Control Scheme for Leading Power Factor Operation of Single-Phase Grid-Connected Inverter Using an Unfolding Circuit

Abstract

A single-phase grid-connected inverter with an unfolding circuit consists of a first-stage dc/dc converter, which generates fully rectified sinusoidal waveforms, and a second-stage unfolding inverter, which switches every 180° of line frequency waveform. This converter exhibits low switching loss and high efficiency. Digitally controlled unfolding inverters can be used in numerous applications, such as power supplies to resistive loads and single-phase grid-connected inverters. The operating principle of an unfolding inverter is typically synchronous voltage and current. However, a reactive power supply function with a leading power factor (pf) (lagging pf from the grid perspective) is typically required for grid-connected inverters to suppress the terminal overvoltage. Limited studies have been focused on the operation of inverters with pf less than unity. Such an operation results in large overshoot and oscillation in the output voltage because of the difficulty in coping with large current fluctuations after voltage zero crossing. To this end, we propose a novel control scheme that enables leading pf operation without additional circuitry and overcomes the aforementioned limitations. When the unfolding inverter is operated with leading pf, a special mode called “all-conduction mode” occurs after voltage zero crossing. The proposed scheme implements appropriate dc current control to terminate this mode, resulting in a smooth transition to the normal mode steady state with pf less than unity. The scheme verifiably showcases exceptional efficiency in the conducted simulations and experiments.