A Tripole HVDC System Based on Modular Multilevel Converters

The conventional tripole HVDC system, which uses thyristor converters, has some inherent defects. This paper proposes an improved scheme which uses the full-bridge submodules-based modular multilevel converters to replace the thyristor converters in pole 3, and the converters in pole 1 and 2 are also replaced by the modular multilevel converters based on half-bridge submodules. In order to obtain good performance among three poles, coordination control is proposed. This paper also proposes voltage-inverse control and a control strategy of keeping current zero in pole 3 to suppress overvoltage and to minimize the ground current in transition processes. The performance of the proposed tripole HVDC system is evaluated through PSCAD/EMTDC. Simulation results show that based on the control strategy, the tripole HVDC system can achieve satisfactory response characteristics in transition processes, maintain the balance of capacitor voltages, and minimize the ground current in the meantime.