Reclosing on underground systems

Reclosing on underground systems - the damage to distribution feeder lines

UK network operators are resolving faults on underground feeder lines by closing and energising potentially faulted lines. What is the result?
Published: Mon 11 Sep 2017

In the UK, data from distribution network operators suggest there are, on average, 8,000 faults on underground cables each year.

For a network operator wanting to avert a period of downtime, the reaction is to close in on the fault and see if it holds. And for every one in 10 or 15 times, it does, says Christopher McCarthy, Managing Director, EMEA, at S&C Electric Company.

“As a network manager, you don’t want to lose an opportunity to save time,” McCarthy says. “You think you've fixed the problem and you have to close in to energise the circuit, and the only way to do that is by closing the recloser or the breaker. Every distribution network operator has probably done it at some point.”

The downside of repeatedly closing into a fault is the potential to cause damage to the feeder line, thereby creating a secondary problem and extended maintenance downtime.

Trade journal Energies notes in its March 2016 issue that a key disadvantage of reclosing manoeuvres is “the fact of creating significant and extensive damage in the cable, being the most probable consequence to have to replace it entirely… This circumstance will keep the distribution line out of order for a long time while the cable is replaced.”

McCarthy attests to seeing damage done to underground cables caused by reclosing and closing practices. “Distribution network operators are expecting to close and just restore load service to everybody, but instead they get 6,000 amps of fault current flowing through the line.

“This creates intense heat at the point of the fault and can cause fires. It also puts extra stress on all the power system components leading up to the fault point.”

Pulse closing technology

The problem is compounded for underground feeder lines because a short circuit somewhere along the length of the cable will create a fault arc and mechanical forces similar to those on overhead lines, but without the room for the line to move.

“Passing fault current through a line creates a magnetic field, and it is the polar repelling that creates quite a bit of movement,” explains McCarthy. “Imagine taking a paperclip and bending it back and forth. Eventually, you weaken the metal and it breaks.”

McCarthy describes how S&C has seen many recorded incidents of splices failing on the overhead lines when there are faults, “and distribution network operators will relate to that”. One utility client, he says, had about a thousand splices that were burning or breaking each year and falling down.

McCarthy concedes that distribution network operators don’t want to be closing back into faults but they have had to rely on manual solutions to restore service.

A new development, called pulse closing technology, uses point-on-wave technology to automate fault resolution without line damage. That’s because it uses about 95% less current to test for faults.

The S&C technology, which can be deployed anywhere along the line - not just in a substation - has been used in the North American and Australian markets for 10 years but is being tested in the UK in a non-protection application.

Network operator Western Power Distribution (WPD) late last year completed a study designed to expedite distributed generation connections through monitoring fault-level conditions in real-time.

The goal of the study, as part of the £17m FlexDGrid project to accommodate more low-carbon generation in the UK city of Birmingham, was to gain a practical understanding of how close simulated fault levels are to actual values.

WPD deployed S&C’s Pad-Mounted Style IntelliRupter PulseCloser as part of the field-trial network to manage the connection of a distributed generator, first through an open-loop control and then through a closed-loop control.

The key requirement was for the fault interrupter’s pulse closing technology to deliver a pulse of current and change of voltage that delivered an accurate representation of the system fault level, without disturbing connected customers, states WPD’s close down report ‘Implementation of Active Fault Level Managements Scheme’.

The report concludes that S&C’s active fault level management (AFLM) system, which was tested on a single substation for a 12-month period, “has proven to deliver a reliable and repeatable artificial disturbance on to the 11kV network”.

Although WPD’s 11-unit IntelliRupter PulseCloser trial is not in a protection application, such as on underground feeder lines, S&C’s McCarthy says it provides a good reference that the solution works.

Though it is difficult to quantify the value of pulse closing technology, he uses the analogy of the effect of sleeping and eating well and exercising regularly on a person’s health.

“You know it will extend your life, but you don't know by how much,” McCarthy says. “It's just the right thing to do.”

Distribution line fault solutions - webinar

In a pulse closing technology webinar on Engerati, 'Reclosing on 10-38kV underground circuits', where McCarthy goes into more detail on the use case of the technology on underground feeder lines. He says the audience is likely to have data to support the contention that reclosing on underground circuits is causing more damage.

“There's a lot of energy in the power system, and when it comes out in an uncontrolled way during a fault, it destroys the equipment in the surrounding area,” says McCarthy.

“That's where the pulse leads to better reliability. And by interrupting the faults mid-line instead of at the head of the feeder, it can help the engineers or technicians find where the fault is quicker, too.”

Related Webinar