Research On Grid-connected Control Strategy Of Unified Control Inverter

Nowadays,China’s modern electric power industry technology has gradually moved from the stage of rapid development to the stage of high quality development,and people’s demand for power quality is getting higher and higher.Due to the large number of nonlinear loads connected to the grid,the harmonic content of the whole system increases and the power quality decreases,which has a certain impact on people’s production and life.The traditional passive filter has poor control effect on power quality.Active power filter(APF)has been gradually applied in many fields to realize harmonic compensation of power grid current.However,their disadvantages of single function,high cost and high manufacturing cost have seriously affected their application and promotion.In this paper,through the design of three-phase three-wire parallel APF,the combination of repetitive control technology and PI double closed-loop control technology,based on the design of LCL grid-connected filter,to achieve the requirements of active current grid-connected and nonlinear load harmonic governance,to achieve the goal of unified power control.In this paper,the active power filter and grid-connected current optimization technology are analyzed,and a new unified control method is proposed to make the active power filter have both the function of active power grid-connected and active filter.Secondly,the parameter setting of LCL is completed by analyzing the operation condition of single-phase grid connection.In this paper,the mathematical models of LCL grid-connected active power filter in different coordinate axes are established,and the decoupling in d-q coordinate system is completed.The PI double closed-loop controller is designed according to the decoupling control block diagram.On this basis,the double closed-loop PI+repetitive control controller is analyzed and designed in detail.The two control strategies are simulated and analyzed,and the correctness of theoretical analysis is verified by fast Fourier transform(FFT)of grid-connected current and harmonic distortion ratio comparison.Finally,Simulink in Matlab software was used to conduct simulation experiments and build a three-phase parallel control system simulation model.It mainly includes APF active power filter,instantaneous reactive power harmonic detection,double closed loop PI controller,double closed loop PI+repeat controller and so on.For the grid-connected harmonic current,the double closed-loop PI+ repetitive control can reduce the harmonics of the grid-connected current to about 0.6%,while for the same control object,the harmonic distortion rate of the double closed-loop PI controller is 1.1%.It shows that the double closed loop PI+ repeat controller has better harmonic optimization effect on nonlinear load and can provide better waveform quality.