Cellulose Ester Insulation of Power Transformers: Researching the Influence of Moisture on the Phase Shift Angle and Admittance

This study investigates the frequency-temperature relations between the phase angle phi and admittance Y for composites of cellulose, synthetic ester, and water nanoparticles. We determined the activation energy value for the relaxation time of a phase shift angle Delta W-phi approximate to (0.783 +/- 0.0744) eV, which was related to the shift of phi(f) waveforms in higher frequency area with increasing temperature. We found that the position of admittance frequency waveforms in double logarithmic coordinates was simultaneously influenced by the temperature dependence of admittance and its relaxation time. Activation energy values for the relaxation time of admittance Delta W-tau approximate to (0.796 +/- 0.0139) eV and the activation energy value of admittance increment W-Y approximate to (0.800 +/- 0.0162) eV were determined. It was found that all three activation energy values were identical and their average was Delta W approximate to (0.793 +/- 0.0453) eV. Impregnation with synthetic ester resulted in a decrease of activation energy by 0.26 eV compared to the impregnation with insulating oil. This was related to higher dielectric permittivity of the synthetic ester.