UPS设计

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 55, NO. 8, AUGUST 2008 2845 Control of Distributed Uninterruptible Power Supply Systems Josep M. Guerrero, Senior Member, IEEE, Lijun Hang, and Javier Uceda, Fellow, IEEE Abstract—In the last years, the use of distributed uninterrupt- ible power supply (UPS) systems has been growing into the mar- ket, becoming an alternative to large conventional UPS systems. In addition, with the increasing interest in renewable energy integration and distributed generation, distributed UPS systems can be a suitable solution for storage energy in microgrids. This paper depicts the most important control schemes for the parallel operation of UPS systems. Active load-sharing techniques and droop control approaches are described. The recent improvements and variants of these control techniques are presented. Index Terms—Droop method, load sharing, microgrids, parallel connection, uninterruptible power supply (UPS). I. INTRODUCTION D ISTRIBUTED generation (DG) is an emerging conceptto decentralize the management of electricity production. However, DG makes no sense without distributed storage en- ergy systems. Thus, the parallel operation is a special fea- ture of high-performance uninterruptible power supply (UPS) systems [1]–[21]. The parallel connection of UPS inverters is a challenging problem that is more complex than paralleling dc sources, since every inverter must properly share the load while staying synchronized. In theory, if the output voltage of every inverter has the same amplitude, frequency, and phase, the current load could equally be distributed. However, due to the physical differences between the inverters and the line impedance mismatches, the load will not properly be shared. This fact will lead to a circulating current among the inverters that can damage or overload them. The fast development of digital signal processors has brought about an increase in control techniques for the parallel opera- tion of UPS inverters. These control schemes can be classified into two main groups with regard to the use of control wire interconnections [7]. The first one is based on active load- sharing techniques, and the major part of them is derived from control schemes of parallel-connected dc–dc converters, such as centralized [22], [23], master–slave (MS) [24]–[32], average load sharing (ALS) [33]–[41], and circular chain control (3C) [42], [43]. Although these control schemes achieve both good output-voltage regulation and equal current sharing, they need Manuscript received February 16, 2008; revised April 7, 2008. Published July 30, 2008 (projected). J. M. Guerrero is with the Department of Automatic Control and Com- puter Engineering, Technical University of Catalonia, 08036 Barcelona, Spain (e-mail: josep.m.guerrero@upc.edu). L. Hang is with the College of Electrical Engineering, Zhejiang University, Hangzhou 310027, China. J. Uceda is with the División de Ingeniería Electrónica, Universidad Politéc- nica de Madrid, 28006 Madrid, Spain. Digital Object Identifier 10.1109/TIE.2008.924173 critical intercommunication lines among modules that could reduce the system reliability and expandability. The second kind of control scheme for the parallel op- eration of inverters is mainly based on the droop method [44]–[74]. This technique consists of adjusting the output- voltage frequency and amplitude in function of the active and reactive power delivered by the inverter. The droop method achieves higher reliability and flexibility in the physical lo- cation of the modules, since it uses only local power mea- surements. Nevertheless, the conventional droop method shows several drawbacks that limit its application area, such as slow transient response, tradeoff between the power-sharing accu- racy and the frequency and voltage deviations, unbalance har- monic current sharing, and high dependency on the inverter output impedance. In addition, the line impedance is unknown, which can result in reactive power unbalances. This problem can be overcome by injecting high-frequency signals through the power lines or by adding external data communication signals. These communication systems, typically digital, must not be critical and robust. This way, controller area networks, power line communications, or wireless (radio frequency links) are often implemented [75]–[78]. In this paper, a review of the control schemes for the parallel operation of UPS systems and the trends of these systems in DG systems and microgrids are presented. Although the control of standalone UPS inverters was widely studied, it will not be shown in this paper [79]–[86]. This paper is organized as follows. Section II describes the configuration types of distrib- uted UPS systems. Section III analyzes the circulating current problem derived from the parallel operation of UPS inverters. Section IV depicts the active load-sharing techniques, including centralized control, MS control, ALS, and 3C. Section V pro- vides the description of the conventional droop control method, including a power flow analysis. Then, a generalization of the droop method, the virtual impedance loop approach, and the multiloop droop control techniques are described. Finally, in Section VI, a comparison between the control techniques and the conclusions is provided. II. CONFIGURATIONS OF DISTRIBUTED UPS SYSTEMS Distributed UPS systems support UPS units and critical loads flexibly located in an interconnected electrical power network. In order to add reliability and expandability to the system, redundant and parallel UPS systems are usually integrated into the power system. There are two major types of distributed UPS systems (see Fig. 1), i.e., online and line-interactive distributed systems [47]. 0278-0046/$25.00 © 2008 IEEE 2846 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 55, NO. 8, AUGUST 2008 Fig. 1. Distributed UPS system configurations. (a) Online. (b) Line interactive. The distributed UPS systems are highly reliable because of redundancy. It is an advantage to achieve the N + 1 or N + X redundancy in these systems, where N UPS units supply the load, and 1 or X additional units stay in reserve. They are also highly flexible to increase the capacity of the system when more power is needed, by simply adding more UPS units [21]. A. Redundancy N + 1 or N + X UPS Units The redundancy concept consists of having one (N + 1) or more UPS units (N + X) in reserve, and, if some of the rest of the N modules are damaged or disconnected, this/these modules can automatically be connected to supply the functions of that unit. The redundancy can reduce the single point failure. A parallel redundant system can typically provide up to 99.99% availability, which means that the system does not operate for less than 1 h/year. In addition to having extra UPS modules, the parallel redundant system needs to give the operator some measure of system-level functionality. The simplest redundant UPS system is the 1 + 1 parallel redundant, which consists of using a centralized UPS with one reserve module [19]. B. Parallel Operation of UPS Systems The proper parallel operation of the N modules that config- ure the distributed UPS system is crucial. Generally speaking, a paralleled UPS system must achieve the following features [5], [6]: 1) the same output-voltage amplitude, frequency, and phase; 2) equal current sharing between the units; 3) flexibility to increase the number of units; and 4) plug and play operation at any time, also known as hot-swap operation capability. The parallel operation of UPS has a number of advantages, includ- ing thermal management, reliability, redundancy, modularity, maintainability, and size reduction. III. CIRCULATING CURRENT ANALYSIS The output currents of each UPS should be equal or at least proportional to its nominal power rating. The difference Fig. 2. Circulating current concept. between those currents provokes circulating currents among the UPS units. The circulating current (ic) is particularly dangerous at no-load or light-load conditions, since one or several modules can absorb active power operating in rectifier mode, as shown Fig. 2. This current increases the dc-link voltage level, which can result in damage to the dc-link capacitors or in a shutdown due to overload [24]. An analysis of the circulating current can be done by using the equivalent circuit of two UPS units connected in parallel, sharing a common load. The analysis presented in this section will be made using phasors, being only valid under sinusoidal conditions. Following Fig. 3, we can define the circulating apparent power as ∆S ∆= S1 − S2 (1) and, consequently, the active and reactive circulating powers are ∆P ∆=P1 − P2 (2) ∆Q ∆=Q1 −Q2. (3) Assuming that L1 � Zo1 + ZL1, L2 � Zo2 + ZL2, and that the total output impedance XT (L1 + L2) is mainly inductive, GUERRERO et al.: CONTROL OF DISTRIBUTED UNINTERRUPTIBLE POWER SUPPLY SYSTEMS 2847 Fig. 3. Equivalent circuit of two UPSs connected in parallel, taking into account the output impedance of the inverters (Zo1 and Zo2) and the power line impedances (�ZL1 = rL1 + jωL1 and �ZL2 = rL2 + jωL2). then (2) and (3) can be simplified as ∆P ∼= E1E2 XT sin∆φ ∼= E1E2 XT ∆φ (4) ∆Q ∼= V XT ∆E (5) where ∆φ = φ1 − φ2, and ∆E = E1 − E2. Thus, these equations can be expressed in function of the currents instead of the power, being the active and reactive circulating currents, as ∆iP ∼= E1E2 V XT ∆φ (6) ∆iQ ∼= ∆E XT . (7) In conclusion, assuming an inductive output impedance, the active and reactive powers or currents can be controlled by adjusting the phase and amplitude of the output voltage. In order to try to avoid the circulating current, there exist a number of control strategies that can be classified in active load- sharing and droop control techniques, depending on the use or not of communication links between UPS units. IV. ACTIVE LOAD SHARING The first kind of control scheme, named the active load- sharing technique, needs intercommunication links. Although these links limit the flexibility of the UPS system and degrade its redundancy, both tight current sharing and low-output- voltage total harmonic distortion (THD) can be achieved. The following section provides a review of the existing techniques for paralleling inverters available in the literature. The active load-sharing techniques can be classified into four different types, i.e., centralized control [22], [23], MS [24]–[32], ALS [33]–[41], and 3C [42], [43]. Using these techniques, we will generate the current or power reference of each module, which is easy to scale according to its nominal power rating. A. Centralized Control This control technique, also known as concentrated control, is depicted in Fig. 4. It consists of dividing the total load current iL by the number of modules N , so that this value becomes the current reference (i∗j) of each module j [22], [23] i∗j = iL N , for j = 1, . . . , N. (8) Fig. 4. Block diagram of a centralized controller for paralleled UPS system. The current reference value is subtracted by the current of each module, obtaining the current error ∆Ij , which is processed through a current control loop. An outer control loop in the centralized control adjusts the load voltage. This system is normally used in common UPS equipment with several output inverters connected in parallel. Using this approach, it is necessary to measure the total load current iL, so it cannot be used in a large distributed system. Consequently, a central control board is necessary. The control implementation can follow two philosophies. The first one is expressed by (8), and the second is to calculate the current error ∆i = i∗j − ij and to decompose it in direct current error ∆ip and in quadrature current error ∆iq . Finally, ∆ip and ∆iq can be used to adjust the phase and amplitude of the output-voltage reference of each UPS unit. The other possibility is to use ∆i and the output voltage to calculate ∆P and ∆Q instead of ∆ip and ∆iq , as shown in Fig. 5 [6]. B. MS In this technique, the master module regulates the load volt- age. Hence, the master current iM fixes the current references of the rest of the modules (slaves) as i∗S = iM , for S = 2, . . . , N. (9) Consequently, as shown in Fig. 6, the master acts as a voltage source inverter (VSI), whereas the slave works as a current source inverter (CSI) [30]. In this configuration, if the master unit fails, another module will take the role of master in order to avoid the overall failure of the system. There exist different variants of this control scheme, depending on the role of the master. 1) Dedicated: the master is one fix module. 2) Rotary: the master is arbitrarily chosen. 3) High-crest current: the master can be fixed by the module that brings the maximum rms or crest current. Unitrode ICs, such as UC3902 or UC3907, are used to parallel dc/dc converters, implementing the MS strategy in which 2848 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 55, NO. 8, AUGUST 2008 Fig. 5. Block diagram of a centralized controller based on active and reactive power deviation decomposition. Fig. 6. MS control strategy. (a) Block diagram of the system. (b) Equivalent circuit of the parallel UPS system controlled through the MS strategy. the module that brings the maximum current automati- cally becomes the master. This strategy can also be implemented by using average active and reactive powers as P ∗S =PM , for S = 2, . . . , N (10) Q∗S =QM , for S = 2, . . . , N. (11) In this particular case, and by using the highest P and Q average values, a scheme similar to the high-crest current control strategy can be obtained. This control strategy is shown in Fig. 7, and in that case, the master UPS of P and Q can be different modules [28]. MS control is often adopted when using different UPS units mounted into a rack. C. ALS This is a true democratic control scheme in which every module tracks the average current done by all the active mod- ules [33]–[41]. This scheme, shown in Fig. 8(a), is simple to implement by using a single wire, which contains the average current information computed by a resistor connected to the current sensor of every single module. In addition, adjusting the resistor to a proper value, we can parallel converters with different power rating. This control technique starts from an idea applied to parallel dc/dc converters by using current- sharing resistors connected to a common information bus. The current of all modules is averaged by means of a common current bus. The average current of all the modules is the reference for each individual one. This control scheme is highly reliable due to the real democratic conception, in which no MS philosophy is present. In addition, the approach is highly modular and expandable, making it interesting for industrial UPS systems. In general, this scheme is the most robust and useful of the aforementioned controllers. A variant of this technique is the current weighting distribution control [39]. The current reference of each module can be expressed as i∗k = 1 N N∑ j=1 ij , for k = 1, . . . , N. (12) This control approach can be performed by using an inner or an outer current loop. The problem with using an outer loop GUERRERO et al.: CONTROL OF DISTRIBUTED UNINTERRUPTIBLE POWER SUPPLY SYSTEMS 2849 Fig. 7. Auto-MS (highest crest P/Q master) power-sharing control scheme. is that due to that, the voltage loop has a narrow bandwidth, and in order to avoid instabilities, the current loop needs a compensator. As a consequence of the bandwidth reduction of this loop, the current dynamics is very slow, provoking poor current sharing during transients. As usual, another possibility is to use active and reactive power information instead of the current. Thus, we use active and reactive power to adjust the phase and amplitude of each module. Fig. 8(b) shows the block diagram of the average power-sharing technique [41]. Using this technique, each UPS unit controls the active and reactive power flow in order to match the average active and reactive powers of the system by adjusting the phase and the amplitude of its own inner output-voltage reference. The active and reactive power can be obtained through the direct and reactive component decomposi- tion of the output current. The average active and reactive power references of each module can be expressed as P ∗k = 1 N N∑ j=1 Pj (13) Q∗k = 1 N N∑ j=1 Qj , for k = 1, . . . , N. (14) An earlier work uses this method to achieve the power sharing between two UPS modules [40]. This control scheme can be extended to more units by using the active and reactive average power-sharing buses. Notice that this technique does not require any master or slave unit, and only low-bandwidth digital communications are required to achieve good P and Q sharing. Nevertheless, it only acts over the fundamental com- ponent of the output current, misleading the harmonic content. Hence, unbalances between the power stages and the power lines can produce large circulating harmonic current between the units. D. 3C This control scheme, shown in Fig. 9, consists of the current reference of each module taken from the aforementioned mod- ule, forming a control ring [42]. Note that the current reference of the first unit is obtained from that of the last unit to form a circular chain connection. This strategy can be expressed through i∗1 = iN (15) i∗k = ik−1, for k = 2, . . . , N. (16) The approach can be interesting for distributed power sys- tems based on ac power rings due to the distribution of power lines [54]. Fig. 10 illustrates the example of a distributed ring- forming UPS system. There are two lines in order to achieve bidirectional communication and to increase system reliability. 2850 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 55, NO. 8, AUGUST 2008 Fig. 8. ALS control schemes. (a) Average current sharing. (b) Average power sharing. GUERRERO et al.: CONTROL OF DISTRIBUTED UNINTERRUPTIBLE POWER SUPPLY SYSTEMS 2851 Fig. 9. Block diagram of the current chain control (3C). Fig. 10. Communication links of the current chain control (3C). The current limitation control is a variant of the 3C. In this case, the load voltage is controlled by the master module, whereas the slave modules are only for sharing the load current. Except for the master module, the current command of the slave is generated by its previous module and limited in amplitude [43]. In this scheme, any module can be the master (dedicated, rotating, or high-crest current). The connection of all control circuits can form a circular chain connection such that every module may become the master. Fig. 11. Equivalent circuit of a UPS inverter connected to a common ac bus. V. DROOP CONTROL METHOD The second kind of control scheme, named the droop control method, is able to avoid critical communication links. The absence of critical communications between the modules im- proves the reliability without restricting the physical location of the modules [44]–[74]. In the literature, the droop method is also called independent, autonomous, or wireless control. The droop method is based on a well-known concept in large-scale power systems, which consists of drooping the frequency of the ac generator when its output power increases. In the case of parallel-connected UPS inverters, the active and reactive powers supplied to the ac bus are sensed and averaged, and the resulting signals are used to adjust the frequency and amplitude of the UPS inverter output- voltage reference. The droop method achieves higher reliability and flexibility in the physical location of the modules since it only uses local power measurements. A. Active and Reactive Power Droop Control Traditionally, the inverter output impedance is considered to be inductive due to the high inductive