Research On Key Technology Of Diode-clamped Three-level Photovoltaic Grid-connected Inverter

With the development of economic globalization,the human demand for energy is increasing rapidly.The reserves of traditional energy sources such as petroleum and coal have been decreasing all over the world,The environmental problems such as air pollution and water pollution have attracted the attention of the governments of various countries.All countries are urgently carrying out energy transformation and achieving sustainable development.The sun light everywhere,solar energy are covered in most parts of the world,and it has the advantages of no transportation,unlimited reserves,easy access and so on,it can be called the most ideal new energy in the world.The sun light can not be directly supplied to human storage,so it need to be converted into electrical energy through the photovoltaic cells,and then through the inverter supply to the user.In this paper,the PV grid inverter systems adopts two stage conversion structure,and both the front-end combiner box and the back-end converter are studied in depth.Firstly,a new type of photovoltaic power optimizer is designed,which is embedded into traditional combiner box to form an intelligent combiner box with MPPT function.Every branch of intelligent power optimizer uses the dual-BOOST structure,this circuit structure can effectively reduce the output current ripple.The MPPT control of photovoltaic cells is studied,and the variable step resistance increment method is adopted.It can be seen from the Simulink simulation that the optimizer by this method can make photovoltaic cell rapidly and stably to achieve the new maximum power point after the sudden change of the illumination intensity,and ensure current equalization in the inductances.Secondly,the working principle of diode-clamped three-level inverter is analyzed,and the mathematical model is established,The space vector pulse width modulation(SVPWM)method is adopted in the control mode.For three-level inverter dc neutral point voltage balance problem,the balanceable factor method is used to make the neutral point voltage fluctuate in the controllable range.The grid-connected control is based on the Voltage Oriented Control(VOC)method.The voltage outer loop and current inner loop constitute a double closed loop system,and the parameters of the PI controller are tuned.Through the Simulink simulation analysis,it can be known that the grid connected current and grid voltage are in phase with the same frequency,and the output current harmonic content is low.Finally,the experimental research on the intelligent combiner box and the PV inventer is carried out respectively.The experimental waveforms and data are obtained,which verifies the feasibility of the control theory and the reliability of the system.