Numerical Modeling of needle-grid electrodes for negative surface corona charging system

Abstract

Surface potential decay measurement is a simple and low cost tool to examine electrical properties of insulation materials. During corona charging stage, needle-grid electrodes system is often used to achieve uniform charge distribution on the surface of the sample. There is not much work that describes the effects of the needle grid geometry and voltage levels on the surface potential characteristics. Hence in this work the dynamic surface charge formation has been investigated through simulation. Using COMSOL Multiphysics axisymmetric finite element models have been developed to simulate the gas discharge. A well-known hydrodynamic drift-diffusion model was used. The model consists of a set of continuity equations accounting for the movement, generation and loss of charge carriers (electrons, positive and negative ions) coupled with Poisson's equation to take into account the effect of space and surface charges on the electric field. The boundary conditions for the needle-grid-electrode are similar to the needle-electrode except in convection and diffusion model for electrons, the grid electrode top surface was set to convective flux, the sides and bottom was insulation/symmetry; the needle grid was set to flux and its amount was controlled by a secondary emission flux equation. The simulation was performed under the following conditions: the needle electrode was set to -6500V and the grid electrode -1000V. It has been found that an initial impulse current appears at 0.6µs charging and Trichel pulse can be observed for longer charging time. The predicted electric field variations with time (around the impulse current appeared time) along the symmetry axis can be plotted to prove the corona phenomenon. The effect of adding a grid electrode can be clearly seen from the logarithmic plot of electron evolution. Finally, surface charge density on the sample has been obtained for further discussion.

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