Page 48 - Energize August 2022

TECHNICAL

Auto-reclosers and sectionalisers

in distribution networks

A continuous and reliable supply of electricity of acceptable quality is imperative in modern electrical
networks. Continuous power delivery of standard quality, in the most economical manner, requires
a system of monitoring and management over the entire network. Distribution networks are the

most susceptible to failure, making the management of failure detection and restoration essential.
Reclosers and sectionalisers play an essential role in this function.

by Mike Rycroft, Energise

ost of the faults which occur in a distribution network are • Birds, reptiles or small animals bridging between an energised line
transient in nature and need not result in disconnection and grounded surface
Mof the network. Reclosers and sectionalisers are important • Tree branches touching energised lines
components in distribution networks, to handle such faults, increase • Switching surges which flash over an insulator
the availability of the network and reduce maintenance call outs.
The economic and safety benefits of deploying automatic circuit The need for a trip-and-reclose function is self-evident. If the line can
reclosers (ACRs) in network assets are well known. Reclosers are be tripped momentarily, a subsequent reclosure would very likely be
used throughout the power distribution system, from substation successful as the fault would have self-cleared. The ARC, in providing
to residential utility poles. They range from small reclosers for this trip-and-reclose function, virtually eliminates prolonged outages
use on single-phase power lines, to larger three-phase reclosers on distribution systems caused by temporary faults or transient
used in substations and on power lines up to 38 kV. Reclosers save overcurrent conditions. This results in a significant decrease in
considerable time and costs, as they permit power to be restored the failure rates of the upstream loads, which reduces the various
automatically, after only a short interruption. Where outages require reliability indices, namely system average interruption frequency
repairs, reclosers minimise the outage area and help to quickly locate index (SAIFI), energy not supplied (ENS), etc.
problems. Reclosers and sectionalisers are used together in various ARCs are commonly associated with rural distribution networks
combinations, depending on the requirements of the network. in the MV range. The recloser has developed from its original form,
The emergence of distributed generation in distribution a standalone automatic switch, to a fairly complex component of
networks has complicated the protection of overhead networks the smart grid, offering communications facilities, monitoring and
and requires intelligent reclosers and sectionalisers as well as the reporting, phasor detection and usually incorporates microprocessor
reconfiguration of networks to achieve the required availability of controls and other digital features.
supply. The recloser assembly consists of the fault-breaking primary
unit, typically pole-mounted, and a control cabinet mounted near
Auto-reclosers (ARC) the ground level (Figure 1). Both three-phase and single-phase units
A recloser is a device used to mitigate nuisance tripping on are used, depending on the secondary distribution operation and
distribution lines and, in newer versions, to report fault conditions. protection philosophy.
ARCs are most commonly found in overhead networks where only Vacuum breakers, together with solid insulation, are the most
around 7% of faults are permanent, and where the vast majority of common choice for systems currently on the market. The ratings
overhead faults – around 80% – are transient and will disappear in of reclosers vary from 2,4 kV to 38 kV for system voltage, from
less than a second. The remaining faults are semi-permanent and 10 A to 1200 A for load current and from 1 kA to 16 kA for fault
are typically caused by animals or branches bridging the power current.
lines. Even these faults, however, usually burn away and clear in a Early recloser units did not have any remote communication
relatively short time. facilities. To enhance the system monitoring and restoration facilities,
reclosers are now commonly equipped with remote communication
Most faults on overhead distribution lines will, by their very nature, protection and control units.
remove themselves from the electric line if the power is removed
before the lines are permanently damaged. Examples of temporary Recloser components
faults include: Explanation of the components of an auto-recloser:
• Windblown conductors touching one another • The switch is required to close automatically and requires a motor
• Lightning surges flashing over an insulator drive or other electromagnetic device.

energize | August 2022 | 46