Transformerless Grid Connected Control of Wind Turbine Based on H-MMC

A high-power wind turbine has low rotational speed and a relatively low terminal voltage level. Thus, the wind energy conversion system (WECS) is commonly connected to the grid by an isolated boost transformer, which increases costs and room greatly. In this article, the hexagonal modular multilevel converter (H-MMC) is applied so that the high-power wind turbine can be directly connected to the medium-voltage ac grid. It has the advantages of transformerless grid connection and low ripples of capacitor voltages. As the bridge arm voltage of H-MMC contains both the low-frequency component induced by a wind turbine and the power frequency component induced by the grid, the voltage ripples of submodules (SMs) are reduced. Due to the large differences in the voltage level between the wind turbine and the grid, the low-amplitude-modulation voltage of the wind turbine side is easily obliterated by the high-amplitude-modulation voltage of the grid side, which will cause distortion of the bridge arm current. Therefore, auxiliary SMs with variable reference voltage are adopted to improve the tracking accuracy. The MATLAB/Simulink simulation and the RT-LAB hardware-in-the-loop experimental results verify the feasibility of the proposed topology and the effectiveness of the control strategy.