Extended stable boundary of LCL-filtered grid-connected inverter based on an improved grid-voltage feedforward control

For the LCL-filtered grid-connected inverter, it has been reported that the digital time delays will narrow the stable region of current control loop when the inverter-side current is used for implementing the feedback control. A sufficient stable condition is that the filter resonance frequency should be designed under one-sixth of sampling frequency. However, the low resonance frequency leads to a comparatively large filter inductance or/and capacitance. To extend the stable boundary to the region above fs/6, this paper proposes a novel voltage feedforward scheme for the LCL-filtered inverter. Theoretical analysis is then provided to validate its feasibility and stability. Compared to other widely used active damping strategies, no extra sensors are needed because the filter capacitor voltage, which is used for voltage feedforward control, is also sampled for phase-locked loop in this paper. Simulations and experimental results are provided for verifying the theoretical analyses.