Why is kinematic viscosity important for transformer oil cooling?

Kinematic viscosity determines how easily transformer oil flows through cooling ducts, windings, radiators, and pumps. Low viscosity improves natural convection and forced circulation, allowing heat to be transferred efficiently from windings to cooling surfaces. If viscosity is too high, oil flow becomes sluggish, hot spots develop, and insulation aging accelerates. Viscosity is especially critical at low ambient temperatures, where thick oil can delay circulation during startup and reduce cooling performance under load.