Investigation of long term stability under AC voltage stress of syntactic foam

Insulation materials of the future are confronted with a wide range of requirements such as lower weight and higher operating temperatures. Syntactic foam represents an alternative to conventional insulation materials. It consists of a polymer matrix as epoxy resin or silicone (called binder) and hollow spherical particles with a diameter in the range of 30-100 μm (microspheres) which are mixed into the binder. Syntactic foam features good electrical properties and distinguishes itself with a very low density at low material costs. Furthermore, variations of the filling degree and the kind of the microspheres used provide options to adapt a wide range of material properties for special applications. In this study syntactic foam based on epoxy resin with coated polymeric microspheres is used, which features high breakdown strength under short term ac stress. However, insulation materials have to withstand the electrical voltage stress over their complete lifetime. Therefore the long term stability under ac stress is investigated for different compounds of syntactic foam. The study includes the variation of the filling degree of microspheres and the influence of different diameters of the microspheres on the long term stability as well as the influence of additional silica particles in the syntactic foam. Partial discharge measurements are performed to discuss the influence of the different syntactic foam parameters. These investigations result in a lifetime model for syntactic foam under ac stress. This allows adjusting the material properties for each special application, which leads to the best performance of the insulation materials to provide a high reliability.