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Monday 16 March 2015

What is Auto-Reclosing and its schemes used in Power System?

80 to 90% of faults on transmission lines are of transient in nature, as voltage level increases, the percentage becomes low, as of insulator flash over etc.

If line is tripped and time is allowed for the fault arc to de-ionize, reclose of the circuit breaker will result in the line being successfully re-energized. Auto reclosing is carried out by auto reclose relay that may be separate or built-in protection relay. Auto reclosing helps in improvement in continuity of supply. Auto reclosing is never carried out on transformers.

For transmission lines up to 132 KV, 3 pole tripping / 3 pole closing is used for all types of faults in zone-1. For faults in zone-2 and above, auto reclosing is blocked.

Most Commonly used Auto-Reclose Scheme

For high voltage and extra high voltage lines, single pole tripping/ single pole closing, three pole tripping, three pole closing and lockout scheme is used. For phase to phase faults, three pole tripping, three pole closing and lockout is used.

Dead Time (System)

The time between the fault arc being extinguished and the circuit breaker contacts remaking.

Reclaim Time

The time following a successful closing operation, measured from the instant the auto-reclose relay closing contact make, which must elapse before the auto-reclose relay will initiate a reclosing sequence in the event of a further fault incident.

Lock-out

A feature of an auto-reclose scheme which after final tripping of the circuit breaker, prevents further auto reclosing.

High speed reclosing

If a circuit breaker is automatically reclosed with in one second after a fault trip operation, it is called high speed reclosing.

Low speed reclosing

If a circuit breaker auto-reclosing is delayed for a time in access of one second, it is called low speed reclosing.