The Research On Key Issues Of Single-phase Soft-switching Grid-connected Photovoltaic Inverter

Energy consumption is increasingly growing with the development of human society.The environmental impact of fossil energy usage and its finite reserves have caused increasing public concern in recent years.As one kind of renewable energy,solar energy has been widely used in recent years.In PV generation system,PV inverter plays an important role as an interface between PV cell and the utility grid.Single-phase inverter is the main topology applied in low power and medium power distributed PV generation system.Efficiency and power density are the most important indicators of single-phase PV inverter.Currently the mainstream single-phase PV inverters on the market are all hard-switching topologies.The switching frequency is generally designed less than 20kHz to reduce the device switching loss,which means high efficiency and power density cannot be obtained.The switching loss in the hard-switching circuit can be effectively reduced with soft-switching technology,which makes soft-switching technology a feasible solution of high efficiency and high power density.The study of single-phase soft-switching inverters has been carried out for many years,in which a class of resonant DC link topology with active clamp has the simplest circuit structure.In single-phase active clamp resonant DC link inverters,the principle of Zero-Voltage-Switching is to resonate the DC link voltage of switch bridge to zero by the auxiliary resonant circuit.The auxiliary switch must be accurately controlled to synchronize with the commutation action of switch bridge.A novel Zero-Voltage-Switching Sinusoidal-Pulse-Width Modulation scheme(ZVS-SPWM)with resonant circulation energy control algorithm is proposed,by which zero-voltage turn-on is realized on all the switches.The switching loss is reduced and switching frequency is increased,resulting in higher efficiency and power density.The operation principle of the single-phase active clamp ZVS-SPWM inverter is analyzed emphatically.The soft-switching condition is derivated,and the design guideline of the single-phase active clamp ZVS-SPWM inverter topology is proposed.A common mode current model of single-phase active clamp ZVS-SPWM inverter is established in this paper to study the feasibility of its application in PV generation system.The influences of the ZVS-SPWM,common mode inductor,and parasitic capacitance of PV panels on the common mode loop impedance are studied.A common mode filter consisting of common mode inductor and bypass capacitors is designed,which suppresses the leakage current of single-phase active clamp inverter.So that it can be applied to transformerless grid-connected PV power generation system.The loss model of the single-phase active clamp ZVS-SPWM inverter is also established in this paper.The hardware parameters are optimized according to the loss model,and the effects of the super-junction MOSFET's output capacitance and the parallel resonant capacitor are analyzed emphatically.Since the dead-time effect is increased in the single-phase active clamp ZVS-SPWM inverter.An improved ZVS-SPWM is proposed by establishing the switching cycle average model of the output voltage,which effectively reduces the lower harmonic of the output current and improves the power quality.Based on the research work of this paper,a 3kW single-phase soft-switching grid-connected PV inverter is designed.The 3kW PV inverter consists of two power conversion stages:one is hard-switching boost circuit and the other is single-phase active clamp ZVS-SPWM inverter.A 30kW distributed grid-connected PV generation system is established for the long-term PV generation test of ten 3kW single-phase soft-switching grid-connected PV inverters.The reliability of single-phase soft-switching grid-connected PV inverter is verified.