Frequency Adaptive Proportional-Repetitive Control for Grid-Connected Inverters

The repetitive control (RC) scheme, which can achieve zero steady-state error tracking of any periodic signal with a known integer period, is widely employed in grid-connected inverters to mitigate feed-in current harmonics. However, the conventional RC (CRC) scheme cannot exactly compensate harmonics with fractional period caused by grid frequency variations. Furthermore, the small RC gain of the CRC scheme causes slow transient response. In this article, a proportional-repetitive control (PRC) scheme, which can accommodate a larger RC gain than the CRC scheme, is proposed to provide fast transient response for RC systems. In order to enhance the frequency adaptability of the PRC scheme with a fixed sampling rate, the Thiran all-pass filter, which is a type of infinite impulse response filter for approximating the fractional delay (FD), is used as the FD filter to achieve fast online tuning of the FD and the fast update of the coefficients. The proposed frequency adaptive PRC (FA-PRC) scheme provides grid-connected inverters with a control solution with excellent dynamic performance and accurate frequency adaptability to eliminate harmonic distortion even in the presence of grid frequency variations. A comprehensive series of experiments have demonstrated the effectiveness of the proposed FA-PRC scheme.