GE MiCOM P40 Agile, Technical Manual, Page: 284 / 962

2 INTRODUCTION TO AUTORECLOSE
Approximately 80 - 90% of faults on transmission lines and distribution feeders are transient in nature. This means
that most faults do not last long, and are self-clearing if isolated. A common example of a transient fault is an
insulator flashover, which may be caused, for example, by lightning, clashing conductors, or wind-blown debris.
Protection functions detecting the flashover will cause one or more circuit breakers to trip and may also remove
the fault. If the source is removed, the fault does not recur if the line is re-energised.
The remaining 10 – 20% of faults are either semi-permanent or permanent. A small tree branch falling onto the
line for example, could cause a semi-permanent fault. Here the cause of the fault would not be removed by
immediate tripping of the circuit, but could possibly be burnt away during a time-delayed trip. Permanent faults
could be broken conductors, transformer faults, cable faults or machine faults, which must be located and
repaired before the power supply can be restored. In many fault incidents, if the faulty line is immediately tripped
out, and time is allowed for the fault arc to de-ionise, reclosing the circuit breakers will result in the line being
successfully re-energised.
Autoreclose schemes are used to automatically reclose a circuit breaker a set time after it has been opened due to
operation of a protection element. On EHV transmission networks, Autoreclose is usually characterised by high-
speed single-phase operation for the first attempt at reclosure. This is intended to help maintain system stability
during a transient fault condition. On HV/MV distribution networks, Autoreclose is applied mainly to radial feeders,
where system stability problems do not generally arise, and is generally characterised by delayed three-phase
operation with potentially multiple reclosure attempts.
Autoreclosing provides an important benefit on circuits using time-graded protection, in that it allows the use of
instantaneous protection to provide a high speed first trip. With fast tripping, the duration of the power arc
resulting from an overhead line fault is reduced to a minimum. This lessens the chance of damage to the line,
which might otherwise cause a transient fault to develop into a permanent fault. Using instantaneous protection
also prevents blowing of fuses in teed feeders, as well as reducing circuit breaker maintenance by eliminating pre-
arc heating. When instantaneous protection is used with Autoreclose, the scheme is normally arranged to block
the instantaneous protection after the first trip. Therefore, if the fault persists after re-closure, the time-graded
protection will provide discriminative tripping resulting in the isolation of the faulted section. However, for certain
applications, where the majority of the faults are likely to be transient, it is common practise to allow more than
one instantaneous trip before the instantaneous protection is blocked.
Some schemes allow a number of re-closures and time-graded trips after the first instantaneous trip, which may
result in the burning out and clearance of semi-permanent faults. Such a scheme may also be used to allow fuses
to operate in teed feeders where the fault current is low.
When considering feeders that are partly overhead line and partly underground cable, any decision to install
Autoreclose should be subject to analysis of the data (knowledge of the frequency of transient faults). This is
because this type of arrangement probably has a greater proportion of semi-permanent and permanent
faults than for purely overhead feeders. In this case, the advantages of Autoreclose are small. It can even be
disadvantageous because re-closing on to a faulty cable is likely to exacerbate the damage.
Chapter 11 - Autoreclose P446SV
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