On-line measurement of pulsed electric-field of insulator surface in vacuum based on Kerr effect

Abstract

Summary form only given. It is well known that the vacuum flashover voltage of an insulator bridging two electrodes is much lower than the breakdown voltage of the same gap without the insulator. This reduction is generally attributed to the insulator surface charging resulting in the modification of inter-electrode field on insulator surface. So, a measurement on surface field and the related surface charging makes sense. Accordingly, an online electro-optical measurement system on insulator surface field, based on Kerr effect, was established. In this work, a high voltage pulse of 100 ns in pulse width and 100 kV in amplitude was stressed on a hollow insulator of 1.5cm high, by which two plane-parallel circular electrodes were bridged. Both insulator and electrodes were positioned in a test cell filled with propylene carbonate as Kerr liquid, and the inner space of insulator was pumped into vacuum. Since the insulator was a hollow one, and Kerr liquid was separated from vacuum by a thin insulator wall of about 1.6mm, the measured electric field close to the liquid-insulator surface is assumed to be equivalent to that close to the vacuum-insulator surface. This test cell was installed between two optical polarizers oriented orthogonally, of which each polarization direction was set 45°with respect to that of electric field stressed on insulator <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> . YAG-laser pulse of 10 ns in FWHM as testing light passed through these two polarizers and cell to detect the Kerr effect, and the intensity distribution of transmitted light was finally recorded by a digital camera. A synchronous control subsystem was also set up to ensuring that the laser pulse can reach the cell during the voltage pulse being stressed on insulator, hence an online measurement was performed. Several images exhibiting the transmitted light intensity distribution resulting from Kerr effect are obtained, which show a fringe pattern in the electrode-gap area. By comparing with the similar measurement without insulator between electrodes, a fringe distortion and shift is observed obviously close to the liquid-insulator interface area, implying a field modification on both liquid-insulator and vacuum-insulator surface. Accordingly, an existence of vacuum-insulator surface charging is also proved.