Hierarchical coordinated adaptive droop control for hybrid HVDC with cascaded multi‐infeed MMC inverters

The hybrid HVDC with cascaded multi‐infeed MMC inverters will be put into reality soon in China, but its MMC inverter in constant DC voltage control would become a rectifier when disturbance happens. Thus, to solve such problem and enhance the stabilities, the adaptive droop control is proposed for MMC inverters and a hierarchical coordinated adaptive strategy is designed, which are realized through three parts. The first coordinated strategy is to maintain the inverter side DC voltage in constant by shifting the MMC inverters’ droop curves, such that the operation variation will not change the DC voltage in droop control, where the power transferred by the hybrid HVDC would be varied if voltage deviation occurs; the second part is to realize MMC inverters’ accurate power control considering the practical project and power dispatch, which is achieved through the droop control reference adjustment. Based on the above coordinated strategies, the third strategy is designed to make MMC inverters and LCC rectifiers coordinate with each other for the power fluctuates and fault situations, which is used to avoid inverters’ overload operation. The simulation results validate the effectiveness of the proposed strategy finally.