Coordinated start-up strategy for MMC-MTDC system

Coordinated start-up strategy for MMC-MTDC system was proposed to start the system rapidly and smoothly while avoiding large electric shock and even start failure caused by mismatches between stations. Firstly, three pre-charging modes by AC side, DC side and both AC and DC sides respectively were pointed out. Based on their charging mechanisms, a method to identify the pre-charging mode for MMC was designed and a closed-loop voltage-balance charging strategy was advanced which can precharge the capacitor voltage of sub-module to its normal level. Secondly, the coordinated start-up strategy using DC voltage's ramp control method for parallel connected MMC-MTDC was proposed based on analyzing the effective cooperation plan for stations' starting sequences. Finally, simulation using a model of three-terminal MMC-MTDC system in Matlab/Simulink was conducted. The results show that the proposed closed-loop voltage-balance charging strategy can adapt all three pre-charging modes automatically and charge the sub-module's capacitor to its normal level stably, and the coordinated start-up strategy accomplishes a smoothly unlocking for the MTDC system. ©, 2015, Power System Technology Press. All right reserved.