Abstract：In order to improve the compensation effect of LCL-type shunt active power filter，a frequency

adaptive decimal delay repetitive control based on IIR filter was proposed. The resonance suppression loop of LCL

was designed through root locus and the design result was used as a new control object for repetitive control. The

IIR filter used could approximate the decimal delay caused by frequency changes，thereby matching the resonant

frequency of repetitive control with the fundamental and harmonic frequencies of the power grid. The adaptive

implementation diagram，the phase characteristics of the approximation part，and the design method of the

compensator in sequence were presented. Finally，the stabilities of the system were analyzed. Simulation and

experimental verification show that the proposed method can improve the steady-state tracking performance and

THD of the system.

      Key words：active damping；approximation；repetitive control；maximum flat group delay；harmonic