Why are oil-filled transformers preferred for high-voltage outdoor applications?

Oil filled transformers are preferred because oil provides both strong dielectric insulation and effective cooling, enabling compact designs at high voltage ratings. Outdoor installations often face high load cycles and ambient temperature swings, and oil cooling systems handle these stresses well. Oil also allows for robust insulation coordination for bushings and internal clearances. With proper sealing, conservators, and maintenance, oil filled designs offer high reliability and long service life compared with many dry type alternatives at equivalent voltage levels.