AbstractConstruction of 275‐kV and 500‐kV substations in densely populated areas is thought to be necessary in the future to maintain a stable supply of electricity. The use of gas‐insulated transformers is now considered for substations because such transformers are non‐flammable and nonexplosive. Since the completion of the 77‐kV gas‐insulated transformer in 1979, we have continued research to develop gas‐insulated transformers applicable to a higher voltage level up to 500 kV. As the results of the research, 275‐kV, 300‐MVA a liquid cooling‐type gas‐insulated transform