Since the 1880s, tramways have made use of overhead contact wires to supply them with their lifeblood – electricity. Since the earliest German trials and the subsequent work of US pioneer Frank Sprague that need has remained the same. Yet while trolley poles have, almost universally, given way to pantographs, a reliable, uninterrupted flow of electric power from the lineside substation to the moving tram remains the key to mobility.
The most popular method of tramway and light rail electrification is to employ an energised contact wire suspended along the line of the tramway onto which a pantograph mounted on the roof of the vehicle is pressed. This contact is a sliding surface, with a force of around 100 newtons (10.5kg/22lb) and a tramcar speed of up to 80km/h (50mph).
The power circuit of any direct current-supplied rail system has six fundamental elements: the substations, which supply direct current at the line’s designated nominal voltage (normally 600V or 750V); the positive conductor (the overhead line); the positive distribution network; the load (the vehicle); the negative conductor (the rails, through which the current is returned to the substations); and the negative distribution network. In most cases the rails are bonded together at regular intervals, and bonded or welded at joints, to provide as low a resistance path as possible for the return current.
The past decade has seen massive advances in traction supplies that allow the removal of the overhead line – such as onboard energy storage, magnetic or inductive pickup or switched third rail systems. While these technologies are now relatively well-developed and proven in many cities, they are still mainly used for relatively short sections of line rather than whole systems. Even then their use is determined by various factors such as topography, geography, aesthetic appeal, height clearance for special events, economics and, often, political factors.
Yet as each new system is different, any common practice and ‘standardisation’ is more likely driven by market forces than by common characteristics.
Anyone involved in light rail OLE design needs to study historical and worldwide practice to gain a full appreciation of the technology. ‘Re-inventing the wheel’, or working entirely from first principles, is seldom appropriate in a field with a rich abundance of previous experience. Railway principles cannot be indiscriminately applied. There is, however, plenty of scope for imaginative and artistic enhancement based on established principles.
KEY DESIGN PRINCIPLES: Creating an elegant, efficient and cost-effective overhead power supply is as much an artistic endeavour as it is an engineering one, with a few key principles that require observation: Avoid visual impact clutter wherever possible, through rationalising and sharing of facilities. If equipment is required in the public space, it should either be disguised, or made into a feature; Visual impact of lineside equipment can be further minimised by integration into existing or new buildings and structures where possible or placement underground; Colour, design and equipment placement can be a key element where stakeholders can be engaged early to help generate positive project support.
Similarities and differences
Tramway and light rail overhead designs share the basic laws of physics with their main line railway counterparts, but require more detailed design and calculation due to their operating environment and development. Other infrastructure is often not present at the start of the scheme – even the proposed alignment may not be available until obstructing buildings have been demolished or new structures created – and the gradients involved are also likely to be steeper with more severe vertical and lateral curves.
Finally, any overhead system will be in full public view so must be designed as part of the overall project aesthetics and ‘fit’ seamlessly within its surroundings. This is particularly relevant in cities with a strong heritage or cultural identity where visual intrusion is an important consideration. This integration is largely achieved through the colour, shape, or style of key components, but by far the biggest effect is achieved by minimising the amount of electrical equipment that cannot be cost-effectively hidden or buried underground.
Service speeds and desired headways also make a difference as higher-speed suburban sections (often using old railway rights of way), can relinquish aesthetic considerations so the finished system may exhibit more of the features associated with the main line railway when it leaves built-up areas.
On a typical design-and-build project there will be many interfaces that the overhead designer must manage. As well as the obvious interactions with trackwork and civil disciplines, many other groups will become involved – local authorities will have an input to the location, style and colour of street items, heritage groups will be involved in attachments to historic buildings, and the emergency services will have a say in emergency isolation procedures.
It might be expected that tramway overhead line is an electrical design concept, but in reality this is a fairly straightforward task once a few basic parameters are known.
The first is to establish the required along-track resistance. This is predominantly a simulation process, where substation positions and line resistance are decided, taking into account the desired service density and the loss of power along the line due to electrical resistance. The number of substations required is directly related to the planned frequency and the amount of overhead line to be provided. Simply put, a greater number of substations means less copper but as substations can be expensive, both in terms of equipment and land usage, a balance between the two will give minimum system cost and optimum operational flexibility.
That said, substations are usually required at intervals of 2-3km along the route. Each is a fully-enclosed building with cabling ducted underground; due to internal heat generation, it is necessary to provide for ventilation. It is also possible to place a substation in an easily accessible location within an existing or new building, and wherever reasonably practicable this is preferable. If a new substation building is required, it should be sited to integrate into the surrounding landscape.
Wiring the ‘Up’ and ‘Down’ lines separately allows the flexibility of single-line operation in the case of a failure; but for tramways in tight urban locations this is not normally possible without major traffic disruption and so the opportunity is taken to bond together both lines of overhead. This provides parallel paths for the current and can reduce the overall weight and bulk of the overhead line requirement.
The overhead must be separated along its length by insulators and isolator switches to allow for maintenance and emergency working. This is achieved by splitting the line into sections, separated electrically by insulators that can be powered independently.
System and layout design
Once the approximate power requirements are established, the next thing is to design the basic overhead line system. For tramways the key principle is to minimise the equipment in the air, reducing the loads (both static and ice/wind loads) on the poles and foundations while also reducing the risk of theft and vulnerability to vandalism or terrorist acts.
To this end most designers adopt simple trolley wire, with or without automatic tensioning.
It is desirable that the contact wire should be as level as possible, without ‘hard spots’, so that wear of both wire and pantographs is minimised. Hard spots can result in pantograph bounce, and arcing. The contact wire should be flexibly mounted, using span wires or shorter bowstrings from bracket arms.
All this is more important the higher the operating speed. On high-speed sections, catenary suspension is best; this minimises the number of poles required, but can be visually more obtrusive. The catenary also carries current, so reducing resistance. In street-running sections, speeds are lower and simple suspension from span wires suffices. The contact wire can be suspended from the span with short ‘bridles’ to give a softer suspension, but this may require additional register arms to maintain lateral wire position. In city centre locations where the tram movements are slower, a hanger fitted on the span wire is often sufficient.
Clearly there must be at least one contact wire above the line and one 150mm2 wire is normally sufficient for a 750V dc system running up to 80km/h (50mph). To achieve a single contact wire, a parallel feeder is likely to be required; this is laid underground. Twin 107mm2 section contact wires not only increase cost and visual intrusion against the sky, but challenges around maintaining even wear also increase maintenance cost.
Other considerations include defective pantographs that can damage long lengths of overhead line, requiring time and resource to repair, during which the service is interrupted; conversely, a defect on the overhead line can damage the pantographs of passing trams. This risk of one system damaging the other in operation means that the pantograph and overhead contact system should be designed and maintained as one entity.
Once the alignment and trackwork layouts are available, a start can be made on drawing a more exact position for the contact wire. The wire shape need not follow all the curves of the trackwork; instead a series of chords is formed between support points.
The limits of the position of the wire are defined by calculations looking at the acceptable movement on the pantograph head, allowing for track tolerances, vehicle suspension and support structure movements, and wire displacement due to atmospheric conditions. The output is the maximum lateral deviation of the wire from the track centre line at support points and at mid-span areas, depending on the position of the pantograph relative to the track/vehicle geometry.
It would be convenient to have one length of overhead contact wire stretched from one end of the line to the other, but this is impossible for today’s longer lines. Overhead line must also be sectioned electrically (typically in 1000-1500m lengths), terminating off the track line to poles or other structures.
Tensioning is achieved either by weights placed inside poles or by gas or spring tensioners; these have the advantage of lower cost and reduced maintenance. If auto-tensioning is used, overlaps ensure continuity of contact with the tram’s pantograph. Modern innovations include systems that offer remote monitoring and adjustment for exceptional circumstances such as extreme temperature changes.
In practice, there will be many revisions to the alignment in any new-build project as the design proceeds. Platform and substation locations may change and overbridge details may change; these alterations all require compromises in the overhead line design. As such, very close co-operation is required between contractors throughout the design and installation for an optimum outcome.
At each support point the contact wire must be attached to poles, span wires (either stranded galvanised steel, stainless steel or synthetic materials) or structures, which can be some distance from the track bearing in mind that the further away the fixing is, the higher it will have to be on the structure to which it is being fixed. If long span wires or pull-offs cross an urban space, which may be required from time to time for temporary features such as Ferris Wheels, temporary block-mounted poles can be substituted.
The positioning of permanent poles can be particularly challenging, with spacing being a key consideration. Locations must be found where they are not likely to be knocked down or damaged – the presence of large, fixed obstacles near road and tramway intersections can significantly worsen the consequences of a collision between a road vehicle and a tram – and a balance needs to be found so that they can be placed as far apart as possible to limit installation cost, but not so far apart that the wires sag excessively.
Intelligent and considered design means that poles can be multi-use and enhance the urban realm by providing additional functions such as supporting street lighting, road traffic signals and signage. To maintain safety, suitable distances of usually a metre or more are necessary between high-voltage tramway equipment and other equipment and accessories mounted to the poles.
Poles must also be placed in optimum locations relative to bridges and stops, due to the wires’ design in these areas, but elsewhere the intention is to achieve an even spacing. Where there is a strong visual axis along the length of a street, the rhythm of the poles should be considered for the street as an entity.
The number of supports can be minimised by means of ‘bridling’, a technique which may also be used in the conventional horizontal plane or the less conventional vertical plane to enable fixings to be made to buildings clear of architectural or decorative features.
The number of poles can also be reduced by supporting two sets of wires from one pole, placed between tracks. This solution results in a well-balanced design that puts all overhead equipment in the track area
and away from pavements and walkways. As the poles are evenly loaded, they may be of modest size. The next best approach is to use double-track bracket arm, where one pole supports both lines from one side, although this must necessarily be of larger diameter and height as it is loaded unevenly.
Span wires need to be positioned where high road vehicles will not hit them, and should be schemed so that if individual wires are damaged or poles are knocked down the tramway operation should be able to continue without allowing the wire to fall so low that it becomes a hazard for pedestrians.
In locations where large convoys crossing the tramway’s path are expected, solutions need to be employed to raise and lower the contact wire. An obvious choice is to use onboard traction energy storage to remove the requirement for overhead contact wires entirely, but if this is not practical or cost-effective then technology exists for extendable support structures that allow the wires to be raised and lowered by a few metres.
At complex junctions, the skill of the designer is in supporting the contact wires with minimum support structures to both reduce the possibility of obstruction of sight lines and for aesthetic appeal.
Building fixings are always preferable for overhead wire support in urban environments, but the major challenge here is one of consent and legal formality. Permission needs to be obtained from the building owner and detailed surveys are required to prove that the structure is suitable for the required loadings. If many such fixings are desired, the process of obtaining approvals from different building owners will necessarily require agreements and interfaces with different surveyors and solicitors, often creating a cumbersome and time-consuming process.
So if wall fixings are to be used, they must be agreed in principle by the client or main contractor in the very early project design stages, undertaking an outline or reference design in advance. Further opportunities for building fixings and for overhead line rationalisation and improvement should continue to be sought throughout the life of an urban tramway. Property owners, for example, may develop a more positive attitude towards the tramway when it is in operation and benefiting them. In particular, standards achieved on extensions should be retrospectively applied to existing installations.
There are a number of foundation options for poles. The preferred option is usually steel piles, although reinforced concrete or pre-cast units that are dug into the ground are suitable alternatives that require less extensive civil works. Pole foundations are dimensioned on the basis of the forces applied by the contact wires, any additional low voltage equipment (public lighting etc), the geotechnical properties of supporting ground conditions and buildings and structures in close proximity.
If the pole base is to be bolted onto the foundation or adjustment – with advantages for future replacement – disguising the base and protective covering of the bolts must be addressed. There are all manner of ways of doing this, including decorative ornamentation and the application of street furniture surrounds.
For aesthetic and technical reasons it is important that poles never lean, or appear to lean, towards the track. They should be installed with positive ‘rake’, leaning away until their loads are applied, pulling them near to, but never beyond, vertical.
Where overhead clearance is limited, particularly under road or pedestrian footbridges, it is often sensible to fix the contact wire directly to the structure or to install a rigid overhead bar. This may also be used where the wire is required to follow a severe vertical track curve. The cost of lowering track, especially in city streets where you may find conflict with under-street utilities apparatus, has to be balanced against the cost of potentially complex future maintenance. The bar may be a standard contact wire backed up by a steel or aluminium structure, or it may be a separate conductor rail of stainless-capped aluminium. In either case the transition to conventional contact wire must be made with care.
Wire profile and hazard analysis
The next job is deciding the vertical profile of the wire. This is done with reference to the standards that define the required wire heights above the rail level on street, in pedestrian areas, and segregated line, and then there are occasional complications of allowing abnormal loads to move under the wires. These heights are relative to the track, which has its own inherent gradient profile, and the wire gradient relative to the track is a feature on which the pantograph performance depends, defined in outline in international specifications.
Registration arms set the horizontal position of the contact wire, the stagger, which is alternately either side of the centre line of the track by a set distance of usually between 75-200mm on straight track and 200-350mm on curves. The purpose of this is to ensure even pantograph wear and account for factors such as changing day and night-time and seasonal temperatures and conditions.
The result is a complex set of calculations to produce a set of wire heights at each support point. There is now enough information to produce the layout plans, showing support points, wire heights and staggers, and the position of poles, wall fixings and special supports, switchgear and ducting schematics. Modern CAD and BIM systems allow the overhead line to be entered onto the main civil interface drawing as a separate layer so interfaces can be shared with other disciplines to address potential conflicts at an early stage.
The design process is accompanied by a comprehensive hazard analysis that identifies the possible risks associated with the equipment during installation and operation. Such incidents might include the breakage of a span wire, a pole being toppled or damaged, or an act of vandalism or terrorism. Such analyses form a strong justification of the design and a good source of reliability predictions.
Safety, materials and decoration
Insulation: The overhead line for most tramway systems is energised at either 600V or 750V dc and it is obvious that all the live conductors must be properly insulated from earth and from the running rails. Tramways will usually use a double insulated system which simplifies the bonding requirements and provides the opportunity to work live on the finished installation. This practice also simplifies and speeds up emergency responses.
Modern insulators use glassfibre or composite cores with silicone moulded sheathing, and cast stainless steel crimped end fittings. These are often combined with stainless steel rope to produce a complete insulated span segment, or a support stitch. The position of these insulators is chosen to reduce the chances, should components fail, of danger to pedestrians from live equipment.
The alternative of insulating synthetic ropes avoids the need for extra insulators, and these have been used in many systems across Europe in the past few decades. Cost savings of around 20-30% per km can be seen from these examples through the reduction in additional components and a consequent saving in installation time. Certain experience has shown that these ropes can suffer from greater environmental degradation in coastal and heavily polluted environments, and so their use must be carefully considered.
Where pedestrian bridges run over overhead lines, suitable barriers such as walls, fences or railings must be positioned to avoid contact with live parts of the system. These must respect local or international standards.
Isolation: As mentioned earlier, the overhead contact wire is divided into electrical sections that can be individually switched on and off. This allows part of the network to be shut down due to an emergency or for planned work.
Such sectioning locations are related to the track layout, in particular near junctions and emergency crossovers. These isolator switches may be mounted at the pole top, with linkages down to a handle at shoulder level, but the resulting assembly is cumbersome so it is preferable to mount such equipment in a lineside cubicle that can be more easily disguised with the cables running in ducts to the poles. Where a substation is located near a sectioning point, the associated switchgear will be located within the substation building.
The section insulators themselves are the most visual of all the overhead components, and create more maintenance effort and wear and tear than any other item on the line, so there are great practical advantages to reducing their mass and size.
Finials: Finials prevent water ingress into hollow poles and come in an almost infinite array of shapes, designs and colours, from the traditional ‘spike and ball’ arrangement to a design that may reflect a local theme or the operator’s logo or corporate shape.
Materials: To reduce maintenance, components are best made from
non-corroding materials such as composites or stainless steel, or should be galvanised, and, for insulators, plastics or glass-fibre reinforced plastics or composites.
Remote monitoring: Bluetooth and wireless technology has seen the advancement of technologies for the remote monitoring of the height and stagger of the contact wire. Portable or hard-wired laser-based solutions remove the need for physical interaction with the energised wires and can be used in any weather conditions, increasing safety and reducing maintenance costs.
Finding a way forward
From the above considerations, it is easy to see that overhead line design, installation and maintenance is a complex process with many disciplines and interfaces. The final design will inevitably therefore be a compromise between civil and electrical requirements; engineering excellence and reasonable cost; first cost and lifecycle maintenance; and artistic and political aspirations.
Negotiating the minefield of competing requirements and achieving an installed system that is at the same time robust and visually appealing is not an easy one. Yet with careful consideration, engagement and plenty of forward planning, tramway and light rail overhead line equipment can indeed be a pleasant addition to any cityscape, both enhancing its unique identity and providing a reliable power supply for decades to come.
Note: This article is a compilation of material from various contributors, to show international practice, and previous TAUT articles, including two excellent pieces from former Brecknell Willis Chief Engineer David Hartland (1999 and 2011). Thanks are due to LRTA members David Holt and David Gibson for their additions.
Feature originally published in October 2017 TAUT (958).