Hybrid DC Transmission System Based on LCC-FBMMC

Hybrid DC transmission systems are mostly used in offshore wind power systems or oil platforms, which have good development prospects in large-scale power transmission, renewable energy grid connection and grid interconnection. Existing systems based on Line Commutated Converter (LCC) and Half-Bridge Modular Multilevel Converter (HBMMC) cannot achieve bidirectional power transmission or meet the situation where short-term power reversal transmission is required for power supports. These kinds of systems do not have the capability of clearing the short circuit fault on the DC side either. Take this situation into account, this paper proposes a multi-terminal hybrid DC transmission system using LCC on the rectifier side and Full-Bridge Modular Multilevel Converter (FBMMC) on the inverter side. By changing the polarity of the voltage on the DC side of the FBMMC, power reversal is achieved. Then the paper analyzes the operating principle and control strategies of LCC and FBMMC. The Simulink model was built to verify the system's ability to reverse the power flow. Finally, the simulation results show that, compared with the existing hybrid DC transmission system, the system proposed in this paper can meet the need of power reversal and has the capability of fault ride-through on the DC side.