313 / 2018-09-25 23:57:31

Study of Low Frequency Domain Relaxation of Oil-paper Insulation

oil-paper insulation,low frequency domain relaxation,double potential well model,conductance process

Dielectric response technology is a useful method on state detection and properties characterization of dielectric materials, and was applied to polymer, oil-impregnated insulating paper and ceramics. However, for the deficiency of explanation on frequency domain spectrum and the lack of research of the connection between measurement result and microcosmic mechanism, its further application is limited, especially to oil-impregnated insulating papers. In the past research, it is believed that the low frequency part of the dielectric loss of oil-paper insulation is mainly composed of the conductance process, while the high-frequency part is mainly composed of the polarization process. However, a large number of experiments have shown that the low-frequency part of the dielectric loss contains not only conductance process, but also other components.
In this paper, the phenomenon of low frequency domain relaxation of oil-impregnated insulating papers, neglected in previous researches, was discussed in detail. Firstly, a mathematical method to distinguish the relaxation loss from conductivity loss based on Debye model was proposed; secondly, a sealed and temperature-controllable test device was designed to realize the measurement of frequency dielectric spectrum; thirdly, based on the test results of frequency domain spectrums under different ambient temperatures, low frequency domain relaxation curves were calculated by the mathematical method mentioned before; finally, the double potential well model was employed to explain the mechanism of temperature action, and the barrier height and inherent relaxation time were calculated by curve fitting.
Based on the above research, it was proved that, for oil-impregnated insulation papers, dielectric loss in low frequency consists of two parts: conductance process and low frequency relaxation. The test result also shown that the conductance loss decreases as frequency increases, while the low frequency domain relaxation curve shows a loss peak. As a result, the low frequency domain relaxation takes up a larger proportion in low frequency loss as frequency increases, even more than conductance loss. According to the test result under different ambient temperature, the low frequency relaxation curve pans to high frequency as ambient temperature increases. The double potential well model, in which the relaxation process is considered as the movement of charged particles between two barriers, was employed to explain test result. Based on the double potential well model, the relationship between characteristic frequency of loss peak and ambient temperature was calculated by curve fitting. Specifically, the characteristic frequency of loss peak increases as ambient temperature increases exponentially. The barrier height, E0, of this low frequency domain relaxation process is about 1.005eV, and the inherent relaxation time, τ0, is about 4.02×10-13s.
According to the comparison between low frequency domain relaxation process and conductance process, the low frequency domain relaxation loss cannot be neglected for oil-impregnated insulating papers. It modified the traditional interpretation model of frequency domain dielectric spectrum of oil-impregnated insulating paper. Besides, this relaxation process is closely related to the micro structure of insulating papers, and so, it is expected to be employed for insulation detection of oil-impregnated power equipment, such as aging diagnosis.