Impedance Characteristic Analysis and Stability Improvement Method for DFIG System Within PLL Bandwidth Based on Different Reference Frames

The phase-locked loop (PLL) will have a negative impact on the stability of doubly fed induction generators (DFIG) system under inductive weak grid. Some PLL-less methods have been presented to remove the PLL such as switching the synchronous reference frame to the virtual reference frame or alpha beta-frame. Based on the equivalent single-input single-output impedance model, it is found that the DFIG systems implemented in the virtual reference frame or alpha beta-frame are more stable than synchronous reference frame. The reason is that the stator voltage will disturb the reference current calculation of the virtual reference frame and alpha beta-frame, then introduce a reference calculation matrix G(cal). This matrix will keep the phase of DFIG system always -90 degrees around fundamental frequency, then narrow the negative resistance band and enhance the phase margin (PM). In order to further illustrate the stability improvement caused by G(cal), this article proposes a stability improvement method by emulating the G(cal) in synchronous reference frame, so as to improve the PM while retaining the advantages of PLL under unbalanced or harmonically distorted voltage, and fault ride through. The experimental results validate the aforementioned conclusions.