Typical autoreclosing scheme is not easy to de ne 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 suf cient 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 de ne 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 ve 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.
Auto-reclosing scheme in radial network
In typical medium voltage overhead network construction is radial type and does not cause any
additional requirements for the autoreclosing scheme in addition to the mentioned air de-ionization time
and the capacity of the circuit breaker which should be the dictating magnitudes for the autoreclosing
scheme. Also typically medium voltage overhead line consists of only consumers and no power
generation which leads to that the most stable supply continuity is the main objective.
Figure. 5.5.4. - 123. Typical rural radial medium voltage network construction.
Typically rural radial medium voltage network consists of short cable connection from substation to the
overhead line and relatively long overhead line which typically ends to consumer. The overhead line
part can feed from basically any location residential, farming etc. consumers which connect with 20 kV
/ 0.4 kV distribution transformers to the medium voltage. Overhead line can have multiple branches
and it usually in the countryside goes through forest areas in between of the consumers. In longer lines
is possible to isolate areas of the overhead line with line disconnectors at least in branches.
In this type of application is normally used two shot (one high speed and one delayed) autoreclosing
which are started by earth fault protection or overcurrent protection. Short circuit protection is used for
interlocking of the autorecloser in case of clear short circuit fault in the line.
AQ-F255
Instruction manual
Version: 2.00
219
© Arcteq Relays Ltd
