Electrical switchgear engineers NOJA Power today highlights technical features of its OSM range of Automatic Circuit Reclosers (ACR) that enable utilities to improve distribution grid feeder reliability.
NOJA Power’s OSM15, OSM27 and OSM38 ACRs incorporate Temporary Time Addition (TTA) functionality as standard. This feature enables utilities to increase the number of ACRs on a feeder and replace load-break switches (“sectionalisers”) with full fault protection devices. In addition, NOJA Power’s ACRs’ use of solid dielectric insulation and vacuum arc quenching help utilities meet their environmental responsibilities by eliminating the polluting oil and Sulphur Hexafluoride (SF6) gas used in load-break switches. (SF6 is the most potent ‘greenhouse’ gas with a global warming potential of 23,900 times that of CO2.)
In conventional installations, the number of ACRs on a single feeder is limited due to the different Time Current Characteristic (TCC or ‘grading’) setting for each upstream unit which ensure 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 breakers policy normally dictates that the devices trip after a maximum of one second if there is a fault current on a given feeder, to protect expensive equipment. Because the minimum time difference between operations of subsequent ACRs is around 200-to-400 milliseconds, this one-second restriction limits the number of ACRs on a single feeder to a maximum of two-to-four devices. Such a limit can be a problem on long feeders with limited number of ACR’s because a single fault may cause a loss of supply for a significant number of customers.
Traditionally, utilities increase the number of sections on a feeder by adding load-break switches to work in conjunction with the ACRs. However, load-break switches have a number of drawbacks, including no fault break or protection capability and the use of non-environmentally friendly oil and SF6 gas.
Because a load-break switch can’t open under a fault current, its operation must be coordinated with the upstream ACR so that it opens during the dead time of the ACR’s final reclose operation. To ensure safe coordination engineers extend the dead time of the ACR’s final operation to allow time for the load-break switch to operate. This slows down the fault protection process potentially increasing the disruption to consumers. In addition, the use of load-break switches raises equipment damage and safety risks because there is no interlocking mechanism between the ACR and load-break switch to guarantee coordinated operation.
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 virtually 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 (see “About Temporary Time Addition” below).
The TTA feature of NOJA Power’s ACRs allow utilities to dispense with load-break switches and replace them with environmentally-friendly full fault protection devices while retaining or improving the granularity of the sectionalisation of a feeder. Moreover, because ensuring coordination of ACR operation by grading can be a complicated and time-consuming process, the engineering overhead for a feeder using ACRs with TTA is also reduced.
“NOJA Power ACRs with TTA relieve a protection engineer of the conventional limitation of the number of ACRs on a single feeder and the engineering is simplified because the units have identical TCCs - removing all the complexities of coordinated grading,” explain Shane Gorman, NOJA Power’s Australia-Pacific Sales Manager. “Furthermore, 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.
“In a conventional system where ACRs and load-break switches work together, the latter is designed to open only after the ACRs has cycled a number of times in its attempt to clear the fault. Consequently every customer on a feeder suffers a number of momentary interruptions even if their section of the line is healthy,” continues Gorman. “By using NOJA Power ACRs with TTA to replace the load-break switches, customers on the healthy section of feeder suffer only a single momentary interruption. And a fully functioning TTA scheme allows for an increase in the number of sections so that fewer customers are affected during an outage. That can make a big difference particularly if the feeder is a long run in a rural area that’s prone to storms and exposed to many trees.”
Units from NOJA Power's OSM range of medium-voltage (15, 27 and 38 kV) ACRs (see “About the NOJA Power OSM range” below) 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.
References: 1. "Direct Global Warming Potentials", Intergovernmental Panel on Climate Change. 2007.About Temporary Time Addition
NOJA Power’s Temporary Time Addition (TTA) feature works in conjunction with the company’s Voltage Reclosing Control (VRC) feature. All the Automatic Circuit Reclosers (ACR) on the feeder are set up with the same Time Current Characteristic (TCC).
In the event of a fault current, all the ACRs upstream from the fault location open simultaneously. VRC then prevents the downstream ACRs reclosing until each unit detects a live voltage on its line side bushings to ensure that the device operates to close only after each upstream device has operated first. The unit closest to the substation closes first and then each subsequent downstream device closes and also detects whether the downstream section of feeder is healthy. If the feeder within the ACR’s section is healthy, the TTA feature is implemented to delay the ACR from operating and ensure the device that is closest to the fault operates to open first, leaving the upstream customers with power and communicating the location of the fault to the utility’s engineers. (See figure 1.)