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Frequency domain spectroscopy (FDS) is employed to evaluate the moisture content and aging state of oil-paper insulation in power transformers. Oil-paper insulation samples with different moisture content and aging states were primarily prepared in the laboratory. A method was proposed based on a study of the influence of the aging state and moisture content on the frequency domain dielectric property of oil-paper insulation. This method distinguishes the influence of moisture content and aging state on FDS through the imaginary part of complex relative permittivity ( εr″). Relevant characteristic parameters that characterize the moisture content and aging state of oil-impregnated pressboards were also extracted. Results reveal that the extracted characteristic parameters are sensitive to moisture content in the range of 10−1 Hz to 101 Hz and in the range of 10−3 Hz to 10−1 Hz for the aging state. An obvious exponential relationship exists between the maximum values of the characteristic parameters in the given frequency ranges and degree of polymerization and moisture content. The new method can be utilized for a non-destructive condition assessment of oil-paper insulation of transformers. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In order to distinguish the influences of moisture and aging on the frequency domain dielectric response of oil-paper insulation and better apply frequency domain spectroscopy (FDS) to assess the insulation condition of power transformers, the oil-paper insulation samples with different moisture contents and different aging states were prepared in the laboratory. The FDS of the samples were tested and a group of characteristic parameters were extracted from dissipation factor (tan?) curves which could be used to assess the moisture content and aging states of oil-paper insulation respectively. The quantitative relationship among characteristic parameters, degree of polymerization (DP) and moisture contents (Km.c) was accurately established. The observations show that the proposed characteristic parameters are sensitive to the moisture in 10-3-102Hz, while the aging states influence the characteristic parameters in 10-3-10-1Hz. Meanwhile, an exponential relationship equation which could be used to assess the oil-paper insulation condition was established among the characteristic parameters, DP and the moisture content. Finally, the evaluation technique proposed in this paper was used to diagnose the insulation condition of several field transformers in this way, and its validity was preliminary and reasonably verified. [ABSTRACT FROM AUTHOR]