Study on DFIG Dissipation Energy Model and Low-Frequency Oscillation Mechanism Considering the Effect of PLL

Considering the dynamic characteristics of phase-locked loop (PLL), the effect of interaction between doubly fed induction generator (DFIG) and power grid on system low-frequency oscillation is revealed from the perspective of dissipation energy. First, the dynamic energy model of DFIG with PLL is derived, and the component that varies nonperiodically is extracted and defined as the dissipation energy. Second, the dissipation energy is decomposed into the free dissipation energy and DFIG-grid coupled dissipation energy. On this basis, the free dissipation intensity and coupled dissipation intensity are defined, which, respectively, characterize the immanent damping of DFIG and the DFIG-grid interaction degree. Thus, the contribution degree of DFIG to electromechanical oscillation is quantified for evaluation, and the mechanism of interaction between DFIG and power grid is further revealed. Finally, hardware-in-the-loop simulation tests with real-time digital simulator of the IEEE 10-machine 39-bus system verify that when the oscillation frequency of PLL is close to electromechanical oscillation frequency, strong coupling and resonance exist between DFIG and grid, and negative dissipation effect on the oscillation of DFIG will result. Thus, DFIG, as an oscillator, continuously injects dynamic energy to the grid, causing the system to oscillate to instability.