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This paper proposes a new approach for obtaining parameters of the zero-sequence equivalent circuit of three-phase three-legged three-winding YNynd transformers (i.e., transformers with two wye windings and a third delta stabilizing winding). The methodology proposed in this paper includes a complete set of onsite field measurements using commercial equipment (traditionally used for onsite transformer leakage reactance tests). An easy-to-implement determination of the zero-sequence behavior of the YNynd transformer is achieved by means of separating the equivalent T-circuit into two different circuits, one of them to be used when the delta stabilizing winding is closed and the other when this is open. The effect of the resistive component of the zero-sequence impedances is also analyzed and quantified. Onsite measurements on a 45/16.05/10-kV 25-MVA power transformer and their comparison with high-voltage factory tests are presented to demonstrate and validate the proposed methodology.