Impedance-Based Stability Analysis and Stabilization Control Strategy of MMC-HVDC Considering Complete Control Loops

Impedance-based stability criterion is effective for analyzing the oscillation phenomenon in modular multilevel converter-based high-voltage DC (MMC-HVDC) transmission system, and modeling MMC impedance is the critical step. However, few papers have modeled MMC impedance with comprehensive consideration of time delay and outer-loop control at the same time. The stabilization strategies to improve system stability are not considered during impedance modeling, and the influences of stabilization strategy on impedance are not analyzed mathematically, either. In this article, an accurate sequence impedance model of MMC is derived with the consideration of time delay and complete control loops. Based on this impedance model, the negative damping caused by time delay is pointed out and the weak stability of grid-tied MMC system is analyzed. Then a stabilization control strategy is proposed to suppress the system oscillation. In the meantime, taking the proposed stabilization strategy into account, impedance modeling is revised to study the influence of this strategy on MMC impedance. Furthermore, based on this revised impedance model, an optimal design method for controller parameters of the stabilization strategy is proposed to maximize the phase margin of the interconnected system. Finally, the effectiveness of the accurate impedance model, the proposed stabilization control strategy and optimal design method are verified by simulation results.