Application of gas insulated transformers to underground substations in Japan

Concentration of population and business activities result in high electricity demand in urban areas in Japan. Underground or indoor substations are required to comply with the increasing demand for electricity. Electric utility companies in Japan studied this situation and came to conclude that underground or indoor substations applying gas insulated transformers (GIT) together with gas insulated switchgears (GIS) would be the best solution. Not only 22/33 kV and 66/77 kV but also up to 275 kV class GIT have been applied to underground or indoor substations. More than 100 units and 6000 MVA GIT are owned by electric utility companies in Japan and they are operating satisfactory. This paper presents examples of GIT applied to underground or indoor substations and experiences operation and maintenance in Japanese electric utilities.