Fault-Tolerance Control for Power Equalization of Cascaded NPC/H-bridge Inverter System

Cascaded NPC (Neutral Point Clamped)/H-bridge (CNHB) inverter is one of Multi-level inverters, and widely used for a large-capacity industrial motor driving. When a fault occurs in the power modules in CNHB, the neutral point shifting control which is commonly used for fault-tolerance control has a disadvantage that power imbalance is occurred between the power modules even though it can operate continuously in case of failure. This paper proposed advanced fault-tolerance control to achieve power equalization of power modules in case of malfunction. The proposed control method is select the limit value of the maximum reference voltage for neutral point voltage shifting control so that root mean square (RMS) values of the output phase voltage of each module are equal and power imbalance is reduced. Power characteristics with proposed control method are analyzed mathematically. Proposed control method is verified by simulation based on 10[kW] scale of CNHB.