Research On Time Delay Compensation And Current Control Strategy Of LCL Type Grid-connected Converter

As a high-order filter,LCL has the advantages of small size and good high-frequency ripple attenuation performance,etc and is widely used in grid-connected converters.However,the LCL filter has resonance problems,and the resonance will affect the stability;In addition,the one-shot delay introduced by digital control also challenges the design of the current loop.Based on the above two issues,this paper takes the current loop of LCL-type grid-connected converter based on proportional resonance controller(PR)as the research object,and studies its delay compensation and current control strategy.The main research contents are as follows:Firstly,the stability criterion of the current loop of the LCL-type grid-connected converter with complex frequency response is clarified.Based on the study of the open-loop zero-pole distribution of the resonant controller in different discrete modes,the optimal discretization method for PR is determined and the discretization model of current loop was established.Based on this,the optimal stability criterion of the current loop of the LCL-type grid-connected converter based on the PR controller is analyzed.By optimizing the distance from the Nyquist curve to the critical point,An optimal parameter design method for the current loop of LCLtype grid-connected converter is proposed.Secondly,the method of the LCL-type grid-connected converter current loop resonance suppression damping is studied.For undamped control,the stability conditions of undamped control and its robust performance in the case of grid-side impedance fluctuations are analyzed;For passive damping control of capacitor series resistance,the design requirements of the damping resistance are determined through the study of the Nissl curve of the controlled object under different damping resistance;Then,the active damping of capacitive current feedback is researched,and its control model is equivalent to the form of damping inner loop plus current outer loop.The study found that under the action of virtual impedance,when the virtual resistance is positive at the resonance frequency,An optimal damping feedback coefficient exists in the damped inner loop,and it is proposed to determine the optimal coefficient by the root locus.Thirdly,the generation mechanism and influence of control delay and dead time delay are analyzed.Aiming at the problem that the phase lag of the control delay at the control frequency leads to a decrease in system stability,PR provides a delay compensation angle at the control frequency to compensate and The best given method of delay compensation angle is analyzed.In addition,the dead time will cause output voltage deviation and affect the output current quality and Aiming at the problem of inadequate compensation accuracy in open-loop compensation,this paper discusses the dead-time closed-loop compensation method,That is,the effect of dead time on the output current is eliminated by introducing a compensator in the current loop.Finally,a 30 k VA experimental platform was built.According to the above research content,the correctness of the theoretical analysis was verified through simulation and experiments.