Short-time Fourier and wavelet transformsfor fault detection in power transformers during impulse tests

A novel approach using the short-time Fourier transform and wavelet transform (time–frequency analysis tools) for fault detection during impulse testing of power transformers is described. The neutral and/or capacitively transferred currents which are recorded during an impulse test can be directly analysed with this approach. These currents are considered to be evolving in time, i.e. as nonstationary signals, especially when there is a fault. Results from simulation studies are presented wherein the fault condition is modelled as a fast decaying transient superposed on the neutral current. A comparison of the two transforms is made to assess their ability to detect as small a fault as possible and other implemenational issues. Advantages of these methods over the conventional transfer-function method are demonstrated, and it appears that the wavelet transform is better suited for this task.