Abstract: Active power filter（APF）is one of the main ways to eliminate harmonic quality problems in power

grid，and LCL filter is commonly used to connect APF to power grid. In order to suppress the resonance problem

caused by LCL filter，active damping based on capacitor current feedback is a common method. Active damping

can effectively suppress the resonance caused by LCL filter，which can be equivalent to the resistance parallel to the

filter capacitor under simulation control. However，due to the delay of capacitance current sampling，the virtual

resistance will be negative，showing negative damping characteristics，which will affect the system stability.

Therefore，the problem was studied from the perspective of duty cycle prediction，and three methods for

capacitance current sampling delay compensation were proposed. The transfer function of the system was

calculated for each method，and the corresponding virtual impedance was analyzed. It can be seen from the results

that the second compensation method has the best effect. Finally，taking the three-phase LCL type active power

filter as an example，the feasibility of this controller was verified by simulation and experiment.

      Key words: active power filter （APF）；digital delay；capacitance current；duty cycle prediction