Single-Phase partial power unfolding inverter for photovoltaic string application

Partial power converters, also know as fractional or differential power converters among other terminologies, is a very attractive concept that has gained an increasing attention during the last decades. Through a rearrangement of the traditional power conversion structures, the converter only processes a fraction of the total power while the rest is directly transmitted to the load. Consequently, a significant reduction in size and cost of the converter is achieved through the use of low-power ratings devices, allowing also higher efficiencies. Partial power converters have been proposed for various applications, such as photovoltaic (PV) systems. In PV applications single-phase grid-connected PV inverter are composed usually by two-stage power conversion, a full power DC-DC converter and a hard-switched high frequency PWM inverter. Although, different DC-DC partial power converter have been proposed as a first stage, most of the existing solutions deals with constant DC-DC regulation voltage, then the DC-AC grid tied inverter stage still requires to perform all the inversion through hard-switched PWM strategy. In this paper a partial power unfolding dc-dc stage is proposed, which generates a rectified sinusoidal current, leaving only the unfolding duty to the inverter stage. As a result, only one hard-switched PWM partial power stage composes the system, increasing the efficiency. The proposed partial power inverter is applied on a 3.3kW PV string inverter. Simulation results are provide to verify the interest of the proposed partial power DC-AC inverter.