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World's First Large-Capacity Gas-Insulated Transformer

275 kV-300 MVA large-capacity nonflammable transformers with high-pressure SF6 gas used for both insulating and cooling medium, have been completed.

World's First Large-Capacity Gas-Insulated Transformer

In 1967, Toshiba delivered nonflammable 66 kV-3,000 kVA transformers, using sulfur hexafluoride (SF6) gas instead of insulating oil for the first time in Japan, to the underground substation facility of The Dai-ichi Mutual Life Insurance Company (currently The Dai-ichi Life Insurance Co., Ltd.). The use of SF6 gas as an insulating medium started to spread widely and rapidly in the field of switchgears due to its high insulating performance and its advantage of nonflammability. In the case of transformers, however, use of SF6 did not expand like switchgears because of the quite weak cooling capability. As the result, the application of SF6 for transformers was limited within small transformers only. On the other hand, needs of large capacity gas insulated transformers for underground substations in urban area were getting higher and higher considering its excellent safety features. So the development of a largecapacity gas-insulated transformers (Large GIT) was a focus of expectations.

Early in 1980s, the development of large-capacity nonflammable transformers started in United States with the capacity around 300 MVA. But this plan was cancelled due to fiscal tightening by the U.S. government. In Japan, inspired by the plan above, the development of Large GIT has started in 1983. At that time, SF6 gas was believed to have insufficient cooling capacity by itself to realize Large GIT, so the concept of using fluorocarbon liquid as cooling medium was examined. Toshiba started its development with “Separate Cooling Structure”, applying independent metal cooling panels with fluorocarbon liquid between layers of sheet type winding insulated by PET film, which was originally tried by GE once. Transformer manufactures in Japan, each worked on development of Large GIT with its own invented structure and mechanism.

In 1989, Toshiba has completed its development as the world’ first Large GIT, with ratings of 154 kV-200 MVA, for the Asahi Substation of Tokyo Electric Power Company (TEPCO). Then, in 1990, Toshiba has expanded the application to the rating of 275 kV-300 MVA for TEPCO’s Shin-Sakado Substation.

Although Toshiba has realized Large GIT, the special structure with the built-in cooling panel to cool the coils, and the application of very expensive fluorocarbon liquid, raised the cost of Large GIT three times higher than that of the conventional oil type transformers. Considering commercial base continuity, Toshiba started further development of direct cooling with SF6 gas itself. In order to increase the cooling capacity, a development team was formed in Toshiba’s own Laboratory, and they started the research on measures to improve the cooling properties of the gas, the flow of gas inside the transformer coils, and the new structure to change the flow deviation of gas. These efforts resulted in a number of achievements and have reached the completion of 275 kV-300 MVA Large GIT with only SF6 as insulating and cooling medium in 1994 for TEPCO’s Higashi-Shinjuku Substation. It has realized with combination of technologies such as the boosting of gas pressure, the adoption of a highly heat-resistant insulating material that allowed the operation temperature higher, the development of a large-capacity high-pressure gas blower, and equalization of the temperature of the coil through detailed analysis of gas flow control in the coil. The rating of Large GIT developed here is one of our standard models and have been on sales since that time.

In the same year, Toshiba also commercialized 275 kV-150 MVA gas-insulated shunt reactors and delivered it to TEPCO’s Katsunan Substation. This marked a new era of gas-insulated transformers and shunt reactors that did not require a special system for cooling. As the result, Toshiba has realized “Oil-less substation” with gas-insulated equipment of this type and it has been replacing the oil type underground substation equipment as the mainstream of TEPCO’s underground substations. These gas-insulated systems have been adopted by other electric power companies in Japan as well, and also started to be applied in other countries, such as Australia, England, Russia, United States and Canada, as a key component for the safe and reliable underground substations in urban areas.

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