A Hybrid Modular Multilevel Converter With Novel Three-Level Cells for DC Fault Blocking Capability

A novel hybrid, modular multilevel converter is presented that utilizes a combination of half-bridge and novel three-level cells where the three-level cells utilize a clamp circuit which, under dc side faults, is capable of blocking fault current thereby avoiding overcurrents in the freewheel diodes. This dc fault blocking capability is demonstrated through simulation and is shown to be as good as the modular multilevel converter which utilizes full-bridge cells but with the added benefits of: lower conduction losses; fewer diode and semiconductor switching devices, and; fewer shoot-through modes. The semiconductor count and conduction loss of the proposed converter are reduced to around 66.5% and 72% of that of modular multilevel converter based on the full-bridge cells respectively, yielding lower semiconductor cost and improved efficiency. Dc fault ride-through operation is realized without exposing the semiconductors to significant fault currents and overvoltages due to the full dc fault blocking capability of the converter.