Siemens 7SR18 Solkor, Manual

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7SR18 Applications Guide
Unrestricted Page 48 of 63 ©2018 Siemens Protection Devices Limited
Section 8: Control Functions
8.1 Auto-reclose Applications Guide
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 function attempts to minimise the
loss of supply to the customer and reduce the need for manual intervention.
The function supports up to 4 Auto-reclose (AR) sequences. That is, 4 x Trip / Recloses (shots) followed by a Trip
& Lockout. A lockout condition prevents any further attempts, automatic or manual, 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 faults and External control).
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 AR sequence and cause unnecessary lockouts.
The Sequence Fail Timer provides an overall maximum time limit on the AR operation. It should therefore be
longer than all the set delays in a complete cycle of AR 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 AR 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 AR 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: -
•
When any fault occurs, the relay will trip instantaneously and then reclose.
• If this does not clear the fault, the relay will do the same again.
•
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.
•
This Trip will Lockout the AR sequence and prevent further recloses.
It is important that all the relays in an AR scheme shadow this process – advancing through their own AR
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 AR scheme must be set
with identical Instantaneous and Delayed sequence of trips.
Figure 8.1-1 Sequence Co-ordination