The Influence of Zero Sequence Currents on Phase Current Reduction in Asymmetrically Tapped Systems

This paper addresses the counter-intuitive phenomenon of phase current reduction in a three-phase system with asymmetrical tap changer positions. Specifically, the study examines cases where a transformer's phase B tap is stuck and is unable to switch while phases A and C are at lower taps. The results from simulations show that increasing the tap position on phase B, which would normally be expected to increase the current in that phase, leads to a reduction of current in phase A. It is demonstrated that this behavior can be attributed to the generation of a significant zero sequence current component. It flows through the earth and neutral path and, by its interaction with the other phase currents, reduces the magnitude of the current on phase A. This paper provides both theoretical and simulation results to illustrate that unbalanced tap settings are a common cause
of zero-sequence currents, and their effect on individual phase currents. The implications of this phenomenon on power system performance and protection are also discussed.