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 de traction systems. Electrical supply substations (ESS's) in modern metrorail and railway de 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, de-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 de short-circuit current is important for correct rating and tuning of protection devices and power equipment (high-speed circuit breakers, isolator snitches, switchgears, de-bus bars, etc.). An analytical solution of the short-circuit behavior is given for zero fault impedance currents: phase currents of the transformer, transient short-circuit de current, approximate expression of the de short-circuit current, and accurate expression of the steady-state de short-circuit current. The general calculation method used in the paper is extensible to any conversion unit with a double secondary transformer.