abstract: The lifetime of a transformer is essentially determined by the life of its insulation system which is a time function of the temperature defined by its thermal class. A large quantity of studies and international standards have been published indicating the possibility of increasing the thermal class of cellulose based materials when immersed in natural esters which are superior to traditional mineral oils. Thus, a transformer having thermally upgraded Kraft paper and natural ester dielectric fluid can be classified as a high temperature insulation system. Such a transformer can also operate at temperatures 20C higher than its mineral oil equivalent, holding additional loading capability without losing life expectancy. This thesis focuses on evaluating the use of this feature as an additional capability for enhancing the loadability and/or extending the life of the distribution transformers for the Phoenix based utility - SRP using FR3 brand natural ester dielectric fluid. Initially, different transformer design options to use this additional loadability are compared allowing utilities to select an optimal FR3 filled transformer design for their application. Yearlong load profiles for SRP distribution transformers, sized conventionally on peak load demands, are analyzed for their oil temperatures, winding temperatures and loss of insulation life. It is observed that these load profiles can be classified into two types: 1) Type-1 profiles with high peak and high average loads, and 2) Type-2 profiles with comparatively low peak and low average load. For the Type 1 load profiles, use of FR3 natural ester fluid with the same nominal rating showed 7.4 times longer life expectation. For the Type 2 load profiles, a new way of sizing ester filled transformers based on both average and peak load, instead of only peak load, called “Sustainable Peak Loading” showed smaller size transformers can handle the same yearly peak loads while maintaining superior insulation lifespan. It is additionally possible to have reduction in the total energy dissipation over the year. A net present value cost savings up to US$1200 per transformer quantifying benefits of the life extension and the total ownership cost savings up to 30% for sustainable peak loading showed SRP distribution transformers can gain substantial economic savings when the distribution transformer fleet is replaced with FR3 ester filled units. / Dissertation/Thesis / Envirotemp FR3 Fluid Brochure / Envirotempe FR3 Fluid Data Sheet / Masters Thesis Electrical Engineering 2018