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Although fundamental frequency switching has been the preferred method for high-power medium-voltage multilevel converters, the technology of current generation power semiconductors has made it possible to use higher rates. In utility-connected applications, where a restricted modulation index range is usually sufficient, a main requirement is for very tight control over individual harmonics. For these reasons, this paper considers the application of selective harmonic elimination PWM (SHEPWM) with 5- or 7-level converters switching at low multiples of the fundamental frequency. An alternative approach for the calculation of multiple solutions of the firing angles is outlined, and is then applied in a modulation scheme that inherently equalizes the power flow from and to the individual DC energy sources. Results from computer simulations for a 7-level energy conversion system are presented in this paper.
Vassallo et al. (Thu,) studied this question.