Estimation of a circulating component of a three-phase Y-δ transformer

This paper proposes an estimation algorithm for the delta winding current of a Y-Delta transformer during the transient state as well as in the steady state. A delta winding current of a three-phase Y-Delta transformer consists of a non-circulating component and a circulating component. The former can be estimated directly from the line currents, but the latter can not. Applying the Kirchhoff´s voltage law on the delta winding gives a first order differential equation in terms of delta winding currents. The delta winding currents are estimated by solving the equation. The various simulation results for the magnetic inrush, over-excitation and an external fault indicates that the algorithm can estimate the delta winding current very accurately even during the transient state.