree in electronics engineering, and the Ph.D. degree in power electronics from the Universitat Politècnica de Catalunya, Barcelona, Spain, in 1997, 2000, and 2003, respectively. From 1998 to 2004, he was an Assistant Professor with the Department of Automatic Control, Universitat Politècnica de Catalunya. In 2004, he became a Senior Lecturer at the same university, where he teaches courses on digital signal processing, control theory, microprocessors, and renewable energy. Since 2004, he has been responsible for the Sustainable Distributed Generation and Renewable Energy Research Group, Escola Industrial de Barcelona. His research interests include photovoltaics, wind energy conversion, and compressed-air devices applied to uninterruptible power supplies, storage energy systems, and microgrids. Dr. Guerrero is an Associate Editor for the IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS and the IEEE TRANSACTIONS ON POWER ELECTRONICS. He is involved in several IEEE Industrial Electronics Society (IES) Committees and usually chairs and organizes sessions at IEEE IES and IEEE Power Electronics Society conferences. Javier Uceda (M’83–SM’91–F’05) was born in Madrid, Spain, in 1954. He received the M.Sc. and Ph.D. degrees in electrical engineering from the Universidad Politécnica de Madrid (UPM), Madrid, Spain, in 1976 and 1979, respectively. Since 1986, he has been a Professor of electronics with the División de Ingeniería Electrónica, UPM. He has published several books and more than 300 papers in journals and conference proceedings. He has been a member of the Editorial Board of the European Power Electronics and Drives Journal. His research interests are power electronics, particularly, high-frequency high-density power converters. Dr. Uceda is currently a Senior AdCom member of the IEEE Industrial Electronics Society. He was a member of the Steering Committee of the European Power Electronics and Drives Association. He was the Technical Program Committee Chairman of the IEEE Power Electronics Specialists Conference in 1992 and the General Chairman of the European Conference on Power Electronics and Applications in 1995.
