Electromagnetic emissions from rolling stock are currently a major concern for train manufacturers and railway infrastructure operators. Rolling stock is required to meet specific harmonised EMC standards and must therefore be tested appropriately.
However, these standards do not completely address interoperability issues resulting from rolling stock interference with signalling systems. These include GSM-R transmissions, and train-borne balise transmission modules (BTM) used with balise-based systems including ETCS and ERTMS. In addition, these standards do not cover representative worst-case conditions – transients generated by the rolling stock or infrastructure such as those caused by discontinuities in the feeding and track circuit systems.
Even in trains that are tested fully to the EN50121 series of EMC standards and pass, problems can arise when in service. If this should occur, the technical solutions are not always straight forward. In addition, the question of who is responsible for the lack of compliance can be difficult to answer and agree on; for example is the rolling stock manufacturer responsible due to excess emissions, or the infrastructure manufacturer for inadequate immunity?
Resolving this is especially difficult if everything has been tested to accepted standards. The length of time it takes for the field testing to solve this kind of problem and to go through the certification process again, varies between three months and twelve months, and may require a large investment by whichever party is ultimately deemed responsible.
In addition to the challenges of responsibility and technical solutions, railway infrastructure operators can suffer loss of service (availability reduction) caused by the rolling stock electro-magnetic incompatibility with the rest of the railway system. To take the worst case example, this could be safety critical signalling systems.
The TREND project (Test of Rolling Stock Electromagnetic Compatibility for Cross-Domain Interoperability) was therefore established with an objective of addressing this situation by:
1. Designing a test setup that enables the harmonisation of rolling stock approval tests for electromagnetic compatibility (EMC) which focuses not only on interferences with broadcasting services but also on railway signalling systems;
2. Identifying and designing possible cross acceptance test sites and/or methods that reproduce representative worst case conditions for both steady state and transient phenomena; i.e. a universal test setup which can be used for full system testing. These representative worst case conditions will be obtained from a combination of experiences from the project members such as those of York EMC Services and from electromagnetic modelling of the rolling stock, rail and feeding infrastructure.
The TREND project has been split into four main research areas – Spot signalling systems (such as BTM), track circuits, GSM-R and broadcasting services (including TV, Radio, Wi-Fi and GSM).
Two years in
As of October 2013, the project is in month 24 of a total of 30 months, up to which point there have been some major achievements stemming from the completed Work Packages, the sections into which EU Seventh Framework Programmes (FP7) are divided. Work Package 2 brought together some examples of situations where the current EMC standards had proved insufficient in ensuring compatibility.
The project consortium has a great deal of EMC experience and all members had some example situations to draw upon. From the results of Work Package 2, the areas of concern regarding the compatibility of rolling stock were identified, leading to the four research areas mentioned above being defined.
The next task, under Work Package 3, was to undertake a thorough investigation of the current EMC standards for railways. After a detailed analysis of the standards from the point of view of interoperability and ease of testing, the following conclusions were drawn including points which remain unaddressed in the analysed standards:
» On board emission testing – arising from concern about hand held transmitters on board the train and in the station environment;
» Categorising and measuring transient emissions – both conducted and radiated transient emissions are not accounted for in any of the reviewed standards;
» Neutral sections – identified as a problem area, particularly in Sweden, where neutral sections are not treated correctly in the analysed standards. EN 50121 makes a specific point of avoiding neutral sections;
» Radiated emission limit – the emission limits in EN 50121-X can be interpreted as being too high, allowing interference on radio systems at large distances from the railway.
» Frequency range – both the lower and the upper frequency limits are insufficient to take into account the changing electromagnetic environment;
» Conducted emissions from substations and/or passing trains should be included in EN50121-2;
» Up to date testing – it is thought that the standards do not keep up to date with advances in technology, as evidenced by interference problems with GSM-R systems.
These points were then used to drive the remainder of the TREND project, with the aim of creating a measurement environment or set of environments in which some of the points identified can be addressed.
Work packages 4 and 5 involved the electro- magnetic simulation and derivation of the worst case scenarios that could be encountered in the railway environment. As part of Work Package 4, a measurement campaign was carried out in Sweden which involved looking at the worst case scenarios within the TREND research areas. One such area is pantograph arcing which was investigated by deliberately icing the catenary to provide almost continuous pantograph arcing. This was measured line-side and via an appropriately placed GSM-R antenna and the results fed back into the electromagnetic model in order to increase the latter’s usefulness.
As the TREND project enters the final six months, all partners are focused on the creation of test environments and measurement techniques in order to achieve the project objectives. It is hoped that the development and understanding of pantograph arcing and time domain measurement of transient emissions will be realised. The remaining work packages allow the development of novel test methods that will help to address some of the issues raised earlier on in the TREND project. The outcome from the remaining project time and the culmination of the project objectives will be a set of test methods and/or environments that recreate the representative worst case scenarios regarding rolling stock emissions that could affect interoperability.
Report by Dr Rob Armstrong, consultant engineer, York EMC Services