Increased number of voltage steps for cascaded H-bridge multilevel inverters using 3-level operation in each half-cycle

Some renewable energy sources provide dc electrical energy. For example, photovoltaic sources, fuel cells, and the rectified output of some turbine driven sources, produce dc voltages. If the outputs of these devices are to be fed to the ac power grid, a power interface is required. The cascaded H-bridge multilevel inverter is an electronic interface that can connect multiple dc energy sources to an ac power system. In this paper we present an analysis to demonstrate the degree to which the total harmonic distortion of a cascaded H-bridge can be reduced by having some H-bridges provide 3-level voltage contributions in each half-cycle of the stepped output voltage waveform. Such 3-level operation produces a very significant distortion reduction, the result of an increase in the number of steps.