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This paper describes the measurement setup and modeling technique used for obtaining a linear wide band frequency-dependent black box model of a two-winding power transformer, for the purpose of calculation of electromagnetic transients in power systems. The measurement setup is based on a network analyzer, shielded cables, and a connection board. The setup is demonstrated to give a consistent data set where the effect of the measurement cables can be eliminated. The accuracy of the data set is increased by using a combination of current measurements and voltage transfer measurements. A rational approximation of the admittance matrix is calculated in the frequency domain in the range of 50 Hz to 1 MHz and subjected to passivity enforcement, giving a stable model which can be included in electromagnetic transients program (EMTP)-type simulation programs. The accuracy is validated both in the frequency domain and in the time domain.