I'm writing this during snatched moments on shift at work, so it may be < 100 % coherent at points . . .
Two different scenarios come to mind:
The first is automatic high-speed reclosing of circuits due to, for example, lightning strikes; after a brief interval for ionized gas paths to dissipate in the wind, a circuit will commonly be automatically placed back on potential by one circuit breaker via a reclosing relay utilizing under-voltage plus time supervision [UV + T], although there are also straight short time, long time, and other reclosing schemes. Once the circuit has been successfully placed on potential, the reclosure of the rest of the circuit's "diameter" breakers is generally by dedicated synchro-check relays that supervise the closure process in such a way that the phase angle, the delta V, the slip frequency, etc. across the breaker must all be within limits before closure is permitted. Note that the aforementioned synchro-check relays supervise both automatic and manual breaker closing, elaborated on in the following:
During manual switching, programmable synchro-check relays, commonly one per voltage level, provide a control overlay to the dedicated synchro-check relays. These PSRs provide the required telemetered quantities to the TSO; the ones my utility uses include a close block feature that denies closing permission to the closing circuitry while manual adjustments are being made to facilitate breaker closure. Once the required parameters have been brought within limits, the block close is removed and the relay will close the breaker. Use of this feature allows for very smooth switching, often much smoother than the rougher restoration to service that can occur during storms where by design only the wider limits of the dedicated synchro-check relays are respected.
When simply closing a loop, the phase angle across the closing device will typically be a standing one, non-adjustable.
Where placing parallel circuits containing one or more quadrature boosters in service, the standing phase angle can be adjusted to very nearly zero, allowing for very smooth returns of circuits to service.
When synchronizing electrical islands, "rotating scope" quantities will be encountered, and the TSO will direct the adjustments of frequencies and voltages as required as required on both sides of the synchronizing breaker to facilitate smooth synchronization.
Finally, a "synch bypass" feature allows for the closure, with discretion, of breakers with no relay supervision at all, i.e. where a ring bus at a station is already closed except for one breaker and the voltage source used for supervision has failed.
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
