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A tale of two metals

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On the face of it, copper wins hands down on electrical conductivity and it would seem that aluminium’s only advantage is its low density at just 30% of copper.

But, and there’s a large ‘but’, copper is, and always has been, very expensive. By any standard, metal prices are volatile – sometimes wildly so. Copper has gone from an historic low in 1999 to today’s high level, but it has always been much more expensive than aluminium. In fact, the ratio is generally about 3:1 which has a couple of significant impacts on the railway industry.

First of all, of course, there is the cost of raw materials in any signalling scheme involving power distribution. But secondly, there is the continuing high cost of disruption because of attempts, successful or otherwise, by villains to steal copper cable. High metal prices mean high scrap prices and for many the temptations are just too great.

As several articles in The Rail Engineer have outlined, there are strategies to mitigate the risks of theft. Including identifying strips in the cables, the use of Smartwater, burying cables at depth, there are all sorts of measures but not everywhere can be covered.

The cunning plan

Tahir Ayub, Network Rail’s senior design authority engineer working within the national signalling innovations group in Infrastructure

Projects, is on the case. Last December you may have read our article on his successful launch of Network Rail’s strategy to use class II based signalling power supplies which reduces the copper required in distribution systems by a third.

In the concluding seconds of our interview with Tahir, he alluded briefly to an ambition to eliminate the remaining copper and that he had a ‘cunning plan’. We were sworn to secrecy because, at that stage, the plan had not been announced to the industry. He shared the overall strategy at a signalling suppliers’ conference late last year. Since then he’s been working to firm up on detail and so we can now reveal what’s in his plan.

“This is a particularly ambitious project. Our driver is firstly to reduce our cost base in line with the Regulator’s target of a year-on-year reduction in signalling costs. That is set at around 5% and, if we’re able to deliver our plan to eliminate copper, then we may make a contribution of around 20% to the target. So this presents a significant business opportunity for reducing the cost of our overall signalling systems. That’s our ‘tier one’ benefit. We’ve also been living with the menace of cable theft and, although we’ve adopted a range of offsetting measures, nobody has had a discourse about getting rid of copper altogether. This is probably because it’s not as easy as it sounds.”

Real-life, long term experiment

Picture of Prototype Soft Start Circuit [online]Indeed it is not as simple as it sounds. Switching over to aluminium has been tried already. Back in the seventies, oil prices hit the roof and many other commodity prices followed suite. The same imperatives worked on the railway industry then and the aluminium solution was tried out. The railways were not alone. The general power industry embarked on the same course. It is easy to say that mistakes were made then, but the move was a bold one at the time.

Some of the original, solid core cables are still in use today and effectively the railways have benefited from the practical results of a real-life, long term experiment. There have been problems with material weathering, water ingress and connectivity issues. There is galvanic action where the two metals connect. This, along with thermal movements, leads to high-resistance connections and reliability issues.

The outside world set about solving all the issues, but seemed to concentrate on high voltage cabling. This includes anything above 1,000 volts. So for high distribution voltages – 400kV, 33kV, 11kV, 6kV – aluminium is still widely used, and used successfully.

New system design strategy

“What we’re proposing is a new solution that integrates aluminium. The cable is only one aspect of the overall solution. It’s an important part, but there’s a wide raft of other design methods, process improvements, products and tools that go around it that make the whole thing happen.

“We can’t get rid of copper by just replacing it with aluminium. That’s what’s different about this workstream. It’s a complete system overview and overhaul that allows us to look at the removal of copper. There’s a new set of design rules that we’re going to develop which will be underpinned by a new system design strategy. As part of our engineering work, we will prepare a case which demonstrates that what we’re proposing is safe, operable and maintainable and will give an asset life exceeding 40 years.”

Cables sizing involves making sure that there is sufficient capacity to take the required current, but there are also issues involving the ability to react to faults and the need for protecting devices to operate correctly. With cables of anything up to 20km long, this can have the effect of increasing the cable size needed.

An aluminium cable is naturally larger than its copper equivalent. To give an idea, the equivalent of a 50mm2 copper cable is a 95mm2 aluminium one and, as cable sizes increase, so handling and installing difficulties increase. Cables above 120mm2 or 150mm2 become more impractical to install.

Supply point regulation

Tahir’s team has come up with a range of measures that offset the increase in cable size. This includes not only new cables but also new protection devices, voltage regulators, booster transformers, soft starts for point machine motors and signalling transformers with enhanced inrush characteristics.

One of the challenges for the railway network is that the supply voltage from a utility is typically 400 volts but, depending on where that supply transformer is positioned, there can be variations. For example, if it’s near a steel works or an industrial network, there can be significant fluctuations in voltage. So instead of it arriving at 400 volts, it may only be 380 volts. That’s still within the tolerances of the supplying utility but those supply variations really have a significant impact 20km down the railway. The hope is to offset and control such fluctuations by providing regulation devices at supply points so, regardless of the supply voltage, there will be a pre-defined voltage to start the distribution. In that way, the losses in the system can be controlled.

While losses along long lengths of cable cannot be avoided, having sets of strategically positioned booster transformers is being proposed as one way of further offsetting the increase in cable sizes.

Soft-start circuits

Point machines and level crossings are a challenge too. When called by the signalling system to set a route, some of these larger signalling loads cause significant start surges which can exceed three times the running current. Level crossings typically have four barriers and some routes may demand up to a dozen or so machines to operate all at the same time.

Cleveland Cable Aluminium Cable [online]
Photo: Cleveland Cable Co Ltd.
Stranded Aluminium, XLPE, FGT, PVC 0.6/1 KV
Existing power system design methodology has to ensure the voltage at location cases for all signalling objects remains in tolerance in the presence of frequent surges from motor starting. This leads to an increase in the size of signalling feeder cables and power source equipment to allow for the presence of the surges. There are also long term operational impacts which result in stress and wear on the associated DC contactors and motor brushes.

Tahir is proposing that point machines and level crossing barriers are fitted with a soft- start device in the motor circuits to mitigate against over-sizing of the power system.

A longer life for motor brushes and motor contactors, together with reduced power distribution cable and power source size, is a target worth consideration. A couple of the suppliers are excited because there may be global opportunities for this.

In some places, the initial power surge for point machines is provided by a bank of batteries. These need to be charged and replaced when their condition deteriorates. New battery chargers are being considered along with, possibly, super-capacitors. This is technology that is appearing in hybrid cars and buses allowing them to accelerate without draining their batteries. There could be some opportunities to look at what’s going on in those sectors and draw some lessons.

Another piece of kit that can demand a high initial power surge is a transformer. The ambition is to move towards even lower inrush transformers so that, when power is switched on and strings of transformers all start up, there is not the need for a large cable size just for the initial surge – before even any load is taken up.

Flexible cables

Back in the seventies, the aluminium cables were solid core construction and pretty inflexible. They were difficult to install, especially at under-track crossings. This led to difficulties making good connections and so to reliability issues. Water used to find its way into cables and cause further problems.

The new generation of cable may be a combination of stranded and solid conductor – to make them more flexible – and will incorporate a technology known as ‘water blocking’. This is something used as a matter of course in high voltage and telecommunications cable, but it doesn’t seem to be used in low voltage power cables. The technique involves using a gel that is sealed within the cable as well as a water blocking tape.

The new flexible and weatherproof cable is, of course, very light so it is possible to get much more cable on a drum of the same weight. This increases the length of cable on a drum which increases the amount of cable that can be laid in possessions.

Connection solutions

So, does this cable actually exist? Well yes, it does. A number of suppliers are currently at the late stage of finalising their developments. “I think it’s worth making the point that this is a cable that has not been seen before, and that it embraces much of the technology that has been applied in high voltage aluminium cables which we’re now introducing into low voltage cables. We’re looking to develop cables from 16mm2 to 150mm2.”

A range of specific connection solutions will be developed along with the various cable sizes. That wasn’t done when aluminium was installed in the seventies when the connection devices were the same for both copper and aluminium. The idea now is to use some of
the jointing and termination technology that’s available for high voltage cables and OLE applications and adapt it for the low voltage equipment. Nothing new is being proposed. It’s a straight migration from what is available at present in the conventional power industry.

The suite of solutions to support the introduction of aluminium even includes the cabinets containing terminations or other devices. These have to be designed to accommodate the new connecting devices and must take into account the flexibility characteristics of the new cable.

Test site

Where will the cable be – quite literally – rolled out? Tahir hopes to carry out an industry trial at the Network Rail Leicester test site. That will give everyone the opportunity to get a feel of the new solution including all the new products. Soon after that it may appear on a couple of projects both of which are developing aluminium options as a shadow scheme, before being rolled out through the infrastructure.

Three messages

To conclude this tale of two metals, Tahir has some final messages.

The first is to the project managers across the industry: “You’re all being challenged to reduce your costs, so this is something that’s relatively quite simple that delivers huge cost savings. We in the Network Rail IP Signalling Innovations Group are going to develop a whole power solution which will be supported by a number of products, so you don’t have to take any risks.”

The second is to the supply chain: “We are seeking collaborative partners and manufacturers to support the development of enhanced unarmoured cable, aluminium termination and connection solutions, booster transformers, voltage regulators, battery chargers, super capacitors, DC soft starts and pluggable solutions for 650V power systems”.

And then to the villains who steal Network Rail’s cables (and who probably don’t read this magazine): “Don’t bother with this cable. It just ain’t gonna be worth nickin’ at all!”

Rail Engineer is the leading independent quality monthly magazine for engineers, project managers, directors and leading rail executive decision makers. Head to www.railsubs.com to make a free subscription to RailEngineer magazine or one of its sister publications.



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