On June 27th, 2023, lightning struck a transmission line about one mile from a substation. Shortly after the first lightning strike, the high-impedance bus differential relay cleared the bus at the substation even though there was no apparent fault on the bus. About half an hour after these two events, the percentage-restrained bus differential relay attempted to trip the bus again, even though the bus was still de-e energized from the initial events.
In this paper, the protection engineers unravel the mystery of what cleared the bus initially and why. It also explains what caused the percentage-restrained bus differential relay to subsequently attempt to trip the bus when it was already de-energized.
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