Research On Grid-connected Control Strategy Of Photovoltaic System

Energy plays a vital role in the sustainable development of today's society.With the rapid development of the global economy,the problems of energy shortage and environmental pollution are becoming more and more serious.The governments of various countries are gradually aware of the importance of the development of renewable energy industry.As a new renewable energy,solar energy is inexhaustible,clean and pollution-free.It is a recognized green energy in the world.As an effective way to make full use of solar energy,photovoltaic grid connected power generation is considered to be one of the most potential power generation methods in the twenty-first Century.However,in the complex and changeable external environment,the output power of photovoltaic array is not stable,so it is necessary to operate near the maximum power point by appropriate control strategy.In addition,it is necessary to convert the direct current output from the photovoltaic array into alternating current through grid-connected inverter and filter out the high harmonic in it before it can be merged into the grid and sent to thousands of households.In this thesis the LCL two-stage three-phase photovoltaic grid-connected generation system is taken as the research object,the MPPT control strategy and the inverter control strategy are deeply analyzed.The specific research contents are as follows:Firstly,the working principle and mathematical model of photovoltaic cell are introduced.According to the mathematical model,the general engineering simulation model of photovoltaic cell is established in MATLAB simulation software.At the same time,according to the parameters provided by the manufacturer,the output characteristic curve of the photovoltaic cell is analyzed,and the influence of light intensity and ambient temperature on the photovoltaic cell is studied.Secondly,the working principle of MPPT control algorithm is introduced,and the advantages and disadvantages of four common control algorithms are compared.Aiming at the problem that the single control algorithm can not take into account the tracking accuracy and the response speed,a fuzzy control MPPT combination algorithm based on perturbation observation is analyzed,and a new combined MPPT algorithm,which is based on voltage self-optimization,is proposed.In the MATLAB simulation environment,the curves of the maximum output power point of various algorithms are compared and analyzed when the external conditions change.Theresults show that the combined algorithm proposed in this thesis can effectively adapt to the change of external conditions,not only can fast track the maximum power point,but also the oscillation is small when the steady state is reached.Then the common types of three-phase photovoltaic grid-connected inverter are introduced.The LCL two-stage three-phase voltage type photovoltaic grid-connected inverter is selected as the research object,and the spatial state equation of photovoltaic grid-connected inverter in three different coordinate systems is deduced.The space vector pulse width modulation technology is also briefly introduced.Finally,the advantages and disadvantages of common inverter control techniques are analyzed in detail.Aiming at the peak amplitude of LCL photovoltaic grid-connected inverter in resonance,a dual current loop quasi-PR control strategy based on grid-connected current feedback and capacitive current feedforward is proposed.The overall simulation model of three-phase photovoltaic grid-connected generation system is established in the MATLAB simulation software.Through comparative analysis,the rationality of the control strategy adopted in this thesis is verified.