Stability of DC-link voltage affected by phase-locked loop for DFIG-based wind turbine connected to a weak AC system

The paper investigates the stability of DC-link voltage, in electromagnetic time scale, of doubly fed induction generator (DFIG) based wind turbine attached to a weak AC system. In weak AC system, the dynamic of phase-locked loop (PLL) is coupled with DFIG converters´ control. A reduced order model is proposed for stability analysis of DFIG DC-link voltage. Eigenvalue analysis shows that 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. 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 PLL 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.