Evaluation of structure influence within the context of water diffusion and surface current of glass reinforced polymer composites for HV insulation

Abstract

This article deals with the issue of selected polymer composites consisting of a thermoset matrix and reinforcing glass fibers. These composites are designed for high voltage applications as an electrical insulating material with high mechanical durability. Three composites with a significantly matrix, specifically, epoxy, polyurethane and polyester resin, are selected with an emphasis on usability in the technical practice. Key properties of tested composites are dielectric and mechanical properties and from this point of view the homogeneity of prepared specimens is very important. An important influencing factor, which is a frequent cause of deterioration of material properties, is a higher level of moisture or even direct contact with water leading to the diffusion of water into the material structure. The main subject of presenting research is therefore the assessment of the water diffusion impact on processed material in the relation to the resistance of the tested specimens to the applied high voltage. Tested composite structures are observed by optical microscopy and the computed tomography is used for analysis of the material volume. Purely surface dielectric properties in the delivered state were evaluated based on the measurement of surface current at a constant direct or alternating test voltage. Results of surface current measurement for all composites in the delivered state show minimal differences between composites. The minimal dependence, primarily for composite with polyurethane, of the water diffusion test results on the internal structure and on the character of voids within tested specimens is determined. The lowest level of voids was found for epoxy matrix composite, but this material did not pass the water diffusion test. Differences of the HV resistance obtained from the water diffusion test are most likely caused mainly by different chemical resistance of resins against boiling slightly alkaline aqueous solution.