Linear thermal expansion coefficients for an epoxy/glass matte-insulated solid cast transformer

Abstract

Glass matte/epoxy-reinforced composites provide high-quality electrical insulation, structural integrity, and environmental protection in solid cast transformers. The thermal expansion characteristics of those composites are very important; the thermal expansion must be compatible with the conductor of the transformer in order to minimize stresses and prevent decohesion between the composite and the copper. The glass matte orientation and loading greatly influence the thermal expansion characteristics of the composite. A section was removed from a glass matte/bisphenol A epoxy-insulated, copper conductor wound cylindrical transformer coil. The linear expansion coefficients of the glass matte/epoxy composite were determined by differential dilatometry for three mutually perpendicular orientations with respect to the cylindrical coil. The expected reduction in thermal expansion of the epoxy in the tangential and axial directions due to the glass matte, which produced improved thermal expansion compatibility with the copper windings, was demonstrated. The measured linear thermal expansion coefficients were compared with theoretical values derived from a model for thermal expansion of a two-dimensional isotropic composite filled with fibers randomly oriented in a plane. An alternate composite system used for solid cast coil transformers, consisting of a cycloaliphatic resin filled with silica flour, was also investigated for comparison.