Transformer with improved natural circulation for cooling disc coils

An electrical transformer comprising a tank containing liquid insulating coolant in which is immersed at least one vertically oriented magnetic core element and associated surrounding flat coil transformer winding, said flat coil winding being comprised by a stack of equal sized flat coils vertically spaced from one another to define a plurality of horizontal cooling ducts therebetween, a pair of continuous duct walls respectively mounted inside and outside said flat coil winding to define inner and outer vertical cooling ducts of predetermined width in the horizontal direction for upward flow of said coolant by natural circulation, and a plurality of vertically spaced, peripherally continuous partial flow barrier inserts mounted in said inner and outer vertical cooling ducts in staggered relation without intervening barrier inserts at a uniform vertical spacing of several of said flat coils between each pair of staggered partial flow barrier inserts, each of said inserts extending only partially across its respective vertical cooling duct, and said inserts alternately partially blocking said vertical cooling ducts and forcing flow of said coolant through said horizontal cooling ducts.