6 December 2016, Brisbane Australia – Utility world, it’s high time we have a little discussion. It seems that lately, a certain artefact of the very nature of our work has become the subject of much scrutiny and misjudgement. In a world where health and safety’s omnipresence permeates our every day’s ambitions and achievements, there has been an unfair critique of a capability of many of our electricity workhorses. In a world where risk mitigation is beginning to trump engineering practicality, it is our duty to remind the new generation of our great industry exactly why we designed our networks the way we did. For the humble circuit breaker, it is time to discuss Automatic Circuit Reclosing. Let us remember the very reason why fault re-energisation was considered in the first place, over 70 years ago.
Why is it, that when electricity networks are designed, there is such an extraordinary amount of attention paid to protection pickup levels and grading? For such extravagant resources, (and highly paid experts), there must be exceptional value in getting your protection settings right. But if the very goal of a protection scheme is to interrupt fault current, how could it possibly be considered intelligent, or even sane, to close a circuit breaker shortly after it has successfully broken current? Why do we consider that reclosing is of fundamental importance to intelligent network operation?
The answer is very simple. Like the success of Vanilla Ice, Dexy’s Midnight Runners and Devo, most network faults are transient. Despite what a cursory tour of YouTube’s ever growing back catalogue of spectacular catastrophic high voltage electrical asset failures appears to indicate, the truth is that the vast majority of distribution network faults are simply intermittent. There are a staggering array of causes for these faults: from flailing vegetation fronds brushing overhead conductors as they wave carelessly in the wind, to birdlife or other critters, right through to tree branches which fall across conductors. All of these temporary problems can be solved by a simple interruption, re-energisation, and interruption cycle. it is exceptionally important to remember why we designed things to work this way in the past.
The concept of reclosing allows for fault re-energisation - the capability to give a physical kick to the fault on the line, providing a little momentum to the fallen stick which has perched itself across parallel conductors, or in the worst case, delivering the final Coup De Grace to the unfortunate stunned possum who dared cross from earthed pole to conductor. Reclosing a circuit breaker which is designed to handle temporary fault currents allows for maximum opportunity to rectify the temporary fault. If the fault is truly a permanent fault, your circuit breaker simply proceeds to lockout. While your field operations crew grumbles into a field vehicle after your dispatch call, you can rest assured that your design providing the best opportunity for your organisation to stay in business, whilst providing the best safety for your people. After all, if it really was a serious fault, your cleverly included High Current Lockout element will bypass your reclose configuration and trip your circuit breaker straight to lockout.
The truth of the matter is, no matter which part of the utility industry you are in, you need to get paid. And that money only comes from one place - your end customer. It doesn’t take university qualifications to understand that if your customers’ lights go out, you’re losing cash. Outages cost a fortune, especially in utility economic climates which advocate penalties for service providers which fail to guarantee reliability. All of this is compounded by the cost of sending crews to all ends of the network, simply to hunt for a fault which was intermittent. Automatic Reclosing is a devastatingly effective strategy, where in most cases a single interruption prevented by a recloser would cover the cost of the entire equipment procurement and installation. Therefore, when you consider safety and risks, having switchgear which is robust and engineered to handle temporary overcurrents is not only recommended, it is absolutely engineering best practice, especially if that equipment is designed with Arc Fault control. When a circuit breaker is instructed to close, you are essentially giving your network the best chance it has for fast, effective reliability.
For every great rule, there is an exception. Auto-Reclosing is no different, a magnificent capability with brutally effective simplicity, but there are some days of the year where a little extra caution should be heeded. In the utility industry, decision making is often greatly influenced by the tension in the relationship power dichotomy of Principal Engineers and Executive Accountants. Auto-Reclosing is usually a win-win scenario, but on days where conditions indicate a high bushfire risk, this win-win scenario starts to become a bit risky. Engineers will almost always share the accountants’ advocacy for implementing reclosing, but on the fire days, an engineer will start to consider that maybe a single shot to lockout is all that is necessary.
In the wake of Australia’s devastating Black Saturday bushfires in 2009, where 173 people tragically perished, mitigating fire risk is at the forefront of distribution network design. Following this event, the reports released by the varying inquests indicated that when considering network reliability, reclosing is the way to go. However, from a bushfire mitigation perspective, the best advice was for reclosing suppression. In practicality, the very best method is for remote SCADA initiated suppression of reclosing functions in circuit breakers during high fire danger days on risky feeders, with reclosing used for the other 11 months of the year.
“As more automation and intelligence are built into networks our reclosers are getting more intelligent to respond to the requirements of the smart grids of the future”, reports NOJA Power Group Managing Director Neil O’Sullivan. “However, simple auto reclosing clears 90% of faults on overhead networks and is still one of the most effective means of increasing reliability on overhead distribution networks today.”
Ultimately, the behaviour of your circuit breakers should be based on reliability and safety, and that compromise should not consist of a watered down version of our original intent. Industry best practice says that Reclosing should be your core behaviour, and If bushfire mitigation is your necessity, use your remote SCADA communications to suppress your Reclosing. There is no need to be ashamed of your arcs and sparks – when it isn’t fire season, use the very power you are charged with delivering to strike the objects that dare get in your way of reliability. It’s time to remember why our grandfathers invented reclosing Circuit Breakers and Automatic Circuit Reclosers. They are the distribution networks’ core building block because they promote reliability, give you bushfire mitigation strategies and are incredibly cost effective.