Tuning and Assessment of Inner Control Loops in an Islanded Nanogrid with Harmonic and Unbalanced Loads

This paper presents the design and tuning of proportional-resonant (PR) controllers in discrete domain using SISOTOOL feature in MATLAB. The controllers are applied to an AC nanogrid system that comprises of a 2-level voltage source inverter (2L VSI) and 3-level T-type (3L T-type) inverter system. The system is tested with various types of loads, including linear, non-linear and unbalanced loads, within the PLECS environment. The response of the controller system is visualized in SISOTOOL and validated through stability analysis, which includes Bode and Nyquist plots. Additionally, control system requirements such as rising time and settling time are assessed. The PLECS simulation results using discrete PR controllers are analyzed in terms of stability and harmonic mitigation.