Electric field interaction in the inter-phase region of transformers

Large three-phase power transformers normally consist of three cylindrical coil sets (phase windings) assembled side by side in line. The high voltage winding of each phase is generally the outermost coil. The space between the phase windings is called the inter-phase region. Insulating sheets of pressboard, are placed between the phase windings to form dielectric barriers to prevent flashover between the phases. The insulation design is often a multi-layer arrangement of boards and oil gaps. A small number of transformers have been affected by the phenomenon of creeping discharge on the surface of the pressboard barriers. The mechanism of this phenomenon is not well understood. The paper describes the test facility which has been developed to model the inter-phase barrier region. Two independent electric fields are used to re-create the electric field profile in the barrier region. A discharge source is introduced into the resultant electric field and applied onto the pressboard surface to explore the effect of the electric field, both along and perpendicular to the barrier, on the discharge. Initial results show that the magnitude and density of discharge is affected by the phase relationship between the interacting fields.