Enhanced Dielectric Response Methods for the Characterization of Dielectric Materials
Abstract: Dielectric response measurement is widely used for condition assessment of high voltage equipment and for characterizing candidate materials. Measurement with high accuracy and reliability is important in all stages of the lifetime of high voltage equipment, from manufacture to replacement decision. However, several deficiencies in the commonly used measurement methods may affect the reproducibility and accuracy of results. This thesis focuses on two issues: the contact problems between the surfaces of sample and electrode in the measurement of solid insulation samples, and the non-linear phenomenon of voltage-dependent permittivity occurring in measurements on insulating liquids such as oil and oil-paper insulation.For reducing the contact problems, this thesis introduces an easily realized electrode arrangement for non-contact measurements. The performance of the electrode arrangement is evaluated in terms of the edge effect and the measure-guard capacitance that can influence results if there is deviation of the potentials of the guard and measure electrodes. The non-contact and contact methods are compared based on the error-sensitivity analysis and experimental data from frequency-domain spectroscopy (FDS). The differences are investigated further, with attention to contact pressure. By using the non-contact method, the contact problems can be reduced, although error sensitivity is likely to be higher. In addition, the non-contact method can decrease the influence of the pressure applied by the electrode compared with the contact measurement. Therefore, the non-contact method can be an alternative to improve the accuracy and reliability of FDS measurements.Regarding FDS measurements of oil and oil-impregnated pressboards, the non-linear phenomenon has been studied using combined AC and DC voltages as stimuli. The complex permittivities are compared between the cases with and without the DC bias, considering the influence of the voltage amplitude and polarity and the temperature. It is seen that using the DC bias in the FDS measurements of oil and oil-impregnated pressboard can significantly decrease the voltage dependence of the complex permittivities caused by the limited numbers of ions in the liquid being blocked at boundaries. Properties of the space-charge polarization in the oil are calculated and discussed based on the DC-biased measurements. A numerical calculation of space charge polarization is done to analyze the experimental results and explain the permittivity change in terms of ion transport. It is concluded that using combined AC and DC voltages can reduce misinterpretation of results because of the voltage dependence caused by the ion movements, and can also provide a method to obtain the contribution of complex permittivities due to the space charge polarization in the oil.
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