Analysis of the electric field distribution on insulating supports of dry-type transformers under high temperature

One of the critical parts for the insulation design of dry-type transformers is the correct inclusion of the insulating supports between the high-voltage winding and the metallic clamps at the top and bottom ends. Breakdown along the surfaces of such supports has been observed once the transformers are in operation and therefore over-dimension is commonly adopted, adding cost and size to the transformer. In this work, a study of the parameters that affect the breakdown voltage in the insulation supports of a standard dip and bake dry-type transformer is performed. The electric field distribution on different profiles of these supports is modelled using the finite element method. By means of laboratory tests, the effect of the high operation temperatures of the transformer on the voltage breakdown in the supports is also investigated. In addition, modifications to the design of the currently used insulating support are evaluated, aimed at reducing its length without increasing the possibility of breakdown.