再エネ出力制御：GW後の運用変更を読む

What Golden Week 2021 Actually Showed Us

For solar plant owners in Japan, the weeks around Golden Week have become a useful stress test for the grid. In 2021, there was widespread speculation that output curtailment (出力制御) might reach Hokkaido or Tohoku. In the end, it did not. Curtailment was implemented only in the Kyushu area, as it had been since the practice began. That outcome is worth understanding, because it tells us where the real constraints sit — and where they don't, yet.

The first curtailment in Kyushu took place on Saturday, 13 October 2018, on the mainland. It did not begin in spring, when solar generation peaks, but in autumn. The reason lies in the supply mix: nuclear restarts at Genkai Units 3 and 4 in May and July of that year pushed baseload supply up sharply, while solar and thermal output were actually lower year-on-year. Comparing April and October 2018 during the 12:00–13:00 window, demand rose about 102% while supply rose about 108%. Curtailment happens when supply exceeds what the grid can absorb — and that imbalance can be created by any large source, not only solar.

The lesson for owners outside Kyushu is not to assume they are safe forever. It is that curtailment risk is a function of local supply mix, transmission capacity, and demand — factors that shift with every nuclear restart, every new interconnection, and every policy change.

The 2021 Rule Change and Why It Surprised Operators

In April 2021, Kyushu Electric Power Transmission & Distribution changed how it applies curtailment rules, and the effect on designated-rule (指定ルール) operators was immediate. Many reported curtailment being called on consecutive days — something that had not happened in the prior year. Others expected that a shift from rotating (輪番) to uniform (一律) curtailment would mean more frequent calls but a lower curtailment rate per event. That expectation did not hold.

The reason for the change was arithmetic. Under the old rules, the number of curtailment calls was projected to exceed 30 for the year, which the previous operating method could not accommodate. The old-rule call count for April through 6 May was roughly 8–9 in 2020 and 9–10 in 2021 — nearly identical. But the full-year old-rule count for FY2020 came to 22, which suggests that without the rule change, FY2021 would have seen a significant increase in old-rule curtailment in autumn and the following March.

One striking detail: more than 20 new projects were commissioned under the old rules between 2020 and 2021. New capacity keeps entering a system whose curtailment framework is already under strain. For owners, this means the rules governing your plant depend heavily on when your project was certified — and those rules can be revised.

Reading Curtailment Through a Duration Curve

To understand why the 2021 change felt so abrupt, it helps to use a duration curve (デュレーションカーブ). This is simply a dataset reordered from smallest to largest, with time removed from the horizontal axis. If you take Kyushu's 2020 electricity demand — 24 hours × 365 days — and sort it by magnitude, you get a curve that shows how often demand sits at each level.

The shape matters. Demand varies by season, and the sorted curve reveals how many hours fall below a given threshold. When supply from solar, nuclear, and other sources is layered on top, the hours where supply exceeds demand become visible as the tail of the curve. Curtailment is called in those hours. Under the old rotating approach, the burden was spread across operators; under the new uniform approach, more operators are affected at the same time, which is why consecutive-day calls appeared.

This is a simplified, individual view rather than an official methodology, but it explains the operator experience: the total volume of surplus hours did not necessarily explode — the way those hours were allocated across the fleet changed.

Will Transmission Upgrades Solve It?

A common hope is that interconnector (連系線) reinforcement will reduce curtailment. It will help, but it is not a complete answer. Curtailment in Kyushu emerged from a combination of nuclear restarts, seasonal solar peaks, and limited export capacity to other regions. Adding transmission capacity addresses the export constraint, but it does not change the timing of surplus generation within the region, and it does not resolve constraints that appear at the local distribution level.

Japan's 2030 energy mix targets imply continued growth in renewable capacity, which means the conditions that produce curtailment will persist in some form even as the grid is strengthened. Upgrades also take years to plan and build. Owners should treat transmission reinforcement as a medium-term mitigant, not a near-term fix.

What This Means for Your Plant

If you own a solar plant in Japan — particularly under the old rules, or in a region adjacent to Kyushu — the practical takeaways are these. First, your curtailment exposure depends on your certification timing and rule category, not just your location. Second, curtailment can be triggered by changes elsewhere in the supply mix, such as nuclear restarts, that have nothing to do with your plant. Third, rule changes can alter the frequency and pattern of calls without changing the total surplus energy much.

This is also relevant to the 10-year inspection and to any reassessment of your business plan. Curtailment losses affect revenue projections, and lenders and buyers increasingly ask how a plant has been affected historically.

The most useful first step is not to speculate about national policy. It is to start with accurately understanding your plant's condition — your rule category, your historical curtailment record, your regional grid's supply mix, and any announced transmission or rule changes. That baseline is what allows you to model risk realistically and to make informed decisions about operations, upgrades, or divestment.