Overload Distribution Transformer With Natural Ester and Aramid-Enhanced Cellulose

The load of a distribution transformer can be daily or seasonal fluctuant, which submits the transformer to the risk of overload through thermal stress. In this research, it is demonstrated that the aramid-enhanced cellulose in natural ester, as a high-temperature insulation system, is a solution by which the overload capability of a transformer can be improved. First, the thermal and ageing model stipulated in IEEE C57.91 Annex G, which was proposed to evaluate the Kraft and thermally upgraded Kraft, has been modified to incorporate the ageing rate benchmark derived from Arrhenius law, so the improved model can now cover various combinations of insulation systems. The top liquid temperatures, hot spot temperatures and equivalent ageing factors of transformers with aramid-enhanced cellulose in natural ester and Kraft in mineral oil under different load scenarios generated from field loading data were analyzed. The results showed that the overloading capability of the transformer with aramid-enhanced cellulose was improved, as its equivalent ageing factor was 13 percent of the equivalent ageing factor of the transformer with Kraft under the heavy overload scenario. Furthermore, an overloading test and field test were also conducted for a prototype transformer with aramid-enhanced cellulose and natural ester. The measured top liquid temperature and hot spot temperature and the calculated equivalent ageing factor after the overloading test verified that the transformer with aramid-enhanced cellulose operated well below the safety thresholds. Additionally, the results from the transformer upon installation in the field proved that the transformer also operated well under an overloading status.