Small Signal Stability Analysis of MMC-HVDC Grid-connected System and Optimization Control of Zero-sequence Circulating Current Controller

The high voltage direct current (HVDC) transmission technology based on modular multilevel converter (MMC) is widely used in the offshore grid-connected wind power generation projects, and the small signal stability problems of the grid-connected system have been attracted great attention. This paper focused on the influence of the zero-sequence circulating current (ZSCC) controller on the stability of the offshore wind power generation MMC-HVDC grid-connected system. The closed-loop frequency domain model was derived by means of complex vectors and harmonic state space firstly. Then the paper developed the single-input-single-output (SISO) impedance model of MMC which considering the ZSCC controller. In addition, the impedance model of the offshore wind farm and the distribution network were established. Based on the impedance model, the influence law and mechanism of the ZSCC controller on the stability of the offshore wind power generation MMC-HVDC grid-connected system were explored. The analysis results show that when the output power of wind is too large, the system is prone to oscillation. The application of the ZSCC controller equivalently increases the bridge arm resistance and enhancing the system damping, which effectively suppressing the resonant peaks of MMC impedance in island control mode. Finally, from the point of view of enhancing the damping of the system, the parameter optimization scheme for ZSCC controller was designed, and the correctness of the theory and the feasibility of the scheme were verified by simulations.