Diagnosis of Moisture in Transformer Insulation - Application of frequency domain spectroscopy

This thesis presents application of dielectric response measurements for identifying condition of high voltage insulation in power transformers. Results of such measurements, incorporated with different modelling techniques, allow to estimate moisture content in solid part of transformer insulation. For this to be accurate, a good understanding of the electrical behaviour of oil-impregnated pressboard/paper insulation is necessary. Calibrating measurements for correlating the dielectric response data and the moisture content in insulation were carried out. Furthermore, numerous field investigations were also performed for studying the feasibility on application of response measurements under different field conditions. The work presented in this thesis was mainly focused on frequency domain spectroscopy (FDS). FDS measurements (1E-4 Hz – 1E6 Hz) were performed on well-controlled oil-impregnated pressboard/paper samples. Results of these were used to form master curves of the dielectric responses in a broad frequency range (e.g. 1E-7 Hz – 1E6 Hz). It had been observed that the presence of moisture influenced the permittivity of impregnated pressboard and paper at very low frequencies (