Research On Control Strategy Of Grid Connected Inverter Under Unbalanced Grid Voltage

With the increasingly prominent energy problem,the development and utilization of new energy is becoming more and more important,which has become a major problem to be solved in today’s society.In recent years,China has vigorously developed photovoltaic power generation and wind power generation,so the research of grid connected inverter has attracted great attention from the academic circles.However,in the actual operation,due to the influence of the access load,fault,resonance and other factors,unbalanced grid voltage will occur and affect the operation of the inverter,and then affect the safe operation of grid connected power generation.Therefore,the control strategy of grid connected inverter under unbalanced grid voltage is needed.Taking the grid connected inverter under unbalanced grid voltage as the research object,this paper analyzes the characteristics of the mathematical model of grid connected inverter in the abc/αβ/dq coordinate system,and designs the multi-objective optimization control strategy of grid connected inverter in theαβ coordinate system according to the characteristics that the mathematical model of grid connected inverter in the αβ coordinate system and on the α-axis and β-axis is strictly symmetrical and independent,and there is no strong coupling term.In the case of unbalanced grid voltage,the existence of negative sequence component of voltage will affect the operation of inverter.In this dissertation,the positive and negative sequence components of grid voltage are separated by improved second-order generalized integral method and controlled independently,which lays the foundation for the subsequent research on the control strategy of grid connected inverter.Secondly,in the case of unbalanced grid voltage,the output current of the inverter is unbalanced and the output power fluctuates.Based on the mathematical model of grid connected inverter in coordinate system,the current reference command value is derived to achieve three control objectives of balancing output current,restraining active power rate fluctuation and restraining reactive power fluctuation.Combined with the relationship and characteristics of the three control objectives,the optimal regulation coefficient k is introduced to unify the reference current expression to realize the coordinated control among the control objectives.Finally,in order to make the grid connected inverter work in the best state and achieve the best output performance,a multi-objective optimization function is constructed to optimize the k value.The multi-objective function is optimized by particle swarm optimization,fruit fly algorithm and wolf colony algorithm,and the simulation model is built on MATLAB/Simulink,focusing on the separation of positive and negative sequence,the influence of unbalanced grid voltage on inverter,three different control objectives of grid connected inverter,the coordinated control among the three control objectives,and the multi-objective optimization based on particle swarm optimization,fruit fly algorithm and wolf colony algorithm The multi-objective optimization control strategies of grid connected inverter based on particle swarm optimization,fruit fly algorithm and wolf colony algorithm are verified and compared.The simulation results show that the multi-objective optimization control strategy based on wolf colony algorithm has the fastest convergence speed,high efficiency,high stability,better control accuracy and global convergence,strong global optimization ability and computational robustness,which verifies the feasibility and effectiveness of the control strategy designed in this dissertation.