Estimation of Off-Nominal Parameters of Three-Winding Transformers Using Lagrange Interpolation

Three-winding transformers equipped with on-load tap changers are widely used in power systems. The impact of tap-changer control across the entire range of regulation from the maximum tap to the minimum tap needs to be taken into account in some power system analyses. This article demonstrates that described in literature and applied in some computer packages certain modeling assumptions for three-winding transformers operating on the off-nominal taps, are incorrect. Consequently these assumptions can lead to significant errors. The new method for the calculation of tap-dependent values of transformer equivalent impedances is proposed. The calculations for sample transformers are provided.