Areva MiCOM P125, Technical Manual

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Application Guide P12y/EN AP/D32
MiCOM P125/P126 & P127 Page 35/74
4. DESCRIPTION AND SETTING GUIDE OF THE AUTORECLOSE
FUNCTION (P126 & P127)
An analysis of faults on any overhead line network has shown that 80 - 90% are transient in
nature. A transient fault, such as an insulator flashover, is a self-clearing ‘non-damage’ fault.
This type of fault can be cleared by the immediate tripping of one or more circuit breakers to
isolate the fault, and does not recur when the line is re-energised. Lightning is the most
common cause of transient faults, other possible causes being clashing conductors and wind
blown debris. The remaining 10 - 20% of faults are either non-permanent (arcing fault) or
permanent.
A small tree branch falling on the line could cause a non-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 that 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 de-ionise, re-closure of the circuit breakers will result in the line
being successfully re-energised. Autoreclose approaches are employed to automatically re-
close a switching device at a set time after it has been opened due to operation of protection
where transient and non-permanent faults are prevalent.
On HV and MV distribution networks, the autoreclose function is applied mainly to radial
feeders where system stability problems do not generally arise. The main advantages to be
derived from using autoreclose can be summarised as follows:
•
Minimises interruptions in supply to the consumer.
•
Reduces operating costs - less man-hours in repairing fault damage and the possibility
of running substations unattended. With autoreclose instantaneous protection can be
used which means shorter fault durations which gives rise to less fault damage and
fewer permanent faults.
As 80% of overhead line faults are transient, elimination of loss of supply from such faults, by
the introduction of the autoreclose function gives obvious benefits. Furthermore, autoreclose
may allow a particular substation to operate unattended. In the case of unattended
substations, the number of visits by personnel to re-close a circuit breaker manually after a
fault can be substantially reduced, which is an important consideration for substations in
remote areas.
The autoreclose function gives an important benefit on circuits using time-graded protection,
in that it allows the use of instantaneous protection to give 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, thus lessening the chance of damage to the line, which might otherwise cause a
transient fault to develop into a permanent fault.
Using short time delay protection also prevents blowing of fuses and reduces circuit breaker
maintenance by eliminating pre-arc heating when clearing transient faults.