Boron nitride enhanced electric insulation paper for extending transformer thermal life

• Oil-paper-boron-nitride system promises doubling of transformer thermal life. • Novel material combination has three times better through-plane thermal conductivity, 25% higher dielectric strength, 30% smaller relative permittivity, and 40% higher tensile strength than conventional oil-paper insulation. • Micrometer scale boron nitride is key to the improved thermal conductivity. • Microfibrillated and nanofibrillated cellulous help maintain mechanical strength by adding hydrogen binding sites to replace those displaced by boron nitride. Increasing the thermal conductivity of the oil-paper insulation system can significantly extend the thermal life of transformers by enabling effective heat removal, which reduces high temperature-induced insulation degradation. This research shows that the addition of nominally 25 µm diameter boron nitride particles can achieve sufficient increase in through-plane thermal conductivity of insulation paper to an extent that doubles the transformer thermal life. Importantly, the dielectric breakdown strength can also be enhanced by adding boron nitride particles to the developed insulation paper. Moreover, applying lignin containing cellulose microfibrils into the paper can compensate for the paper strength loss due to the disruption of hydrogen bonding by the addition of BN particles. Building on these findings, we outlined a pathway for a boron nitride-based enhanced insulation system with outstanding through-plane thermal conductivity, enhanced dielectric strength, an appropriate dielectric constant and the needed tensile strength. Additionally, thermal aging experiments showed that the proposed material can have a reasonable thermal life under transformer operating conditions. Overall, this research shows that the mix of thermal, mechanical, and dielectric properties can be successfully tuned to achieve a beneficial insulation system which can significantly enhance the transformer life. As the summary, the proposed material has three times better through-plane thermal conductivity (0.76 W/m·K vs. 0.2 W/m·K), 23 % higher dielectric strength (84.6 kV/mm vs. 65.3 kV/mm), 35 % greater tensile strength (71.26 N·m/g vs. 45.91 N·m/g) and 33 % smaller relative permittivity (3.7 vs. 5.5) than the commercial insulation paper with similar thermal degradation rates.