  The widespread deployment of inverter-based resources (IBRs) renders
distribution systems susceptible to transmission-level faults. This paper
presents a comprehensive analysis of the impact of transmission-level faults on
3-phase and 1-phase distribution IBR operation. To evaluate distributed IBR
tripping across various phases and locations on a distribution feeder, we
conduct simulations of both symmetrical and unsymmetrical transmission faults
at progressively greater electrical distances on a real-time transmission and
distribution (T&D) co-simulation platform. The IBR power-to-load ratios (PLRs)
at 50%, 100%, and 300% are considered to emulate low, medium, and high IBR
conditions. Our results indicate that, while 1-phase and 2-phase faults
typically trigger fewer IBR trips when compared to 3-phase faults, a
significant power imbalance arises from the tripping of 1-phase IBRs on the
affected phases. The imbalance can result in significant power quality problems
and unintended equipment tripping. It may be necessary to design
fault-ride-through mechanisms specifically tailored to 1-phase IBRs to help
mitigate the power imbalances caused by unbalanced faults.
