Abstract
In recent years, the doubly fed induction generator (DFIG) operates in a weak grid, rather than a strong grid due to the high proportion of wind energy into the power grid. The impedance interaction between the DFIG system and series and parallel compensated weak grid might cause the subsynchronous resonance (SSR) and high frequency resonance (HFR) in the DFIG system, respectively. Phase locked loop (PLL) is a popular grid synchronization technique, and the high bandwidth PLL can cause resonance at middle frequencies in the DFIG system. However, the impact of PLL types and their controller dynamics on the resonance in the DFIG system are not adequately researched. The impact of the PLL controller with different types, such as synchronous reference frame (SRF) and Lead/Lag PLL, is studied in this paper to fill this gap. Additionally, an improved PLL is proposed, which can guarantee the high phase margin and decrease the likelihood of the resonance at middle frequencies in the DFIG system under a weak grid. Moreover, the phase margin of the DFIG system impedance with an improved PLL is less sensitive to its controller parameters. Simulation and experimental results verify the effectiveness of the proposed method.
Highlights
The cost-effective variable speed operations and partial size power converter make the doubly fed induction generator (DFIG) prevalent in a modern wind power plant [1]
The subsynchronous resonance (SSR) [5,6,7,8,9,10,11] and high frequency (HFR) [12,13,14,15,16] may occur, when the DFIG system operates under a series and parallel compensated weak grid, respectively
Reactive power, and torque for the DFIG system operated under weak grid employing the low and high bandwidth phase locked loop (PLL) (KpPLL = 500, KiPLL = 5000), respectively
Summary
The cost-effective variable speed operations and partial size power converter make the doubly fed induction generator (DFIG) prevalent in a modern wind power plant [1]. In the literature [18,19,20,21], the impact of PLL on the harmonic stability and impedance modeling of the grid-connected converter under a weak grid has been reported. The authors in [25] have proposed the symmetrical PLL for single input single output (SISO) impedance modeling to improve the stability of the grid-connected converter under a weak grid. The impedance modeling usually ignores the PLL impact, due to the lower bandwidth when compared to the high bandwidth of the PI controller of the closed-loop rotor and grid current. The impact of PLL on impedance modeling and resonance in the DFIG system under a parallel compensated weak grid is studied.
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