FRT dynamic analysis and stability enhancement of MMC-HVDC connected offshore wind farms employing voltage drop control

Voltage drop method is generally accepted for the fault ride-through (FRT) control of VSC-HVDC connected offshore wind farms. However, it may lead to the dc-link voltage fluctuation and further deteriorate the dynamic stability. This paper reveals above phenomenon for the first time and analyses its coupling mechanism. The instantaneous power of the MMC arm and phase unit are derived and it indicates that the dc current component induced by the offshore voltage drop leads to the fundamental-frequency ripple in the dc-link voltage. Two available internal control schemes are presented for MMC stations to improve the FRT dynamic stability by suppressing the dc-link fluctuation. Simulation results show that the presented energy-based method can suppress the fluctuation of dc-link voltage effectively and improve the stability of offshore grid through the FRT period.