DC Voltage Compensation Strategy for Parallel Hybrid Multilevel Voltage-Source Converter

The hybrid multilevel voltage-source converter, combining semiconductor switches and power module cells, is a new family for VSC-HVDC applications. The hybrid multilevel converter in parallel arrangement (PHMC), in terms of converter investment, is very adaptable for cable transmission systems. However, the dc voltage of the converter, synthesized by the output voltage of three-phase cell stacks, is strongly coupled with ac voltage variations, which influences the stability of converter operation. This paper proposes a dc voltage compensation strategy for the parallel hybrid multilevel converter. This strategy, implemented by introducing an isolation period, can effectively cope with the dc voltage deviation, especially under ac network faults. Simulation results in PSCAD/EMTDC verify the effectiveness of the proposed strategy under various circumstances.