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| Effect of Water Content on Low Frequency Dielectric Relaxation of Epoxy Resin Impregnated Paper |
| Zhang Hongliang1, Huang Ning1, Liu Peng2, Jin Hai1, Peng Zongren2 |
1. College of Electrical and Information Engineering Lanzhou University of Technology Lanzhou 730050 China;
2. State Key Laboratory of Electrical Insulation and Power Equipment Xi' an Jiaotong University Xi' an 710049 China |
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Abstract Epoxy composite material is the main insulating material of UHV DC dry bushing capacitor core, in which epoxy resin impregnated paper is composed of epoxy resin and corrugated paper. The molecular chain of the two materials has hydrophilic groups and is easy to absorb water, resulting in the deterioration of the insulation performance of epoxy resin impregnated paper, which has a negative impact on the safety of the equipment. In this paper, two kinds of epoxy resin and epoxy resin impregnated paper samples were prepared, and the water absorption characteristics of the two samples were measured under different humidity conditions. The relaxation strength, relaxation time and activation energy of σ polarization and interfacial polarization of cellulose in epoxy resin impregnated paper were obtained by using a variety of dielectric spectrum characterization forms and dielectric relaxation models. The effect of moisture on the low frequency dielectric relaxation of epoxy resin impregnated paper was studied.
In this paper, the epoxy resin and epoxy resin impregnated paper were first placed in five different relative humidity environments of 12%, 33%, 43%, 75% and 100%, and the water absorption characteristics of the two samples were measured and given. It was found that the time required for the two samples to reach the saturated water state was 7 days and 2 days, respectively, and the saturated water content of epoxy resin impregnated paper was higher.
Then, the low frequency dielectric properties of epoxy composite materials under different humidity were studied by using three dielectric spectrum characterization forms of "complex permittivity", "complex conductivity" and "complex modulus". It can be seen from the three dielectric spectra that the influence of moisture on the low-frequency dielectric properties of epoxy resin is small, but the influence on the low-frequency dielectric properties of epoxy resin impregnated paper is obvious.
HN model, complex AC conductivity and complex electric modulus were used to fit and analyze the dielectric spectrum information. The low-frequency dielectric relaxation of epoxy resin impregnated paper composite includes two processes: σ-polarization of impregnated paper and interface polarization of epoxy/impregnated paper. The real part of the complex AC conductivity does not show a plateau in all humidity conditions, indicating that the epoxy and epoxy-impregnated paper samples containing water do not have DC conductivity processes throughout the sample in the test frequency range.
Finally, the influence mechanism of low-frequency dielectric relaxation process of epoxy-impregnated paper is analyzed. With the increase of moisture content, the σ-polarization activation energy required by carrier transition to the deep trap in epoxy-impregnated paper will decrease, resulting in shorter corresponding relaxation time. Meanwhile, moisture will increase the σ-polarized dielectric relaxation strength, thus enhancing the σ-polarization process of epoxy-impregnated paper. Moisture also increases the DC conductivity of impregnated paper, greatly reduces the time constant of interfacial polarization of epoxy/impregnated paper at room temperature, and enhances the dielectric relaxation strength of interfacial polarization.
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Received: 15 December 2023
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2025, Vol. 40 
