Frequency-domain measurements can be used for obtaining a wideband model of a power transformer via its terminal admittance matrix. This paper shows that the current sensor used in the measurements can significantly affect the elements of the obtained admittance matrix by its insertion impedance. The insertion impedance influences diagonal and off-diagonal elements, often leading to asymmetries in the matrix for transformers having a high-voltage ratio. Formulae are developed which are capable of removing these effects. The procedure is demonstrated for a distribution transformer. © Copyright 2012 IEEE - All rights reserved.