The compensation performance of active power filter(APF)is improved by using repetitive control(RC). For the harmonic characteristic of a three-phase rectifier

as 6k+1 order harmonics are positive sequence component

while 6k-1 order harmonics are negative sequence component

a selective repetitive controller is proposed. This controller is designed to compensate for the 6k±1 order harmonics in the dq reference frame. In addition

the delay time of the proposed repetitive controller may not be an integer and will reduce the compensation performance. In order to eliminate the N/6 beat delay effect

an IIR filter based on Thiran approximation is introduced to approximate the desired fractional delay. In this paper

the composite control structure of PI controller in series with the proposed repetitive controller is used

and the theoretical derivation and stability analysis of the composite control system are carried out

till the detailed design method of the controller is provided. Finally

the effectiveness and superiority of the proposed method are verified by simulation. This work is supported by National High Technology Research and Development Program of China(863 Program)(No.2014AA052001).