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Flying-Capacitor-Based Chopper Circuit for DC Capacitor Voltage Balancing in Diode-Clamped Multilevel Inverter

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Flying-Capacitor-Based Chopper Circuit for DC Capacitor Voltage Balancing in Diode-Clamped Multilevel Inverter View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Queensland University of Technology ePrints Archive IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 57, NO. 7, JULY 2010 2249 Flying-Capacitor-Based Chopper Circuit for DC Capacitor Voltage Balancing in Diode-Clamped Multilevel Inverter Anshuman Shukla, Member, IEEE, Arindam Ghosh, Fellow, IEEE, and Avinash Joshi Abstract—This paper proposes a flying-capacitor-based chop- causing a violation in the safe operating limits leading to in- per circuit for dc capacitor voltage equalization in diode-clamped verter malfunctioning. Therefore, balancing of the dc capacitor multilevel inverters. Its important features are reduced voltage voltages is required under all conditions, which determines both stress across the chopper switches, possible reduction in the chop- per switching frequency, improved reliability, and ride-through the safety and efficiency of DCMLIs [1]–[3]. capability enhancement. This topology is analyzed using three- Two possible solutions of the voltage imbalance problem and four-level flying-capacitor-based chopper circuit configu- exist: 1) installing of voltage-balancing circuits on the dc side rations. These configurations are different in capacitor and of the inverter [8]–[16], [23]–[25] and 2) modifying the con- semiconductor device count and correspondingly reduce the de- verter switching pattern according to a control strategy [17], vice voltage stresses by half and one-third, respectively. The de- [18], [26]–[38]. The latter is definitely preferable in terms of tailed working principles and control schemes for these circuits are presented. It is shown that, by preferentially selecting the cost, as the former requires additional circuits and power hard- available chopper switch states, the dc-link capacitor voltages can ware, which add to the system cost and complexity. For appli- be efficiently equalized in addition to having tightly regulated cations involving only reactive power [like static compensators flying-capacitor voltages around their references. The various op- (STATCOMs)], the switching pattern modification strategies erating modes of the chopper are described along with their pref- can be used for voltage balancing. However, voltage balancing erential selection logic to achieve the desired performances. The performance of the proposed chopper and corresponding control would influence reactive power control if priority were given to schemes are confirmed through both simulation and experimental voltage balancing [8]. Moreover, the switching pattern modifi- investigations. cation strategy cannot be used to control the capacitor voltage, Index Terms—Chopper, diode-clamped multilevel inverter except at low modulation indexes [24], [36], [37]. For higher (DCMLI), flying capacitor, four-level, multilevel, three-level. level inverters, the voltage balancing through switching pattern modification strategies limits the output voltage to 50% of the I. INTRODUCTION maximum [25]. Furthermore, a medium- or high-voltage power MONG THE multilevel voltage-source-inverter configu- converter intended for installation on a utility grid is required to A rations, the diode-clamped multilevel inverter (DCMLI) be more reliable and robust against line faults and transients. is widely accepted for applications in high-power drives and It is also to be noted that, for applications like a unified utility systems [1]–[22]. It possesses some of the desirable power flow controller (UPFC), where two such converters are features like the following: 1) the dc capacitors can be easily connected back to back, the switching modification strategy precharged as a group; 2) switching control is easiest; and necessitates the two converters to operate at a fixed ac voltage 3) the protection circuit required is least complex among the ratio for the capacitor voltage equalization [10]. This constraint multilevel inverters, etc. [1]–[7]. Moreover, by using a DCMLI, would seriously limit the flexibility of UPFC. Therefore, extra the multilevel voltage outputs are easily obtained with a low- capacitor voltage control circuits are preferably used in practice cost string of the dc capacitors. However, these features, for [1], [2], [7]–[15]. more than three levels, are achieved at the expense of the There are many articles available in literature discussing divergence of dc capacitor voltages, resulting in the collapse the extra control circuits for dc capacitor voltage balancing in of some and rise of others due to the nonuniform power drawn DCMLIs, which are mostly bidirectional buck–boost choppers from them [1]–[3], [8]–[18], [23]–[38]. This may result in poor [8]–[16], [23]–[25]. In [8]–[12], such circuits are successfully quality voltage outputs, affecting the control performance and tested when DCMLI is used for STATCOM applications. Simi- lar chopper-based capacitor voltage balancing is achieved when Manuscript received March 14, 2009; revised June 11, 2009; accepted DCMLI is used for UPFC applications as reported in [13] and July 8, 2009. Date of publication August 21, 2009; date of current version June 11, 2010. [14]. The performance of the back-to-back-connected five-level A. Shukla is with ABB Corporate Research, 721 78 Vasteras, Sweden diode-clamped converters supported by a pair of buck–boost (e-mail: [email protected]). choppers for dc capacitor voltage balancing is found to be A. Ghosh is with the School of Engineering Systems, Queensland University of Technology, Brisbane, Qld. 4001, Australia (e-mail: [email protected]). effective as well [15], [16]. A similar buck–boost chopper was A. Joshi is with the Department of Electrical Engineering, Indian Institute of implied for unidirectional current control for the dc capacitor Technology Kanpur, Kanpur 208 016, India (e-mail: [email protected]). voltage balancing in [23]. A balancer circuit based on the three- Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. level diode-clamped chopper configuration is reported in [24]. Digital Object Identifier 10.1109/TIE.2009.2029527 It has the advantage that lower rated devices may be used in 0278-0046/$26.00 © 2010 IEEE 2250 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 57, NO. 7, JULY 2010 figure, the inverter circuit [Fig. 1(a)] is not shown but assumed to be connected to the chopper. The resistances R1 and R2 in Fig. 1(b) represent the winding resistances of the corresponding inductors. Using this circuit, the capacitor voltages are con- trolled within a band by transferring the extra energy from the overcharged capacitor to an inductor and then transferring it back from the inductor to the undercharged capacitor. This chopper consists of two parts. In the upper part, L1 is used to exchange the energy between Cd1 and Cd2 using SC1, SC2, DC1, and DC2, while in the lower part, L2 exchanges the energy between Cd3 and Cd4 using SC3, SC4, DC3, and DC4 [Fig. 1(b)]. By using this circuit, it is possible to transfer the Fig. 1. (a) One phase leg of a five-level diode-clamped inverter and charge both from Cd2 (Cd3) to Cd1 (Cd4) or from Cd1 (Cd4) to (b) conventional chopper for dc capacitor voltage balancing in DCMLI. Cd2 (Cd3). However, in a steady-state operation of DCMLI, the C C TABLE I voltages of d1 and d4 always tend to increase while those of SINGLE-PHASE FIVE-LEVEL DCMLI SWITCHING SCHEME Cd2 and Cd3 tend to decrease [1]–[3], [8]–[18], [23]–[38]. There are many schemes available in the literature for con- trolling the chopper in Fig. 1(b) [8]–[15], [28]. The easiest approach is single-pulse control in which a band is set across the reference capacitor voltage, so that, if a capacitor voltage comes out of this band, the corresponding chopper operation is initi- ated. This implies that, when the capacitor voltages are within the defined bands, the switches SC1−SC4 in Fig. 1(b) remain the chopper compared with that in the conventional buck–boost open. Note that two independent chopper controllers are used choppers. to control the capacitors Cd1 and Cd2 and Cd3 and Cd4, respec- In this paper, a flying-capacitor-based chopper circuit is tively. The detailed operating principles of this chopper and dif- proposed to overcome the voltage imbalance problems among ferent control schemes can be found in [8]–[16], [22], and [23]. the dc capacitors of a DCMLI. As compared to the conventional chopper, it requires additional power semiconductor devices of III. FLYING-CAPACITOR-BASED CHOPPER CIRCUIT lower rating and capacitors. For generalization purposes, the proposed topology is analyzed using two configurations which In the conventional chopper in Fig. 1(b), the semiconduc- are termed as three-level and four-level choppers, respectively. tor devices are subjected to half of the net dc-link voltage, The various control schemes and switching decision rules of the when not conducting. Therefore, these devices need to be at chopper devices are derived to regulate the capacitor voltages least of twice the voltage rating compared with those of the close to the reference value in this topology. The principles main devices in the five-level inverter circuit. Similarly, for presented are verified by both detailed PSCAD/EMTDC sim- higher level inverters, the ratio of the required voltage rating ulation and experimental investigations for a five-level inverter. of chopper semiconductor devices to that of the inverter main devices correspondingly increases. This higher voltage rating requirement of the chopper devices contradicts with one of the II. DCMLI AND CONVENTIONAL main motives of using a multilevel inverter, which states that, in CHOPPER CONFIGURATIONS a multilevel inverter, smaller rating devices are used to produce A single-phase five-level DCMLI is considered here, and its a correspondingly higher voltage output. Therefore, it can be schematic is shown in Fig. 1(a). Two similar additional phase said that, by using the conventional chopper, the advantages of legs connected to the same dc bus would be required for a a multilevel inverter are not fully exploited as the higher rating three-phase inverter.
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