Dielectric Properties of Natural Esters and their Influence on Transformer Insulation System Design and Performance

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. Considerable work has been done and is in progress to establish these criteria for Kraft insulation in natural ester fluids. The permittivity of natural ester fluid and various Kraft insulation materials including diamond pattern paper (DPP), low density pressboard and high density pressboard impregnated with natural ester is presented. A test program to determine the design criteria for interfacial stress (creep stress) is presented