Impedance Modeling and Stability Analysis of Bipolar MMC-HVDC System with Islanded PMSG-Based Wind Farm Integration

Bipolar connection is always adopted in modular multilevel converter-based high voltage direct current (MMC-HVDC) systems to improve the power capacity and system reliability. However, most research about the stability of the MMC-HVDC system with islanded PMSG-based wind farm integration focused on the monopolar connection mode, and the stability of the bipolar MMC-HVDC systems is still worth further studying. This paper develops the equivalent SISO impedance models of islanded PMSG-based wind farm and bipolar MMC-HVDC system with double U/f droop control and constant U/f-P/Q control, respectively, considering the effects of frequency coupling characteristics, the initial phase of reference frame, and the impedance coupling of metallic return at the dc side. And then a contrastive study between the effects of two bipolar coordinated control methods on the impedance characteristics and the stability of the interconnected system is carried out, including the common and special control loops in the two methods. Finally, the detailed time-domain simulation results in Matlab and hardware-in-the-loop results verify the accuracy of the proposed impedance models and the effectiveness of the stability analysis conclusions.