Simulation of trichel pulse in air and study of distribution of trichel pulse intervals

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Simulation of trichel pulse in air and study of distribution of trichel pulse intervals

 

Author: Chan, Ka Wing
Title: Simulation of trichel pulse in air and study of distribution of trichel pulse intervals
Degree: M.Sc.
Year: 1996
Subject: Corona (Electricity)
Electric discharges
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Pages: iii, 121 leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1219785
URI: http://theses.lib.polyu.edu.hk/handle/200/2850
Abstract: When an appropriate negative voltage was applied to the point of a hemi-spherical concentric electrode system in an electronegative gas, a pulsating corona current is observed in the external circuit. The current waveform consists of pulses having a very fast rise time and short duration, with a long period of relatively low current between the pulses. In air the pulses are extremely regular and are called Trichel pulses, after G.W. Trichel, who first reported the phenomenon. In pure SF6, similar pulses occur but they are irregular. In non-attaching gases such as pure nitrogen, only a continuous corona current without pulses is observed. Although many authors have studied the mechanism of the formation of Trichel pulses in air, oxygen and SF6, they confined their studies to the first pulse, neglecting the subsequent formation of pulses. In this dissertation, the first Trichel pulse was started from one seed electron which left the cathode of hemi-spherical concentric electrode system and started the Townsend ionization process. The second pulse was assumed to be started by the combination of field emission, photoemission and positive ions bombardment onto cathode. A programme has been developed to give theoretical predications of the development of the current and the distributions of space charge and electric field in negative corona, or Trichel current pulse in air at various pressure. A hemi-spherical concentric electrode with inner radius 1.5mm and outer radius 30mm was used in the simulation. The simulation was based on the accurate numerical solution of Poisson's equation and the Townsend ionization process. In the simulation, the gap between cathode and anode was divided into 2 regions with a number of cells. Since most Townsend ionization took place in region 1, the cell separation in region 1 was small compared with the cells in region 2. In region 2, the cell separation was larger for efficient simulation since zero Townsend ionization occurred. In conventional approach in [10], fixed cell separation and unique simulation time step were used under different voltage and pressure combinations. But it was obviously not an appropriate and effective approach since the mean free path (m.f.p.) of electrons and ions were proportional to temperature and inversely proportional to applied pressure. At low pressure, long m.f.p. would result in long simulation time without significant improvement in accuracy if the cell separation was not adjusted accordingly. In the simulation, the cell separation in region 1 was set to be equal to a fixed number of m.f.p. of electrons so that the cell separation would automatically be adjusted at different pressure. Moreover, constant time step was used in conventional simulation in [1], [10], [28], and it was not an effective way in an abrupt change of electric field. Drift velocity was equal to the product of mobility and electric field strength. Abrupt change of electric field implied that abrupt change of drift velocity would result as electrons travelling across the gap. Since Townsend ionization took place in the high field region, small time step resulted in high accuracy in high field region but inefficiency in low field region as no Townsend ionization occurred. In the simulation, dynamic timing approach was developed and each simulation time step was equal to the travelling time of electrons, which was closest to the cathode, across the cell. In this way, the travelling time of electrons closest to cathode was the smallest figure compared with the travelling time of electrons in the rest of gap and positive ions in the whole gap. To verify the accuracy of the simulation programme, experimental data were retrieved in the High Voltage Laboratory and analyzed. Comparison showed a close agreement at different voltages and pressures. Moreover, the fully informative display of the simulation can help students and research workers understand the complicated process in the development of Trichel pulse in air as a teaching aid.

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