Research On Robustness Of Digitally Controlled LCL-type Grid-connected Inverter And Multi-inverter-paralled System In The Weak Grid

The grid-connected inverter is an indispensable interface between the renewable energy devices and the power grid,in which,inverters are often adopted to convert the dc voltage to high quality ac voltage for feeding the ac current to inject into the ac grid.In order to improve the stability and reliability of distributed power generation system,grid-connected inverter is required to present excellent performance.However,due to the long-distance transmission lines and a large number of transformers between the grid-connected inverters and the power grid,a wide range variation of inductive grid impedance exist in practical applications,which will lead a stricter stability requirement for the digitally controlled LCL-type grid-connected inverter.Furthermore,with the gradual expansion of the scale of distributed power generation system,the inverter system develops from single-module to multiple-module,and the coupling phenomenon of multi-module inverters make the stability analysis more complex.Therefore,the problem of enhancing the robustness of digitally controlled LCL-type grid-connected inverter and multi-inverter-paralled system in the weak grid cannot be ignored.This paper first investigates the digitally controlled LCL-type grid-connected inverter in the weak grid,which adopt single inverter-side inductor current control,single-grid side inductor current control,or grid-side inductor current feedback combined with capacitor-current-feedback active-damping.Then,the effect of the digital-control-delay on the stability of the system using different control methods is analyzed,and the constraint conditions of system stability are revealed.The analysis results show that compared with the first two single-loop control systems,the allowable design range of the LCL filter resonance frequency has been greatly extended after the capacitive current active damping is added,but the system still has a resonance frequency design forbidden zone at one-sixth of the sampling frequency.Therefore,for the grid-connected inverter with capacitor-current-feedback active-damping,in order to weaken the adverse effects of the digital-control-delay,a physical deduction method is proposed to connect the R-C-R virtual impedance in parallel with the filter capacitor,which is equivalent to introducing a simple delay-compensated phase-lead link in the capacitor-current-feedback path.In doing so,the boundary frequency of the virtual-positive-resistance-region can be increased.In addition,the resonant frequency forbidden region for the LCL filter can be eliminated,such that the inverter robustness against the grid impedance variation can be significantly improved.Then,a design procedure is further presented for the delay-compensated phase-lead link as well as the closed-loop parameters.Finally,the simulation and experimental waveforms of a 1 k VA LCL-type grid-connected inverter are presented to verify the effectiveness of the proposed control scheme and parameter design procedure.Next,this paper focuses on the multi-module LCL-type grid-connected inverters parallel system with different parameters in the weak grid.By establishing the Norton equivalent model and multi-input multi-output matrix of the system,the coupling mechanism of the multi-module system is revealed.Then,the system stability is divided into two parts: self-stability and interactive-stability.At the same time,the equivalent grid impedance perceived by each module is derived,and a stability criterion for multi-module inverters system based on equivalent allocation of grid impedance is proposed.In addition,the adverse influence of digital-control-delay on the stability of multi-module inverters system in the weak grid is analyzed,and the phase-lead delay compensation method is applied to multi-module system to improve the system robustness against grid impedance variation.Finally,the prototype of two-module LCL-type grid-connected inverters system are built.Then,the correctness and feasibility of the theoretical analysis are verified by the experimental results.