Stability and Performance Analysis of Grid-Connected Inverter Based on Online Measurements of Current Controller Loop

The amount of grid-connected three-phased inverters is increasing rapidly. In a weak grid, the non-ideal grid impedance decreases the control performance and can even compromise the system stability through load effect. The stability assessment of the inverter-grid interface has been assessed extensively through state-space and impedance-based methods. The current work presents stability analysis method based on the load-affected loop gain of the innermost control loop, which includes the effect of phase-locked loop and grid impedance. The stability analysis is carried out by assessing modeled and measured loop gains using the Nyquist criterion, step responses, and system closed-loop poles. The stability issues originating from grid impedance or too high phase-locked loop bandwidth are accurately predicted by examining the innermost control loop.