by the magnetizing inductance and non-linear behavior of the iron core, while at higher frequencies, eddy currents effectively display the magnetic circuit; consequently, the winding inductances are determined by local leakage fluxes [7]. It is feasible to detect small changes in the bushing circuit parameters using FRA approach, and it is possible to monitor the condition of the bushing via FRA [8].
Further improvements can be achieved by using signal processing and signal analysis techniques which may enhance the difference between the transfer functions. In this paper, a new method, which is based on Short Time Fourier Trans-form (STFT), is proposed. The sensitivity of fault detection can be improved by using STFT. A time at which a resonant frequency component occurs, can be used as a relative change factor to help distinguish the various types of failure. Transfer functions are obtained from input current and output volt-age. Comparison between the transfer functions is evalu-ated, also.