Thermal characteristics of a meta-aramid and cellulose insulated transformer at loads beyond nameplate

Advancement in meta-aramid-a synthetic, aromatic polyamide in which the phenyl molecules are connected in the meta orientation-paper and pressboard technology has lead to the production of a high temperature insulation system that can be used in power transformers, replacing cellulose materials in the windings, thereby eliminating the cellulose temperature limitation. A medium-power mineral oil-immersed transformer was constructed with a hybrid insulation system comprised of meta-aramid paper and pressboard insulated windings and cellulose bulk insulation. Fiber-optic temperature sensors were installed for direct reading of oil and conductor temperatures at key points throughout the transformer. The transformer was tested and thermal constants were determined in accordance with PC57.119/D12 for both ONAN and ONAF cooling modes at load levels up to 137% of the maximum nameplate rating of an otherwise identically designed all cellulose insulated unit. Temperatures predicted by equations in the ANSI/IEEE C57.92 Loading Guide were compared with measured values. Anomalies in the data are investigated and explanations offered.