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This letter investigates the resonance characteristics and stability problem caused by the interactions of multi-parallel LCL-filtered inverters. Compared to single grid-connected inverter, the multi-inverter system presents a more challenging resonance issue, where the inverter interactions may excite multiple resonances at various frequencies. This letter proposes a modeling and analysis method based on the current separation scheme. It reveals that an interactive resonant current that circulates between the paralleled three-phase inverters may arise, depending on the current distribution and the system resonance characteristics. This is different from the common resonant current injected into the grid and has yet been discussed before. The analysis with the root loci in the discrete z-domain are carried out with two stability constraints identified, which correspond to the conditions of triggering the interactive resonant current and common resonant current, respectively. Simulations and experimental results verify the theoretical analysis.Index Terms-Multi-parallel inverters, LCL-filter, resonance interaction, stability analysis.

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This paper comprehensively analyzes the stability of a grid-interfacing inverter with LCL-filter in the discrete domain, where the LCL-filter, along with the controller, are modeled in a polar coordinate. System open-loop and closed-loop poles are analytically studied and expressed in the z-domain.Through the poles movement and distribution analysis, the relationship between system stability and the ratio of resonance frequency over sampling frequency is mathematically revealed and calculated as well as the system control gain limit. Moreover, this paper demonstrates that grid-voltage feedforward regulator would significantly alter the inverter stability in a weak power system. By means of Jury stability criterion, the stability status under different filter resonance frequency is given. The selection of resonance frequency and filter parameters makes a considerable difference on system behavior. Finally, to improve the robustness against grid inductance variation, a conservative design recommendation of filter parameters and control gain is given. Through the tests on a laboratory-scale prototype, the theoretical analysis is validated by experimental results.