Introduction To 3-Phase Autoreclose - GE MiCOM P40 Agile Technical Manual

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Chapter 13 - Autoreclose
2

INTRODUCTION TO 3-PHASE AUTORECLOSE

It is known that approximately 80 - 90% of faults are transient in nature. This means that most faults do not last
long and are self-clearing. 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.
A transient fault, such as an insulator flashover, is a self-clearing 'non-damage' fault. The flashover will cause one
or more circuit breakers to trip, but it may also have the effect of clearing the fault. If the fault clears itself, the fault
does not recur when the line is re-energised.
The remaining 10 – 20% of faults are either semi-permanent or permanent. A small tree branch falling on the line
could cause a semi-permanent fault. Here the cause of the fault would not be removed by the immediate tripping
of the circuit, but could 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 supply can be
restored.
In the majority of fault incidents, if the faulty line is immediately tripped out, and time is allowed for the fault arc to
deionise, reclosure of 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 HV/MV distribution networks, autoreclosing is applied mainly to radial feeders, where system stability problems
do not generally arise. The main advantages of using Autoreclose are:
Minimal interruption in supply to the consumer
Reduction of operating costs - fewer man hours in repairing fault damage and the possibility of running
unattended substations
With Autoreclose, instantaneous protection can be used which means shorter fault durations. This in turn
means less fault damage and fewer permanent faults
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 autoreclosing, 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 auto-
reclosing 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 autoreclosing are small. It can even be disadvantageous
because re-closing on to a faulty cable is likely to exacerbate the damage.
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P14x
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