Study Of Digital Current Control Technology For LCL-typed Grid-connected Inverter

In order to make the power of the new energy system connect to the grid stably and efficiently,the high-order filter is already widely used in the grid-connected which connects new energy system and the grid.But the high order LCL typed filter has a resonant peak which may influence the interfacing topology performance of grid-connected inverter.The traditional resonance suppression problem is studied and analyzed in simulation domain,however,with the rapid development of digital signal processing technology and digital chip,the influence of delay link in digital control on system stability is already inevitable under the circumstances that analogy control is replaced by digital control,so the discrete mathematical modeling of grid-connected inverter considering delay link,digital single current control and digital double current active damping control are analyzed and discussed systematically in this paper.The discrete mathematical model of the LCL-typed grid-connected inverter is established and analyzed by adopting the direct digital method considering the digital delay elements such as computation,sampling.And the system stability with single-loop control based on grid-side current is comparatively analyzed in continuous domain and discrete domain,which explains that the digital delay changes system stability.Thus the influence on stability of the ratio of the sample frequency and resonance frequency was mainly analyzed based on the grid-side current control when the digital delay was set one-step-delay and the frequency ratio range was obtained,meanwhile the parameters of the controller was designed.Only grid-side current digital single-loop control cannot stabilize the system under low ratio of resonant frequency and sampling frequency,which causes the deficiency of Robustness of the digital grid-current single-loop control.Thus,the digital capacitor current feedback active damping method is further studied to enhance the damping,and the influence of digital delay which brought by capacitor current feedback on the active damping method is analyzed.Meanwhile,a combination method of predicted duty cycle with increased zero-pole to compensate control delay is proposed aiming at the influence,which effectively compensates the system inner loop digital delay and enable the active damping method to enhance damping effectively.The design of the method is simple and the stability of thesystem is enhanced.Effectiveness and feasibility of the proposed method is proved by simulation.Meanwhile,the experiment platform of grid-connected inverter system is built in this paper,and the problems discussed in this paper are verified by experiments which enhances its persuasiveness.