Transmission & Distribution Technology
No room for errors
Stefan Kramer looks at a fuse-saving circuit breaker to minimise outages.
Stefan Kramer observa un disyuntor de ahorro de fusibles para minimizar las interrupciones.
Stefan Kramer untersucht einen Leistungsschutzschalter, der Sicherungen schützt und so Ausfälle minimiert.
R
ural electricity distribution is less reliable, generates less revenue and is more expensive to operate than urban networks. Long line lengths, high fault frequency and
the extra time taken to find and repair faults, results in poor performance and, in more and more countries, regulatory penalties. Outages require line crew attendance and this incurs high operating costs due to long drive times to find the fault and access the site. All these factors have significant implications on the scale and type of investment that network owners are prepared to make on their rural networks. Low cost of purchase, installation and ownership are key drivers for investments to improve network reliability. Most rural medium voltage networks are
configured with the primary feeder protected by a circuit breaker or recloser, whilst a fuse protects the spur line. When a fault occurs, the fuse operates to clear the fault. If the fuse protection is graded correctly with the upstream recloser, the recloser will never need to operate on a spur line fault. Tis means only the customers on the faulted spur line experience an outage. Te problem with this configuration is that the fuse blows on all faults, both permanent and transient, causing downstream customers to always experience a sustained outage and always requiring a line crew to patrol the line and replace the fuse incurring substantial operating costs for the network owner. In most cases this sustained outage is unnecessary as the fault is transient.
A fuse that has blown will drop down and provide a visual indication to passing line crews to the faulted line. When in the dropped down position the fuse provides a genuine electrical isolation due to the large air gap. A fuse has no electronics or intelligence and therefore no capability to record historical data about fault events or reliability data. Without communication functionality, it cannot communicate device status remotely. It makes no contribution to the formation of an intelligent grid. When a live-line crew is working downstream of a fuse, the operating time of the fuse is dependent upon the fault level. As such, normal practice is to use the feeder line recloser with a hot line tag setting as the protection for the live-line crew. Even the fastest recloser protection will allow 2-3 cycles of current to flow, which is adequate to cause significant burns to an operator in case of an accident.
Search and repair Whilst fuses possess a low capital cost, up to 80 per cent of fuses blow unnecessarily. Fuses are quick and easy to install on-site. But a line man or crew, in an average rural environment may take hours to travel, patrol the line for potential fault, search for and repair the blown fuse, costing the utility hundreds of dollars for a single fuse operation. Drop out sectionalisers are single-phase devices used in place of the spur line fuse and are partnered with the feeder line recloser. When a fault occurs on the spur line, the drop-out sectionaliser does not have a fault interrupting capability of its own and relies upon
Fig. 1. Performance with Fusesaver in case of temporary faults. In this case, the fault disappears after closing and the power supply is restored. The fuse did not operate, and the Fusesaver is ready for the next fault. Only the customers on the affected spur experienced an interruption in power during the Fusesaver’s dead time, while all other customers on the feeder, including nearby spurs, did not even notice its operation – in less than one cycle.
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