Effect of moisture and thermal degradation on the activation energy of oil-paper insulation in frequency domain spectroscopy measurement

Frequency domain spectroscopy (FDS) is widely used to diagnose the condition of transformer oil-paper insulation. However, FDS measurements are highly temperature dependent. The master curve technique is the most common method to remove the temperature effect on FDS, but the reliability of this method is largely dependent on the value of the activation energy, which can be affected by moisture content and thermal degradation degree. To understand the effects of these two factors on the activation energy, FDS measurements at different temperatures were performed on oil-impregnated pressboards with different thermal degradation degrees and moisture contents. The activation energies of the pressboards were calculated using the Arrhenius equation. Results show that they vary between 0.88 and 1.06 eV along with thermal degradation degree and moisture. The two-way ANOVA indicates that the effects of these two factors on activation energy are not significant at 95% confidence level when the degree of polymerisation (DP) changes from 415 to 1300 and when the moisture does not exceed 4%. The average activation energy (0.95 eV) can be utilised to generalise the Arrhenius equation to better eliminate the temperature effects. Finally, FDS measurements on a model transformer are performed to verify the effectiveness of the proposed technique.