Research On Control Strategy Of Grid-connected Inverter Under Unbalanced Grid

The progress of society and the further development of industrialization have not only caused a serious shortage of fossil fuels,but also caused serious impact on the environment and climate.Therefore,it is urgent to develop and utilize new energy sources that can replace traditional fossil fuels and accelerate the transformation of energy structure.In recent years,the installed capacity and power generation of new clean energy sources such as wind and solar power at home and abroad have been increasing as a proportion of the total installed capacity and power generation.However,the merits of grid-connected inverter control technology will directly affect the access rate of new energy generation and the quality of power delivered to the grid,and in actual grid operation,the three-phase voltage is often unbalanced due to singlephase faults,three-phase load imbalance and line non-full-phase operation,which brings negative sequence voltage in the operating system,thus leading to negative sequence current and harmonics in the grid-connected current of the grid-connected inverter.When the negative sequence current and harmonics are serious,it will not meet the grid connection condition,which directly affects the access rate of new energy generation,and also causes the fluctuation of the output power of the grid-connected inverter and the fluctuation of the DC bus voltage.Therefore,it is of great significance to study the control strategy of grid-connected inverter under unbalanced grid to improve the access rate of new energy and the output power quality of inverter.This paper focuses on the control strategy of grid-connected inverter under unbalanced grid.First,the mathematical models under different coordinate systems are established according to the main circuit structure of the grid-connected inverter,and the traditional constant power control under the ideal grid is introduced in detail,and the influence of unbalanced grid on the traditional grid-connected inverter control is analyzed.Then,for the influence of unbalanced grid on inverter control,a reduced-order generalized integrator is used to separate the positive and negative sequence of voltage,and an adaptive reduced-order generalized integrator is proposed to solve the problem that the reduced-order generalized integrator cannot accurately separate the positive and negative sequence when the grid frequency fluctuates normally.Next,the single objective control of the grid-connected inverter under unbalanced grid is simulated and analyzed.A multi-objective coordinated control method is proposed for the problem that the single-objective control cannot control the three objectives of grid-connected current balance,active power and reactive power without fluctuation at the same time.By establishing the multi-objective function and combining the particle swarm algorithm to optimize the multi-objective function,it can output the optimal coordination coefficient according to the unbalance situation of the grid and the actual operation needs,so that the grid-connected inverter can work in the optimal state.The simulation model of the grid-connected inverter is built in Simulink,and the proposed positive and negative sequence separation control and multi-objective coordinated control methods are simulated and verified.The TMS320F28335-based grid-connected inverter experimental platform was design,and the main circuit,control circuit and each software program required for the experiments were introduced in detail,and finally the effectiveness of the proposed control strategy was verified through experiments.