This book deals with the fault analysis and diagnostics of power transformers. In order to improve electric power quality, the sources and causes must be known before appropriate mitigating action can be taken. Insulation failure within transformer is considered to be one of the most important causes of total failure of power transformers. Any distortion in wave shape of recorded current and voltage signal shows the existence of fault in the transformer winding. These transients depend on the extent of failure. Transfer function approach is generally used as a diagnostic tool. Though, this method is quite efficient, detection of minor faults, particularly with regard to its location is difficult, since variation to such minor faults is minute in nature. Wavelet analysis is a relatively new method for studying power system transients. A theoretical and experimental simulation has been carried out for 61 MVA, 11.5/230 kV Transformer, with a view to determine the localization of fault, %change in the frequency in relation to % change in length of winding is examined. Time ~ frequency characteristics of the transients existing are computed using wavelet transform technique.