Space-Frequency Analysis and Experimental Measurement of Magnetic Field Emissions Radiated by High-Speed Railway Systems

This paper shows that in the radiated emission spectrum of high-speed railway systems, a low-frequency range exists in which the magnetic field space-frequency distribution is determined, to a large extent, by the railway infrastructure features. To prove this, electromagnetic modeling of a suitably long stretch of the railway infrastructure is developed, and an ad hoc experimental campaign is carried out for comparison. Modeling and experimental measurements refer to a 50 km stretch of the Italian 2 × 25 kV ac railway system. Infrastructure simulation is obtained by resorting to a simplified approach based on multiconductor transmission line. To this end, short segments of uniform lines are cascade-connected with lumped networks representing equipment and electrical devices placed along the line, and the radiated magnetic field is evaluated from the corresponding noise-current distributions. Since the emission source, i.e., the train, has unknown electrical properties, it is represented as a unit-current source. Thus, the proposed model provides a transfer ratio between the radiated magnetic field and the current source. As a result, the maximum values of train current corresponding to magnetic field limits provided by the International Standards can be readily derived. An optimized measurement procedure is used to show that in the frequency band 60-120 kHz, transient emissions due to random and unpredictable phenomena are negligible. In such a frequency range, where continuous and persistent emissions are present, the structure of time-frequency measurement data is readily explained, and the role of the railway infrastructure on the radiated magnetic field is unambiguously identified.