Modeling And Performance Analysis Of Digital Control System Of Three-phase Grid-connected Converters

With the development of energy technology,distributed generation and energy storage based on renewable energy have been widely used.As the interface among distributed generation systems,energy storage systems and the grid,three-phase grid-connected converters can directly affect the power quality and operation status of the whole system,which thus makes them become the research hotspot of recent years.However,with the popularity of digital control,digital delay in digital control systems poses an effect on the traditional control methods of grid-connected converters,damaging the control performance or even triggering instability.Besides,when conucting analysis in three-phase grid-connected converter systems,most of the exsisting research adopts approximate modeling methods for simplifying the modeling process and system model,which leads that the inaccurate model cannot reflect the properties of practical systems in some applications,such as the systems with low switching frequency.By building the accurate model of three-phase grid-connected converters with digital control,this paper analyzes the influence of digital delay on the system control performance,based on which,corresponding methods are adopted to improve its performances.The detailed research content includes:(1)The control strategies and digital control models of three-phase grid-connected converters with LCL filter are investigated.Firstly,the topology,average model of LCL-filtered three-phase grid-connected converters are analyzed.Based on that,an invesitagation about the current control strategies-the stationary frame(abc/??)control with Proportional Resonance(PR)controller and the synchronous rotating frame(dq)control with Proportional Integral(PI)controller is conducted,where the digital control models of the two strategies are built in both continuous and discrete domains.The complex-vector modeling method in dq frame and its corresponding analysis tools are introduced in detail.On the basis of the digital models,the two control strategies are comparatively discussed.(2)The axis cross-coupling of the current control model in dq frame is studied.Based on the complex-vector model built in dq frame,double-sided frequency response and pole-zero map are employed to analyze the effects of two different coupling terms,which are generated by digital delay and filters,respectively.In the meanwhile,the traditional decoupling methods(phase compensation for the coupling cause by digital delay and state feedback decoupling for the coupling caused by filters)are also evaluated.To tackle the problems of the existing decoupling methods in systems with low-switching frequency,this paper proposed an improved decoupling control method,which can effectively enhance the decoupling effect.In the meanwhile,the dynamic performance can also be improved.All the performed analysis and the proposed method are verified by the simulations and experiments conduced in MATLAB/SIMULINK and dSPACE 1103.(3)The effects of digital delay on the resonance of LCL filter and active damping are analyzed.A study about how the inherent phase lag introduced by digital delay affects the suppression of LCL resonance is carried out.Capacitor-current feedback(CCAD)and grid-current feedback active damping(GCAD)methods are taken as an example to analyze the influence of digital delay on active damping.The model-free delay compensation techniques are also introduced,which are successfully applied to the grid-connected converter to improve its performance of resonance suppression.(4)The influence of non-ideal grid with harmonics on the grid-connected converters is stuied as well.Harmonic impedance is utilized to illustrate the principle of the exsisting methods for suppressing the grid harmonics' effects-resonant controller and grid voltage feedforward.The effects of digital delay on these two control strategies are also investigated.It is found that the suppressed frequency range of the resonant controller is limited by digital delay.For the grid voltage feedforward,its suppression performance is influenced.