Study On Digital Single-Loop Control Scheme Of Grid-Connected Inverters With LCL Filter

The grid-connected inverters are the most universal interfacing topology forconnecting distributed power generation systems with the utility grid. The LCL Filters areapplied widely in the distributed power generation system because they show a goodperformance on suppression of high-frequency harmonics. The grid-connected inverterwith LCL filters adopted digital single-loop current controller has advantages of simple，low cost and reliable due to without extra sensor. However LCL filter has a resonant peakwith the filter resonance frequency which may raise oscillations and even instability if thecontroller is not suitably designed. This paper focuses on the investigation of systematicanalysis of digital single-loop control based on grid-side current feedback. The researchissues mainly include: the modeling of system, the design of controller parameters andfilter parameters, resonance suppression and adaptation performance to change of gridimpedance.The continuous-time model and discrete-time model of grid-connected inverter withLCL filter which contain delay link have been analyzed and implemented. In order toanalyze and design digital control system by the direct digital method, the delay of thefull-bridge PWM converter has been analyzed in detail under a variety of samplingmethods and modulation methods, and the system continuous-time model contains delayhas been established. And the discrete-time model contains delay has been deduced bymeans of discretization. In order to analyze the influence on digital control system by gridvoltage, the accurate discrete model of system output impedance has been derived, and hasbeen linearized approximation for convenient.Universal quantitative stability and stability margin analysis of digital single-loopcontrol based on grid-side current feedback in the discrete domain are demonstrated in thispaper, and the parameters design method based on stability margin is proposed. Thecomparison of two single-loop current control strategies is analyzed. The stabilizingmechanism of the digital single-loop current control system based on grid-side currentfeedback has been revealed. By means of Nyquist diagram based on discrete time model ofthe system, the quantitative analyses of stability and relative stability with digital Pcontroller, PI controller and PR controller have been carried out. And the stable range ofcontroller parameters and the resonant frequency ratio have been derived under a certain gain and phase margin. So it could be ensured to meet the requirements of practicalengineering applications. Furthermore, the vector margin is introduced to evaluate therelative stability of control system. The quantitative analyses of vector margin with digitalP and PI controller have been also derived. In this paper a controller design method basedon stability margin is proposed. The design principle is to optimize medium frequencyband characteristic to obtain the filter and controller parameters under a certain stabilitymargin. So it could be ensured to have excellent control performance and robustness.The optimization design methods of LCL filter parameters combined with the controlstrategy is proposed in the paper. The traditional design methods of LCL filter parametershave the random of parameters and trial-and-error problem. The design method is tooptimize the resonant frequency of LCL filter based on control strategy. According to thedesign method, the design process could be simplified obviously. This paper also proposesan inductance design principle based on grid current harmonic standards. Furthermore, thedesign principles of ratio of inverter-side inductance to grid-side inductance have beenanalyzed. The design procedure of digital single-loop current control strategy has beendemonstrated, and the design method could be applied to other control strategy. The effectsof grid impedance variation on single-loop control system have been analyzed in detailbased on the change of stability margin. In order to adapt to the change of grid impedance,the LCL filter parameters could be optimized by suitable choice of inductance ratio.The performances of single-loop control, dual loop control and full state feedbackcontrol have been compared. The stability and relative stability of dual loop controlstrategy based on capacitor current inner feedback have been analyzed, and the quantitativerelationship of phase margin with controller parameters and filter parameters have beencarried out. And the stability, pole assignment method and grid voltage feed-forwardcontrol of full state feedback control strategy have been analyzed. The comparativeanalyses of single-loop control with dual loop control and full state feedback control havebeen implemented. The comparative issues include the scope of filter parameters,especially the scope of resonance frequency ratio and controller performance, and thecomparative principle is to compare the stability margin of the system under differentcontrol strategies under the same control bandwidth.