Anisotropy of the dielectric properties of laminated epoxy insulation subjected to water absorption [power generator insulation]

This paper describes experimental measurements carried out to study the dielectric properties of composite dielectric materials (glass cloth/mica/epoxy) used for the insulation of stator bars in high power generators. Due to the structure of the material, composed of a stack of layers, its dielectric properties are nonisotropic. If the material is polluted by water, water diffusion is much faster in the direction parallel to the layers. Dielectric measurements also show that the degradation of properties due to water (increase of DC conductivity, of dielectric losses) is more marked when the electric field direction is parallel to the layers.