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.