Enhancing Transformers Loadability for Optimizing Assets Utilization and Efficiency

The limited loadability of mineral oil filled transformers leads the users to select the transformer size necessary to meet the required peak demand; this causes the nominal rating to be higher than necessary for the typical demand resulting in transformers that have low annual average loading. The association of a transformer using high temperature insulation system, having both the solid (thermally upgraded Kraft paper) and liquid insulation (natural ester dielectric liquid) thermal classes higher than conventional, with the strategy of defining the nominal rating based on the conventional temperature rise limits lead to transformers holding a “reserve capacity”. This significant additional capacity without accelerated life consumption of the paper and no oil degradation enhances the loadability of the transformers and can be used to handle yearly peak demand by means of a smaller transformer. Brazilian utility CELESC adopted this concept for twelve natural ester filled transformers, which were installed and monitored in service for a period of one year. The reduction of the no-load losses of the smaller transformer resulted in 22.7% reduction of total dissipated energy in the period. Detailed results are presented in this study. Besides the advantage of improving system efficiency, the reduction of the nominal rating of the transformers also brings savings in transformer purchasing, and reduces environmental impact, mitigates the risk of fire, which are important issues for the utility.