Variable Frequency Repetitive-Resonant Combined 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 achieve zero steady state error for each periodical signal with known period, is not able to track or reject variable frequency periodic signals. In fact, digital RC weakness is related to performance degradations when it deals with variable frequency references. In other words, existing RC algorithms require an integer number of samples every 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 outputs 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 constant delay line, regardless of the desired output frequency in both grid-tied and stand-alone operation.