Why is dehydration of transformer oil necessary during maintenance?

Dehydration removes dissolved and free water that reduces dielectric strength and accelerates paper insulation aging. Moisture can enter through breathers, leaks, poor handling, or temperature cycling. Even if BDV looks acceptable, high moisture increases partial discharge risk and can trigger failures during switching surges. Vacuum dehydration also helps remove dissolved gases and improves insulation reliability after repairs or oil filling. Dehydration is typically performed when moisture ppm rises, BDV drops, or trending indicates worsening dielectric losses and increased risk to the insulation system.