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Section 4: Control Functions
4.1 Auto-reclose Applications
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Automatic circuit reclosing is extensively applied to overhead line circuits where a high percentage of faults
that occur are of a transient nature. By automatically reclosing the circuit-breaker the feature attempts to
minimise the loss of supply to the customer and reduce the need for manual intervention.
The function supports up to 4 ARC sequences. That is, 4 x Trip / Recloses followed by a Trip & Lockout. A lockout
condition prevents any further automatic attempts to close the circuit-breaker. The number of sequences selected
depends upon the type of faults expected. If there are a sufficient percentage of semi-permanent faults which
could be burnt away, e.g. fallen branches, a multi shot scheme would be appropriate. Alternatively, if there is a
high likelihood of permanent faults, a single shot scheme would minimise the chances of causing damage by
reclosing onto a fault. In general, 80% of faults will be cleared by a single Trip and Reclose sequence. A further
10% will be cleared by a second Trip and Reclose. Different sequences can be selected for different fault types
(Phase/Earth/Sensitive Earth faults).
The Deadtime is the interval between the trip and the CB close pulse being issued. This is to allow for the line to
go 'dead' after the fault is cleared. The delay chosen is a compromise between the need to return the line to
service as soon as possible and prevented unnecessary trips through re-closing too soon. The Reclaim Time is
the delay following a re-closure before the line can be considered back in service. This should be set long enough
to allow for protection operation for the same fault, but not so long that two separate faults could occur in the
same Autoreclose (ARC) sequence and cause unnecessary lockouts.
The Sequence Fail Timer provides an overall maximum time limit on the ARC operation. It should therefore be
longer than all the set delays in a complete cycle of ARC sequences; trip delays, Deadtimes, Reclaim Time etc.
Generally this will only be exceeded if the circuit-breaker has either failed to open or close.
Since large fault currents could potentially damage the system during a prolonged ARC sequence, there are also
settings to identify which protection elements are High-sets and these can cause an early termination of the
sequence.
Where a relay is to operate as part of an ARC scheme involving a number of other relays, the feature attempts to
clear any faults quickly without regard to normal fault current grading. It does this by setting each Trip element to
be either Delayed or Instantaneous. Instantaneous Trips are set to operate at just above maximum load current
with small delays while Delayed Trips are set to suit actual fault levels and with delays suitable for current
grading.
A typical sequence would be 2 Instantaneous Trips followed by a Delayed Trip & Lockout:
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When any fault occurs, the relay will trip instantaneously and then reclose.
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If this does not clear the fault, the relay will do the same again.
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If this still does not clear the fault, the fault is presumed to be permanent and the next Trip will be
Delayed and so suitable for grading with the rest of the network. Thus allowing downstream
protection time to operate.
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The next trip will Lockout the ARC sequence and prevent further recloses.
It is important that all the relays in an ARC scheme shadow this process – advancing through their own ARC
sequences when a fault is detected by an element pickup even though they are not actually causing a trip or
reclose. This is termed Sequence Co-ordination and prevents an excessive number of recloses as each
successive relay attempts to clear the fault in isolation. For this reason each relay in an ARC scheme must be set
with identical Instantaneous and Delayed sequence of trips.
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© 2018 Siemens Protection Devices Limited
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Figure 4-1
Sequence Co-ordination
7SR10, 7SR11, 7SR12 Applications Guide
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Chapter 7 Page 29 of 41
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