Until 1980s, Air Insulated Switchgear (AIS) had been used in almost all medium voltage substations. And due to the technological development of the insulation design, the size of the switchgear became smaller and the installation area was reduced. But when smaller switchgear was demanded because of the rapid increase of land price, Cubicle Type Gas Insulated Switchgear (C-GIS) using SF6 gas as an insulation medium was adopted. After COP 3 was held in Kyoto, Japan in 1997 and SF6 was specified in the greenhouse gas, the products not using SF6 were required. Then, in 2002, Toshiba developed Solid Insulated Switchgear (SIS).
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