Although the Japanese transmission and distribution system has been integrated at the national level since 1958, it comprises two geographically separate networks each operating at different frequencies. The Chubu, Hokuriku, Kansai, Chogoku, Shikoku and Kyushu EPCo networks covering Kyushu, Shikoku and southern Honshu operate at 60 Hz whilst the Tokyo, Tohoku and Hokkaido systems supplying Hokkaido and northern Honshu operates at 50 Hz.
In order to facilitate interconnection and maximise the opportunity for power exchanges between companies there are two frequency converter stations, one at Sakuma which is owned and operated by EPDC and the other at Shin-Shinano which is owned and operated by Tokyo EPCo.
In Japan electricity is currently transmitted nationally at five main AC voltages; 500, 275, 220, 187 and 154 kV as well as at voltages up to 77 kV and others at between 110 and 132 kV. A 250 kV DC line links Hokkaido and Honshu joining the Hokkaido and Tohoku EPCo networks. Between 1951 and 1999 the transmission network almost doubled from 43,427 route km to 89,433 route km as the system was both extended and upgraded to encompass higher voltages.
Currently 500 kV lines are operated by six EPCos; Tokyo, Chubu, Kansai, Chugoku, Shikoku, and Kyushu which form a backbone network in Shikoku and southern Honshu. Overall this system comprises 5,156 route km of overhead lines with a total circuit length of 9,957 km.
In the Hokkaido, Tohoku and Hokuriku supply area the maximum transmission voltage is 275 kV and in Okinawa the maximum is 132 kV. Overall the 275 kV networks, including those within the Tokyo, Chubu, Kansai, Chugoku, Shikoku, and Kyushu EPCo supply areas, total some 7,790 route km with a total circuit length of 15,415 km. Of this a total of 7,478 route km (14,566 circuit km) was overhead and 312 route km (849 circuit km) was underground.
The 250 kV link joining the Hokkaido and Tohoku EPCo networks comprises a total of 167 route km of double circuits, made up of 124 km of overhead lines and 43 km of underground cable. With a total circuit length of 335 km (248 km overhead and 87 km underground), the interconnect operates between Kamikita and Hakodate AC/DC converter stations on Honshu and Hokkaido
