Stability of Stator-Flux-Based Controlled DFIG with Weak AC Grids

This paper explores the potential instability of a weak AC grid connected doubly-fed induction generator (DFIG) system with a stator-flux based active power control. A small-signal model allowing for the weak grid connection of DFIG is first presented, which can explicitly characterize the dynamic interaction relations among the control loops such as the active power control, stator voltage control, and synchronizing PLL. Then a decomposition analysis of overall model is conducted at different conditions, including constant active power current, constant reactive power current and closed-loop stator voltage control, to explore the possible instability. It will be demonstrated that the coupling between active power current and stator voltage will introduce an additional positive feedback loop over the PLL, whose gain gets larger with grid strength decrease, thus will result in instability. For constant reactive power current condition, a pertinent stability margin is analytically derived. And on this basis, the impact of stator voltage control is analyzed by complex torque coefficient method. Detailed time domain simulations are conducted to verify the analysis.