Current Balancing Control Approach for Paralleled MMC Groups in Hybrid LCC/VSC Cascaded HVDC System

A high voltage direct current (HVDC) system with conventional line commutated converter (LCC) and multiple paralleled modular multilevel converters (MMCs) in hybrid cascaded connection at the inverter side, known as a hybrid LCC/VSC cascaded HVDC (HC-HVDC) system, effectively integrates the advantages of LCC, as well as MMC technologies. However, due to the master-slave control mode adopted in the project of paralleled MMC stations, current imbalance issues would inevitably occur among paralleled MMCs in the case of power change, DC voltage drop and short-circuit fault conditions, which could result in overcurrent in MMCs or even slow down the fault recovery process. To realize current of MMC group distributed evenly, a power-based compensation control is proposed in this paper. First, imbalanced current between power-controlled and voltage-controlled MMCs is deduced, then the imbalanced current is transformed into active power, and finally, obtained active power is compensated to the active power outer loop control. By compensating the active power reference value, the proposed control approach can achieve current balancing of paralleled MMCs under different conditions. The HC-HVDC system, based on the Baihetan-Jiangsu project, which is currently under construction, is developed in PSCAD/EMTDC to validate effectiveness of the proposed control approach. Results show that, power-based compensation control can effectively balance currents of paralleled MMCs, accelerate system recovery process, and improve system dynamics.