Cascaded Multilevel Converters with Non-Integer or Dynamically Changing DC Voltage Ratios

Shuai Lu
Keith Corzine, Missouri University of Science and Technology

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Multilevel converters made of cascaded cells (MCCC) offer a high number of voltage levels with a given switch count. Many variants of the MCCC topology have been introduced over years. Mostly, only integer DC voltage ratios between the cascaded converter cells have been applied to its PWM modulation. This is mainly due to the non-uniform distribution of voltage vectors with the non-integer DC ratios. It poses difficulty to the conventional SVM modulators relying on locating the equilateral "triangle" made of three nearest vectors enclosing the reference vector. In this paper, an original modulation method is introduced to operate with any DC voltage ratio between cascaded converter cells, integer or non-integer; and even with a dynamically varying ratio; while the normal PWM output fast-average is not disrupted. The space vector analysis and the detailed simulations verify the modulation method. The new concept of MCCC PWM with non-integer DC voltage ratio offers great flexibility in its practical operation, particularly in MCCC with single DC source where the auxiliary inverter cell DC-link capacitor voltage can be regulated at arbitrary value online. Its wide range of practical applications is briefly discussed in this paper.