The protection element also provides a non-sensitive area to avoid instability due to small asymmetries and unbalances that can generally be present in the network system. This condition is represented by the characteristic reproduced below, where the hatched area is the tripping zone.
Introduction
An analysis of faults on any overhead line network has shown that:
− 80 - 90% are transient in nature,
− the remaining 10-20% of faults are either non-permanent (arcing fault) or permanent.
A transient fault is a self-clearing ‘non-damage’ fault. This type of fault can be isolated and
cleared by the immediate tripping of one or more circuit breakers, and does not recur when
the line is re-energised. The most common cause of transient faults are lightning, insulator
flashover clashing conductors and wind blown debris.
The immediate trip will not clear a non-permanent or permanent fault, and the use of the
recloser may be necessary to clear it. A small tree branch falling on the line could cause a
non-permanent fault. 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.
Most of the time, if the faulty line is immediately tripped, and the fault arc has sufficient time
to de-ionise, reclose of the circuit breakers will result in the line being successfully reenergised. Autoreclose schemes are used to automatically reclose a switching device once a
time delay has elapsed and starting after the CB has opened.
On HV and MV distribution networks, the autoreclose function is applied mainly to radial
feeders where system stability problems do not generally arise. Using the autoreclose
minimises time of interruption and reduces operating costs.
Automatic autorecloser allows a substation to operate unattended:the number of visits to
manually reclose a circuit breaker is substantially reduced. This feature constitutes therefore
an important advantage for substations supervised remotely.
On circuits using time graded protection, the automatic autorecloser 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 and to develop the transient fault into a permanent fault.
Using short time delay protection prevents blowing of fuses and reduces circuit breaker
maintenance by eliminating pre-arc heating when clearing transient faults.
The next figure shows an example of 4 autoreclose cycles (maximum numbers of allowed
cycles) to the final trip (in the following diagram, td1, td2, td3, td4 = dead time 1, 2, 3 and 4
timers, tr = Reclaim time, O = CB open and C = CB closed).
