where An and fn are, respectively, the magnitude and resonant frequency location for the fingerprint. Ai and fi are the magni-tude and resonant frequency location for all other simulated conditions [9,11].
By using STFT, a third factor can be considered, the rela-tive change in time (Dt), when a resonant frequency compo-nent occurs. This factor, Dt, can be used by technicians, while making decision about changes in the winding structure of a transformer. The Dt can be written as: