Fault ride-through coordinated control for offshore symmetrical AC fault of wind farms integrated MMC-HVDC system

The traditional AC fault ride-through (FRT) control will lead to a significant decrease or even reversal of the active power transmitted to the onshore grid, when an offshore symmetrical AC fault occurs to the transmission lines of wind farms integrated MMC-HVDC. It severely affects the power balance of onshore grid. Therefore, this paper proposes a control strategy based on the coordination between wind farms (WFs) and windfarm side MMC (WFMMC) to achieve the positive and maximum active power transmission during FRT period. Firstly, four FRT operating modes of the offshore system in the traditional control are clarified and the mechanism of active power reversal is analyzed. Secondly, a FRT analysis model based on multi-WFs and WFMMC is established and the mechanism of the coordinated control is analyzed. An approach to determine the optimal operating point of the analysis model is elaborated in detail. In this approach, the control objective is the positive and maximum transmission of active power, the controlled variables are the amplitudes and phase angles of WFs currents and the amplitude of WFMMC voltage, the constraints include the current amplitude and active power of WFs as well as the voltage amplitude and current amplitude of WFMMC. Then, an implementation method based on communication is proposed. The factors influencing active power transmission are analyzed and the robustness against information errors is also discussed. Finally, the superiority of the proposed strategy is verified based on PSCAD/EMTDC simulation and the real time experimental platform.