Comparison of different DC-link voltage balancing methods for cascade H-bridge multilevel converter

Cascade H-bridge multilevel converter can be directly connected to the medium voltage grid, which can effectively improve the voltage stability of the power system. However, the imbalance of the DC-link voltage of each H-bridge module due to the power loss difference is a challenging issue, which would seriously affect the output performance of the cascade multilevel converter. Software based scheme is preferable to solve such problem because it requires no more hardware and is easy to implement. This paper compares the performance of two software based schemes, namely the negative-sequence voltage injection and zero-sequence voltage injection methods, employed in balancing the DC-link voltage of the cascade converter. It is concluded that the negative-sequence voltage injection scheme can provide stronger ability to balance the DC-link voltage with negative-sequence current injection to the grid, while the zero-sequence voltage injection scheme can balance the DC-link voltage without zero-sequence current injection. The simulation results verify the comparative analyses in this paper.