Impedance Modeling and Stability Analysis of Grid-Connected DFIG-based Wind Farm with a VSC-HVDC

A new type of subsynchronous oscillation (SSO) has been observed recently in double fed induction generator (DFIG)- based wind farm integrated via voltage source converter based HVDC (VSC-HVDC) system. However, the mechanism of this emerging oscillation is not entirely understood. In this paper, the impedance models of DFIG with and without considering the phase locked loop (PLL) dynamics are both derived. Then the impedance-based simplified equivalent circuit of the multiple DFIGs interfaced with VSC-HVDC system is established. This model can be further represented as the RLC series resonance circuit to quantify the start-oscillating condition intuitively. The theoretical analysis results show that DFIGs behave as an inductance in series with a negative resistanceat the resonance point, whose interaction with wind farm side VSC (WFVSC) (regard as a resistance-capacitance) constitutes an equivalent RLC resonance circuit with negative resistance. Therefore, the oscillation tends to occur due to the negative damping. In addition, the impact of various factors including number of gridconnected DFIG-wind turbines, wind speed, and parameters of PI controllers and PLL on the SSO characteristics are analyzed based on the proposed simplified model. Finally, the correctness of the theoretical analysis is validated by both the time-domain simulation and hardware-in-loop (HIL) experiments.