Impedance-Based Analysis and Stability Improvement of DFIG System Within PLL Bandwidth

In this article, we analyze the negative impact of phase-locked loop (PLL) on the stability of the doubly fed induction generators system. The small-signal disturbance of PLL mainly affects the coordinate transformation of the rotor current and introduces a negative resistance within PLL bandwidth. Two impedance reshaping methods are carried out to weaken this negative resistance. A virtual impedance in the conventional current control is added to the first reshaping method. The second reshaping method completely removes the coordinate transformation of the rotor current by switching the control target from current to power. The difference between these two reshaping methods is analyzed based on the equivalent single-input single-output impedance model. The experimental results validate the effectiveness of these two reshaping methods and the superiority of the second reshaping method at a lower short-circuit ratio.