Reduction of magnetizing inrush current in a delta connected transformer

Abstract

Magnetizing inrush current in power transformer creates severe power quality problems. Energization of the transformer from the delta side of a delta-star transformer does not allow the control of neutral resistor at the time of switching the supply for reducing the inrush current. Controlled switching at peak value of the voltage may reduce inrush current in one winding of the transformer, but the line currents are not reduced. Delayed and controlled energization of the third phase may reduce inrush winding currents in two windings and only one line current is reduced, but the inrush currents in the other two lines become very high [11]. The inrush current may be reduced either by inserting suitable value of resistance connected to each of the line wire or by combining the controlled switching at peak value of the voltage in one winding and resistance switching in the third phase. The resistances are shorted within a very short time after switching on the delta winding. Simulation studies of delta side energization of a delta-star transformer depict the results under various operating conditions. Optimum resistance value is decided to get a quick decay of inrush current, low voltage drop and losses before it is shorted.