PURPOSE: To acquire protection characteristic and make easy the connection at the installation field by sealing insulation gas within a connecting duct and also connecting a gas insulation type arrester provided between unit transformers to a lead conductor within the connection duct.

CONSTITUTION: The unit transformers 1, 2 have the similar constitution wherein the transformers 7, 7' themselves winding parallel windings 5, 6 and 5', 6' to the main leg of the 4-leg iron cores 4, 4' are housed within the tanks 3, 3' together with insulation oils 8, 8'. The wingings 5, 6 and 5', 6' are respectively connected in parallel and the winding of a single phase is formed by connecting thems in series through a conductor in the connection duct 9. The duct 9 comprises insulation spacers 17, 17 at both ends thereof and is insulated from the earth by sealing the insulation gas 19. Moreover, an arrester 16 is installed between the transformers 1, 2, connected to the duct 9 by the insulation spacer 21 through the arrester duct 20, and thereby a conductor 22 is connected to a lead wire 18. The distance between the arrester 16 and transfermers 1, 2 are shortened, acquiring the protection characteristic. Connection at the installation site can also be simplified.