Design and control of an LCL-type single-phase grid-connected inverter withinverter current feedback using the phase-delay method

In this study, a novel single-phase grid-connected microinverter system and its control applications are introduced for solar energy systems. The proposed system consists of two stages to transfer solar power to the grid. In the first stage, an isolated high-gain DC/DC converter is used to increase low solar panel output voltage. In the second stage, an inverter is used to supply a sinusoidal current to the grid. Moreover, a proportional resonant controller is adopted to reduce grid current total harmonic distortions (THDs) and an LCL filter is used to provide better harmonic attenuation. However, the ratio between the sampling frequency $f_{s}$ and the resonance frequency $f_{res\, }$should be greater than 6 in the LCL filter with the inverter current feedback for a stable system. In order to obtain higher phase margins, the sampling frequency should be increased, which increases the inverter switching frequency. The present study shows that $f_{s}/f_{res} $can be lower than 6 by placing a phase delay on the inverter current feedback path to guarantee adequate stability margins. The effectiveness and feasibility of the proposed method are confirmed by the experimental test results based on a 300-W laboratory prototype.