Grid-supporting Characterization of MMC-HVDC System for Offshore Wind Farm Under Different Control Modes: A Comparative Study

When transient processes such as large disturbance faults occur in the power grid, system frequency fluctuations will affect the normal operation of a wind farm. In severe cases, it will cause a large-scale wind turbine to go off the grid. By imitating the inertia support ability of the synchronous machine, the grid-forming control strategy makes the converter have the frequency modulation ability. However, most of the current studies focus on one single converter. There is no literature to discuss whether the inertia of the wind turbine still has a frequency support role after being transmitted by the MMC-HVDC system decoupled from DC. To fill this gap, in this paper, the general grid-connected control method is used to simulate and compare the frequency support capacity of the joint system under the four control structures under the combination of two pairs(grid-following wind farms, grid-forming wind farms, MMC-HVDC with inertial support and MMC-HVDC without inertial support). It is concluded that the frequency support effect is the most obvious when the MMC-HVDC with inertial support is operated in coordination with the grid-forming wind farms.