Stability Enhancement of Single-Loop Inverter-Side Current Feedback Controlled Grid-Connected Inverters with LCL filters

Grid-connected Voltage Source Inverters (VSIs) equipped with LCL filters are attractive in utility applications for their better power quality features, while the inherent LCL resonance may limit the current control loop bandwidth or even threaten system stability. Simple but effective single-loop current control strategies have been studied for obtaining a stable system, where either the inverter-side current or the grid-side current can be chosen as the feedback control variable. In this paper, the single-loop inverter-side current feedback (ICF) control is studied. It has already been proved that, due to the time delay effect, a stable grid-connected inverter with single-loop ICF control can be achieved only when the LCL resonance frequency is below one-sixth of the system sampling frequency (P6). To solve this limitation, this paper proposes an improved single-loop ICF control for widening the stable region exceed f(s)/6. The proposed method is implemented by directly adding a second order lead-lag compensator cascaded with the current regulator. The added compensator aiming at compensating the time delay effect is designed based on the Pade approximation. By conducting the continuous s-domain analyses, the boundary of the widened stable region can be calculated. Moreover, parametric influences of the proposed single-loop ICF control are studied in the discrete z-domain. Finally, the effectiveness of the proposed method is validated by the experimental results.