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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]

An input-series–output-series inverter system is suitable for the large-capacity high-voltage ac power applications. If such a combined system comprising a number of small-capacity inverters is utilized in the high-power grid-connected power generation to replace a single high-capacity inverter, the voltage stress of the module would be greatly reduced. Additionally, the reliability of the whole grid-connected system could be effectively heightened owning to its flexible assembled-connection architecture. The principal issues for this grid-connected combined system touch upon multiple control objectives, including power balance, high power factor of the grid current, suppression of LCL resonance spike, and redundant operation of the system. Above all, the control variables are attentively screened to obtain a concise scheme to achieve the first three fundamental demands. Furthermore, the control units are dispersed into each module and a hot-swap technique is proposed to actualize the last objective, acquiring the superior reliability of the system. Finally, a three-module prototype is fabricated and the experimental results verify the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

Input-series-output-parallel (ISOP) inverter system is very suitable for high dc input voltage and large ac output current power conversion applications. If this topology is applied to the grid-connected occasion, a bulky-capacity grid-connected inverter can be built with multiple small-capacity standardized inverter modules, which can significantly improve the reliability of the system operation and facilitate system design. In this paper, we investigate an ISOP LCL-type grid-connected inverter system, and put forward an optimized way for the topology of combined LCL filters first. Then, the inverter-side inductor current is carefully chosen as the controlled variable and thus a concise distributed control method is proposed to achieve multiple control objectives. On the basis of the presented distributed control, the redundant control technique is also provided to heighten the reliability of the grid-tied ISOP inverter indeed. Finally, a three-module prototype is built and the experimental results validate the effectiveness of the raised strategy. [ABSTRACT FROM AUTHOR]

Input-series-output-parallel (ISOP) inverter system is very suitable for high input voltage and large output current power conversion applications. One of many merits of this assembly system lies in that its characteristic of multimodule series–parallel combination can significantly improve the reliability of the operation. To address this point, redundancy should be realized for the whole system. However, the existing methods for the ISOP inverter system all belong to centralized control, which restricts the modularity of the system. From the above perspective, this paper proposes a new scheme to achieve both power balance and distributed configuration according to the conception of compound control. Also, the relationship of control loops is analyzed and the design procedure of them is given. Based on the fully modular system actualized by the distributed control, the hot-swap technique is then raised to get a redundant system with superior reliability. Here, the way of bypassing other than cutting off is adopted to fulfill withdrawal of the faulty module from the system due to series connection at the input terminal. In addition, the detailed timing sequence of system operation is provided to ensure the smooth transition during the hot-plugging transient. Finally, a three-module prototype is built and the experimental results validate the effectiveness of the presented strategy. [ABSTRACT FROM AUTHOR]