Improving Voltage Support in Islanded AC Microgrids During Unbalanced Short-Circuit Faults

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Abstract
In the event of a short-circuit fault in an islanded ac microgrid, grid-forming power inverters activate a resistive virtual impedance to protect themselves against over-current, whereas grid-feeding power inverters are tasked with providing voltage support. In this scenario, the sequence impedances that are responsible for voltage support exhibit significant variability depending on the values of both the virtual and short-circuit impedances. State-of-the-art control schemes do not take this variation into account, resulting in voltage support that could be enhanced. This article presents a control scheme designed for grid-feeding inverters that improve voltage support during unbalanced short-circuit faults. The scheme is based on an online measurement of the sequence impedances and the selection of the most appropriate current injection technique for voltage support using the measured impedance values. A theoretical analysis is also included, which was performed to verify the stability of the microgrid with the proposed control. Finally, experimental results from a laboratory microgrid are presented for validation purposes.

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