DC Power Control Strategy of MMC for Commutation Failure Prevention in Hybrid Multi-Terminal HVDC System

This article presents a control strategy for a modular multilevel converter (MMC) to prevent commutation failure of a line-commutated converter (LCC), forming a three-terminal hybrid HVDC transmission system, where one LCC sending end is connected to the large generation and two receiving ends (LCC inverter and MMC) are located near the load center. This configuration, one of the potential options, has been proposed to strengthen Korea electric power transmission system through the optimized use of existing assets and rights-of-way, extremely challenging to secure. The MMC power control strategy has been developed to regulate the AC voltage and the extinction angle of the LCC inverter. This indirect yet effective active and reactive power control of the LCC inverter terminal helps prevent the commutation failure (CF) of the LCC in emergency and maximize the benefits of the costly planning option. By establishing a theoretical foundation for this power control problem and relationship among the control parameters, we quantify the active power reference for MMC to secure the desired LCC extinction angle. A coordinated strategy has been developed for the AC filter, the on-load tap changer of a transformer, along with the MMC control to lower the risk of CF and its catastrophic impact on the whole power system. The validity and performance of the proposed control methods are demonstrated for the real Korea electric power planning cases using a real-time power system simulator.