M MICROGRIDS ARE BEING DEPLOYED AT A RISING RATE, PRIMARILY as a means of increasing power system resilience. Commonly, a microgrid today includes at least some inverter-based resources (IBRs), and many microgrids have modes or conditions under which they are entirely energized by IBRs. Most microgrids today are deployed on radial distribution circuits, but it is conceivable that they could also be considered for deployment on secondary network systems. Under faulted conditions, IBRs have significantly different behavior from rotating machines, and this difference introduces new challenges for microgrid protection system design. The fault current properties of IBRs create a significant difference in the fault current profile between grid-connected and microgrid-islanded modes and between IBR-dominant or rotating machine-dominant configurations. These variations also impact the conventional phasor-domain short circuit analysis used to design such systems. This article is the first in a two-part series on the influence of IBRs on microgrid protection. In part one, the focus is on microgrids deployed on radial circuits. This article discusses some of the challenges related to the protection of IBR-based microgrids and presents some ongoing research and solutions in the area. The different controls Influence of Inverter-Based Resources on Microgrid Protection Part 1: Microgrids in Radial Distribution Systems Digital Object Identifier 10.1109/MPE.2021.3057951 Date of current version: 19 April 2021 36 ieee power & energy magazine 1540-7977/21©2021IEEE may/june 2021