Effect of impregnant on the electrical and mechanical properties of polypropylene paper

It has been recognized for some time, that oil impregnated cellulose paper will not be a useful insulating material for cable carrying in excess of 500 kV because the thickness of the insulation layer would become excessive and the dielectric losses associated with such thick structures would be prohibitive. Also, removal of the considerable heat generated in such a structure would become very difficult if not impossible. To overcome this problem, it has been suggested that the cellulose paper be replaced by some synthetic material possessing better dielectric properties than does cellulose paper. Paper-like structures obtained from such polymeric materials as polyphenylene oxides
¹
and polyolefins
²
have shown considerable promise as replacements of cellulose paper in cable insulation. While polyphenylene oxides exhibit many attractive properties for the contemplated use in oil filled cables, they are not yet available as an industrial product with fixed specifications. Polyolefins are commercially available in large quantities and at relatively low cost. In view of these considerations and the encouraging results reported by Buehler et al.
²
a more detailed study of polypropylene paper has been undertaken and the results are reported in the following sections.