Impedance Model and Stability Analysis of Three-phase Grid-connected Inverter in Vehicle-to-grid

The cooperation between electric vehicles and the power grid makes electric vehicles be a kind of distributed power source. Electric vehicles are connected to the grid through grid-connected inverters, which plays an important role in regulating the operation of the power grid by changing the charging and discharging cycles. The increase of inverters causes the interaction about the inverter and the grid can cause harmonic oscillations that threaten their stability and lead to serious distortions in the grid current. To address this issue, this paper establishes a three-phase grid-connected inverter output impedance model considering the effects of current loop, phase-locked loop (PLL) and DC voltage loop with the dq-frame linearization method. This model illustrates the causes of frequency coupling in a grid-connected system. Besides, this paper uses the generalized Nyquist criterion (GNC) to grasp the effects of frequency coupling characteristics on system's stability. Results show that considering the frequency coupling characteristics can lead to more accurate results of stability analysis for the grid-connected system, and further analyze the stability change rules of the grid-connected system affected by different grid impedance. Finally, simulation results verify the validity of the above conclusions.