A9FC671Cd01 - Distribution of electric fields in a rod to...

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Distribution of electric fields in a rod to plane gap at the onset of negative corona L.C. Thanh, B.E., M.Eng. Sc, Ph.D. Indexing terms: Corona, Electric fields Abstract The distribution of electric fields in close proximity to a rod cathode at the onset of a negative corona are calcu- lated using the charge-simulation technique. In this method, the highly stressed cathode is modelled by a number of line charges and point charges. As the potentials due to these simulated charges satisfy Laplace's or Poisson's equation inside the space under consideration, the solution is unique. The results are applied to calculate the onset field strength and its distribution along the axis of symmetry. The onset of the corona is defined as the transition from a nonselfsustained discharge to a selfsustained one, and it takes place when the cathode field strength reaches a critical value. The theoretical study is carried one step further by calculating the corona-current waveshape produced predominantly by electrons. A comparison is also made between calculated and measured values near fields and current waveforms. 1 Introduction The electric field in the vicinity of a rod cathode is evaluated for a rod to plane electrode system by using the charge-simulation technique. The results of the field calculation are applied to the evaluation of the corona onset field strength and voltage, and subsequently, to calculate the magnitude and length of the corona current. Generally, electric fields in the proximity of the cathode of a rod to plane gap could be calculated by the finite-difference technique, 2 the successive-image technique, 3 and the charge-simulation technique. 4 The charge-simulation technique is used here since it is reasonably simple, requires much less computing time than the other two methods and gives good accuracy. The criterion of the corona onset is based on Townsend's theory, with a modification being made to suit the condition of corona onset in a nonuniform field. The transition from a nonselfsustained discharge into a selfsustained one is determined by the availability of secondary electrons emitted from the cathode. This is somewhat different from the condition in a uniform field where Townsend's criterion is attained when the conduction current increases rapidly to infinity. Finally, the corona-current waveshape is calculated by taking into account the quenching effect of negative-ion space charge. The field of the space charge becomes sufficiently strong to stop the ionisation in the vicinity of the cathode when the applied potential is higher than the onset value by a few hundred volts. V=i 0; Fig. 1 Diagram of hemispherically capped rod to plane electrode system with image of rod electrode Paper 8264 S, first received 24 th October and in revised form 15th November 1978 Dr. Thanh is with the Department of Electrical Engineering, Ballarat College of Advanced Education, Mt. Helen, Victoria 3350, Australia 270 0020-3270/79/030270+06 $01-50/0 2 Electric field strength
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A9FC671Cd01 - Distribution of electric fields in a rod to...

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