To deal with the overvoltage

the mechanism and restraining strategy of the sending-end overvoltage in offshore wind farm-flexible HVDC transmission system under grid fault are studied. Firstly

the mathematical model of the grid-side and valve-side voltage under grid faults is established. The influence mechanism of MMC control strategy to the grid voltage is analyzed

and the influence of the over-modulation problem caused by the overvoltage on the control performance is studied. On this basis

an overvoltage restraining strategy is proposed

which can restrain the overvoltage without introducing negative sequence currents

and can accelerate the voltage recovery speed after the faulty line is cut off. Finally

the offshore wind farm-HVDC transmission system is established in the PSCAD/EMTDC environment

and simulation studies are conducted. The research results show that the mathematical model proposed in this paper can reflect the fault characteristics accurately. In addition

the proposed overvoltage restraining strategy can achieve the desired performance

which can benefit the system stable operation during the grid fault and recovery periods.