IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 55, NO. 1, JANUARY 2008 21 Individual Voltage Balancing Strategy for PWM Cascaded H-Bridge Converter-Based STATCOM Jon Andoni Barrena, Student Member, IEEE, Luis Marroyo, Member, IEEE, Miguel Ángel Rodríguez Vidal, Member, IEEE, and José Ramón Torrealday Apraiz Abstract—This paper presents a new control method for cas- caded connected H-bridge converter-based static compensators. These converters have classically been commutated at fundamen- tal line frequencies, but the evolution of power semiconductors has allowed the increase of switching frequencies and power ratings of these devices, permitting the use of pulsewidth modulation techniques. This paper mainly focuses on dc-bus voltage balanc- ing problems and proposes a new control technique (individual voltage balancing strategy), which solves these balancing prob- lems, maintaining the delivered reactive power equally distributed among all the H-bridges of the converter. Index Terms—Multilevel converter, power conversion, static compensator (STATCOM). NOMENCLATURE ν GRID Grid voltage. V GRID RMS value of the grid voltage. i f Output current. I f RMS value of the output current. ν H i Output voltage of the i th H-bridge. V H i RMS value of the fundamental component of ν H i . ν inv Inverter output voltage. V inv RMS value of the fundamental component of ν inv . Vc i DC-bus capacitor voltage of the i th H-bridge. P i Output active power of the i th H-bridge. Q i Output reactive power of the i th H-bridge. N Number of series-connected H-bridges. ϕ Phase angle between the grid voltage and the output current. φ Phase angle between the fundamental compo- nent of the inverter output voltage and the output current. Manuscript received November 30, 2006; revised July 16, 2007. This work was supported in part by the GENEDIS II project of the ETORTEK strategic program of the Basque Government. J. A. Barrena, M. Á. Rodríguez Vidal, and J. R. Torrealday Apraiz are with the Department of Electronics, Faculty of Engineering, University of Mondragón, 20500 Mondragón, Spain (e-mail: jabarrena@eps.mondragon.edu; marodriguez@eps.mondragon.edu; jrtorrealday@eps.mondragon.edu). L. Marroyo is with the Department of Electrical and Electronic En- gineering, Public University of Navarra, 31006 Pamplona, Spain (e-mail: luisma@unavarra.es). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIE.2007.906127 subscripts d and q Direct and quadrature axes of the output-current- oriented reference frame. AV Average value. superscripts ∗ Reference value. I. I NTRODUCTION R ECENTLY, interest in electric power quality has in- creased due to more restrictive regulations on this area. This interest has been conducive to the development of multi- ple equipment, which could improve the energy transmission capability of the transmission grids and the quality of the voltage waveform in the connection point. These devices are known as flexible ac transmission system (FACTS), which are based on state-of-the-art high-power electronics. Much work has been done on these FACTS for a high-voltage transmission grid, as well as in distribution grids where these devices are known as distribution FACTS (DFACTS) or “custom power” devices [1]. A static compensator (STATCOM) is basically one of the shunt-type FACTS controllers, and DSTATCOMs are the distribution network STATCOMs. There are some varia- tions of the STATCOM, but their composition is basically the same. One STATCOM is composed of one inverter with energy storing capacitors on its dc side, inductances and a coupling transformer on its ac side, and a control system, and it is connected in parallel with the power grid, as shown in Fig. 1. The STATCOM controls the reactive-power flow in the electric line, injecting or absorbing it. This reactive-power output of the converter is controlled by varying the amplitude of the output voltage [2]. The evolution of existing power semiconductor switches (GTO, IGBT) and the appearance of new ones (IGCT, IEGT, etc.), combined with the utilization of new inverter topologies, have allowed the increase of power and voltage ratings of electronic converters. This means that, in some cases, even the coupling transformer is not necessary, and the inverter could directly be connected to medium voltage levels [3]. Applications where only the reactive power is required, such as the STATCOM application that is studied in this paper, do not need any energy source. Therefore, in this kind of application, the cascaded H-bridge multilevel topology presents several advantages compared to other multilevel topologies [4]–[6]. Fig. 2 shows one phase of the cascaded H-bridge multilevel converter with N H-bridges that are connected in series. The 0278-0046/$25.00 © 2008 IEEE