In order to overcome the problem that the maximum power output of photovoltaic string systems cannot be outward in partial occlusion condition

this paper presents a grid-connected PV system based on quasi-Z-source inverter topology and multi-level modular cascade PV inverter structure

which can achieve module-level maximum power point tracking. The quasi-Z-source inverter has fewer active devices

lower energy conversion series and higher reliability compared with the traditional photovoltaic inverters cascaded DC-DC to DC-AC. This paper shows how the quasi-Z-source inverter cascaded multi-level PV system works and describes the closed-loop control strategy. A power-current dual closed-loop structure has been used to achieve grid-connected control. The power control loop ensures the quasi-Z-source inverter DC link voltage being constant and the current control loop controls the net current waveform to reduce harmonic currents and improve the quality of grid. The simulation results show the effectiveness of the control methods.