Some low frequency EMC issues in electrified railways

This paper describes some of the techniques that have been developed to define the maximum permissible current levels for various types of track circuits and other lineside equipment e.g. Reed Frequency Division Multiplex (FDM) Systems. In most cases a combination of analysis, computer modelling and onsite testing is employed. BRR (a part of AEA Technology plc) has developed techniques for modelling infrastructure of electrified railways. These models have been used to provide technical support for rolling stock safety cases, which are required by Railtrack before new rolling stock can operate on electrified lines. Basically the modelling process involves the identification of all relevant current carrying conductors and evaluation of their electrical parameters. In the case of track circuits the model is too complex to search for a closed form algebraic solution. Hence a computer model is used and, wherever possible, validated by on-site testing. It is necessary to make some simplifying assumptions to determine the structure of the model and in the evaluation of the parameters. Examples of applying modelling techniques to determine the maximum permissible current levels for DC and high voltage impulse (HVI) track circuits are included. Determining the limits of induced voltages in line side cables is described as an example of the use of analytical techniques and validating the method of analysis by on-site testing