  This letter discusses the 18-20 Hz oscillation event at 05:30 am on November
21, 2021, in Kaua`i's power system following the trip of an oil power plant. As
far as the authors are aware, this is the first report of a transmission
system-wide subsynchronous oscillation driven by inverter-based resources
(though the system in question is relatively small). In this letter, we
leverage two data-based methods-the dissipating energy flow method and the
sub/super-synchronous power flow method-to locate the sources of the
oscillation. Also, we build an electromagnetic transient model of the Kaua`i
power system and replay the 18-20 Hz oscillation. Finally, we propose two
mitigation methods and validate their effectiveness via numerical simulation.
