Evolution of the electric field due to forced flow in oil pressboard ducts

The electrical insulation system of power transformers is a combination of mineral oil and pressboard materials. We have measured the influence of forced oil flow on the electric field stress in oil pressboard insulation system with applied DC potential using Kerr electro-optic effect. The data show a variation in the electric field strength in flow direction, depending on velocity, time and applied voltage. The observations are explained by variation in space charge distribution on the pressboard.