A new high-speed digital relay protection of the 3-kVdc electric railway lines

In Italy, the increase of railway traffic and of the power demand by the trains, especially the high-speed trains, has caused a continuous increase of the currents through the electric traction lines rated at 3 kVdc. As a consequence, in some cases, the load currents may approach the value of the fault currents for faults located far from the supply stations. The detection of the faults in such a situation has become difficult for the electromechanical protections at present in operation in Italy in the lines above. In addition, transient phenomena related to the train absorption, frequently cause untimely trippings of the protections above. For such a reason, in the years 1999-2000, a field test campaign was performed in the 3-kVdc lines Campoleone-Cisterna and Farneta-Montallese in Italy, in order to investigate the phenomena that lead to the untimely trippings above. On the basis of the field test campaign, it was found that such an inconvenience is related to the inner characteristics of the electromechanical protections in use. A digital protection, operating on new protective algorithms, was thus set up and thereafter put in operation in the years 2003-2004. This paper presents the basic principles of the new protection above and the results of the first two years of operation.