Dynamic Frequency Support and DC Voltage Regulation Approach for VSC-MTDC Systems

When an additional frequency control is implemented in the voltage source converter-based multi-terminal high voltage direct current (VSC-MTDC) system, the DC grid is capable of responding to a frequency disturbance in the AC system. However, the original additional frequency control may cause the DC voltage to exceed the limit when providing power for a severe frequency disturbance, threatening the security of the DC system. A novel dynamic additional frequency control strategy for the VSC-MTDC system is developed based on the relationship between the DC voltage and the frequency droop coefficient. A dynamic frequency droop coefficient is designed to adaptively adjust the support power of the DC grid, balancing the frequency regulation of the disturbed AC system and the voltage stability of the DC grid. A DC voltage recovery method based on multi-converter cooperation is proposed to cope with the DC voltage deviation caused by the additional frequency control. Simulations validate the advantages and satisfactory performance of the proposed method during power disturbances with different severities and for the process of DC voltage recovery.