Transient and steady-state short-circuit currents in rectifiers for DC traction supply

This paper presents some significant extension and analytical formalization of the knowledge on transient and steady-state short-circuit currents in DC traction systems. Electrical supply substations (ESSs) in modern metrorail and railway DC systems are equipped with 6- and 12-pulse diode bridge rectifiers. Large existing plants are equipped with 6-pulse converters, but the 12-pulse option is preferred in the design of new plants in the parallel configuration. Such a converter presents a good compromise between number and voltage/current rating of semiconductor devices, complexity and rating of transformers, DC-side voltage ripple, and AC-side current harmonic content. In this paper, the author considers the features of 6- and 12-pulse conversion units in terms of transient and steady-state short-circuit currents on zero fault impedance, accurate determination of the transient DC short-circuit current is important for correct rating and tuning of protection devices and power equipment (high-speed circuit breakers, isolator switches, switchgears, DC-busbars, etc.). An analytical solution of the short-circuit behavior is given for zero fault impedance currents: phase currents of the transformer, transient short-circuit DC current, approximate expression of the DC short-circuit current, and accurate expression of the steady-state DC short-circuit current. The general calculation method used in the paper is extensible to any conversion unit with a double secondary transformer