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Due to the existence of grid impedance, there is coupling resonance problem which may cause the multi-inverter grid-connected system to become unstable. This article focuses on universal mechanism for multimodule grid-connected system composed of LCL-type inverters. By establishing the multi-input multi-output matrix of the multi-inverter system, the system stability is divided into two parts, i.e., the self-stability and the interactive-stability. Then, based on elaborate deduction for the interactive-stability, the equivalent allocation mechanism of grid impedance is raised and a stability criterion for multimodule inverters system is proposed. In addition, on the basis of exploring the effect of the digital-control delay on the stability of the multi-inverter system in the weak grid, the phase-lead compensator combining with the optimal capacitor-current-feedback coefficient is adopted to improve the system robustness against grid impedance variation. Finally, the effectiveness of the theoretical analysis is verified by experimental results. [ABSTRACT FROM AUTHOR]