Constant Delay-Line Frequency Adaptive Repetitive-Resonant Control for Grid-Tied and Intentional Islanding Operations

Repetitive Control (RC) enables inverters to inject, at fundamental frequency, high quality sinusoidal current into the grid or to provide low THD voltages in off-grid applications. However, digital RC, which can obtain a zero steady-state error for each periodic signal with a known period, gets its performance significantly degraded in the case of periodic signals with variable frequency. In other words, RC algorithms require an integer number of samples per period, while in some industrial applications the number of samples per period could be a non-integer time-varying value. Therefore, RC would lead to a significant quality degradation of the provided tracking capabilities when the grid frequency or the off-grid desired frequency changes. According to that, a newly RC structure with adaptive decoupled sampling is proposed to work with a RC implemented with constant delay line, regardless of the desired output frequency in both grid-tied and stand-alone operation. After the FPGA implementation, the experimental campaign highlights the effectiveness of the proposed control architecture.