Peculiarities of Measurements of AC Traction Current Asymmetry in Rail Lines

Abstract

Up to half of failures in the operation of rail circuits and automatic locomotive signaling at the sections of railways with ac electric traction occurs due to the disturbances caused by the traction current asymmetry in the rail lines. Since the primary source of this asymmetry is the asymmetry of resistances of rail threads within the rail circuits, the measurements of the resistance of elements of each rail thread make it possible to not only define the traction current asymmetry, but also to find the elements, the resistances of which go beyond the limits of the norm. The traction current asymmetry can also be measured directly; however, the measurement results depend as well on the conditions of their being carried out—on the temperature of rails and the magnitude of alternating traction current in them. This is explained by the fact that the asymmetry of resistances of rail threads of rail line is formed owing to an asymmetric increase in them in the transient electrical resistances in the joints of rail links and in the choke bridges. These transient resistances depend on ambient temperature and traction currents in rail threads slightly. As a result, when decreasing the temperature of the rails of P65 type from +40 to –40°C, the asymmetry of resistances of rail threads taking into account their mutual inductance increases by 2.2—2.4 times, while increasing the traction current in rails, for example, by three times causes almost the same asymmetry increase. The influence of mutual inductance of rail threads with the contact wire leads to a decrease in the traction current asymmetry in the rail line. The mutual inductance of rail threads with high-voltage lines of longitudinal power supply that are placed on the contact line supports can increase or decrease this asymmetry, depending on which of the rail threads has less resistance. All these factors should be taken into account both when measuring the asymmetry of resistances of rail threads or the asymmetry of alternating traction current in them and when extrapolating these data to other operating conditions of the rail traction network by the rail temperature or by the magnitude of currents in rail threads, contact wires, and high-voltage lines.