Breaking the trade-off: sustained outages vs short interruptions
Christopher McCarthy (managing director - EMEA) at S&C Electric Company discusses dealing with outages.
1st May 2018 by Networks
Wireless connectivity, electric vehicles, and renewable generation are changing how the electric industry works. This equipment is less tolerant to interruptions of any duration. As IoT and renewable technology continues to expand, these concerns will accelerate. In this digital age consumer expectations for reliability are higher than ever.
Power outages are more than nuisances. They cost electricity users real money. The US Department of Energy found that whilst sustained outages cost the US a total of $26 billion each year, short interruptions were costing twice as much as that – $52 billion according to the Lawrence Berkeley National Laboratories report. Yet, as an industry, we are typically only addressing averaged sustained outages, and our success measurements are not adequate to drive infrastructure changes that modern consumers expect.
Industry regulations helped drive sustained outages down in the UK. Power cuts are down 48 per cent in number, and 58 per cent in length since Ofgem introduced interruption incentives for network companies to improve reliability in 2001. However, a corresponding increase in short interruptions is an unfortunate, unintended consequence of these great gains in reducing sustained outages.
If all faults were persistent, meaning they caused damage to the distribution network and required the crew personnel to drive to the site and repair damage, then designing the protection system would be easy. You could simply start with large fuses near the substation, and use progressively smaller fuses as you get further away. When a fault occurs, the closest fuse blows to isolate the problem area.
But, it’s not that simple, because most faults – typically 80 to 90 per cent – would be transient if they were interrupted and given a few seconds to clear. This is why reclosing exists – for most faults if you can interrupt and wait a few seconds before re-energising the circuit, the faults would no longer exist.
There are three methodologies that utilities have to manage transient faults: Fuse-saving, fuse-blowing, or automatic sectionalising links. Each of these comes with a significant trade-off.
Fuse-saving can reduce sustained outages from transient faults, but the high number of short interruptions will keep knocking customers and generation off-line. Changing recloser settings to allow fuses to respond to first swings it too far the other way. Now, faults on spurs don’t bother customers on other spurs, but even simple faults cause extended outages, because crews must drive out and replace fuses.
Use of sectionalising links has a similar result as fuse-saving, with lots of interruptions to all customers on the circuit.
But, there is a simpler solution for the overhead network. S&C’s TripSaver II single-pole recloser offers a simple, no maintenance, long-life solution, that can be installed instead of fuses or sectionalising links, and breaks the trade-off between the sustained outages and short interruptions by reducing both.
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