Impact of Control Interaction of Wind Farm with MMC-HVDC Transmission System on Distance Protection Adaptability under Symmetric Fault

In an offshore wind farm connected with a high-voltage direct current (HVDC) transmission system based on modular multilevel converter (MMC), a symmetric fault on the outgoing line at the sending end (SFOLSE) exhibits complex controlled characteristics in the fault current, which can undermine the reliability of relay protection. Detailed analysis of the control interaction between the wind farm and the MMC sending station (MMCSS) is conducted to ascertain the fault current characteristics. Considering the constraints imposed by the existence of a stable operating point (SOP) during SFOLSE, the phase angle difference distribution law for short-circuit currents sourced from both the wind farm and MMCSS is analyzed. Furthermore, the influence of control interaction on the reliability of distance protection is discussed. The results show that the additional impedance exhibits specific distribution characteristics under the influence of control interaction. In addition, the setting ratio of the dq-axis current reference for wind farm distance protection is analyzed, and the impact of wind farm control on the adaptability of distance protection under the constraints of the grid-connected guideline is evaluated. The main risk scenarios of misoperation are clarified, and the correctness of the analytical results is validated through PSCAD time-domain simulations.