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Automatic Circuit Reclosers for Single-Phase Earth Faults on Distribution Grids

Sept. 15, 2015
ACRs provide flexibility for utilities looking to minimise service disruptions in the event of single-phase earth faults on isolated or high-impedance systems by grounding the faulted phase rather than shutting down an entire feeder

NOJA Power highlights how modern Automatic Circuit Reclosers such as its OSM single triple ACR can assist utilities in protection schemes for isolated or high-impedance networks to minimize disruption in the event of electricity feeder faults.

Utilities are implementing smart grids––advanced, computerised electricity distribution networks––to both improve efficiency and minimize disruption to consumers in the event of faults such as those caused by caused by lightning strikes strikes or falling tree branches. By employing modern distribution automation such as NOJA Power’s OSM series ACRs, utilities are able to implement smart protection schemes that, for example, isolate short sections of a faulty feeder and re-route power to ensure as many consumers as possible remain connected to the grid in the event of a fault.

Single Triple Recloser used for faulted phase earthing within a substation in Australia

Utilities employing isolated or high-impedance three-phase systems (three-phase grids without a direct neutral to ground connection) are now enhancing these smart protection schemes by introducing new techniques to deal with single-phase earth faults while ensuring continuity of supply.

In isolated or high-impedance systems, single-phase earth faults can result in fault currents that while relatively low are still sufficient to cause hazards such as arcing. NOJA Power is working with customers to demonstrate how they can use OSM series ACRs to implement single phase earth fault grounding in a more effective way than was previously possible. For example, NOJA Power’s OSM single triple ACR can be sited at a substation to independently ground each phase of a three-phase line in the event of a single-phase earth fault. Grounding the phase reduces its electrical potential and eliminates arcing hazards without the need to shut down the feeder.

Another advantage of the OSM single triple is realised in networks that employ long feeders serving rural areas. The voltage potential on a conductor subject to a single-phase earth fault can rise due to capacitive coupling with the other conductors increasing the likelihood of arcing. By grounding the defective conductor, at a suitable distance along the length of the long feeder, using an OSM single triple ACR, the build-up of voltage can be significantly reduced.

NOJA Power’s OSM single triple ACRs continuously monitor voltage and current on all six bushings and NOJA Power is now developing hardware to enable these ACRs to detect fault currents as low as 0.2 A. This capability, coupled with new software algorithms, will enable the ACRs sited at intervals along a feeder to narrow down a fault location when a single feeder has been isolated at the substation by a device such as a ground fault neutraliser (GFN) – enabling maintenance crews to be quickly directed to the fault.

“NOJA Power’s auto reclosers have proved popular devices for our customers implementing smart grids,” explains Neil O’Sullivan, NOJA Power’s Managing Director. “An auto recloser performs voltage measurement on all six bushings, current measurement on all three phases, bidirectional protection and extensive power quality and data logging capability all contained in a stainless steel enclosure that forms the only solid dielectric unit with controlled internal arc fault venting on the market.

“When complemented by the RC10 control and communications cubicle, these features make NOJA Power’s auto-reclosers very adaptable allowing them to be employed in many applications beyond straightforward three-phase circuit protection. NOJA Power is committed to working with our customers to help them implement innovative schemes such as single-phase earth fault grounding in order that they can maximise the potential of their ACRs.”

The OSM single triple ACR comprises three OSM38/27/15 single pole ACRs (see “About the NOJA Power single phase ACR” below) mounted on a single bracket. The product offers coordinated individual ACR protection of each phase in a three-phase system mounted on a single platform for simplified installation. The OSM38 single pole ACR features a rated maximum system voltage of 38/27/15 kV and a rated continuous current of 800 A.

NOJA Power’s OSM series ACRs are fundamental elements of smart grids and provide a comprehensive suite of automation features. The OSM series has been fully type-tested by independent laboratory KEMA in the Netherlands to ensure long life and reliability under the harshest environmental conditions. Since their introduction, the OSM series ACRs have been installed by utilities in over 84 countries around the world. (See “About the NOJA Power OSM series” below.)

The RC10 is a supervisory control and data acquisition (SCADA)-ready controller that provides a directional overcurrent, earth fault and sensitive earth fault relay, auto reclosing relay, instantaneous metering, event log, demand logger and remote terminal unit (RTU) for remote control in a single package.

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