Multi-Terminal Coordinated Frequency Support Strategy for Alternating Current/Direct Current Hybrid Voltage Source Converter-Based Multi-Terminal Direct Current System

The VSC-MTDC system incorporating hybrid AC/DC interconnections exhibits characteristics distinct from those of a purely DC-connected system during the process of grid frequency regulation. To address this issue, this paper proposes a novel frequency support approach for AC/DC hybrid VSC-MTDC system. Firstly, a frequency control model of VSC-MTDC system incorporating wind power integration is constructed. The mechanisms by which VSC-MTDC system and AC interconnected system participate in frequency regulation are analyzed. It has been clarified that power fluctuations will occur in areas connected by AC tie lines. Then, a re-identification method for disturbed areas is investigated, which can alter the operational state of VSCs in such area grid to mitigate power fluctuation that may happen in the exist approaches. In order to enhance the frequency support capability of the VSC-MTDC, a frequency deviation degree judgement factor is designed for the disturbance side VSCs to adaptively adjust the frequency droop coefficient. In addition, an adaptive voltage droop coefficient that based on the power margin is devised to achieve a more rational power allocation for the non-disturbance side VSCs. The proposed method is validated through simulation on Matlab/Simulink. The results demonstrate that compared to existing adaptive control methods and conventional control strategies, the approach can effectively reduce power fluctuation and improving the frequency regulation capacity for disturbed area.