Equivalent Circuits of the 27.5 kV Electric Railway External Power Supply System
Abstract
In analyzing the short circuit fault currents in electric railway traction networks, it is necessary to take into account the impedance of the external three phase power supply system. This impedance is taken into account in the form of a so called input impedance of traction substations. This input impedance is calculated using the equivalent circuit of the three phase power supply system jointly with the traction substation power transformer; this equivalent circuit may be drawn in the star configuration (traditionally) or in the delta configuration (non traditionally). The phase impedances of the same elements of the power supply system and transformer in the delta shaped equivalent circuit are a factor of three larger than their values in the star shaped equivalent circuit, and it should be noted that this ratio, and also the choice of the equivalent circuit kind (star or delta) do not depend on the power transformer winding connection diagram. The traction substation input impedance values for the star and delta equivalent circuits are identically equal to each other; however, the star shaped equivalent circuit is a preferred one. The commonly adopted method, according to which the external power supply system is taken into account separately and independently for each of traction substations, is simplified in nature. When being used for certain traction substation power supply arrangements, e.g., in the case of a ring shaped power supply from the high voltage longitudinal power supply line, this method yields noticeable errors in calculating the distribution of short circuit fault currents in the traction network between adjacent traction substations. It is proposed to determine the power system’s remote part impedance in the traction substation input impedance from the parameters of a short circuit fault on the buses of master traction substations. In so doing, the equivalent impedances of the longitudinal power supply lines connecting the master substations with the intermediate ones are additionally taken into account. Relevant formulas for different power supply arrangements are given.
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