Corona pulse rise time measurements between point plane electrodes in atmospheric air

Abstract

Extremely fast rate of rise voltage and current pulses (rate of rise in range of few tens of nanoseconds) are of great importance in recent years. They have applications in electrostatic Discharges (ESD) and Electromagnetic interference. Also, in medical sciences, fast rate of rise voltages with rise time in range of few nanoseconds are used for study of electroporation effects in biological cells. In view of these considerations experimental investigations have been carried out using point plane gaps for the positive polarity of the point electrode in atmospheric air under fair weather conditions for determination of corona pulse voltage rise time at instant of corona inception. The radii of tip of hemispherically capped cylindrical electrode varied from 0.5mm to 1.5mm and the gap spacing from 10mm to 50mm (approximately). Also, the phenomena have been investigated using electroplated brass electrodes, the electroplated materials being Nickel, Chromium and silver. The minimum rise time of corona pulse obtained in these experiments is 35 nanoseconds. In all these experiments the input voltage was increased continuously (rate of rise 2KV/Sec) from near zero value upto occurrence of corona inception voltage and results are characterized as steady state corona inception. Another set of experimental investigations has been carried out by applying overvoltage with extremely fast rise voltage to a point plane electrode system which is kept energized at a steady potential by applying a steady DC voltage of magnitude equal to 65% value of the steady corona inception voltage. The peak value of overvoltage corresponds to magnitude of voltage, which can cause 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> electron multiplications (threshold multiplication for streamer formation) close to the point electrode (highly non uniform field electrode configuration). The calculation of field intensities and electron multiplication have been made using ANSYS software version 9.0 (multiphysics) and numerical method. These investigations have shown that the rise time of pulse can be decreased from 80 nanoseconds which is the value recorded for the steady state corona inception phenomena. The reduced value in rise time can be 30 nanoseconds (approximately) for the case of overvoltage applied to point plane gap for hemispherical tip diameter of 1mm and gap distance of 10mm in atmospheric air under fair weather conditions.