Electric field at sharp edge as a criterion for dimensioning condenser-type insulation systems

Inside the oil-paper insulation of high voltage condenser-type bushings and instrument transformers, conducting surfaces or capacitive shields have been in use for many years to control electric field distribution. Traditional insulation design methods take into consideration dielectric stresses in axial and radial directions, but experience shows that partial discharges occur in the vicinity of capacitive shield edges and can severely affect the expected life of oil-paper insulation. In this paper, a criterion for dimensioning condenser-type insulation systems is presented, based on maximum electric field at sharp edge of capacitive shield. Maximum permitted value of the electric field at the shield's edge was obtained through numerous experimental tests and numerical field calculations based on the finite element method (FEM).