Recently, a multilevel cascaded converter fed with a single dc source has been presented. An analysis of the steady state working limits of this type of converter is presented in this paper. Limits of the maximum output voltage and the minimum and maximum loading conditions for a stable operation of the converter are addressed. In this paper, a way to achieve any dc voltage ratio (inside the stable operation area of the converter) between the H-bridges of the single dc source cascaded H-bridge converter is presented. The proposed dc voltage ratio control is based on a time domain modulation strategy that avoids the use of the inappropriate states to achieve the dc voltage ratio control. The proposed technique is a feed-forward modulation technique which takes into account the actual dc voltage of each H-bridge of the converter leading to output waveforms with low distortion. In this way, the dc voltage of the floating H-bridge can be controlled whilst the output voltage has low distortion independently of the desired dc voltage ratio. Experimental results from a twocell cascaded converter are presented in order to validate the proposed dc voltage ratio control strategy and the introduced concepts.

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Abstract-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 HBridge 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.