Observer-based virtual impedance modelling for enhanced fault current limitation in converters with grid-forming open-loop voltage control

This paper addresses open-loop grid-forming (GFM) control of inverters, which enables fast dynamic response but poses challenges for current limitation during faults. Implementing virtual impedances in this context requires the time derivative of measured currents, amplifying noise. Conventional workarounds avoid ideal differentiation but perform poorly, especially under asymmetrical faults. To overcome these limitations, an observer-based approach is proposed that accurately estimates current derivatives without explicit differentiation. The method is evaluated through PSCAD time-domain simulations and frequency-domain analyses, comparing the behavior of different virtual impedance implementations and demonstrating improved fault performance for open-loop GFM control.