Influence of the geometric orientation of cellulosic fibers on the electrical behavior of pressboard under DC stresses

Impregnated pressboard is currently one of the insulating materials under investigation for DC applications at the University in Schweinfurt. In HVDC (high voltage direct current) equipment, AC and DC stresses have to be regarded and therefore, permittivity as well as electrical conductivity play a role. Especially the conductivity is dependent on various parameters and one of the parameters, the geometric orientation of cellulosic fibers is in the focus of this contribution. Thus, air-impregnated pressboard sticks have been prepared and the conductivity has been determined in two configurations: Paper layers orthogonal and parallel to the electric field. It has been found that the anisotropic electrical behavior of oil-impregnated pressboard reported in literature can also be found for air-impregnated pressboard. Moreover, a physical explanation with respect to the transport paths in pressboard is given. This opens a new field of further research on the way of achieving desired pressboard conductivity values.