The Kanmon connection lines sway gracefully over the water and serve as the only power transmission cables linking the main islands of Kyushu and Honshu.
The pylons are nondescript, and the cables are each only about 3 centimeters in diameter. But they were essential in averting a crisis that could have left millions of residents without power during a particularly bone-chilling winter day.
Based on this success, there appears to be a relatively easy way to prevent summer blackouts, as warned by the industry ministry and regional utilities, even if the nation has no nuclear reactors online.
However, such a plan depends on whether the ministry and the utilities are willing to move outside the normal thinking of the electric power industry concerning excess power supplies.
So far, it appears they are not.
The central government and Kansai Electric Power Co. (KEPCO) recently repeated their warnings of power shortages in the summer if two suspended reactors at KEPCO's Oi nuclear power plant in Fukui Prefecture are not brought back online soon.
A nuclear reactor in Tomari, Hokkaido, is now the only one of Japan's 54 reactors currently running, and it is scheduled to be shut down for regular inspections on May 5.
The industry ministry said April 9 that power supply could fall up to 19.6 percent short of peak summer demand across KEPCO's service area if no reactors are restarted. KEPCO estimated its power supply capacity this summer could fall 13.9 percent short of last summer's peak demand.
On the day of peak demand last summer, amid the crisis at the Fukushima No. 1 nuclear power plant, KEPCO procured about 1.2 gigawatts of excess power from other regional utilities.
Although that was less than 5 percent of KEPCO's peak-day power supply capacity, the regional utilities have an even higher level of excess electricity to share.
KEPCO has "connection line" links with four other utilities--Chubu Electric Power Co., Hokuriku Electric Power Co., Shikoku Electric Power Co. and Chugoku Electric Power Co.--with a combined transmission capacity of more than 29 gigawatts, far exceeding the power supply shortage of 5.5 gigawatts envisaged by KEPCO for this summer.
But the problem on sharing this electricity stems from what the regional utilities consider their supply leeway.
"We have an approximately 12-percent supply reserves (power supply leeway) with respect to the actual figures of last year, when our customers saved power use by about 1 gigawatt," Chubu Electric President Akihisa Mizuno told a news conference on March 27.
That means Chubu Electric alone will have a supply leeway exceeding 3 gigawatts.
However, utilities are generally ambiguous about electricity supplies that can be procured during emergencies.
"We cannot casually estimate the amount of excess power supply available from other utilities because all utilities meet peak demand in the summer," an official at a power utility said.
In the winter, Kyushu Electric Power Co. managed to overcome a supply crisis by procuring excess power from other utilities through the Kanmon connection lines. One of those power-supplying companies was Tokyo Electric Power Co., operator of the Fukushima No. 1 plant, with which Kyushu Electric has no direct connection.
During normal times, the connection lines, which cross the Kanmon Straits and run parallel to the Kanmon Bridge, are exclusively used to send power generated at Electric Power Development Co.'s coal-fired thermal power plants in Nagasaki Prefecture from Kyushu to Honshu.
The flow of power was reversed for half an hour from 7 a.m. on Feb. 3, sending a record 1.41 gigawatts of power--more than the output capacity of a typical nuclear reactor--from Honshu to Kyushu.
"It was almost as astonishing as if Saudi Arabia was importing oil," an industry source said.
The problem started when Kyushu Electric's Shin-Oita thermal power plant in Oita, which has an output capacity of 2.295 gigawatts, shut down around 4 a.m. The supply of liquefied natural gas, which fuels the largest thermal plant in Kyushu, was suspended due to a rudimentary mistake involving frozen pipes.
The intense cold in Kyushu, which the Fukuoka District Meteorological Observatory said occurs only once a decade, increased winter power demand to a record 15.38 gigawatts in Kyushu Electric's service area between 6 and 7 p.m. on the previous day.
All six of Kyushu Electric's nuclear reactors had been offline since Dec. 26.
The emergency supply of excess power by other utilities across Japan helped Kyushu Electric overcome the triple crisis--no running reactors, surging demand and a troubled power plant.
In fact, the Kanmon connection lines can transmit 5.56 gigawatts of power, but the "operational capacity" for the Kyushu-bound flow of power is limited to only 0.3 gigawatt. That is because overshooting that limit raises the risk of unexpected blackouts due to frequency drops if the connection lines are interrupted.
The power flow on that winter day was 4.7 times as large as the threshold value.
Utilities have reiterated that a failure to restart reactors could result in blackouts, but they have never mentioned the final resort of using emergency supplies of excess power beyond the "operational limits."
Under the logic of the power industry, the regional monopolies should not rely on "reserves" of excess power supplied by other utilities because they have no way of telling how much excess power is actually available.
The March 2011 accident at the Fukushima No. 1 nuclear plant, however, changed that picture. As fewer and fewer reactors remain online across Japan, it is becoming more common for power companies to receive excess power from other utilities that have a greater supply leeway.
The industry ministry set up a study group in February to draw up rules for the flexible use of power connection lines across Japan.
However, the government and KEPCO appear determined to give the green light to an early restart of the two reactors at the Oi nuclear plant.
(This article was compiled from reports by Tomoki Yasuda and Kaname Ohira.)
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