Fractional Order Feedforward Odd-Harmonic Repetitive Controller for Grid-Tied Inverters in Microgrids

The harmonic suppression performance of the odd-harmonic repetitive controller (ORC) will decrease when the microgrid frequency fluctuates due to power imbalance. This article presents a novel fractional order feedforward odd-harmonic repetitive controller (FO-FORC) for grid-tied inverters in microgrids. The feedforward ORC (FORC) with a feedforward channel has a much higher open-loop gain and a wider bandwidth at odd-harmonic frequencies than conventional ORC (CORC) without a feedforward channel. Therefore, the FORC can more efficiently suppress the odd harmonic of grid current while using fewer storage units. Nevertheless, the ratio of the sampling frequency of the FORC to the microgrid frequency can only be an integer, which degrades the harmonics suppression performance of the FORC with a fixed sampling frequency when the fundamental grid frequency changes in a relatively wide range. In order to settle the problem, based on a Thiran infinite impulse response (IIR) filter, this article proposes the FO-FORC with frequency adaptability to ensure that the resonance peaks of FO-FORC maintain at the real disturbance signal frequencies by online adjusting only a few coefficients of low-order IIR filter. The stability analysis, parameters design, and performance estimation are given. Comparative experiments verify the feasibility and superiority of the presented FO-FORC.