Dielectric properties of natural esters and their influence on transformer insulation system design and performance — An update

There is considerable knowledge of transformer insulation system design based on the use of cellulose insulation used in conjunction with mineral oil. This knowledge is based on over 100 years of transformer design and manufacture. Insulation design is based on the stress distribution between the solid insulation, in this case Kraft, and the fluid. In power and distribution transformers the stress is distributed in accordance with the permittivity of the various insulation components. The insulation designer must determine the stress in the fluid, in the solid insulation and along the interface. Design curves have been established for mineral oil, which give limits to the allowable stress at each of these critical areas. This paper presents the results of a test program to develop these criteria for Kraft insulation in natural ester fluids. This paper is an update to a paper presented at the IEEE T&D Conference in Dallas Texas in 2006. At that time we had not completed all of the dielectric tests. These have now been completed and are presented in this paper.