Methods for distributing traction power to light rail vehicles (LRVs) have remained largely unchanged for over a century, which can make it a challenge to incorporate them into modern urban environments.
The primary methods still consist of overhead contact lines (OCL) and ground-level conductors, with the latter being used only in fully segregated systems. They have remained unchanged mainly due to their relative simplicity and concomitant reliability, yet are at a disadvantage as mature technologies in a rapidly modernising urban environment.
The question that I hear with increasing frequency 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.
There are several futuristic solutions to this issue. The problem with the future, however, is that is keeps turning into the present without these future solutions actually coming to fruition. 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
This system involves a continuous primary circuit comprised of 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 into electrical energy.
The ground-level components are not visible and are unaffected by weather, and the cable conduits are powered only when the LRV is present, making it safe for pedestrians and other vehicles while mitigating the effects of EMC/EMI emissions. As there is no direct contact between the power source and the LRV, the traction components are not subject to wear.
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 vulnerable to adverse weather conditions, including standing water, ice and snow, as well as natural (sand and leaves) and man-made contaminants.
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.
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 needing to change the LRV.
There are several competing systems, each designed to accommodate a specific application. Technical specifications include a 20-second recharge, and between 500 to 2,500-metre wire-free running. 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.
Whether this means overhead wires will be a thing of the past on light rapid transit (LRT) systems depends upon several factors, not the least of which is funding.
The challenge for infrastructure owners and operators alike is to find the right system solution to suit their specific 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 at 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 decline, reliability will increase, and we can look forward to a safer, more visually attractive public transport system.