Impregnation Study of Porous Elements of High-Voltage Components

The impregnation of HV components is important if not critical for reliable operation. Fine windings, porous insulation such as kraft paper and pressboard, and similar elements need to be impregnated for good thermal conductivity, mechanical strength and, in the case of insulation, to minimize internal discharges. Although widely used, there appears to be little systematic work done to explain the important factors governing the basic process when a filled, thermosetting compound such as a filled epoxy is used in a pressure impregnation. In this paper we have derived an expression for the depth of penetration of a thermosetting compound into a porous substrate such as kraft pressboard. The equation is extended to the case of a filled thermoset where filtration of the filler occurs at the substrate surface. The expression derived relates filter cake permeability, substrate permeability, applied pressure and the viscosity-time relationship of the impregnating. filtered compound. Data are presented showing good agreement with the derived expression.