Novel Approach for Reducing Transformer Inrush Currents: Laboratory Measurements, Analytical Interpretation and Simulation Studies

Transformer inrush currents can lead to a reduction of transformer lifetime and inadvertent tripping of relays. This paper investigates a novel approach for minimizing the inrush current with a potential application in circuit-breaker (CB) control strategies without independent-pole operation and residual flux estimation. For the analyzed transformer, the worst-case inrush current is halved compared to the rapid-closing switching strategy. Measurements of inrush current transients are performed on an unloaded 11-kV distribution transformer varying disconnection and connection instants systematically. This reveals a characteristic pattern in the extremal value of the inrush current as a function of switching times. The pattern is reproduced with simulations and extended to alternative winding configurations. A condition for minimum inrush currents, consistent for all phases and winding configurations, is identified and explained physically. The impact of the current chopping capability of the CB is important and is discussed in this paper.