Since the 1970s, Tokyo Electric Power Co. (TEPCO) has been extending its extra-high-voltage (EHV) transmission network in the center of Tokyo, Japan, to supply the increasing power demand. From the inner and outer circle of bulk power transmission systems that surround Tokyo, many of the 275-kV and some of the 500-kV power cable circuits are installed in the city’s metropolitan areas. The majority of the EHV substations in these areas are located in the basement of buildings to maintain environmental harmony with the adjacent streets.

The practice of installing underground substations also is a business decision providing an effective use of the city’s land. It minimizes the long-term loss of tenant income to the property owners that would occur if the substation was sited at ground level. Alternatively, there are instances within the city where a redevelopment project leads to the need to replace an existing underground substation with a larger-capacity substation to supply the increased demand for power.

In the construction of an underground substation in dense urban areas, the most important consideration is disaster prevention. Another consideration is decreasing the floor-to-ceiling height of the underground substation, which can reduce the total cost of construction significantly. In these situations, the installation of large-capacity gas-insulated transformers (GITs) in underground substations is a key factor. Apart from the reduction in total construction costs, underground substations with GITs are designed to provide a reliable level of disaster prevention and a stable electric power supply to the city.

TEPCO, increasing capacity
The configuration of the 275-kV and 500-kV transmission system that supplies power to Tokyo.

Features and Advantages

The large-capacity GITs are designed with high gas pressures (0.53 MPa-g [77 psi-g]) to improve the insulation and cooling capacity. However, GITs with a capacity not exceeding 100 MVA use comparatively low gas pressures (0.12 MPa-g [17 psi-g]).

The GIT’s core and winding consist of a simple structure similar to that used in conventional oil-immersed transformers. The core and winding are cooled and insulated by high-pressure SF6 gas. Taken from the lower side of the tank, the gas cools the core and winding, is collected in the upper part of the tank and is returned to the radiator and cooled. The transformer has a disc-type winding, and the internal gas flows in a zigzag line by the gas stop collar. An excellent polyethylene terephthalate (PET) film, which is relatively heat-proof when compared with the insulating paper, is applied to the stranded winding insulation.

TEPCO, increasing capacity
The schematic structure of a 275-kV, 300-MVA GIT.
TEPCO, increasing capacity
The winding structure of a 275-kV, 300-MVA GIT.
TEPCO, increasing capacity

Large-capacity GITs have several advantages:

• Only nonflammable materials are used; hence, there is no need for special firefighting facilities in the substation.

• GITs have excellent nonexplosive performance. In the event of an internal failure, the design and structure enclosing the compressible gas are superior to that used in the design of oil-immersed transformers. Therefore, nonflammable and nonexplosive characteristics make GITs the safest large transformers.

• Unlike oil-immersed transformers, GITs are a conservator-free structure, reducing the overall height of the transformers. In practice, this results in a 2-m to 2.5-m (6.5-ft to 8.2-ft) reduction in the height of the transformer chambers. Also, it is not necessary to install an oil basin or bung under the transformer. In addition to reducing the substation construction costs because of the lower height and smaller area required for the transformer chambers, there is no potential hazard from oil spills.

• GITs can be installed together with gas-insulated switchgear in the same room because similar SF6 gas pressures are used in these components. This can provide high compactness and flexibility in the design layout of the substation.

•Gas-insulated on-load tap changers (OLTCs) with vacuum interrupters are used on GITs. This also contributes to the nonflammable characteristic of GITs and makes maintenance work easy. Moreover, the installation of large GITs in suburban areas with mountainous districts and in environmental protection areas has removed the anxiety of oil spills. Substations equipped with GITs have been positioned to supply railway and private organizations.

TEPCO. increasing capacity
Example of substation layout with a direct connection to GIS.