Photo/IllutrationSmoke rises from the No. 4 generator of Hokkaido Electric Power Co.’s Tomato-Atsuma thermal power plant in Atsuma, Hokkaido, on Sept. 6 after a powerful earthquake hit. (Provided by Hokkaido Electric Power Co.)

An emergency electricity supply mechanism was activated immediately after the Sept. 6 earthquake in Hokkaido, but power still ended up being cut off to the entire island for some unknown reason.

Hokkaido Electric Power Co. has said damage to its Tomato-Atsuma thermal plant caused a chain reaction involving a supply-demand imbalance that led to the blackout that affected all households in Hokkaido.

But the company has not revealed that it received an emergency power supply from utilities on the main island of Honshu right after the magnitude-6.7 earthquake struck at 3:07 a.m.

An analysis of data from the government’s Organization for Cross-Regional Coordination of Transmissions Operators (OCCTO) showed that the emergency supply had helped restore the supply-demand balance in power in Hokkaido.

But that equilibrium was lost after 18 minutes, and the northernmost main island was plunged into darkness.

According to OCCTO and Hokkaido Electric, the No. 2 and No. 4 generators at the Tomato-Atsuma thermal plant automatically shut down immediately after the temblor hit, instantly severing a supply of 1.3 gigawatts.

The three-generator plant, with a capacity of 1.65 gigawatts, is located near the quake’s epicenter in the hard-hit town of Atuma and covers half of the power needs in Hokkaido.

With the two generators shut down, Hokkaido Electric received an emergency supply of a maximum 0.6 gigawatts from Honshu side in an attempt to ride out the crisis.

Hokkaido Electric also stopped supplying power to some areas to reduce demand and help restore the equilibrium.

These steps apparently worked; a supply-demand balance had been restored by 3:11 a.m., according to the OCCTO data.

Power plants in Hokkaido Electric’s areas of operation are arranged to automatically shut down in a prolonged supply-demand imbalance to prevent glitches in equipment from occurring. Such shutdowns can lead to widespread blackouts.

Although the emergency measures had restored the balance, the remaining No. 1 generator at the Tomato-Atsuma plant--and other power plants in Hokkaido--came to an abrupt halt at 3:25 a.m., resulting in the blackout that spread across the island.