Characterization of a railways system overhead transmission line excited by a lightning electromagnetic field

We present a new field-to-line coupling model for the analysis of overvoltage on overhead dc traction power lines of modern electric mass-transit systems, such as Light Rail Transportation Systems (LRTS), in the case of a lightning strike in the vicinity of the line. The developed model is essentially based on a mathematical procedure involving suitable expansions of the induced currents and of the scattered voltages by means of a series of Gegenbauer polynomials. Such a procedure allows the integro-differential system describing the fieldto- line coupling problem to be solved with faster performances in comparison to more classical “full-Maxwell” approaches (e.g. MOM, FDTD) for a given accuracy. This feature makes it possible to find, in a reasonable time, a frequency domain representation of the contact line in terms of a matrix of impedances and open circuited voltages in response to several lightning strikes, taking into account the effect of radiated power, which is not modelled by the classical transmission line approach. This phenomenon is usually quite relevant for typical contact line geometries, since they are characterized by transversal dimensions that are not negligible with respect to the relatively short length of each overhead section. This representation is suitable for a future integration (in a case using modal analysis) in a time domain model that can be analyzed by means of a circuit simulator. The evaluation of the effects of several lightning strikes with different impact points and characteristics will allow a statistical assessment of the protection level offered by different technical solutions to be performed (by way of example, the different location of the surge arresters).