Study On Dual-Mode Photovoltaic Power Generation System In Micro-Grid

In recent years,due to the depletion of traditional fossil energy sources,the development and utilization of new energy represented by solar energy and wind energy have gradually become the focus of the world.As a carrier for new energy generation and transmission,the micro-grid system not only improves the flexibility of power system operation,but also brings considerable economic benefits to users.However,with the increasing of photovoltaic power capacity,the traditional grid-connected photovoltaic power generation system can no longer meet the requirements of the micro-grid operation due to the limitation in capacity and cost of the energy storage device.This paper takes a two-stage photovoltaic power generation system as a research object.From the aspects of photovoltaic cells,photovoltaic inverters,and dual-mode control strategies,a detail study of the island/grid-connected dual-mode photovoltaic power generation system in micro-grids is carried out.In this paper,according to the photovoltaic cell mathematical models,its power output characteristics are firstly analyzed in detail,and an improved conductance increment algorithm based on the integral regulator is proposed for the defect of the traditional conductance increment algorithm in the fixed disturbance step.This is used as a power control strategy for photovoltaic cells in operation mode of grid-connected to achieve maximum power tracking control.According to the requirements of the micro-grid in island mode,a load power tracking algorithm for photovoltaic battery is proposed to match the output power of the photovoltaic cell with the load power in the micro-grid(within range of maximum output power)so as to meet the conditions of the internal power balance of the micro-grid with operation in island.By integrating the above two algorithms,a dual-mode power control strategy for photovoltaic cells is formed.Through simulation,it is proved that the control strategy can effectively control the output power of the photovoltaic power generation system in different operating modes,that is,the maximum power output in grid-connected mode and the load power output in island mode(within range of maximum output power),so as to achieve safe and efficient operation of the micro-grid.For the trend of photovoltaic power generation systems moving toward high voltage and high power applications,this paper introduce three-level flying capacitor-type photovoltaic inverter.According to the requirements of different operating modes of the micro-grid,this paper makes a detailed analysis into the control strategies of dual mode photovoltaic inverter,that is,P/Q control strategy in grid-connected mode and V/f control strategy in island mode,based on its mathematical models.In order to further improve the performance of photovoltaic power generation system,this paper selects the modulation scheme for a three-level flying capacitor-type photovoltaic inverter by comparing the effects of different three-level carrier modulation strategies on the harmonic effects.And the scheme is improved so that it can maintain the voltage balance of the flying capacitor and ensure the normal operation of the inverter.The paper also discusses the switching process of dual-mode inverter and adopts corresponding measures to reduce the impact of switching.At the end of the article,through the integration of the front and rear photovoltaic power generation systems,a complete two-stage,dual-mode photovoltaic power generation system was formed.With the aid of modern computer simulation technology,a comprehensive simulation platform including a photovoltaic array model,a three-level flying capacitor-type inverter model,a dual-mode control strategy model and a micro-grid model was constructed on MATLAB/Simulink.By means of the simulation platform,a series of operation conditions including load switching,power supply switching,illumination variation and mode switching are simulated,which effectively verify the correctness and feasibility of the dual-mode photovoltaic power generation system constructed in this paper,and to establish a solid foundation for future research.