The second largest blackout in the history of TEPCO (The Tokyo Electric Power Company, Inc.) hit central Tokyo area at about 7:38 a.m. on August 14, 2006. It was caused by a floating crane on a barge going upstream on a river on the eastern edge of the city.

The workers on the boat did not realize that the 33 meter crane was raised too high, so it hit TEPCO's 275 kV double circuit transmission lines that run across the river.

As a result of the accident the transmission lines were short-circuited and the wires damaged. The relay protection operated and tripped both lines.

Power transmission was interrupted, resulting in a blackout that affected about 1.4 million households and offices during a very hot summer day. The transport network was disrupted, with passengers unable to use two lines on the city's largest subway operator, Tokyo Metro, for an hour to 90 minutes.  There were no reports of injuries. The affected areas included the central part of Tokyo, northern part of Yokohama and western part of Kawasaki in Kanagawa, Ichikawa and part of Urayasu in Chiba.

TEPCO immediately started trying to restore power supply to its customers. A Power Station continued to supply power until 7:58 a.m. after successful operation of a System Integration Scheme for the islanding, when it was automatically shut down because the balance between the generation and load was lost due to increased demand in the morning. About 174,000 households and offices were blacked out as a result 59 minutes after the initial event, at 8:37 a.m., all substations were restored and resumed power transmission. Electricity supply to all TEPCO customers, except for two high-voltage customers, was restored at 9:55 a.m.  Supply to the two high-voltage customers was also restored at 10:44 a.m.

At this point in time, electricity supply to about 1,376,000 out of a total of 1,391,000 households and offices hit by the blackout was resumed, and the number of those customers hit by the blackout decreased to about 15,000, due to distribution equipment failure or incorrect settings. As for about 15,000 households and offices mentioned above, the power supply to them was interrupted by malfunctions of TEPCO's power distribution equipment, which were triggered by the floating crane that contacted the transmission lines.  As a result of resolution of these malfunctions, the number of TEPCO customers suffered from the blackout decreased to about 200 at 10:44 a.m.  After four hours and forty-two minutes from the occurrence of the accident, power supply was fully restored at 12:20 p.m.

Biography

Shinichi Imai was born in Osaka in Japan, on January 11, 1964. He joined Tokyo Electric Power Company, Inc. after he earned his Master degree from the Osaka University in 1989. After 12 year experience in power system operation & protection, his current position is Manager of Benchmarking group in the Power Network Division in TEPCO. His special fields of interest include power system protection, power system stability and Special Protection System. He is member of IEEE and IEEJ (The Institute of Electrical Engineers of Japan). He has been involved in international technical activities such as IEEE PSDP and PSRC since 2004. He is a Vice Chairman of working group on Undervoltage Load Shedding in the IEEE Power Systems Relaying Committee. He is registered Professional Engineer of State of California.