Frequency behavior and its stability of grid-interface converter in distributed generation systems

This paper presents a state-feedback model to predict the frequency behavior of the grid-interface converter which uses phase-locked loop (PLL) techniques to synchronize with the grid. The frequency positive-feedback mechanism in the converter system is proposed and quantified in the model. The nonlinear behavior of the PLL under the weak grid condition 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, large reactive power variation, and weak grid tends to destabilize the converter frequency operation. The proposed model can help study the electric power system or microgrid operation under a large penetration of renewable energy resources with the power electronic interfaces.