PURPOSE: To reduce the number of connections and reduce dimensions by arranging the complex connections of gas-insulated bus-bars between containers properly.

CONSTITUTION: A phase-isolated oil-filled transformer is composed of phase transformers 1 and an LVR (Loaded Voltage Regulator) 2 and each transformer 1 and the LVR 2 are connected by gas-insulated bus-bars 3. Branch bus-bars 4 for connection between the transformers 1 and the LVR 2 are used as the insulated bus-bars 3. Two gas-filled bushings or spacers a1, a2, b1, b2, c1, c2 are provided to an outlet for connection of each transformer 1 and connected to conductors 5a, 5b, 5c of the bus-bars 3 which run parallel to the center axis of the bus- bars 3. Excitor winding terminals a3, b3, c3 of the LVR 2 are also connected to the conductors 5a, 5b, 5c of the bus-bars 3. With this constitution, the number of the connections are reduced and the total dimensions are also reduced.