Parameters of Equivalent Circuits for External and Traction Power Supply Systems during Short Circuits in a Contact Network

Short circuit currents are traditionally calculated in traction ac and dc networks assuming that each of the traction substations has an independent power source. In fact, in most cases a group of traction substations is connected to a single high voltage longitudinal power line that is a component of an external power supply system. Failure to take this into account may lead to errors in calculating distant short circuits reaching several tens of percent, which is entirely impermissible for ensuring reliable functioning of relay protection. Setting up an equivalent circuit for an external electrical supply for calculations of short circuits is a fairly complicated and undesirable procedure, which makes the practical calculations much more complicated. It has been shown that the existing recommendation for the equivalent circuit for a traction substation in a three-phase system of single-phase alternating current with power sources and other components of the system, included in a triangle but not traditionally, in a star, is unjustified and cannot be used for this purpose. A universal equivalent circuit for external electrical supply that takes the features of the system and the connections of inputs for traction substations to a longitudinal electrical supply into account is developed and presented. It is proposed that the resulting resistance of this system be calculated on the basis of known three-phase short-circuit currents at the inlets to two neighboring traction substations. A generalized equivalent circuit for a system of external and traction electrical supply during a short circuit in a contact network for dc and single-phase alternating currents is introduced together with equations for calculating its parameters.