Impedance-Phase and Magnitude Control Method to Improve Stability of Grid-Connected Inverters in Weak Grid

Grid impedance and phase-locked loop (PLL) are critical factors for the stability of the grid-connected inverters (GCIs) in a weak grid. They are the positive feedback control loops formed by PLL in the GCI with grid impedance. It is prone to GCI instability, especially in the case of the higher PLL bandwidth. A novel impedance-phase and magnitude control strategy is proposed to improve stability of GCI with different grid impedance. Moreover, a detailed design of control loop and parameter calculation for the impedance-phase and magnitude control strategy are introduced. First, PLL output impedance is reshaped to broaden the frequency range of the GCI phase-frequency characteristic curve above the −90° line towards the low-frequency band. In addition, current loop output impedance is reshaped to maintain the phase margin (PM) of the GCI near to 45°. Meanwhile, the magnitude of GCI output impedance is also increased significantly. Stability of the GCI in a weak grid is enhanced by adopting the proposed control strategy. Simulation and experimental results verify the analysis and the proposed method.