The transformer manufacturers make use of electromagnetic transient calculations by detailed white-box models to ensure that the transformer will withstand the lightning impulse test during the factory acceptance test. In principle, such models could be used in general electromagnetic transient simulation of overvoltages in the system. One of the objectives of CIGRE JWG A2/C4.52 is to assess the accuracy of the manufacturer’s white-box models in the context of general transient overvoltages that can occur in the system. As part of this activity, extensive measurements have been performed on two three-winding transformers: one three-phase unit and one single-phase unit. The measurements involved voltage transfer between external terminals and from external terminals to three points in the regulating winding. The measurements were performed with alternative points of voltage excitation, grounding conditions and alternative tap settings, giving 64 cases for each transformer. Some admittance measurements were also performed. This paper gives an overview of the measurements that were performed, the measurement procedure, and some initial results. The voltage transfer measurements were mostly performed in the frequency domain and transferred to the time domain via rational function approximation and recursive convolutions. That way, voltage transfer functions have been generated for well-defined excitations, e.g. the standard 1.2/50 µs voltage wave. In addition, initial results for black-box modeling of the three-phase unit is shown. The measurements are currently being used within CIGRE JWG A2/C4.52 for benchmarking of white-box models