Active Damping of LCL Filters with All-Pass Filters Considering Grid Impedance Variations and Parameter Drifts

Active damping is a common way to stabilize the current control of LCL-filtered inverter systems. In this paper, the active damping is realized with digital filters to reshape the open-loop phase response, where no additional sensors are required. A safety region of −180°-phase crossing is first identified within a predefined range of grid impedance changes and LCL parameter variations. Once the phase of the current control loop is reshaped to the safety region, a stable control loop can be ensured. Different digital filters are selected for comparison, which shows that the allpass filter has a unit gain and adjustable phase-lag capability, being a promising solution for phase reshaping. Therefore, the allpass filter is employed to move the −180°-phase crossing into the targeted safety region. By doing so, the all-pass filter-based active damping can be separated from the current controller design, without retuning the PI parameters. Simulations are performed and the results verify that the proposed method can ensure the system stability, while maintaining fast dynamics.