POWER TRANSFORMER FAULT CLEARING

A faulted transformer can be separated from its power source by devices such as circuit breakers, power operated disconnect switches, circuit switchers, and fuses, or by remote tripping of fault-interrupting devices.

In addition to separating the transformer from its power source, due consideration should be given to tripping oil pumps and fans to reduce their possible adverse effects in sustaining or  spreading transformer oil fire and to halt circulation of contaminants in the oil resulting from the arc.

Determination of the type of fault-clearing devices to be used should consider factors such as:

a)Installation and maintenance cost
b)Fault-clearing time relative to fire hazard and repair or replacement costs of the transformer
c)System stability and reliability
d)System operating limitations
e)Device interrupting capability

Relay tripping circuits
Usually a transformer protective relay operation requires a careful inspection for the cause of tripping before any attempt is made to reenergize the transformer. Usually two or more breakers or switching devices must be tripped. Therefore, tripping is usually done by a lockout relay that also blocks closing circuits and must be reset manually.

For a large transformer having several protective relays, two lockout relays and dc power supplies are often used. If both lockout relays trip the same breakers, the differential relays may operate one lockout relay and the sudden-pressure relay and overcurrent relays, including instantaneous units, may operate the second lockout relay for the greatest redundancy.

If the two lockout relays perform different tripping functions, then a different assignment of protective relays to each lockout relay may be desired. The lockout relay contacts may initiate breaker failure relaying and may be supplemented by a small self-reset relay operated in parallel with the lockout relay coil for redundancy.

Relays should be connected to trip fault-interrupting devices that will clear faults in the zone that the relay is intended to protect. For example, the relay devices 87G in Figure 8, 67G in Figure 9, and 51GB and 51GT must trip the transformer high-side (source) circuit breaker in order to clear a ground fault on the low-voltage side of the transformer.