Why are oil cooled transformers preferred for high MVA ratings?

Oil cooling enables much higher heat removal than air cooling, allowing higher power density and stable hot spot control at large MVA ratings. Oil also provides dielectric strength that supports high voltage insulation systems without extreme size increases. For high MVA transmission transformers, dry-type designs would be too large, difficult to cool, and impractical at required voltage classes. Oil cooled designs also enable advanced monitoring such as DGA, which is valuable for critical, expensive assets.