Design Considerations of Digitally Controlled LCL-Type Grid-Connected Inverter with Capacitor-Current-Feedback Active-Damping

The capacitor-current-feedback active-damping is an effective approach for damping the resonance peak of the LCL filter. When the LCL-type grid-connected inverter is digitally controlled, the control delay will be generated. This will result in different behavior of the capacitor-current-feedback active-damping from that with analog control. In this chapter, the mechanism of the control delay in the digital control system is introduced first. Then, a series of equivalent transformations of the control block diagram considering the control delay are performed, and it reveals that the capacitor-current-feedback active-damping is no longer equivalent to a virtual resistor in parallel with the filter capacitor, but a virtual frequency-dependent impedance. A forbidden region for choosing the LCL filter resonance frequency is presented in order to guarantee the system stability. Then, the controller design for digitally controlled LCL-type grid-connected inverter with capacitor-current-feedback active-damping is studied. Since the control delay leads to a phase lag and consequently changes the location of −180°-crossing in the phase curve of the loop gain, the system stability might be guaranteed even without damping the resonance of LCL filter. For this case, the necessary condition for system stability is studied, and the controller design method is presented. Finally, the controller parameters design examples for the grid current regulator with and without the capacitor-current-feedback active-damping are given, and the effectiveness of the theoretical analysis is verified by the experimental results.