A Capacitor-current Real-time Feedback Active Damping Method for Improving Robustness of the LCL-type Grid-connected Inverter

The grid-connected inverter with an LCL filter has the ability of attenuating the switching frequency current harmonics.However,the resonance hazard of the LCL filter requires proper damping solutions.As an effective method to damp the LCL-filter resonance,capacitor-current-feedback active damping is equivalent to a resistor in parallel with the filter capacitor in analog control.However,due to the control delay introduced by digital control,capacitor-current-feedback active damping exhibits the characteristic of negative resistor at the frequency range higher than one sixth of sampling frequency.As a consequence,the current loop yields a poor robustness against grid-impedance variation if the resonance frequency is higher than one sixth of sampling frequency.Especially,if the resonance frequency is equal to one sixth of sampling frequency,system will be unstable all the time.To overcome these issues,capacitor-current real-time feedback with reduced control delay is proposed in this paper.In doing so,the characteristic of negative resistor is eliminated,and the current loop has satisfactory performance while dealing with the wide range grid-impedance variation.Moreover,a stable operation is obtained even if the resonance frequency is equal to one sixth of sampling frequency.Experimental results from an LCL-type single-phase grid-connected inverter confirm the theoretical expectations.