Research On Three-phase T-type Photovoltaic Grid- Connected Inverter Based On Repetitive Control

Compared with the traditional two-level converter,the multi-level converter has the advantages of lower output harmonic content and lower voltage stress of the device.It has been widely applied in high-voltage and high-power applications.In this paper,a threephase non-isolated two-stage grid-connected inverter is studied.The inverter system is composed of three-level Boost circuit and three-phase T-type three-level inverter topology.Compared with the traditional two-level Boost circuit and diode clamped inverters,the first-stage three-level step-up circuit can widen the output voltage range of the PV array and the second-stage T-type three-level topology can solve the problems of uneven loss distribution and excessive power devices in the traditional diode clamped to pology.Firstly,the working principle of three-level Boost converter and three-phase T-type three-level inverter is analyzed in detail,and its mathematical model is established.The first-stage converter not only reduces the inductor current ripple by the interleaved control strategy of phase shift 180° of power switch but also achieves the balance of two capacitors on the DC side by adjusting the conduction time of power switch.Simultaneously,a feedforward current control method is adopted to improve the dynamic response speed of the system and reduce the voltage fluctuation of the DC bus.In order to reduce the harmonic content of grid-connected current and enhance the stability of the system,an improved repetitive control is proposed.In order to overcome the disadvantages of traditional three-level space vector pulse width modulation(SVPWM)method,such as complex calculation and difficult sector judgment,a simplified three-level SVPWM algorithm is adopted,which simplifies the operation by decomposing the three-level space vector into a simple two-level space vector.Aiming at the problem of three-level neutralpoint potential fluctuation,the reasons for the unequal capacitor voltage and the influence of vector on the neutral-point potential are analyzed.The action time of the small vector is redistributed by the balance factor allocation method and the voltage balance of the two capacitors on the DC side is realized.Finally,a two-stage photovoltaic grid-connected inverter simulation model is built by Matlab/Simulink simulation software.The control strategy and modulation algorithm are simulated and verified.Simultaneously,the hardware and software of the photovoltaic inverter system is designed and a 20 kW three-level inverter experimental prototype is built on the basic of theoretical analysis and simulation research.The validity and effectiveness of the proposed control method is verified by the comparative analysis of experimental results.