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

Doktorsavhandling, 2006

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 (<1E-1 Hz), whereas its influence on the loss was more pronounced and visible in the whole frequency range below 1E3 Hz. All the measured responses were modelled with terms representing dc conductivity, low frequency dispersion and Cole-Cole polarisation mechanisms with two distinct relaxation time constants. Influences of temperature and moisture content were described by analysing variations in the model parameters. The FDS responses formed a database, which later was used to develop a computer program for interpreting the results of diagnostic measurements in terms of moisture content in the solid part of power transformer insulation. During the field experiments the influences of different conditions that could disturb the measurement were carefully studied and it was shown that guarded measurements between low voltage and high voltage windings (CHL) provide the most reliable data for further analyses. The estimated moisture content and the calculated oil conductivities from the results of FDS measurements were compared with the results obtained from Karl Fischer titration (KFT) and by direct measurements of conductivity of oils sampled from the investigated units. It had been shown that similar values could be attained by both methods when correct temperature values were used i.e. for analyses of FDS data the temperature value during the measurements was applied, whereas for KFT analyses the temperature of the unit during its normal service had to be considered.