Stability Analysis of Grid-connected Inverter in Weak Grid Systems

Distributed Generation may be located far away from load centers due to their use of new energy resource distribution characteristics. In this case, Long distance transmission lines will introduce an equivalent impedance to the grid, so that the grid showing the characteristics of a weak grid. In this paper, the interaction between grid-connected inverter and weak grid is studied by means of impedance analysis in the frequency domain. The experimental results are used to evaluate the stability of grid-connected inverter under different grid impedance condition. In a weak grid system, the increasing grid inductance results in a smaller stability margin of impedances ration. To improve the stability of the grid-connected inverter, the paper introduces a kind of active damping method with a virtual resistor in parallel with the capacitor. The new active damping control scheme without differential factor has a simple algorithm. The experimental results are depicted to demonstrate the correctness of theoretical analysis and the feasibility of the active damping algorithm.