Improved Coordinated Control Strategy of Multi-terminal DC Grids Considering Frequency Adaptation and Fast Stabilization Characteristics

Multi-terminal flexible DC transmission (VSC-MTDC) technology has a wide range of application prospects in large-scale and wide-range multiple types of renewable energy grid-connection and outward transmission. To address the influence of AC-DC coupling on DC voltage and AC system frequency under large unbalanced power disturbances of flexible DC network, an improved coordinated control strategy based on DC voltage sag control is firstly proposed to eliminate the DC network voltage deviation from the rated operation point before and after the disturbances. And then the adaptive sag coefficients are introduced to improve the DC network voltage steady state recovery rate and certain control reliability. Thirdly, the adaptive capacity enhancement of the improved coordination control frequency is analyzed in order to realize the participation of converter station in AC system frequency regulation and control, and to make up for the lack of AC-side FM capacity. Finally, a simulation model of new energy transmission via four-terminal DC grid is constructed based on the PSCAD/EMTDC, which verifies the feasibility and effectiveness of the proposed improved coordinated control strategy and its AC grid frequency regulation.