Research On Direct Digital Control Method Of LCL Grid Connected Inverter

Three-phase two-level LCL grid-connected inverters are the main research objects in this paper.Direct digital control methods are used to control the system under the feedback of inverter-side current feedback(ICF)and grid-side current feedback(GCF).In order to improve the stability margin and parameter adaptability of LCL grid-connected inverters and to suppress the resonance peak of grid-connected inverter systems.Firstly,the circuit structure and characteristics of the LCL grid-connected inverter are analyzed,and the mathematical models of the frequency domain and time domain of the system are established.The influence of the parameters change of the LCL filter on the system characteristics is analyzed.The selection principle of the filter parameter is elaborated.At the same time,the influence of digital control delay on system stability under ICF and GCF feedback control is studied separately.Secondly,based on the consideration of the delay time of digital control,the direct digital control algorithms such as Dahllin algorithm and least-beat non-difference and their design processes are studied.Based on the direct digital control algorithms,the controllers of the system under ICF and GCF feedback are designed.According to the analysis of its characteristics,it can be seen that the traditional direct digital control will make the system have such problems as insufficient stability margin and small parameter adaptability,Therefore,in order to improve the stability margin and parameter adaptability of the system and reduce the influence of the system's inherent resonance peak,in the case of ICF feedback control,the LCL filter is equivalent to an L-type filter(hereinafter referred to as the equivalent method of L)to design and configure the zero poles of the digital controller to optimize the LCL grid-connected inverter system,thereby improving the system's stability margin and parameter adaptability,suppressing the system's inherent resonance peaks,and improving The stability characteristics of the system are described.Finally,the feasibility and effectiveness of the research contents were verified by using the simulation platform built in Matlab/Simulink and the experimental platform with TMS320F28346 as the control core.The experimental results show the correctness of the analysis method and the effectiveness of the direct digital control optimization method.The theoretical analysis and experimental verification in this paper show that the direct digital control method can realize the design of the LCL-type grid-connected inverter controller design.It also illustrates the effectiveness of the equivalent L-type method and the pole-zero configuration method.The stability margin,parameter adaptability,and resonance peak suppression of the system are improved.