Single-phase Non-isolated Buck-Boost Inverter

The inverter is the key in photovoltaic power generation systems. Due to the affection of environmental factors, its output DC voltage is fluctuant. Therefore the inverter should have buck-boost function to ensure stable output. Considering the characteristics of high efficiency and low cost, this paper presents a single-phase single-stage non-isolated Buck-Boost inverter. It has a simple structure and can buck or boost the input voltage.At first, the working principle of single-phase Buck-Boost inverter is analyzed. Because of the bridge structure, the topology can use unipolar SPWM modulation and SVM modulation to achieve the buck-boost function with the constant current source mode. However, the constant current source mode needs a large value of the inductor, so that the cost of the system is increased and the efficiency is reduced, so a pulsating current source program comes into being. Due to the inductor current works in half sine wave at the same time, thereby the value of the inductor decreased significantly. In this paper, the mathematical model is established and the buck-boost ability of inverter is analyzed separately. Three control methods, that PI, PR and pseudo-rotating coordinate system control are used in the constant current source mode. These control methods were applied in passive and active load to verify the topology as a photovoltaic grid-connected inverter is feasible. A single closed-loop PR control is used in pulsating current source mode. Through the theory analysis and simulation studies, in constant current source mode, the control method of the inverter is flexible, the output power is greater, the boost capacity is stronger. In pulsating current source mode, the inverter’s buck-boost capability is balanced, the dynamic and static performance of closed-loop control are superior.Finally, the software and hardware of the main circuit and control system are designed. A experimental prototype is builded and the experimental results have proved that theoretical analysis and control strategy are correct and feasible.