Abstract
In on-site measurement, as the transformer’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’s effectiveness, which indicates the sensitivity of assessed moisture content to the error of the tan <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\delta $ </tex-math></inline-formula> , changes at different frequencies. Moreover, there are some frequencies at which tan <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <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 “characteristic frequencies.” 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%.
Highlights
The oil-immersed transformer is the crucial equipment in power system
This paper proposes a moisture assessment method for oilimmersed paper based on the dynamic characteristic frequency to evaluate the moisture content with the unknown structure of the transformer
This paper proposed a moisture assessment method for oilimmersed paper based on the dynamic characteristic frequency to evaluate the moisture content with the unknown structure of the transformer
Summary
The oil-immersed transformer is the crucial equipment in power system. The major insulation of oil-immersed transformer is oil-paper insulation, and undergoes degradation by several factors, such as thermal, electrical stresses, and especially water content [1, 2]. The frequency spectroscopy of oil-immersed paper can be calculated by the transformer’s structural parameters [15,16,17]. Based on the frequency spectroscopy of oil-immersed paper, the transformer’s moisture content can be evaluated [18,19,20,21,22,23,24]. Few studies have been done on the extraction of oil-immersed paper’s frequency spectroscopy and the moisture evaluation of transformer with unknown insulation construction, which is common in the on-site measurement. This paper proposes a moisture assessment method for oilimmersed paper based on the dynamic characteristic frequency to evaluate the moisture content with the unknown structure of the transformer. The moisture content is assessed according to the frequency spectroscopy of oilpaper insulation without the structural parameters. As ε'paper and ε''paper are both positive in only one of the two solutions, the correct one can be chosen
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