Oil cooled transformers are preferred for high MVA ratings because oil provides superior heat transfer compared to air. This allows compact designs with higher power density and controlled winding hot spot temperatures. Oil also offers high dielectric strength, enabling insulation systems suitable for very high voltages. Dry type transformers would require significantly larger size and complex cooling to achieve similar ratings. For transmission and large substation applications, oil cooled designs deliver better thermal stability, reliability, and long term performance.
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