Design Of Grid-connected PV Inverter Based On TNPC

With the increasingly serious problem of environment and energy,new energy industries such as photovoltaic power generation have attracted more and more attention.Photovoltaic grid connected inverter is the core component of photovoltaic power generation system,which puts forward higher requirements on the conversion efficiency and power quality.In order to improve the conversion efficiency and power quality of photovoltaic grid-connected inverter,this paper studies the optimization design of the hardware and control system.The main contents are as follows:(1)Comparison and selection on topologies of photovoltaic grid-connected inverter.The power losses of two-level and three-level topology are compared and analyzed,and the advantages and disadvantages of various topologies are analyzed,which can provide a reference for photovoltaic grid-connected inverter on selecting high efficiency topology.(2)A prototype of the 20kW TNPC three-level photovoltaic grid-connected inverter is developed,and the design parameters and targets are presented.At the same time,the losses of power devices is calculated,and the calculation results proved the power devices can meet the design requirements.The design process of DC-capacitors and LCL filter is presented.Finally,the harmonic content and efficiency of the inverter prototype are verified by simulations and experiments.(3)Research on algorithm of maximum power point tracking.On the basis of the traditional perturbation observation method,a perturbation method based on variable step size at the maximum power point is proposed.At the same time,the design methods of MPPT parameters,such as initial operating voltage,switching power and variable step size,are discussed.Finally,the experimental results show that the improved method has a good performance,and has a certain effect on the efficiency of the grid-connected inverter.(4)Research on algorithm of anti-islanding detection.In view of the disadvantages of interference for the output power and power quality in traditional reactive power disturbance method,a method based on frequency positive feedback is proposed.This method has a smaller power output interference in normal operation,and it can also increase the reactive power disturbance by the effect of positive feedback after entering the islanding state which can speed up island detection.Finally,experiment prove the method has a small non detection zone and quick detection for islanding,the power output and power quality of the inverter can hardly be affected by using the new method.