Influence of acid on frequency domain dielectric spectroscopy of oil-paper insulation

To investigate the influence of acid on thermal aging and frequency domain spectroscopy (FDS) of oil-paper insulation, formic acid and stearic acid were added in the oil to achieve 0.3mgKOH/g. Testing the solubility parameter after balance in the oil-paper insulation under the conditions which the temperatur is 20°C, 70°C and 100°C and the initial moisture content is 0.2%, 3.0%. The FDS of samples are measured at different temperature, analysising the variation of real part (ε') and imaginary part (ε'') of complex permittivity of samples after equilibration. Experimental results show that formic acid is easily dissolved in paper, while stearic acid in oil. Temperature and moisture content had a small influence on the distribution of the acid in oil-paper insulation. Formic acid has significant effect on ε' and ε'' at low temperature, but stearic acid only has visible influence on ε''. Formic acid and stearic acid have the influence on ε'' and ε' respectively at high temperature. The infuence of acid should be taken full consideration while the FDS were used to estimate the moisture content of oil-paper insulation.