Guest writer: Tahir Ayub
Cost reduction is one of the major transformation themes for Network Rail, and the industry imperative to drive costs down has never been greater. Engineers are working on a range of ways in which costs can be reduced, and the electrical fraternity is playing its part.
The most economical and controllable way of distributing power to the signalling system is to have a dedicated low-voltage power distribution network. In the UK, Network Rail has one of the largest low-voltage power distribution systems outside the power utility organisations. The problem is that it’s expensive – supplying power to signalling installations can be between five and ten per cent of the total capital cost of a resignalling project.
One of the reasons for this high cost is the wiring. Copper currently costs around £4.50 per kilo as a raw material – more once it has been turned into insulated copper wire – and there is a lot of copper in a signalling installation.
To reduce this cost, Network Rail moved from three-core signalling power cables to two-core by removing the dependence on an earth conductor. Now, instead of it being earthed back to the main transformer, electrical equipment is double-insulated and any metal structures are earthed locally. This is called a Class II system and it reduces the amount of cabling, and copper, by a third.
Working together with its suppliers, Network Rail has achieved significant cost reductions, and saved thousands of tonnes of copper, by developing over 400 products from 20 manufacturers in five product categories.
Safety and performance go hand in hand
Class II-based signalling power distribution systems do not rely on automatic disconnection triggered by earth fault current flow. Instead, electrical safety is provided by the selection of suitable Class II equipment which, by its construction, provides the necessary degree of intrinsic safety with regard to electric shock. A local earth is also not required to provide safety under fault conditions.
This is a radical departure from previous railway and industry standards and has required a system- level change affecting numerous sub-systems. Class II systems provide several benefits – reduced CAPEX cost without reducing safety, improvement in personnel safety, reduction in system risk (fewer power supply interruptions), compliance with standards, reduced maintenance burden, greater tolerance of DC corrosion in DC areas, simplified earthing and bonding in AC-electrified areas and the ability to integrate into legacy systems.
Over the last five years, over six million meters of Class II signalling power cable have been installed, generating tonnes of copper savings with commensurate cost efficiencies.
New system design strategy
The next phase is to remove copper from power cables altogether. That’s not as simple as it sounds – although an alternative conductive metal, aluminium, could be used, and is cheaper at around £1.50 a kilo, it’s not such a good conductor. A new approach is needed.
“We can’t get rid of copper by just replacing it with aluminium,” said Nigel Edwards, professional head of power distribution HV/LV at Network Rail. “That’s what’s different about this work stream. It’s a complete system overhaul that allows us to look at the wider implications of the removal of copper. There’s a new set of design rules which will be underpinned by a new system design strategy. As part of our engineering work, we have prepared a case which demonstrates that what we’re proposing is safe, operable and maintainable and will give an asset life exceeding 40 years.”
The proposed solution includes a new, enhanced, unarmoured aluminium cable constructed of either solid or stranded conductors with integral water blocking. This is a brand new design, seeking to address the many challenges associated with aluminium cable. But this cable is only one aspect of the overall solution. It’s an important part, but there’s a wide raft of other things – design methods, process improvements, cable termination and jointing instructions, connectors, switchgear, protection devices, distribution interface transformer assemblies and tools – that go around it and make the whole thing scalable.
Use of aluminium cables
Although cables with aluminium conductors have been manufactured for more than eighty years, they are less commonly used than cables with copper conductors for installations in commercial and domestic premises. The major users of aluminium cables are the electricity generation, transmission and distribution companies, which have been using them at all voltages since the 1930s. Aluminium cables are also widely used in railway traction distribution systems.
Electricity distribution companies widely use aluminium cables in their low-voltage systems at some stage to feed domestic dwellings. Presently, three major UK companies use aluminium rather than copper for all of their new low- voltage cable distribution systems, their feeds to the cut-out fuse in domestic properties.
Cables with aluminium conductors are also used for low- voltage circuits in the UK nuclear industry. There is even a move towards aluminium conductors being used in subsea umbilical cables for the offshore oil industry.
Due to the difference in metal price, there have been attempts to change copper to aluminium before, not always very successfully. When aluminium house wiring cables were introduced in the USA, they were not introduced as a complete system with joints and terminations specifically designed for use with aluminium cables; instead, aluminium cables were used with the existing accessories that were designed for use with copper cables. This resulted in a clear lesson that connections to aluminium conductors are less tolerant of errors than connectors to copper conductors.
British Rail installed aluminium cables in the signalling power supply system in the 1970s, when copper and oil prices were out of control. Although there have been failures due to water ingress, corrosion and at terminations, these failures have generally been attributed to the installation methods used – some of these aluminium cables may have been terminated with lugs intended for use on copper cables, for instance. Notwithstanding this, some of these aluminium cable circuits are still in use and have had an asset life in excess of 40 years, so it has been proved that aluminium can stand the test of time.
Comparing the two metals
On a size for size basis, the resistance of a cable with an aluminium conductor will be approximately 1.6 times that of a cable with a copper conductor. This higher resistance will lead to a larger voltage drop, lower current rating and higher fault loop impedance (lower fault current) than for a copper cable of the same size. The fault current withstand of an aluminium conductor is also approximately 1.5 times lower than that of a copper conductor.
Even taking these differences into account, a like for like equivalent size is still 50 per cent cheaper than a copper equivalent. This makes the aluminium cable commercially very attractive!
The effect of the higher resistance is that a larger conductor size is needed if an aluminium cable is used to replace a copper cable and other circuit parameters are not changed. The conductor size has to be increased by one or two standard sizes. NR/L3/SIGELP/27425 now sets equivalent cables sizes between copper and aluminium that can be used without any further design analysis.
Although aluminium has a higher resistivity than copper, it is also less dense. On a size-for-size comparison, an aluminium conductor will be approximately one-third of the weight of a copper conductor. This eases cable handling, such as pulling or lifting into cable troughs, and allows longer drum lengths of cable where the limiting factor is the weight of the drum.
The modulus of elasticity of aluminium is approximately 60 per cent of that of copper. Thus, on a like-for-like basis, a cable with an aluminium conductor is easier to bend than a copper conductor. Where solid conductors rather than stranded are used, the smooth surface of the solid conductor allows the cable components to slide against each other more easily. Cables with solid aluminium conductors do not tend to spring back after bending, thus making it easier to fit the aluminium cable into the correct position for terminating.
The coefficient of thermal expansion of aluminium is approximately 1.5 times that of copper. If a cable is expected to operate over a wide temperature range in service, usually due to fluctuations in load between zero and the current-carrying capacity of the cable, then additional allowance has to be made for the expansion and contraction of an aluminium cable.
In heavily loaded circuits, the higher coefficient of thermal expansion could also affect bolted terminations, and repeated load cycles are more likely to lead to loosening of the terminations than with copper lugs.
However, cables used in signalling power distribution circuits are generally selected on the basis of voltage drop or a requirement to achieve sufficient fault current rather than the day-to- day operational load. Because of this, the temperature fluctuations during operation are small, so the higher coefficient of thermal expansion of aluminium does not cause an issue at terminations. The effect of temperature fluctuations is further reduced by the requirement of NR/L2/SIGELP/27408 that a minimum cable size exceeding 16mm2 cross-sectional area is used for all circuits, eliminating the effects on the smallest, and therefore most vulnerable, cables.
Copper conductors in cables are almost universally stranded conductors, except for small sizes such as 1, 1.5 and 2.5mm2. In the UK, standard low-voltage cables with aluminium conductors have exclusively used solid conductors since the late 1960s. For conductor sizes up to 25 or 35 mm2, solid circular conductors are specified and, for larger sizes, sector shaped conductors are used.
There are two significant advantages in using solid aluminium conductors when making connections. With stranded conductors, the oxide film has to be broken on every strand to make a good connection. A solid conductor, on the other hand, has only one ‘strand’ with an insulating oxide film.
In addition, a solid conductor significantly reduces the opportunity for water to migrate along the cable in the event of damage to the cable sheath and insulation. The extents of the voids where water could be trapped at a termination are also minimised.
However, the disadvantage of using solid aluminium conductors is that they are less flexible than stranded aluminium.
In Germany, solid conductors are used up to 50mm2 CSA (cross-sectional area) while other countries, including France, Spain, Italy and the USA, specify stranded conductors. In all cases, for low voltage cables, the smaller solid conductor sizes are circular and the larger sector shaped. Although the international standard includes aluminium conductors down to 10 mm2, BS7671 does not permit conductors smaller than 16mm2 in UK installations. To limit the effect of the oxide film on joints and terminations, a minimum stranded conductor of 120mm2 CSA has been selected.
In wet conditions, when a cable’s insulation is damaged, water can enter the cable and travel for some distance between conductor strands and other cable components. If this occurs, long sections of cable may have to be cut out. In high voltage cables, water-blocking compounds are used within the cable construction to limit the extent to which water will travel along the cable after damage.
The presence of water blocking will not have a significant impact on the life of an undamaged cable permanently submerged in water. Over time, moisture will migrate through the sheath and insulation to reach the conductors, although this is generally a matter of decades. Both copper and aluminium cables are equally affected.
The above analysis has driven the development of a brand new enhanced unarmoured cable design with increased insulation and sheath thickness. Solid conductors are available for sizes up to 150mm2 CSA and stranded conductors are available from 120mm2 to 185mm2. Water blocking is a standard feature on both configurations including a new security tape that runs the length of the cable with the legend “Property of Network Rail”.
Two methods of joining or terminating conductors are generally used for low voltage cables; mechanical connectors and compression crimp technology, both used with copper and aluminium conductors and defined in British Standard BS EN 61238-1.
Mechanical connectors are generally range-taking, do not require special tools to make the connections and usually employ shear-head bolts to ensure that they are tightened to the correct torque. While they can be used at both joints and terminations, they are more commonly found in joints. Generally larger than crimp connectors, mechanical connectors will also contain voids between the conductors and the barrel of the connectors which could trap water after any flooding incident.
Crimp connectors require a special crimping tool to make the connection. For smaller sizes, this may be a hand tool but, for larger sizes, hydraulic crimp tools can be used as can hand-pumped hydraulic, foot-operated pump or hand-held battery powered tools. The crimp tools use die sets that have to be selected to match the size of the cable and the type of crimp required. Connectors can be designed such that one die set can be used to crimp connectors onto a range of cable sizes.
Connections to copper conductors are more ‘forgiving’ than connections to aluminium conductors. This is because the oxide film that forms on aluminium is insulating and it forms very quickly. Plain aluminium crimp lugs and connectors have been selected with the barrel filled with grease during manufacture to prevent oxidisation in the bore of the connector. Similarly, the prepared end of the conductor is required to be greased and then scratch brushed through the grease before inserting it in the barrel. The ‘grease’ that is used with aluminium connectors can be a joint compound that contains fine metal particles that will dig through the oxide layer on aluminium conductors. Such compounds are commonly used with overhead line connectors.
Compression lugs for aluminium cables can be either plain aluminium, tin-plated aluminium or bimetallic with an aluminium barrel and a copper palm. The tin-plated and bimetallic lugs are suitable for connection to copper or brass terminals where the atmosphere may be damp and there is a risk of galvanic corrosion between the terminal and the lug. This risk also applies to tin-plated lugs if the tin plating is damaged, therefore bimetallic lugs have been selected as Network Rail’s choice of connector.
The workstream reviewed two types of joint that are commonly used for low voltage cables – heat-shrink and cast-resin joints. The former rely on heat-shrink tubes to provide insulation, mechanical protection and seal against water ingress. Cast resin joints use heat-shrink tubes, tape, spacers or other means to provide primary insulation. A plastic joint shell is then fitted around the assembly and filled with a two-part insulating resin mix. The plastic shell only provides a mould to contain the resin while it cures; it is not relied on for mechanical or electrical protection.
Joints can be aluminium to aluminium or aluminium to copper and may incorporate a connector that allows different conductor sizes to be joined.
Heat-shrink joints are generally smaller than cast-resin joints and can usually be completed more quickly because of the time taken for the resin to cure. However, heat shrink joints are mechanically less robust than cast resin joints and are more prone to water ingress if they are not correctly assembled.
Fuses have well-proven characteristics and are traditionally used to provide both overload and short-circuit protection for cable circuits. However, the disadvantage of fuses for short-circuit protection at the supply point of signalling power distribution systems is the current required for the fuse to operate within a reasonable time and degredation of the fuse over time.
Electronic protection devices or MCCB (moulded case circuit breaker) trip units using Definite Minimum Time (DMT) devices and Inverse Definite Minimum Time (IDMT) curves can also be used to optimise cable size selection by adjusting protection curves around the available short circuit current. This is a key process change on protection design that the workstream recommends for adoption with aluminium cables.
A DMT protection curve monitors the current in the circuit and will trip if it exceeds a pre-set current for a pre-set time. An IDMT device is programmed with an inverse time characteristic such that the time it takes to operate is a function of the magnitude of the fault current. This allows the operating characteristic of the device to be tailored to match the load, overload and fault-withstand characteristics of the circuit.
In signalling power distribution circuits, cable size selection is usually based on voltage drop and achieving sufficient short-circuit current to operate the protection. Thus, the main criteria for the protective device are that it will not operate when inrush currents occur and it is able to detect the low fault levels that are expected. These criteria can readily be met with a DMT device and hence there is no requirement to develop specific IDMT characteristics for use with signalling power distribution systems.
A number of options are being considered to manage the offset in the increase in cable size. These include, 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.
While losses due to long lengths of cable cannot be avoided, having strategically positioned step-up transformers is being proposed as one way to further offset the increase in cable sizes. This will be undertaken by using Distribution Interface Transformers Assemblies (DITA), specified in NR/L2/SIGELP/27419.
The use of four-core cables rather than two-core cables may be appropriate where large conductor sizes are needed as a four-core cable would be slightly more flexible for the same total cross-sectional area. The four-core cable would have two pairs of cores connected in parallel at the terminations, giving the benefit of reduced overall cross-sectional area for the same conductor CSA.
Performance, reliability and maintainability
The engineering case is supported by a Failure Modes Effects Criticality Analysis and the mitigation measures derived from it have shown that, when the correct components are used in an aluminium cable system and the appropriate techniques are used during installation, the performance of aluminium cables is comparable to that of copper cables. The resulting conductor size for aluminium cables will be one or two standard sizes larger than with copper cables if no load optimisation measures are taken. The performance shows relative advantages and disadvantages for both cable types with neither having a single overwhelming advantage or disadvantage.
On a size for size basis, less force is required to bend a solid aluminium conductor cable than one with stranded copper conductors. Larger conductor sizes are required for aluminium cables, so the force required to bend a solid aluminium cable that has similar electrical performance to a stranded copper cable is slightly higher. Because of the reduced flexibility of equivalent solid aluminium conductors, cables with stranded aluminium conductors may be used for sizes equal to or greater than 150 mm2. At these sizes, stranded aluminium conductors are easier to bend than the equivalent size of copper conductor.
Solid aluminium cables do not have a tendency to ‘spring back’ when bent. This aids dressing of the cables at terminations and results in reduced stress on the terminations.
The advantages and disadvantages of using aluminium conductor cables balance out when compared to copper conductor cables for performance during installation. The basic design is the same for cables having copper and aluminium conductors, so the equipment required to install and repair both cable types is effectively the same.
The primary difference is the importance of following the correct procedures when jointing and terminating cables with aluminium conductors. Greater care is required when fitting lugs or connectors to aluminium conductors than copper, so jointing and terminating aluminium conductors correctly is more reliant on the skill and care taken by the installers.
Network Rail has specified a water- blocking layer under the sheath of aluminium cables has been specified. The purpose of this layer is to prevent the longitudinal transmission of water in the event of sheath damage in areas that may become waterlogged or flooded.
Aluminium cables are considered to be less attractive to thieves than copper conductors. It is anticipated that, over time, this will lead to a reduction in the disruption caused by cable theft.
For a correctly designed, installed and maintained aluminium cable system, no factors have been identified that reduce the electrical or operational safety of the system when compared with copper cable systems.
As Network Rail and its suppliers and contractors are all being challenged to reduce costs, this move is relatively simple and can deliver huge cost savings. A copper-elimination learning day is scheduled for the industry on 27 July 2017 so, with technology and processes now mature, there are no unknown risks involved in securing the benefits of delivering a cheaper and less vandalism-prone copper-free distribution system.
This article was written by Tahir Ayub, programme engineering manager for Central Enhancements at Network Rail Infrastructure Projects.