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| Dielectric Response During Thermal-Oxidative Aging of Low-Voltage Cable Insulation Analyzed by Davidson-Cole |
| Wang Jingran1, Wang Yingjie2, Zhuang Hongwei3, Ji Minzun1, Liu Wenfeng1 |
1. State Key Laboratory of Electrical Insulation and Power Equipment Xi’an Jiaotong University Xi’an 710049 China;
2. Suzhou Nuclear Power Research Institute Suzhou 215004 China;
3. Guangxi Fangchenggang Nuclear Power Co. Ltd Fangchenggang 538001 China |
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Abstract The low-voltage(LV) cables in nuclear power plants would be affected by heat and oxygen under actual operation, and the cable materials would gradually deteriorate with the increase of service time. The most common standard traditionally used to evaluate the insulation state of LV cables was the mechanical parameter, mainly elongation at break (EAB), however, the tensile experiments were destructive. Rencently, frequency domain spectroscopy (FDS), as one new non-destructive diagnostic technology, had the advantages of being non-destructive and simple, and was suitable for the characterization of the overall insulation state of the cables. However, the vast majority of studies focused on qualitatively analyzing the variation of the FDS with various influencing factors. Therefore, one scientific dielectric model was used to quantitatively analyze the dielectric response characteristics of LV cables, and the correlation between the dielectric parameters and the physical, chemical, mechanical properties of the materials was discussed.
Firstly, the accelerated aging experiments were carried out on the cross-linked polyethylene (PE)/ ethylene-vinyl acetate copolymer (EVA) specimens at 165℃ to obtain the samples with different degrees of deterioration. The changes in physical and chemical properties of PE/EVA specimens were tested through the Fourier transform infrared (FTIR) spectroscopy and the gel content. The deterioration of the mechanical property was investigated through the EAB experiment, and the dielectric properties were analyzed through the FDS at 100℃. The PE/EVA specimens had two relaxation processes form the analysis of FDS, and the DC conductance was considered at the same time. Therefore, the complex permittivity spectra of the PE/EVA samples with different aging degrees were fitted by the Davison-Cole dielectric model to extract the characteristic parameters of polarization and conductance.
The results showed that the re-crosslinking reaction of the specimens occurs during the whole aging process, which increased the three-dimensional network structure. The carbonyl index of PE/EVA showed a trend of decreasing at the beginning and then increasing, which was caused by the degradation of vinyl acetate groups at the early stage and the oxidation reaction after 504 h. It was found that the relaxation strength Δεα due to the motion of segmental chains, the thermionic polarization strength Δεδ as well as the DC conductivity σdc gradually decreased with the increase of aging time through the Davidson-Cole dielectric model. The decrease of Δεα and σdc slowed down due to the accelerated oxidation reaction after 504 h. The preliminary fitting of the characteristic parameters Δεα, Δεδ, σdc of the dielectric model and the traditional state evaluation criteria EAB showed a positive correlation.
The following conclusions can be drawn from the analysis of experimental and fitting results: (1) The degree of cross-linking, as the key parameter during the aging process, is the most important determinant of the consistency between the dielectric parameters and the mechanical parameter. The increase in the degree of cross-linking of PE/EVA inhibits the migration of ions, so that the DC conductivity σdc and the relaxation strength of the samples decrease with the aging time. The free volume of PE/EVA specimens is reduced due to the augment of the macromolecular network structure, which weakens the movement of the molecular segments. At the same time, the increase of the degree of cross-linking reduces the flexibility of macromolecules of PE/EVA, and then the EAB decreases during the aging process. (2) It is found that the quantitative analysis of the frequency domain spectroscopy can not only characterize the dielectric response mechanism of the materials, but also has a strong correlation with the elongation at break, which can be better applied in the evaluation of the state and life of the LV cables.
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Received: 23 May 2022
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2023, Vol. 38 
