0885-8993 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TPEL.2016.2535481, IEEE Transactions on Power Electronics Impact of Switching Harmonics on Capacitor Cells Balancing in Phase-Shifted PWM Based Cascaded H-Bridge STATCOM Ehsan Behrouzian, Student Member, IEEE and Massimo Bongiorno, Member, IEEE and Remus Teodorescu, Fellow, IEEE Abstract—The purpose of this paper is to investigate the impact of switching harmonics on the instantaneous power distribution in the cells of a cascaded H-Bridge based STATCOM when using Phase-Shifted PWM. The case of high and low switching frequency for the converter cells are investigated and the in- teraction between voltage and current harmonics is analyzed. It is shown that in both cases, this interaction results in an uneven power distribution among the cells in the same phase leg, leading to drifting of the dc-capacitor voltages and thereby the need for proper stabilization control loops. It is also shown that the selected frequency modulation ratio affects the active power distribution among the cells. In particular, the selection of a non-integer frequency modulation ratio helps in providing a more uniform power distribution among cells of the same phase leg, thus contributing to the capacitors balancing. A methodology for optimal selection of the frequency modulation ratio is given. Theoretical conclusions are validated through simulation and experimental results. Index Terms—Multilevel systems, Cascade systems, Static VAR compensators, Reactive power control, Pulse width modulation, Harmonic analysis. I. I NTRODUCTION T HE Modular Multilevel Converter (MMC) technology [1] has proved to be superior to other converter topologies for grid connection in terms of efficiency, footprint, modu- larity and fault tolerance [2]. Recently, H-bridge MMC-based STATCOM, also known under the name of Cascaded H-Bridge STATCOM (CHB-STATCOM), have been introduced in the market [3]. One of the main challenges for this converter topology is to control the DC-link voltages; for each H- Bridge cell that constitutes the phase leg of the converter, the DC-link voltage might diverge from its reference value due to non-uniform active power distribution. This non-uniform power distribution is mainly due to different characteristics of the components of the individual cells and, more in general, any condition that leads to a deviation from ideal conditions [4]. As a result, specific stabilization control loops (typically denoted as an individual balancing control loop) are needed to guarantee a proper operation of the system [5]. The financial support provided by ABB is gratefully acknowledged. E. Behrouzian and M. Bongiorno are with the Department of Energy and Environment, Division of Electric Power Engineering, Chalmers University of Technology, SE 41296 Gothenburg, Sweden (e-mail: name.surname@chalmers.se). Phone: +46-31-7721682, Fax: +46-31-7721633. R. Teodorescu is with the Department of Energy Technology, Aalborg Univer- sity, 9220 Aalborg East, Denmark (e-mail: ret@et.aau.dk). Phone: +45-9940- 9249. R. Teodorescu is 20% with the Department of Energy and Environment, Division of Electric Power Engineering, Chalmers University of Technology. In the recent years, both manufacturers and researches have paid high effort to improve the control and the modulation of this converter topology. For the latter, Phase-Shifted Pulse Width Modulation (PS-PWM) has been extensively investi- gated in the literature [5]. However, not sufficient attention has been given to the investigation of the different harmonic components that are generated when using PS-PWM and their impact on the system performance, in particular in case of non-ideal conditions of the system. The works in [6] [7] investigate the impact of non-ideal conditions on harmonic components, with effort in trying to minimize them and therefore improve the harmonic perfor- mance of the system. In [8] a phase-shift modification solution, aiming at finding online a suitable phase-shift for each carrier in order to improve the harmonic performance of the system is proposed. However, this approach can be impractical in case of modular converters with large number of cells. A carrier- rotation strategy to more uniformly distribute the instantaneous power among the different cells is proposed in [9] [10] [11] [12] when level-shift PWM is used. This modulation technique can also be applied when using PS-PWM, as discussed in [13] [14]. Although the carrier-rotation method can be beneficial from a capacitor balancing point of view, when using PS-PWM this method can negatively impact the system performance by introducing a transient at each carrier rotation. Due to the inherited phase shift between the carriers, it is not possible to perform a carrier rotation when all carriers have completed their cycle at the same time. As a result, the carrier of a cell will be changed before the switching cycle is completed, affecting the duty cycle of the ongoing pulse and thereby leading to distortion. This will occur at each carrier rotation. In [15], the use of a non-integer frequency modulation ratio (defined as the ratio between the frequency of the carrier and the grid frequency) is investigated; this work mainly focuses on MMCs based on half-bridge double-star structure with high number of cells per phase leg, which is typically the case for HVDC systems; the presence of a high number of cells per converter arm allows the use of a reduced switching frequency for the individual cell and at the same time to assume that the line current is a quasi-sinusoidal waveform. For this reason, the work presented in [15] mainly focuses on the interaction between the cell voltage carrier harmonics and the fundamental component of the arm current. However, in case of MMCs with reduced number of cells (such as for STATCOM applications), carrier harmonics in the current must