Press Release
Power Outage Caused by the Earthquake that occurred in Miyagi/Fukushima at 11:34 PM on March 16
[As of 3 AM, March 17]
March 17, 2022
Tokyo Electric Power Company Holdings, Inc.
As a result of the shutdown of power generation equipment caused by the earthquake that occurred in Miyagi/Fukushima at 11:34 PM on March 16, UFR(Under Frequency Relay)*, which is a device that protects the entire power system, automatically tripped thereby causing a power outage. We would like to offer our sincere apologies for the inconvenience this has caused customers residing in the service area of TEPCO Power Grid, Inc., and society as a whole.
At 11:59 PM on March 16, a maximum of approximately 2.1 million households lost power, but power transmission was gradually recommenced and as of 2:52 AM on March 17, power to all households had been restored. The number of households in TEPCO Power Grid, Inc.'s service area that were without power when the maximum number of households lost power is as follows:
<Maximum number of households without power (as of 11:59 PM on March 16)>
| Region/Prefecture | Maximum number of households without power |
|---|---|
| Tochigi | Approx. 120,000 |
| Gunma Approx. | Approx. 80,000 |
| Ibaraki | Approx. 220,000 |
| Saitama | Approx. 300,000 |
| Chiba | Approx. 220,000 |
| Tokyo | Approx. 700,000 |
| Kanagawa | Approx. 310,000 |
| Yamanashi | Approx. 120,000 |
| Shizuoka | Approx. 30,000 |
| All areas | Approx. 2.1 million |
*When multiple generators shutdown simultaneously due to an earthquake, there is a large drop in generator output (supply), which causes a large imbalance with the amount of electricity used by customers (demand), thereby resulting in a frequency decrease. If this frequency decrease continues, other generators working normally begin to automatically shut down ultimately causing the potential for an unpredicted power outage over a wide area.
In order to avoid this, UFR(Under Frequency Relay) automatically activates depending on the output and frequency of generators that have been shut down by the earthquake, and also the supply-demand balance at that time, thereby resulting in a shutdown of a certain amount of power transmission in order to maintain suitable frequency.
