Research on A Novel Single-Phase Forward-Mode Five-level Inverter and Control Strategy

The multilevel converter is a prospective topology for both Medium Voltage and High Voltage Direct Current (HVDC) applications. In high power application, the forward topology is applied to increase the power region. In this paper, a novel forward-mode five-level inverter is presented which combines the features of the conventional multilevel converters and forward topology. The converters can transfer high dc voltage into regulated sinusoidal voltage with low THD at arbitrary frequency. It consists of DC voltage source, DC-link capacitor, forward-mode five-level conversion unit, four-winding high-frequency transformer, cycloconverter, LC filter and output AC load. The working mode of the topology is analyzed, and the small-signal model is established based on the state space averageing method. Voltage balancing is a common problem in multilevel converters. In order to achieve a reliable five-level dc-ac conversion, and overcome the voltage imbalance, a voltage equalization strategy based on transient voltage feedback control strategy is proposed. The feasibility of the topology and the effectiveness of the equalization strategy are confirmed by simulation and experimental results. Meanwhile, the results prove that the converters have four improved advantages, including the lower voltage stress of power switches, the high frequency electrical isolation, the bi-directional power flow and the low THD of output voltage.