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| Dielectric Response Properties and Moisture Assessment of Oil-Paper Insulation Based on Micro-Strip Ring Resonator |
| Wu Ming1, Zhang Daning1, Shao Xianjun2, Yang Zhi2, Li Quanhao1, Zhang Guanjun1 |
1. State Key Laboratory of Electrical Insulation and Power Equipment Xi'an Jiaotong University Xi'an 710049 China;
2. State Grid Zhejiang Electric Power Research Institute Hangzhou 310014 China |
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Abstract In the condition assessment of oil-paper insulation, the dielectric response is a widely used characterization method. Both time domain spectroscopy (TDS) and frequency domain spectroscopy (FDS) test at lower frequencies require long time and cannot reflect damp state of insulation pressboard in time. In order to improve the speed of dielectric characterization and the convenience of experiment, the GHz-band dielectric response technology based on the resonant measurement method gradually has the feasibility of research and the possibility of application in the detection of damp state in oil-paper insulation. The resonant method is to measure the scatter parameters by designing a resonator with some weak couplers. The dielectric characterization of resonant method is carried out at the resonant frequencies, which can indirectly calculate dielectric properties of the upper medium at the GHz frequencies, which has the advantages of convenient experiment, fast characterization, and high testing frequency. Resonators usually have a fixed geometry, and the micro-strip ring is a commonly used resonator.
In the physical structure of the micro-strip ring resonator, the selected substrate is Nelco's NX9320 high-frequency plate, whose thickness is 0.762mm, εr is 3.2±0.4, and tanδ is 0.002 4. Two copper micro-strip lines and a copper ring are attached to the high-frequency sheet base. The characteristic impedance is 50Ω, the copper thickness is 0.5oz, the width of the copper ring and microstrip line is 1.85mm, the inner radius of the copper ring is 25mm, the outer radius is 26.85mm, and the coupling gap is 0.4mm. Above the substrate is a layer of experimental material with known thickness. Two high-frequency SMA interfaces are designed at both ends of the micro-strip lines, which are used to connect the network analyzer. The network analyzer Pico VNA 106 produced by Pico company is used, and the test bandwidth is 1~6GHz. By testing the S21 parameter of the whole resonant device, that is, the insertion loss characteristic, the resonant point is found and its dielectric parameter is deduced.
By analyzing the test results of the micro-strip ring resonant dielectric response, it can be concluded that the dielectric parameter response curve of GHz insulating pressboard is relatively flat with weaker frequency dependence. The dielectric characteristic curves show an obvious gradient with water content, indicating that εr and tanδ of the insulation pressboard at GHz band also increase significantly with the gradual aggravation of damp state. In the obtained dielectric characteristics of 1~6GHz, the dielectric response curves with different degrees of damp conditions show weak polarization peaks and loss peaks between 1GHz and 3GHz. It's pointed out that the polarization process in oil-paper insulation is dominated by dipole steering polarization at higher frequencies, and the dominant charges involved in polarization mainly come from cellulose molecules and water molecules with polarity. According to the Debye model, the real part of the complex permittivity of pure water at 0℃ and 1~100GHz shows a monotonically decreasing process, and the imaginary part shows an obvious polarization peak, indicating that pure water has a certain polarization process at GHz frequencies. Therefore, the dielectric properties of the insulating board obtained in this paper in the GHz frequency band are of great significance to the study of the polarization process in this frequency band, especially the dipole polarization process. As for dielectric response curves of three kinds of pressboard stacks with equivalent water content at 1~6GHz, the uneven damped samples also show a certain polarization peak and loss peak between 1GHz to 3GHz. In addition, in the sample groups with the same equivalent water content, the fluctuation trend of dielectric response curves is the same, but the intensity of fluctuation increases with the increase of the degree of inhomogeneity. There is an obvious gap between the curves of the uneven and even damped samples, especially at the peak, and the gap increases with the increase of the degree of unevenness.
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Received: 30 June 2022
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2023, Vol. 38 
