Page 50 - Energize August 2022

TECHNICAL

Recloser versions
Single-phase
Single-phase ARCs are used to protect single-phase lines such as
branches or taps of a three-phase feeder. They can also be used on
three-phase circuits where the load is predominantly single-phase. Figure 4: ARC operation to lockout
When a permanent phase-to-ground fault occurs, one phase can be
locked-out while service is maintained to the remaining phases.
Single-phase reclosers can be controlled by means of a hydraulic
control (integrated within the recloser tank), or an electronic control
(housed in a separate enclosure) based upon the recloser design.
Figure 5: ARC operation to power restoration
Three-phase
Three-phase reclosers are used on three-phase circuits to improve
system reliability and where lockout of all three phases is required for
any permanent fault, to prevent single phasing of three-phase loads
such as large three-phase motors.

Three modes of operation are commonly used:
• Three-phase trip and three-phase lockout: For any fault (single-
phase-to-ground, phase-to-phase or three-phase), all contacts
open simultaneously for each trip operation. The three phases,
mechanically linked together for tripping and reclosing, are operated Figure 6: Protection curves available (Schneider)
by a common mechanism.
• Single-phase trip and three-phase lockout: Each phase operates ionised gas caused by the lightning strike or flashover to dissipate,
independently for overcurrent tripping and reclosing. If any phase typically 2 seconds or less. If this first reclose is too quick, the ionised
sequences to lockout condition (due to permanent fault), or if gas could cause a restrike of the fault.
lockout is locally or remotely asserted, the other two phases trip If the ARC recloses onto a fault it is fairly certain that the fault
open and lock out. Extended single-phase energisation of three- was not caused by lightning, and most likely by contact with the
phase loads is prevented. conductors. The second and third trips are usually slower inverse
• Single-phase trip and single-phase lockout: Each individual phase time delay curves, which allow fault current to quite literally burn the
trips and sequences to lockout independent of each other. This fault off the network. These inverse time curves trip faster on higher
is primarily for residential loads and/or where single-phasing of fault currents and slower on low fault currents, effectively acting as
three-phase loads is protected by other means. Phase-selective an equipment stress threshold which allows for the maximum energy
relays must be used to control the tripping and the recloser requires to be delivered to a fault so as to burn off the fault without causing
individual closing/tripping mechanisms for each phase. As a physical damage to the network.
result, the single-phase option is more complex and usually more The second and third reclose times typically tend to be longer (in
expensive. the 5 to 10 second range), allowing time for the incinerated debris
to fall away from the line1. If a reclose sequence makes it past the
Recloser operation third trip, the fault is likely to be a permanent bolted fault, at which
Upon detecting a fault, the ARC will open and remain open for a point reclosing does very little anymore and field service becomes
period known as the dead time. At the end of the dead time the ARC necessary.
will reclose. If the fault persists, the ARC will re-open and remain open In city areas where buildings are higher than power lines, the
for a second dead time. If the fault still persists, the ARC will lock out most common transient fault is more likely to be a foreign body on
after the last trip. If the fault clears before the last trip cycle is reached, the network or a fallen conductor. In built up areas, the most common
the ARC remains closed and restores power to the network. deployment is a three shot to lockout system with a first fast trip. The
The ARC may be set to lock out after fewer than four trips, reclose times are in line with what is deployed in the rural lines.
and both the dead time and the trip time may be set to suit local In the case of SEF in urban areas, the ARC is usually set to a single
conditions. Operation is limited a total of four trips, and three recloses. shot to lockout. This is due to the likelihood of SEF being triggered
Dead time is chosen to allow the fault to clear. Dead time is based on by conductors lying on the ground, and this risk is much higher in an
the expected type of fault to be cleared. The ARC will have the option urban environment.
of a number of different tripping curves, and trip time may be selected
to follow any standard fuse or circuit breaker curve or user-defined Sectionalisers
curve. Unlike a recloser, a sectionaliser does not have the capability to
In a rural overhead network, lightning is the most common fault, break fault current. A sectionaliser is a device which automatically
followed by vegetation contacts, so the first trip is typically as fast as isolates faulted sections of a distribution circuit, once an upstream
possible. This is to interrupt the fault before damage is caused to the breaker or recloser has interrupted the fault current and is usually
network. The following dead time needs to be long enough for the installed downstream of a recloser.

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