Introducing DC electrified tramways into city centres means existing establishments may be exposed to a number of issues relating to electromagnetic compatibility (EMC), which include: Stray DC, Radio Frequency Interference (RFI), DC and quasi-static magnetic fields.
Stray DC is the description for traction current that returns to the substation via unintended paths. If not controlled, this current may flow back to the substation via the track ballast, ground and any buried metallic structures or services. Over a prolonged period, corrosion may result, ultimately leading to failure of the structure. This is a well understood problem and is managed through the development and implementation of a stray current management plan.
RFI from trams is controlled by applying good EMC engineering practice through design and build. Compliance with railway EMC harmonised standards is demonstrated by a programme of emissions testing in accordance with BS EN 50121-3-1. The immunity of trams to external sources of RFI is controlled in the same way but with testing of onboard electrical equipment in accordance with BS EN 50121-3-2.
However, the third of the issues listed above, DC and quasi-static magnetic fields, often requires innovative design solutions. Although the EMC Directive 2004/108/EC covers all EMC phenomena regardless of frequency range, the supporting EMC harmonised railway standards (BS EN 50121 series) do not specify limits for DC and quasi-static magnetic field emissions. This presents a problem as compliance with the Directive (and the implementing UK legislation) is a legal requirement. However, with no emission limits specified in the harmonised standards, how do we demonstrate compliance?
For DC and quasi-static magnetic fields, each tram system is considered on a case by case basis. Third parties such as hospitals, universities and research laboratories which may use equipment susceptible to sudden variations in the background DC magnetic field need to be identified. During the planning process, engagement with these local stakeholders is crucial and some may raise formal objections on the basis that it could affect the operation of their equipment.
Once these third parties have been identified, it is necessary to obtain specific susceptibility information about their equipment – for example, the limits on tolerable variation in DC and quasi-static magnetic fields over a specified period of time for each item of equipment. This becomes the baseline specification.
If possible, it is useful to obtain magnetic field emissions data from similar tram/traction systems by taking measurements along the route under various loading conditions.
This data can be used in a model of the traction system to predict the magnetic
field under various load conditions and design options.
With the limits agreed, options for the design are then reviewed and this can typically involve the following:
- Reroute the tram alignment: The amplitude of the DC and quasi-static magnetic fields reduce with distance. This in effect separates the source of the interference (the tram/traction power system) from the sensitive equipment. This may not be practical for all schemes.
- Redesign of conductor configuration: The large loop formed by the overhead contact wire and the running rails produces a correspondingly large magnetic field. Reducing the size of this loop can significantly reduce the amplitude of the magnetic fields. One way of achieving this is to run parallel feeders alongside or underneath the running rails. This option, together with other modifications to the traction power system, might be sufficient to ensure electromagnetic compatibility.
- Provide shielding: Shielding the sensitive equipment may be feasible, either using passive systems (i.e. placing the sensitive equipment in enclosures which are built from materials which attenuate magnetic field variations), or active systems, where the external fields are detected by the active cancellation system and opposing fields are generated to counteract them.
- Relocate affected equipment: If all other options are unviable or not cost-effective, then relocation of sensitive equipment to alternative locations may be considered.
- Reduce the traction current demand: The magnitude of the magnetic field disturbance is proportional to the variation in traction current and hence related to the magnitude of this current. Current limiting via automatic means or an operational procedure, for example a speed limit, may be considered to mitigate a specific case.
- Overhead line free traction: Modern onboard energy storage systems can enable trams and LRVs to run for short distances without contact with the overhead line, possibly providing a viable solution.
However, the additional vehicle cost and complexity may be prohibitive and this will not entirely eliminate electromagnetic emissions, so care would be required to ensure that the requirements were met.
In our experience, all problems arising from the introduction of light rail networks can be resolved through one or more of these solutions. However, as each system must be dealt with on a case by case basis, early engagement between the design team, the client and those likely to be affected by electromagnetic interference is crucial.
Feature originally appeared in Tramways & Urban Transit – July 2015 issue (931).