Analysis and design of a MMC based wind power converter for offshore dc grids

In this paper the modular multilevel converter (MMC) is proposed as the wind power converter for offshore dc grids. MMC has been widely used for high-power high-voltage applications, and it is able to connect a low ac voltage source to a high dc voltage source, which makes it a promising solution for wind power converter in offshore wind farm dc grids. This paper presents the analysis and design of MMCs for direct-drive medium voltage permanent magnetic synchronous wind power generators. The operation of MMC in proposed application is discussed, and it is found the approximately sinusoidal ripple of submodules will distort the output ac voltage. The conventional field oriented control scheme is modified to enhance the dynamic performance that is affected by the distorted ac voltage. The detailed digital simulation on RT-LAB platform confirms the theoretical analysis and control scheme on a 10 MW test system, which connects a 6 kV generator to a 50 kV dc grid.