Impedance Analysis of DFIG System based on DPC without PLL at Fundamental Frequency

The doubly fed induction generators (DFIG) has the resonance issues under inductive weak grid due to the phase locked loop (PLL). Direct power control (DPC) without PLL is a widely studied control strategy, which can remove the PLL. By the equivalent single-input and single-output (SISO) impedance model, it can be found that the phase margin of DFIG system is always sufficient even if the parameters of DFIG and grid change. Then the influence of different reference frames and control targets are both analyzed, and the key influencing factor to improve the fundamental frequency stability for DPC without PLL is revealed, that the power control can abandon the Park transformation and reduce one grid voltage disturbance path. The experiments established in the control-hardware-in-loop (CHIL) validate the above conclusion.