Post-quake power outages intentional, used to avoid massive blackout

The Yomiuri Shimbun
A residential area of Koto Ward, Tokyo, goes dark during a power outage around 11:45 p.m. on Wednesday.

The earthquake that struck late Wednesday night off Fukushima Prefecture caused a massive power outage affecting about 2.1 million homes spreading as far away as the Tokyo metropolitan area.

There was no major damage to Tokyo Electric Power Company Holdings, Inc. (TEPCO)’s power grid, but electrical supply to some areas was purposely cut to prevent sudden blackouts over larger areas.

Following the earthquake, 12 thermal and 24 hydroelectric power plants were shut down for safety checks, mainly in the Tohoku region. That resulted in about 6.3 million kilowatts being lost from the power supply.

To provide a stable power supply, demand (the amount used) and supply (the amount generated) must be equal. When the balance is suddenly thrown off, it could — in a worst case scenario — trigger the safety mechanisms of plants and cause a blackout. Additionally, the TEPCO and Tohoku Electric Power Co. power grids are interconnected, necessitating a unified adjustment to supply and demand.

This time, TEPCO’s safety system automatically activated after detecting a supply shortage over the entire power grid, including in Tohoku, causing intentional power outages in some areas.

According to TEPCO, the safety system automatically determines which areas will lose power. When it occurs late at night as it did on Wednesday, the system adjusts the balance of supply and demand by instantly making combinations, such as entertainment areas with high power demand and office districts with low demand. This also affected whether neighboring areas would also lose power.

A backup power system was activated, and electricity in TEPCO’s service area was restored by 2:52 a.m. Thursday, about three hours after the earthquake. In Tohoku Electric Power’s service area which had severed electrical lines, at one point about 160,000 households were left without power, but all were back on line by 9:41 p.m.