Research On Key Technology Based On T-NPC Three-level Photovoltaic Grid-connected Inverter System

With the ensuing problems of energy depletion and ecological destruction,photovoltaic power generation technology has received widespread attention in the world under the background of international policy advocates for environmental protection and sustainable economic development.T-NPC three-level inverter is the most widely used topology in the field of medium and high power generation.The research hotspot and difficulty lies in how to suppress the neutral-point potential fluctuation and reduce the switching loss.The neutral-point balance strategy can avoid output waveform distortion and switch over-voltage,which is the guarantee of inverter safety;soft-switching technology is the best choice to solve the contradiction between high frequency and low loss in industrial applications,which can maximize efficiency and power density.Therefore,the paper mainly studies key technologies such as neutral-point balance strategy and soft-switching technology of photovoltaic power generation grid-connected systems.(1)Overall Modeling of Double Closed Loop Photovoltaic Grid-connected Power Generation System.Firstly,this paper study the characteristics of photovoltaic cells,and a feasibility analysis was made for the modeling of photovoltaic cells.Then,the relevant working principle,mathematical model and grid-connected control strategy of T-NPC three-level inverter are analyzed,and the calculation methods of relevant parameters are studied.Finally,based on these,taking the voltage and current double closed-loop control model of the three-level inverter as the research object,establishing the simulation model to simulate the experiment.The experimental results show that the double closed-loop control waveform has higher quality and the DC bus voltage is more stable.(2)The NPC three-level inverter neutral-point balancing strategy based on improved auto disturbance rejection controller.Aiming at the neutral-point potential fluctuation problem inherent in the mid-point clamp type three-level inverter,the fluctuation model of the DC side neutral-point potential is established,and a neutralpoint voltage balancing strategy based on the fractional-order auto disturbance rejection controller(FOC-ADRC)is proposed,the control of the voltage outer loop is realized,and the purpose of suppressing system disturbance is achieved.The nonlinear error feedback rate(NLSEF)of the traditional ADRC is avoided.It is necessary to try to solve the three parameters without clear physical meaning by experience,and it is convenient to apply to the neutral-point potential of the three-level inverter for real-time voltage compensation.The prototype experiment verified the efficiency of the method.(3)A T-NPC three-level inverter based on a novel soft switching technique is proposed.Soft switching technology maximizes efficiency and power density and is the best choice for high power and high frequency requirements in industrial applications.However,the current soft-switching technology in the three-level inverter field has few shortcomings,such as a large number of auxiliary switching devices,complicated control,oscillation,and large voltage stress of the switching tube.Therefore,a novel passive soft-switching three-level inverter is proposed.The auxiliary circuit is simple and does not need to change the modulation mode.It can smoothly realize soft-switching of all switching transistors.According to the equivalent circuit diagram of each mode,the working principle and the basic conditions of the realization are detailed,and the experimental argument is given.(4)A 60 kW principle physical prototype based on T-NPC three-level photovoltaic power generation inverter system was built.The hardware parameter calculation basis of the platform,the design of the functional circuit and the corresponding specifications of the selected device are detailed.The prototype is debugged,relevant tests and performance tests are carried out,and the test results show that the principle prototype has good output performance and can meet the requirements of grid connection.