This paper presents a new procedure to characterize an admittance matrix over a wide frequency band, for the purpose of black-box modeling of power transformers. Unlike previous approaches, our setup is based on a commercial sweep frequency-response analysis measurement instrument which is lightweight and considerably less expensive than dedicated high-precision laboratory equipment. The use of 50-Ω termination resistors allows measuring the terminal admittance matrix without the need for correcting for the measurement cables. The measured admittance matrix is next subjected to rational modeling using vector fitting and passivity enforcement. When applied to a 300-kVA distribution transformer, the resulting model accurately reproduces measured time-domain waveforms. © 1986-2012 IEEE.