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Frequency Domain Spectroscopy (FDS) is a powerful tool to study the dielectric properties of oil-paper insulation, which is widely used under high electric field strength. The motion of charge carriers under high electric field strength will cause new dielectric characteristics in FDS curves, from which more information could be extracted for insulation state assessment. In this paper, FD spectra are measured under high electric field strength. The electric field dependence of the loss peak in middle frequency and the characteristics of dielectric loss in low frequency are studied. With the influence of trapping on the motion of charge carriers under electric field being considered, charge carriers hopping polarization model and charge carriers motion dependent conductance model are proposed. The trap energy level of oil-paper insulation decreases when the moisture content increases, thus, the motion characteristics of charge carriers change, leading to decreasing loss peak in middle frequency and inverse dielectric loss characteristics in low frequency. These two models introduce the influence of traps on carrier motion into the dielectric response and enable one to gain a better understanding of the dielectric properties of oil-paper insulation under high electric field strength. [ABSTRACT FROM AUTHOR]

The performance of high-voltage apparatus is seriously restricted by the surface discharge on dielectric materials. A surface charge density distribution scanning system is developed with a dc feedback-type electrostatic probe based on the field-nullifying technique. The residual charge density distribution after the occurrence of a positive leader discharge is measured. Using the 2-D Fourier transformation in the spatial domain and Tikhonov's regularization method, the surface potential distribution and electric field distribution are arrived at. The leader stem and radial lament streamer zone located at the head of the leader can clearly be distinguished from the distributions. There is an obvious knee point that divides the main channel of the discharge into a leader channel and a streamer channel. The knee point is at about 9 kV, which corresponds to about 320 pC/mm^2. [ABSTRACT FROM PUBLISHER]

In this work, samples of oil-immersed paper with different moisture levels and aging condition were studied. Frequency domain spectroscopy (FDS) of the samples was examined at 40 °C under different excitation voltages. A non-linear phenomenon in frequency domain dielectric response with different moisture contents and aging condition was observed. The non-linear phenomenon appears to be related to the hindrance effect of cellulose on the movement of ions and the Poole-Frenkel effect on the concentration of ions. With aging, the nonlinear loss peaks at different frequencies caused by the effect of nonlinear conductance and interface polarization appear in the FDS results. The non-linear phenomenon was modeled and verified experimentally. The characteristic parameters and quantitative method to estimate moisture content and the degree of aging degree of oil-immersed paper can be obtained by the proposed method. [ABSTRACT FROM AUTHOR]

The discharge mode transitions and the characteristics of atmospheric pressure dielectric barrier discharge in pure helium (He-DBD) are studied. Different discharge modes of He-DBD are clarified, including filament-diffuse mixed mode, filamentary mode, diffuse mode, and multichannels coupled mode. The transitions of different modes are explored by changing the gap width and applied voltage. According to the luminous structure of discharge [short exposure (100~300 ns) image], it is found that there should be only Townsend-like and glow-like discharges that appear in the above modes. In Townsend-like discharge, the secondary electron emission coefficient (SEEC) of dielectric is estimated. Based on which, a fluid model coupled with an external circuit is built to explain the He-DBD mode transitions mechanism and the applied voltage drop’s influence on He-DBD. Through the temporal evolution of a luminous structure, a discharge filament is believed to be a contractive state of discharge channel, which should be an unstable glow-like discharge. The radius of discharge channel, changing with gap width and applied voltage, is measured. The discharge contraction is attributed to the nonlinear relationship between Townsend ionization coefficient and electric field. The enhancing interaction between different discharge channels inhibits the appearing of diffuse discharge around discharge filament, leading to the transition from mixed mode to filamentary mode. In the end, the multichannels coupled mode is proven to be glow-like discharge, and its formation mechanism is explained by the overlap of discharge channels. [ABSTRACT FROM AUTHOR]