Measurements of electric field and charge on insulator surface

It is well known that the flashover voltage of an insulator between two electrodes is much lower than the breakdown voltage of the same gap in the absence of insulator. The reduced value of the breakdown voltage is generally attributed to the surface charging of the insulator and resulting inter-electrode field modification. In order to correctly design an insulator, it is desirable to measure the distribution of the electric charge and field on the insulator surface. Consequently, the techniques for the measurements of the electric charge and field on an insulator surface are reviewed. Field-emitted electrons, partial discharge, dielectric interface polarization, variations in bulk or surface conductivity, non-linear field-dependent conductivity, and metallic particles are responsible for the charge accumulation on insulator surface. Three techniques for the measurements of the electric charge and field on the insulator surface are described and compared, including capacitive probe, dust figure and electro-optical measurement based on Pockels effect or Kerr effect.