Influence of geometric structure and material properties on dielectric frequency response of composite oil cellulose insulation

Dielectric frequency response (DFR) is a versatile tool for assessing the properties of practical insulation systems. A real system often consists of combinations of several different materials with, in general, different dielectric properties. The dielectric response of the system will be a function both the structure of the system and the dielectric properties of the individual materials. In this paper we analyse how the materials properties as well as the geometric arrangements affect the response of transformer insulation. The insulation system of power transformers is a typical case of a composite insulation system consisting of two main components: mineral oil and cellulose. To study the influence of the geometry and materials we make use of simple circuit models as well as complex finite element calculations (FEM) with frequency dependent materials properties deduced from measurements on laboratory samples.