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Frequency Behavior and Its Stability of Grid-Interface Converter in Distributed Generation Systems

Fig. 1. Grid-interface converter under study.
This paper presents a state-feedback model to predict the frequency behavior of the grid-interface converter, as shown in Fig. 1, which uses phase-locked loop (PLL) techniques, as shown in Fig. 2, to synchronize with the grid.

As shown in Fig. 3, the proposed model consists of two feedback systems. In addition to the typical negative feedback system, the frequency positive-feedback mechanism in the converter system is proposed and quantified in the model. The nonlinear behavior of the PLL under weak grid conditions can be accurately predicted, and the large-signal frequency stability region can be also estimated by the proposed model. It shows that large penetration of distributed generation (DG) units, a large reactive power variation, and a weak grid tend to destabilize the converter frequency operation. The proposed model can help study electric power system or microgrid operation under a large penetration of renewable energy resources with power electronic interfaces.

Fig. 2. Synchronous reference frame PLL.
Fig. 3. Proposed PLL model.
Fig. 4. Switching converter simulation results ω2 (purple) [5 Hz/DIV], one-phase current ia (blue) [25A/DIV], and proposed PLL model output ω2 (dark blue).
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