Control Strategy of Single Phase Back-to-back Converter for Medium Voltage Drive under Cell Fault Condition

Cascaded H-Bridge (CHB) inverter is the most widely used topology for Medium Voltage (MV) drive system due to the high degree of modularity, easier implementation of medium output voltage, and ability to continuous operation under the cell fault condition. Because each power cell of CHB should have isolated DC source, multi-winding input transformer and three phase Active Front-End (AFE) are generally used for regenerative applications. The whole system can be reduced by replacing the three phase AFE with single phase AFE. However, input power imbalance among three phases under the cell fault condition inevitably occurs, if control strategy of normal operation is employed in system. This paper proposes a control scheme for the cell fault condition of single phase AFE CHB. Not only DC-link voltages of each cell but also grid current are regulated well without imbalance by applying the proposed control scheme to the system, even in the cell fault condition. Simulation and experimental results are provided to verify the effectiveness of the proposed scheme.