Frequency Coupling Characteristic Modeling of DFIG System based on Type-1 Frequency-locked Loop

For grid-connected devices, synchronization scheme is adopted to track the phase of grid voltage, and it has also been reported that synchronization schemes can shape the impedance characteristics of grid-connected devices and lead to frequency coupling phenomenon, thus the system stability will be affected. Recently, a novel type-1 frequency-locked loop (FLL) is proposed to resolve the contradiction of fast dynamic responses and disturbance rejection capability that the widely applied synchronous reference frame phase-locked loop (SRF-PLL) suffers. Therefore, this paper tries to fill in the blanks of analytical modeling and analyzing the frequency coupling characteristic caused by type-1 FLL. In this paper, the analytical model of type-1 FLL will be developed and validated against time-domain simulations. The derived model of type-1 FLL is also analyzed to provide insights into its impedance shaping effect and frequency coupling characteristics. Applying the type-1 FLL to the doubly fed induction generator (DFIG) system, an analytical model that describes the frequency coupling characteristics of type-1 FLL based DFIG system will be developed and validated. Based on the frequency coupling model of DFIG, the influence of type-1 FLL on system stability will also be studied.