Surface discharge dynamics: theory, experiment and simulation

Abstract

The use of solid insulators in electrical generation, transmission and distribution is widespread. However, the accumulation of charge on the insulator surface has proved to be one of the major factors contributing to system failures. This research work is aimed at studying the dynamics of surface discharge in theory, by simulation and experiment.

Different surface charging theories have been reviewed and classified according to electric field uniformity. The focus is on basic processes involved in the formation of positive and negative surface discharges. The experimental work utilises the non-destructive quantitative Pockels technique to measure surface charge density distribution. Practical considerations of the Pockels experiment together with image processing techniques are discussed in detail. Using this technique, various factors which influence the surface discharge dynamics have been studied including the effects of the applied voltage waveform, electrode shape and local gaseous environment. Results obtained using positive/negative square wave, ramp and sinusoidal voltages are reported. The impact of using a mushroom electrode instead of a needle electrode is also analysed. In addition, various insulation gases have been experimented namely dry air, N2, CO2 and their mixtures with SF6. Surface discharge measurements have been performed in these gases at various levels of pressure.

Surface discharge modelling and simulation studies have also been undertaken. The simulation principles are based on a system of coupled hydrodynamic equations consisting of continuity and Poisson's equations. By solving these equations, the movement and interaction of charged particles and transient electric field can be simulated and used to verify the discharge theories and experimental results. Due to the asymmetric filamentary nature of positive surface streamers, the development of a positive surface discharge is separated into two phases. The first phase involves the axial streamer development in the gas gap between the needle electrode and the dielectric surface. This phase is simulated in 2D axial symmetry space dimension by the finite element package COM-SOL. The second phase simulates the streamer propagation in 1D along the dielectric surface by using the field results from the first phase. This part of the model is solved by the accurate flux-corrected transport algorithm. The effects of model parameters on the simulation results are discussed and a comparison with experimental data made. Prior to the simulation of a negative surface discharge, a negative corona discharge model in 2D axial symmetry has been analysed (Trichel pulses). The model behaviour is studied with reference to experimental data as model parameters are varied. When the insulators are introduced, the accumulation of surface charge distorts the electric field leading to the formation of only one discharge current pulse. The simulation charge density distribution is in good agreement with results obtained from the Pockels experiment.

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Identifiers

ORCID for P.L. Lewin: orcid.org/0000-0002-3299-2556

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