Elimination of inrush current of transformers and distribution lines

Repeated switching of distribution transformers take place due to poor generation and load shedding. The transformers mounted on electric locomotives are also regularly reswitched after crossing each buffer zone between two substations. The steady state magnetizing current of a transformer is about 5% of the full load current, but the transient inrush current may be as high as ten times the full-load current. The switching instant decides the nature and magnitude of the switching current and it is used here to control the inrush current. Another method is adopted by placing a capacitor at the secondary side of the unloaded transformer connected at the sending or receiving end of the distribution line. A third method is proposed using the distribution line as a low-pass filter. In the case of a three-phase transformer, when it is switched, inrush current flows in either one or two phase windings. Instead of simultaneous switching of all the phases (windings), each winding of the transformer is switched at predetermined switching instants sequentially. Thus inrush currents are contained to a steady-state level using an instant-controlled switching circuit. Switching of all phases completes within 1/3 or 1/4 of the power-cycle depending upon the mode of transformer configuration and the switching strategy. The switching current is found to be the same as the steady-state no-load current. These schemes are useful for traction transformers as well as for poorly supplied and poorly maintained distribution lines including traction lines which are subjected to repeated switching.