Application Guide - NP900 Series
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A996A
the line is not-energized) and the supply is restored to the line. If the fault is not cleared by
this first autoreclosing cycle (called Shot) then more shots can be applied to the line.
If the fault was not cleared by the time autorecloser closes the breaker and second shot is
applied into the line there can be set either time delay (called Arcing time) in order to burn
the fault causing object from the line or normal protection operating times can be applied.
In autorecloser is selection also if the fault is not present when closing the breaker but
reappears soon after closing the breaker (called Discrimination time, Reclaim time),
Autorecloser either arms another shot or gives final trip command and locks-out. In case
one shot is applied to the line and if it is not successfully clearing the fault autorecloser will
init final trip and will lock out the feeder closing also.
Whether single or multi-shot autoreclosing should be used is matter of the type of
protection, switchgear, circuit breaker, stability requirements, network type, consumer
loads and also local utility knowledge and practices of the network.
Typical autoreclosing scheme is not easy to define since in distribution and transmission
networks these mentioned parameters vary greatly thus affecting directly to the scheme
main parameters, how many shots and how long dead times should be set for the
reclosing scheme and also which protection functions should init the autorecloser.
Minimum times for the Dead Time setting is mostly dependent of the voltage level of the
protected network in order to give enough time for the air to de-ionize after the circuit
breaker is opened. For medium voltage 20 kV to 75 kV Dead Time of 200 ms should be
sufficient when for 110 kV requires about 300 ms and 400 kV requires 400-500 ms Dead
Time. This minimum time is not this straightforward to define since it is affected by other
parameters also like conductor spacing, wind speed, fault type, fault duration etc. The
main purpose of the Dead Time is to allow and give time for the fault location surrounding
air to return to isolating state before the line is re-energized and inhibit the arc from re-
ignite due to heated and ionized air. Also for lower voltage levels the breaker open-close-
open cycle capacity gives restrictions for the minimum Dead Time setting while with
higher voltage levels the de-ionizing time dictates the minimum Dead Time which makes
possible a successful autoreclosing.
In case of evolving faults like transient earth fault turns to multi-phase short circuit or
overcurrent fault different schemes can be built by setting the requests into different
priorities and behaviors. Autorecloser has five independent priority requests for reclosing
and one critical request which halt the recloser in any position it is running when the
critical request is received. REQ1 has the highest priority and REQ5 lowest.
