Frequency adaptive repetitive controller for grid-connected inverter with an all-pass infinite impulse response (IIR) filter

The repetitive controller is widely employed in grid-connected inverters to improve the power quality. Its resonant frequencies are predesigned to the nominal grid frequency and its harmonics to achieve excellent reference tracking and disturbance eliminating. However, the performance degrades when the grid frequency varies. In this paper, a frequency adaptive repetitive controller with a specially designed all-pass infinite impulse response (IIR) filter is proposed to improve the steady-state performance considering grid frequency variation. The IIR filter approximates a fractional delay. By adjusting the delay, the resonant frequencies can be corrected to the actual grid fundamental and harmonic frequencies. The filter design is based on the Thiran formula, which is simple with a closed form, and online real-time computation burden is highly reduced. The IIR filter has a wide band of linear phase region with low order. As the system stability is independent on the IIR filter for its all-pass characteristic, the system synthesis is simplified. Results from a single-phase grid-connected inverter are provided to validate the effectiveness of the proposed control scheme.