| With the rapid development of distributed energy in recent years,the power supply system in the form of micro grid is becoming more and more mature.Micro grid can use the local characteristic energy to form an independent micro grid to provide power for local users.It can not only be connected with the large power grid to form a part of the power generation system,but also can be operated independently as an easy to change power supply,which can meet the diverse needs of users.The islanding operation mode of micro grid needs to be studied specially.Compared with the grid connected operation mode,the inverter with small capacity of micro grid in islanding mode is easily affected by the user side,resulting in the degradation of output voltage quality or the occurrence of micro grid fault.This paper mainly analyzes and discusses the control technology of inverter and parallel inverter technology in islanding mode in the following aspects:First of all,the advantages and disadvantages of three-phase full bridge inverter and half bridge inverter,LC filter and LCL filter are compared,and the appropriate inverter topology structure is selected,and the filter parameters are designed and the mathematical model is deduced,and the control model is established in dq coordinate system.Then the influence of the delay caused by SPWM modulation method and digital control technology on the control system is analyzed,and a double closed-loop control strategy considering digital delay is established: the mature PI controller is selected in the outer voltage loop to realize the tracking of direct flow without static difference;the closed-loop control of inductor current feedback is adopted in the inner current loop,and thesimple proportional regulator is selected to realize the control of the current loop to compensate for the slow dynamic response of the voltage control.Finally,the parameters of voltage loop and current loop are designed by root locus method.Secondly,considering the influence of the harmonic current of the connected nonlinear load on the output voltage quality of the system,PI control can not suppress the high-order harmonic,so the control strategy needs to be improved.Based on the analysis of the principle of repetitive controller,the repetitive controller is improved on the basis of stability,By comparing the performance of the hybrid structure composed of repetitive control and PI control,the PI repetitive controller is selected to form the voltage outer loop,which not only uses the good performance of repetitive controller to suppress harmonics,but also uses PI controller to overcome the disadvantage of poor dynamic performance.Considering the fundamental frequency shift caused by the frequency fluctuation of power grid will further deteriorate the output voltage quality,improve the repetitive internal model,and design a repetitive controller to adapt to the frequency fluctuation to effectively suppress the voltage harmonics when the frequency changes.The parameters of the proposed integrated control strategy are designed,and a simulation model is built to verify the feasibility of the control strategy through simulation analysis.Finally,by analyzing the power transmission characteristics of the parallel system and deducing the working principle of the traditional droop control,the shortcomings of the traditional droop control are obtained.Considering the influence of the control strategy proposed above on the output impedance of the inverter,the virtual impedance method is used to improve the droop control.The advantages and disadvantages of the commonly used virtual impedance method are analyzed.Considering the advantages of the virtual impedance of the second-order generalized integrator when connecting the nonlinear load,the design method of the virtual impedance based on SOGI is given.The parallel system simulation model is established by MATLAB,and the droop strategybased on the virtual impedance of SOGI is simulated to verify the control effect. |
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