Research On Topology And Control Strategy Of High Efficiency Photovoltaic Inverter

With the rapid development of society and increasing energy consumption,research and development of new renewable clean energy sources can solve the problems of fossil fuel depletion and serious environmental pollution.As a kind of clean energy,solar energy is widely used in the development of new energy because of its features of extensive radiation,unlimited reserves,no pollution and convenient utilization.As the core of photovoltaic power generation,the cost and efficiency of inverters restrict the development of photovoltaic power generation.Therefore,it is of great significance to research photovoltaic inverters with low cost and high efficiency and stability.Excellent topology and grid-connected control strategy play a decisive role in improving the stability and conversion efficiency of the inverter and reducing the system cost.Firstly,the commonly used three-level inverter topology and soft switching technology is compared and analyzed in this thesis,and focuses on the improved T-type three-level inverter topology in which passive components are added to the T-type three-level inverter,and its commutation process and output characteristics are introduced in detail.Then,the different characteristics of the topology commutation process in three soft-switching dead-band modes are described.The soft-switching characteristics are verified by simulation and the conclusion that the operating conditions are optimal in the dead-band-free mode is obtained.Next,a mathematical model of the three-level inverter is built,and several grid-connected control strategies are compared and analyzed.The current control of multiple vector proportional integral and the voltage control of the DC-side bus voltage are analyzed.The modulation method of three-level SVPWM is described in detail,and the reason of DC side midpoint level shift and its suppression method are introduced.Finally,the transmission characteristics of L,LC and LCL filters are simulated and analyzed to verify the optimal performance of LC filters.In addition,the design and parameter optimization process of SRF-PLL are introduced.By using a three-level inverter circuit and soft switching technology,the design complexity and volume of the filter are greatly reduced,the switching loss is significantly reduced,and the conversion efficiency of the inverter system is improved.Combined with the multi-vector proportional-integral strategy to control the grid-connected current,the reliability and anti-interference of the grid-connected network in case of grid voltage sudden changes are guaranteed,and makes the system respond quickly,track accurately,efficiently and with low cost.The MATLAB system and its Simulink simulation tool are used to build the overall simulation model of the system,and the simulation analysis is performed.Compared with the PI control and deadbeat control strategies,the MVPI current control strategy used in this thesis has higher conversion efficiency,faster response speed,and higher reliability.SRF-PLL technology realizes the transient response and accurate tracking of the system.The SVPWM modulation can form complementary driving signals with the optimal combination,and the balance factor method is used to eliminate the phenomenon of neutral point potential shift,which further verifies the efficiency of the strategy.