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Multilevel converters have received increased interest recently as a result of their ability to generate high quality output waveforms with a low switching frequency. This makes them very attractive for high power applications. A Cascaded H-Bridge converter is a multilevel topology which is formed from the series connection of H-Bridge cells. Optimized pulse width modulation techniques such as Selective Harmonic Elimination (SHE-PWM) or Selective Harmonic Mitigation (SHM-PWM) are capable of pre-programming the harmonic profile of the output waveform over a range of modulation indices. Such modulation methods may however not perform optimally if the DC links of the Cascaded H-Bridge Converter are not balanced. This paper presents a new SHM-PWM control strategy which is capable of meeting grid codes even under non-equal DC link voltages. The method is based on the interpolation of different sets of angles obtained for specific situations of imbalance. Both simulation and experimental results are presented to validate the proposed control method. I. INTRODUCTION Multilevel converters have become the focus of research in recent years as a result of their suitability for high power applications [1]. Amongst the available topologies are the Neutral Point Clamped (NPC), Flying Capacitor (FC) and Cascaded H-Bridge converters (CHB) [2]. The latter is constructed from a series cascade of three-level H-bridges. This connection enables the converter to produce high quality, high voltage waveforms whilst utilizing low or medium voltage switching devices. This functionality makes this converter an attractive option for grid connected applications such as Uninterruptible Power Supplies, Static VAR compensators, Series and Shunt Compensators etc. [3]-[10]. The use of power electronic converters at high power levels usually demands a reduction in switching frequency in order to ensure that losses caused by the imperfect nature of practical Manuscript

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This study presents an improved voltage balancing strategy which can be employed on multilevel cascaded H-bridge (CHB) rectifiers using low switching frequency. The proposed method balances the voltage of all DC links in a CHB rectifier to a predefined value, without increasing the number of switching transitions. The redundancy states which are available in the CHB rectifier to produce a voltage level are used to balance the DC links in each transition. Moreover, using the well-known selective harmonic elimination-pulse width modulation method, all current harmonic components up to 29th harmonic are eliminated in the three-phase CHB rectifier, whereas the average switching frequency of each switch is limited to 150 Hz. The effectiveness and validity of the proposed strategy is verified by several simulations and experiments that are carried out on a 7-level CHB rectifier.

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