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Metros & Urban Rail

Alternative Traction Power Distribution Systems within the Urban Environment

Ralph Moulang

The means of distributing traction power to light rail vehicles (LRVs) has remained largely unchanged for more than 100 years: overhead contact lines (OCL) and ground-level conductors, the latter being used only in fully segregated systems.


In this article, Ralph Moulang, Aurecon's Principal Overhead Wiring Engineer, discusses how cities can integrate new and existing light rail systems into the urban environment, while meeting the requirement for continuous uninterrupted tractive power supply for electrified rail systems.

They have remained unchanged mainly due to their relative simplicity and concomitant reliability They suffer, though, from the fact that they represent mature technology in a rapidly modernising urban environment.

The question that I hear more often is this: how do we integrate new and existing light rail systems that are safe, reliable and visually enhancing into the urban environment, while meeting the basic technical requirement for any electrified rail system, which is the availability of a continuous uninterrupted tractive power supply.

The Options

A simple internet search will reveal several futuristic solutions to this issue; however, the problem with the future is that is keeps turning into the present.  Fortunately, some of these systems are available now and can be grouped according to the method by which they transfer traction power to the LRV and from the LRV to the traction motors:

  • Electromagnetic Induction
  • Segmented Direct Ground Contact
  • On-board Capacitors / Batteries


• Electromagnetic Induction

This system comprises a continuous primary circuit formed from power cables buried between the rails, producing a strong magnetic field. A power receiver system (coils) is mounted under the LRV, which converts the magnetic field to electrical energy.

The ground-level components are not visible and are unaffected by weather.  As there is no direct contact between the power source and the LRV, the traction components are not subject to wear.This system has been employed by Bombardier as part of their PRIMOVE system, which features 8-meter cable conduits powered only when the LRV is present, making it safe for pedestrians and other vehicles, and mitigating the effects of EMC/EMI emissions.

• Segmented Direct Ground Contact

This system is similar to a traditional ground-level conductor system, as traction power is transmitted along a segmented conductor rail embedded between the rails and transferred to the LRV via a collector shoe. 

The difference and key feature of the segmented system is that each segment is energised and de-energised as the LRV moves along the track, thereby transferring continuous power only where required.  The process is made possible by automated sequential switching of the energised segment.

As this system relies on ground-based contact, it is susceptible to adverse weather conditions, including standing water, ice and snow, as well as natural (sand and leaves) and manmade contaminants.

Both Ansaldo STS’s TramWave and Alstom’s APS system make use of this principle.

  • TramWave uses a box segment system, which houses both the feeder and return cables.  Each segment is mechanically energised by the raising of contact plates fitted along the upper surface of the box.
  • APS operates on much the same principle, but the conductor rail segments are energised by means of electrical switching.


• On-board Capacitors / Batteries

The basic principle is an LRV fitted with a self-contained power source capable of operating a normal service with only short and infrequent recharge requirements.  Thus, the LRV can operate in all environments, since adverse weather or other topographical features shouldn’t inhibit power transfer.

The basic principle is an LRV fitted with a self-contained power source capable of operating a normal service with only short and infrequent recharge requirements.  Thus, the LRV can operate in all environments, since adverse weather or other topographical features shouldn’t inhibit power transfer.

Although huge advances in capacitor and battery technology have been made in recent years, achieving a balance between range and weight remains the key challenge, as it is with the private electric vehicle sector.

For this reason, although many LRVs already make use of on-board storage as part of their system, few employ the principle as the sole means of providing power to the traction motors.  One of the advantages, though, of adopting an LRV fitted with this technology is that as the power unit efficiency increases, the old unit can be replaced without replacing the LRV.

  • CAF’s Rapid Charge Accumulator ACR system can be fitted to their LRVs, which provide sufficient power for normal operation between tram stops and can recharge within 20 seconds
  • Siemens’ Sitras HES the hybrid energy storage system for trams. Sitras HES combines a double-layer capacitor and a battery to enable a tramcar to travel up to 2,500 meters without an overhead contact line
  • Alstom Citadis LRVs in Nice contain NiMH batteries, enabling them to traverse two 500 metre long historic squares, and the Paris T3 LRVs are fitted with 48 supercapacitors as part of their STEEM project, with a 300 metre range.
  • Kinkisharyo e-Brid LRVs employ lithium-ion batteries, and claim a range of up to 8km on the batteries before needing to be recharged.

The common feature of these systems is that recharge can occur at stations or while the LRV is running on traditional OCL, which is the basis of the hybrid approach, whereby a traditional LRV, with standard pantograph, is retrofitted with an on-board storage system.

The Solution

Does this mean overhead wires will be a thing of the past on light rapid transit (LRT) systems? 

That depends upon several factors, not least of them being funding.  The challenge for infrastructure owners and operators alike is to find the right system solution to suit their particular requirements. 

This presents an opportunity for competent engineering consultancies to provide prospective clients with timely advice and impartial recommendations at the future-proofing and planning stages, through to the ability to conduct detailed comparative analysis and system design.  This is particularly important when one realises that most of these system solutions are proprietary, and would require a long-term commitment to a single supplier.

A few examples of practical applications of some of these alternative technologies are:

  • Removal of existing OCL at critical locations, including unsightly ‘spiderweb’ arrangements complex intersections and on tight curves
  • Simplification of feeding arrangements at tram-train crossings
  • OCL free depot yards and maintenance facilities, providing a safer, cheaper and less maintenance intensive system
  • Reduction in the overall visual impact of LRT systems

These systems represent the latest technology in terms of traction distribution for LRVs.  And as is the case with all new technology, if it proves to be effective, costs will reduce, reliability will increase, and we can look forward to a safer, more visually attractive public transport system.

Sed fugit interea, fugit irreparabile tempus, singula dum capti circumvectamur amore, Georgics by Virgil, 29 BC.

"But meanwhile it flees: time flees irretrievably, while we wander around, prisoners of our love of detail."

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