Small-signal Stability Analysis For A Weak-Grid-Connected DFIG-based Wind Power Generation In DC-link Voltage Time Scale

The main features of wind power generation in our country are large scaled, centralized, and generating facilities located far from load center. This weak grid connected situation brings challenges for system power-swing stability, reactive power regulation and fault ride through of wind turbines(WTs). The thesis investigates the stability of DC-link voltage, in electromagnetic time scale, of doubly fed induction generator(DFIG) based WT attached to a weak AC system.This thesis presents a reduced order small-signal model that can be used to analyze the stability of DFIG's DC-link voltage control system, especially under weak AC grid conditions. This model neglects DFIG's flux and fast current control dynamics. However, the effects of operating points, grid strengths and control loops' interactions on system dynamic performance are taken into account. The feasibility and accuracy of the presented model is verified by time domain simulation and eigenvalue comparision with detailed model. Eigenvalue analysis and transfer function analysis are both adopted to analyze system small-siganl stability with the model. Eigenvalue analysis shows that phase-locked loop(PLL), active power control(APC) and reactive power control(RPC) of rotor side converter(RSC) mainly participate in dominant oscillation mode of control system. The interactions between PLL and RSC control damage the stability of DC-link voltage in weak grid. Analogy to the rotor swing mode of synchronous generator, synchronizing and damping components defined by PLL are presented for stability analysis physically. Each control's contribution to synchronizing and damping components is analyzed for different grid strengths and controller gains. It clearly illustrates how PLL and RSC control interact leading to system instability. Time domain simulation of a detailed model was carried out to verify the validity of the analysis above.