Physical based criteria of electrical breakdown in transformer insulation

Breakdown in insulating liquids can be described satisfactorily by the Thomas-Watson-Forster hypothesis. This approach was developed and supplemented concerning criteria of assessment: the size of partial discharge region and the field on-axis at the surface, the electrostatic component of pressure, the wall velocity (mobility), the conditions of propagation. In the paper are suggested the computer models which allow to assess the electrical strength of insulating system under the stress of lightning and switching impulses, AC and long-time test voltage (thousand of hours). It can be used for determining the inception voltage for different uniform configurations. Conductor insulations and barriers disposed between the electrodes are studied in addition to the oil gap between bare electrodes. These models take into account the discharge development on the solid/liquid interface. Experimental and theoretical results are in satisfactory agreement