This thesis presents three condition monitoring techniques for Power Transformers namely interpretation of DGA (Dissolved Gas Analysis) using ANN, APD (Acoustic Partial Discharge) & FRA (Frequency Response Analysis) . While DGA & FRA are offline technique to determine internal faults & winding deformation, APD is a on line technique to determine deterioration of insulation inside the transformer. The speciality of this thesis is the subcategorisation of transformer internal faults based on DGA data available with NTPC R&D along with the types of faults observed after the transformers were opened for detection of fault. Modeling of transformer for the purpose of studying the response to FRA with varying circuit parameters has been carried out. Case studies have been presented with measurements carried out on three transforms at NTPC Faridabad & Dadri. FFT (Fast Fourier Transformation of the signals has been carried out to detect the levels of various frequency components & an attempt has been made to correlate it to the probable faults.