Asymmetrical Cascade Multilevel Converters With Noninteger or Dynamically Changing DC Voltage Ratios: Concepts and Modulation Techniques

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Asymmetrical cascade multilevel inverters offer a high number of voltage levels with a given switch count. For a given topology, the number of levels depends on the configuration of the dc voltage (ratios). This paper deals with the design and control of such converters with noninteger dc voltage ratio, which leads to unevenly distributed space vectors. It describes how to select the dc voltage ratio configurations that yield space vectors as evenly distributed as possible. It describes how to produce an effective PWM modulation that allows generating undistorted current even in the presence of (some) unevenly distributed space vectors. </para>