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

A single-phase grid-connected inverter with an unfolding circuit typically 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 the line frequency waveform. This converter exhibits low switching loss and high efficiency, and the operating principle of the unfolding inverter typically includes synchronous voltage and current. However, limited studies have focused on the operation of inverters with power factor (PF) less than unity. Such operations often result in large overshoots and oscillations in the output voltages. To address this problem, we proposed a novel control scheme that enables leading PF operation without additional circuitry and overcomes the aforementioned limitations in the previous literature. Thus, this paper describes a novel control scheme that enables lagging PF operation. Notably, for a lagging PF, a phenomenon in which the dc voltage inevitably increases immediately after unfolding is known to occur, and herein, this phenomenon is used in reverse to rapidly decrease the dc current. Consequently, we develop a method to land the circuit variables at normal mode steady-state value by using reverse-polarity pulse width modulation (PWM)/forward-polarity PWM combination of the unfolding inverter and a virtual PWM inverter in the proposed controller. The control method is validated using simulations and experiments, and it is found to facilitate the four-quadrant operation of the unfolding inverter.