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New ACRs are improving distribution grid reliability

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article image NOJA Power OSM Single Phase Recloser

The OSM range of automatic circuit reclosers from NOJA Power Switchgear offers Temporary Time Addition functionality and solid dielectric insulation to boost feeder reliability and reduce greenhouse gas emissions by replacing traditional load-break switches.

Electrical switchgear engineers NOJA Power highlights the technical features of its OSM range of Automatic Circuit Reclosers (ACR) that enable utilities to improve distribution grid feeder reliability.

The Temporary Time Addition (TTA) functionality provided as standard in NOJA Power’s OSM15, OSM27 and OSM38 ACRs enables utilities to increase the number of ACRs on a feeder and replace load-break switches with full fault protection devices. The ACRs’ use of solid dielectric insulation and vacuum arc quenching helps utilities meet their environmental responsibilities by eliminating the polluting oil and SF6 gas used in load-break switches.

The number of ACRs on a single feeder in conventional installations is limited due to the different Time Current Characteristic (TCC or ‘grading’) setting for each upstream unit, which ensures that only the closest upstream ACR to a non-recoverable fault locks out. The grading is set such that each ACR reacts more quickly than any upstream unit but slower than any downstream unit.

However, substation circuit breaker policy normally dictates that the devices trip after a maximum of one second to protect expensive equipment if there is a fault current on a given feeder. Since the minimum time difference between operations of subsequent ACRs is around 200-400 milliseconds, this one-second restriction limits the number of ACRs on a single feeder to a maximum of 2-4 devices. This can be a problem on long feeders with limited number of ACRs because a single fault may cause a loss of supply for a significant number of customers.

Load-break switches added by utilities to work in conjunction with the ACRs and increase the number of sections on a feeder may not be sufficient as load-break switches have a number of drawbacks, including no fault break or protection capability and the use of non-environment-friendly oil and SF6 gas.

TTA enables NOJA Power’s ACRs to overcome the limit on the number of units on a single feeder by dispensing with traditional grading and using ACRs with identical TCCs. In the event of a fault current, all the ACRs on a feeder simultaneously open almost instantaneously and certainly well within the one-second delay of the substation circuit breaker. Fault location and lock out (if the fault is not rapidly cleared) is then achieved by coordinated reclosing between all the ACRs starting with the device furthest upstream and then each device downstream in turn.

The TTA feature of NOJA Power’s ACRs allows utilities to dispense with load-break switches and replace them with environment-friendly full fault protection devices while retaining or improving the granularity of the sectionalisation of a feeder.

Shane Gorman, NOJA Power’s Australia-Pacific Sales Manager explains that the ACRs with TTA relieve a protection engineer of the conventional limitation of the number of ACRs on a single feeder while the engineering is simplified because the units have identical TCCs, removing all the complexities of coordinated grading. He adds that the feature allows utilities to dispense with load-break switches and replace them with ‘green’ devices with full fault protection capabilities, which allow implementation of smart grids.

According to Gorman, by using NOJA Power ACRs with TTA to replace the load-break switches, customers on the healthy feeder sections suffer only a single momentary interruption. Additionally, a fully functioning TTA scheme allows for an increase in the number of sections so that fewer customers are affected during an outage.

Units from NOJA Power's OSM range of medium-voltage (15, 27 and 38 kV) ACRs have been installed by utilities in over 80 countries around the world. The ACRs have been subjected to full type testing by independent test laboratories such as KEMA in the Netherlands to the latest standards, underscoring the superior safety and performance of NOJA Power’s protection solutions.

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