The anisotropic dielectric constant of oil-impregnated, laminated compressed wood and the effect on the electric field distribution

Laminated compressed wood is one of the most important insulation materials for liquid-immersed power transformers. It is used for pressure beams, platforms, coil clamping rings, lead and cleat supports, step blocks, and fasteners, e.g., threaded rods and nuts. In many insulation configurations the live part does not directly contact the solid insulation part, but there is an oil gap between these two elements. In such setups the electric field distribution is nonuniform and depends on the permittivities of the involved materials. On the basis of the dielectric anisotropy of laminated compressed wood, it can be assumed that the electric field strength not only changes at the oil-wood interface but also from veneer to veneer and even in the oil. Hence, for planning and constructing insulation systems, it is important to know the permittivities of the different veneers. For mechanical reasons the permittivity of wood parallel to the fibers cannot be directly measured. It has to be determined from measurements on samples of crosswise laminated compressed wood. Using the determined permittivities the effects of the anisotropy can be simulated.