Fixed-Duty-Cycle Control of a Quasi-Z-Source Inverter in a Battery-Assisted Photovoltaic System

This paper presents two simple control strategies for a battery-assisted photovoltaic (PV) system with the three-phase quasi-Z-source inverter (qZSI). The specific control objectives depend on the operating mode of the PV-qZSI system. In the stand-alone mode, the objective is to keep the fundamental RMS value of the ac-side phase voltage at a desired value, whereas in the grid-tied mode, the objectives are to synchronize the qZSI with the grid and achieve unity power factor. The additional common objective is to operate the PV system close to the maximum power point (MPP). This can be achieved in an open-loop manner by assigning a fixed shoot-through duty ratio, thus avoiding any additional sensors, but the inherent drawback of this method is sensitivity to irradiance and temperature variations leading to suboptimal performance. The proposed control strategies were experimentally tested over a reasonably wide range of irradiance levels in order to assess the overall performance of the PV-qZSI system.