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Voltage-controlled voltage source inverters (VSIs) are generally adopted to achieve seamless transfer between islanded mode and grid-connected mode in microgrid applications. However, these two operation modes for VSIs feature different output filters, i.e., LC filters for islanded mode while LCL filters for grid-connected mode, thus leading to different stability performance. In this article, the traditional control strategy based on an outer capacitor voltage loop cascaded with an inner capacitor current active damping loop is investigated in details. The analysis indicates that there exists an inevitable blind area of stability design, where the system cannot be stable under two operation modes at the same time. Therefore, in order to raise available space for system design, this article proposes a stability blind-area-free control design method. The analysis indicates that the proposed stability design is only affected by LC filter parameters under islanded mode. Therefore, the blind area of the traditional control method can be well eliminated, and meanwhile the system robustness against the grid impedance variation under grid-connected mode is improved. The detailed design method is also presented. Simulation and experimental results are finally provided to verify the effectiveness of the analyses and the feasibility of the method. [ABSTRACT FROM AUTHOR]

In this paper, the stability of LC-type voltage source inverter is investigated, with the emphasis focused on the LC resonance. It has been found that the traditional capacitor-voltage inductor-current dual-loop control is unstable when the resonance frequency is higher than one-sixth of the sampling frequency. In addition, due to the coupling between current inner loop and voltage outer loop, it is difficult to design control parameters to ensure the system stability in traditional dual-loop control. In order to develop a systemic design principle, the capacitor-voltage control with inductor-current feedback active damping evolved from traditional dual-loop control is analyzed in detail. The mathematic model is established to reveal the reason of instability, and the stable regions are identified. Moreover, the optimal damping coefficient is also derived to enhance the system damping performance. Finally, the experimental results are provided to verify the effectiveness of the proposed method. [ABSTRACT FROM PUBLISHER]

This paper has presented a stability analysis of a LCL-type grid-connected inverter in the discrete-time domain. It has been found that even though the system is stable when the resonance frequency fr is higher than one-sixth of the sampling frequency ( fs/6), an effective damping scheme is still required due to the potential influence of the grid impedance. With a conventional proportional capacitor-current-feedback active damping (AD), the valid damping region is only up to fs/6. This however is not sufficient in the design process for obtaining a high quality output current and the system can easily become unstable due to the resonance frequency shifting. Considering the resonance frequency design rules of the LCL filter, this paper proposes an improved capacitor-current-feedback AD method. With a detailed analysis and proper parameter design, the upper limit of the damping region is extended to fs/4, which can cover all the possible resonance frequencies. Then, the damping performance of the proposed AD method is studied. It shows that the optimal damping is obtained when the actual resonance frequency is (fr + fs/4)/2. Moreover, an approximate calculation for the optimal damping coefficient R is given. Finally, the experimental results have validated the effectiveness of the proposed AD method. [ABSTRACT FROM AUTHOR]