Modular muti-level converter grid-connected oscillation suppression measures based on virtual impedance

Modular multilevel converters(MMC) are widely used in power systems. However, relevant studies and reports show that MMC may cause oscillation problems by connecting with the grid. The second-harmonic circulating current is caused for the energy exchange between MMC phases under the current conversion process. The unreasonable parameter design of converter will lead to the natural frequency of the MMC approaching the frequency-doubled of grid. Farther, it may cause the bridge arm current oscillation. When the bridge arm resistance is large enough, the operating conditions will be stable for the overdamping in the system. Virtual damping control strategy is applied to stability study, which can improve the damping of the system without increasing the system loss and suppress the oscillation. Therefore, a stability control strategy base on virtual damping is designed to improve the stability margin of the system. In order to verify the effectiveness of the method, the main circuit model of MMC is established by using the form of matrix. Besides, it considers the virtual damping control. In order to verifie the effectiveness of the virtual damping control, the MMC simulation model is established under the Matlab/Simulink . The results show that the proposed virtual damping control can effectively suppress the MMC oscillation problem.