electric distribution networks

The seemingly simple technology of automatically closing circuit breakers after interrupting a fault is brutally effective in increasing network reliability.

However, as smart grid interventions steer the energy revolution, an evaluation of using reclosers on modern distribution networks holds invaluable information.

This article first appeared in ESI-Africa Edition 1, 2019. You can read the magazine's articles here or subscribe here to receive a print copy.

Over the past few decades, distribution network service providers (DNSPs) have witnessed a phenomenon in the worldwide proliferation of automatic circuit reclosers (ACRs). The economic and safety benefits of deploying ACRs in network assets are well documented by global utilities that spend valuable resources deploying reclosers, and then see their balance sheet improve within a year of deploying the device. From an economic standpoint it makes sense, but we are interested in understanding how best to make the most of our engineering installation. It is certainly worth exploring the rationale involved in the actual configuration of ACRs to reach the best possible value from their deployment.

In the old days of network maintenance, a utility operator would receive a dispatch call to visit a tripped field circuit breaker. The first instruction would be to look for any obvious cause of a fault, but then proceed to reset the relay and close the breaker. Cautiously, the operator would go about his task, reset the relay and close the breaker, but to his utter bewilderment the circuit breaker would close and no fault current would be present. Was this a nuisance trip? Was it mechanical failure? Did the relay fail?

The answer to these questions was almost invariably no. The true diagnosis is that up to 80% of distribution network faults are caused by transient effects. The reality is that most circuit breaker operations should only be temporary.

Furthermore, the entire time that it took to drive out to the circuit breaker installation site, customers’ supply lines were cut and the utility was effectively shredding its income. There is a tremendous amount of value that can be captured simply by automating this restoration process. Automatic circuit reclosers are the industrial revolution of the distribution network, automating what used to take hours into mere seconds.

“Very well,” mutters the engaged reader, “but what do I actually set my auto-reclosing times to be?” The truest answer is “it depends”, but we are willing to explore some of the rationale and guidelines behind this configuration. Let’s begin with the simple truths – any sequence of recloses can, at the most, be a total of four shots to lockout. This equates to four trips and three recloses.

We make the differentiation between an ‘Open’ state and a ‘Lockout’ state, in that ‘Lockout’ implies that the breaker is open without any automatic pending close. The first golden rule is that the dead time/open time/auto-reclose time is to allow the fault to clear. This time is selected based on the expected type of fault that is to be cleared.

Examining a rural overhead network, lightning is the most common fault, followed by vegetation collisions. Lightning can cause external flashovers on insulation, initiating an arc that is then sustained by the power frequency voltage on the network. Since lightning is exceptionally quick, the first trip of a rural ACR is typically as fast as possible.

After this first interruption, the following dead time needs to be long enough for the ionized gas caused by the lightning strike or flashover at the fault location to dissipate. This would typically be two seconds or less. If this first reclose is too quick, the ionized gas could cause a restrike of the fault. Since the first trip in our sequence is for lightning strikes, if the ACR then recloses onto the fault we can declare with a fair degree of confidence that the fault was not caused by lightning. At this point, the next most likely candidate is a foreign body on the network such as an animal or a tree branch. This is the reason that the second and third trips in a reclose sequence are usually slower Inverse Time Delay curves, usually of the ANSI or IEC standardised variety.

These slower curves allow fault current through the recloser for a short time interval to, quite literally, burn the fault off the network. These inverse time curves trip faster on higher fault currents and slower on low fault currents, effectively acting as an equipment stress threshold that allows for the maximum energy to be delivered to a fault location to attempt to burn off the fault without causing physical damage to the network. Since energy is being provided to the fault, the second and third reclose times typically tend to be longer (in the 5 to 10 second range), allowing time for the incinerated debris to fall away from the line.

The arcing energy associated with this phenomenon has been the subject of bad press in recent years, but the reality is that networks are designed to sustain the temporary overloads, and ACRs are designed to keep the network in safe operating conditions.

Technologies that purport minirecloses for testing fault presence don’t add any value – we already know the fault is there. We need to clear it, and the way to clear it is to deliver energy safely to the fault site. If you were simply to test for the fault, it would obviously be there; the point is to remove the fault.

Most often if a reclose sequence makes it past the third trip, the fault is likely to be a permanent bolted fault, at which point reclosing does very little anymore and field service becomes genuinely necessary. If the recloser trips three times, utilities generally set their ACRs to go to lockout, or conduct one final fast trip attempt. However, at this point in the sequence we are clutching at straws and it is probably time to send the team to find out what happened on the distribution line.

The sequence in urban areas is similar, but with a few key differences. In general, in cities where buildings are higher than the power lines, the most common transient fault is less likely to be a lightning strike and more likely to be a foreign body on the network. In built up areas, the most common deployment is a three shot to lockout system with a first fast trip. The reclose times are the same as for those deployed in the countryside.

Furthermore, in urban areas utilities often forego the use of SEF Reclosing and simply set it to a single shot to lockout. This is due to the likelihood of SEF being triggered by conductors lying on the ground, and this risk is of course much higher in an urban environment.

There is not much use in reclosing on SEF in an urban environment when the suspected reason would be a cable on the ground. Now, if wildfire risk is even a consideration, it would be best to follow the published ‘Energy Safe’ guidelines, which indicate that Reclosing should be turned off when the fire risk is high. The industry guidelines advocate that reclosing makes sense: it must simply not be used on high fire risk days. No form of reclosing would mitigate this risk and single shot tripping would be the most appropriate protection technique during this period.

“Auto-reclosing is one of the earliest forms of Distribution Network Automation,” reports NOJA Power Group managing director Neil O’Sullivan. “Even in today’s extensively automated smart grids, auto-reclosing is still one of the most effective forms of automation an electricity utility can deploy today.” In summary, the principles behind configuring auto reclosing and the associated timing arises from the practicality of running a distribution network. While every situation has its unique merits making ‘cookie cutter’ standard settings difficult, with a little forethought and field experience it is possible to achieve significant reliability and economic advantages with some fairly simple settings. ESI

About the company

If you wish to comment on or have unanswered questions about this recloser rationale, you are encouraged to give the team at NOJA Power a call. NOJA Power has been manufacturing Automatic Circuit Reclosers for many years and is armed with decades of experience in outdoor circuit breakers. The team will be delighted to share some of their experience to assist you in achieving a safer and more reliable distribution network for yourself and your customers.

www.nojapower.com.au | sales@nojapower.com.au | @nojapower

This article first appeared in ESI-Africa Edition 1, 2019. You can read the magazine's articles here or subscribe here to receive a print copy.

About the company

If you wish to comment on or have unanswered questions about this recloser rationale, you are encouraged to give the team at NOJA Power a call. NOJA Power has been manufacturing Automatic Circuit Reclosers for many years and is armed with decades of experience in outdoor circuit breakers. The team will be delighted to share some of their experience to assist you in achieving a safer and more reliable distribution network for yourself and your customers.

www.nojapower.com.au