Evaluation of High Temperature Operation of Natural Ester Filled Distribution Transformers

Natural ester filled transformers, having a high thermal class of solid and liquid insulation, can operate at temperatures up to 20°C higher than mineral oil equivalents; thus, holding additional loading capacity without losing life expectancy. An American utility - Salt River Project (SRP), wants to evaluate the use of this feature as an additional capacity for enhancing the loadability and/or extending the life of their distribution transformer fleet. Different transformer designing options to use this additional loadability are compared, allowing the utilities to select the optimum design for their application.Yearlong load profiles for SRP distribution transformers, sized conventionally on peak load demands, are analyzed for their oil, hot-spot temperatures and loss of insulation life using a Spreadsheet program. A new way of sizing ester filled transformers based both on average and peak load, instead of only peak load, called “Sustainable Peak Load Design” showed smaller size transformers can handle same yearly peak loads while keeping superior insulation lifespan. It is additionally possible to have reduction in total energy dissipation over the year. This proposed approach fits perfectly for units having a high peak but a low yearly average demand.Transformer units, which already have high average loads, are not suitable for such an approach. However, the use of natural ester filled units in such cases, keeping the same nominal rating, showed longer life expectation. A net present value cost benefit analysis quantifying benefits of the life extension and sustainable peak loading shows SRP distribution transformers can have substantial economic savings when the distribution transformer fleet is replaced with ester filled units.