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In on-site measurement, as the transformer&#x2019;s structure is often unknown, the application of frequency domain spectroscopy (FDS) is greatly limited in moisture assessment of transformer. This paper proposes a moisture assessment method for oil-immersed paper based on the dynamic characteristic frequency even for an unknown transformer structure. FDS tests are performed on the oil-paper insulation, corresponding oil-immersed paper, and mineral oil under different moisture conditions. The moisture criterion of oil-immersed paper and the influence of the structural parameters on the calculated frequency spectroscopy at different frequencies are discussed. It is concluded that the moisture criterion&#x2019;s effectiveness, which indicates the sensitivity of assessed moisture content to the error of the tan<inline-formula> <tex-math notation="LaTeX">$\delta $ </tex-math></inline-formula>, changes at different frequencies. Moreover, there are some frequencies at which tan<inline-formula> <tex-math notation="LaTeX">$\delta $ </tex-math></inline-formula> is not influenced by structural parameters and is close to the measured value, which are regarded as &#x201C;characteristic frequencies.&#x201D; Based on the characteristic frequency, the moisture content of oil-immersed paper is assessed according to the frequency spectroscopy of oil-paper insulation without the structural parameters. The result shows that the assessing frequency changes under different moisture conditions, and the error of the assessed moisture content is less than 0.1&#x0025;.

Moisture Assessment of Oil-Immersed Paper Based on Dynamic Characteristics of Frequency Domain Spectroscopy