Symmetrical Hybrid Multilevel Inverters for Ship Electric Propulsion Drives

Medium voltage direct current (MVDC) based ship integrated power system (IPS) is regarded as the future developing trend of ship power system. Medium-voltage high power inverter for ship electric propulsions is one of the key problem. In order to improve the output voltage, power and harmonic performance, a family of symmetrical hybrid multilevel inverter for open-winding multi-phase motor based propulsion drive is presented in this paper. Each phase of this inverter is composed of a high-frequency multilevel DC/DC converter cell and a high-voltage H-bridge cell. The DC/DC converter is operated at high frequency with low voltage devices and the H-bridge is operated at fundamental frequency with high voltage devices. All the phases are connected to a common DC-link and each drives an isolated winding of an open-winding motor. A symmetrical hybrid five-level topology is taken as an example and the control method is discussed in this paper. Simulation results demonstrate the feasibility of this topology and control method.