Section 4: Control & Logic Functions; Auto-Reclose (79); Overview - Siemens 7SR224 Description & Operation

(1) Description of Operation 7SR224
Section 4: Control & Logic Functions
4.1 A -R
UTO ECLOSE
4.1.1

Overview

A high proportion of faults on an Overhead Line (OHL) network are transient. These faults can be
cleared and the network restored quickly by using Instantaneous (Fast) Protection trips followed by an
automated sequence of CB reclosures after the line has been dead for a short time, this 'deadtime'
allows the fault current arc to fully extinguish. Typically, this auto reclose (AR) sequence of
Instantaneous Trip(s) and Reclose Delays (Dead times) followed by Delayed Trip(s) provides the
automatic optimum method of clearing all types of fault i.e. both Transient and Permanent, as quickly
as possible and achieving the desired outcome of keeping as much of the Network in-service as
possible.
The AR function, therefore, has to: -
Control the type of Protection trip applied at each stage of a sequence
Control the Auto Reclose of the Circuit Breaker to provide the necessary network Dead times,
to allow time for Arc extinction
Co-ordinate its Protection and Auto Reclose sequence with other fault clearing devices.
A typical sequence would be – 2 Instantaneous+1Delayed+HighSet Trips with 1 sec & 10 sec dead
times.
The Autoreclose feature may be switched in and out of service by a number of methods, these are:
79 Autoreclose ENABLE/DISABLE (AUTORECLOSE CONFIG menu)
A keypad change from the CONTROL MODE
Customer programmed function key (using Quick Logic)
Via the data communications channel(s),
From a 79 OUT binary input. Note the 79 OUT binary input has priority over the 79 IN binary
input - if both are raised the auto-reclose will be Out of Service.
Knowledge of the CB position status is integral to the auto-reclose functionality. CB auxiliary switches
must be connected to CB Closed and CB Open binary inputs. A circuit breaker's service status is
determined by its position i.e. from the binary inputs programmed CB Open and CB Closed. The
circuit breaker is defined as being in service when it is closed. The in service status has a drop-off
delay of 2 sec, this delay is known as the circuit memory time. This functionality prevents
autoreclosing when the line is normally de-energised, or normally open.
Auto-Reclose (AR) is started by a valid trip relay operation while the associated circuit breaker is in
service.
The transition from AR started to deadtime initiation takes place when the CB has opened and the
protection pickups have reset and the trip relay has reset. If any of these do not occur within the 79
Sequence Fail Timer setting the relay will Lockout. This prevents the AR being primed indefinitely.
79 Sequence Fail Timer can be switched to 0 (= OFF).
Once a AR sequence has been initiated, up to 4 reclose operations can be attempted before the AR
feature is locked-out. Number of recloses is determined by 79 Num Shots. Each reclosure (shot) is
preceded by a time delay - 79 Elem Deadtime n - giving transient faults time to clear. Separate dead-
time settings are provided for each of the 4 recloses and for each of the four fault types – P/F, E/F,
SEF and External.
Once a CB has reclosed and remained closed for a specified time period (the Reclaim time), the AR
sequence is re-initialised and a Successful Close output issued. A single, common Reclaim time is
used (Reclaim Timer).
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