Switchgear failure at a Rhode Island distribution substation after feeder fault

What happened

In August 2022 a major equipment failure occurred at a distribution substation in Rhode Island, United States. The substation, commissioned in the early 1980s, had a single 115 kV to 13.8 kV load tap changing transformer supplying four 13.8 kV distribution feeders serving several communities and a neighbouring utility. The failure took all four distribution feeders offline and interrupted supply to approximately 9,300 customers, including a hospital and a mental health facility. The operator mobilised more than 50 personnel for restoration, deployed contracted emergency generation, replaced the failed switchgear and progressively restored customers. The state regulator carried out a formal investigation and published its findings.

Contributing factors

According to the published investigation, the initiating event was a phase to phase fault on one distribution feeder caused by a fallen tree. The recloser on that feeder cleared the fault, but the fault persisted for longer than a bolted, low impedance fault would have done, because tree contact produces a higher impedance fault with lower fault current, for which protective relays are set to allow a longer clearing time. The investigation reported that the combination of fault duration and fault location placed significant stress on the upstream substation switchgear. Engineering guidance in use at the time recommended that bolted faults at the end of the protected zone be cleared in under one second, while explicitly allowing longer clearing times for higher impedance faults. The post incident analysis considered whether protection coordination at substations of this design vintage could be adjusted to reduce the stress imposed during such fault clearing events.

Lessons learned

1. Review protection coordination at older substations, since a higher impedance tree fault allowed longer clearing time than a bolted fault and stressed the switchgear.
2. Include high impedance fault scenarios in protection studies at vintage substations, as tree contact produces lower fault current and extends clearing time.
3. Plan restoration with contracted generation and customer switching when a single transformer substation loses all feeders.

More lessons on structural integrity and turbines, electrical protection and isolation, from every incident on the register.

Not yet known

• Switchgear model and age
• Exact duration of customer outages
• Cost of the switchgear replacement

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Source

web.archive.org