Oil filled transformers dominate substation use because they can handle high voltages and large power ratings with efficient cooling and strong insulation. The oil immersion allows compact winding arrangements, high dielectric strength and robust thermal performance, which are critical in grid substations that operate continuously and see fault currents and transients. With radiators, fans or pumps, oil systems support multiple cooling modes. Properly maintained oil insulation provides long service life, and industry standards, monitoring methods and repair practices are well established.