Influence of temperature on phase shift angle and admittance of moistened composite of cellulose and insulating oil

•Precise measurements of basic AC parameters (phase shift angle and admittance)•Frequency-temperature relations between the phase angle and admittance.•Composite of cellulose, synthetic oil and water nanodrops.•Investigation by Frequency Domain Spectroscopy (FDS).•Elimination the influence of temperature on the insulation parameters of transformers. This paper describes precise tests of frequency–temperature relations of the phase shift angle and admittance of a reference sample of composite of cellulose and insulating oil with a moisture content of (5.0 ± 0.20)% by weight. A method of moisturisation the pressboard as close as possible to the natural moisturisation process in power transformers was used, which uses a moisture source from which moisture moves through the insulating oil to the reference sample. A climate chamber was used to ensure long-term temperature stabilisation, at uncertainties not exceeding ± 0.01 K, for measuring the temperature dependence of alternating current parameters with high accuracy. A new method of determining the activation energy of relaxation time of the phase shift angle and admittance was developed, which allowed obtaining low uncertainty values of their determination. The values of activation energy of relaxation time of the phase shift angle ΔWφ ≈ (0.914 ± 0.0085)eV, activation energy of relaxation time of the admittance ΔWτY ≈ (0.866 ± 0.0023)eV, and activation energy of the admittance ΔWY ≈ (0.897 ± 0.0043)eV were determined. Uncertainties of these parameters were ± 0.93%, ±0.27% and ± 0.48%, respectively.