A supply-demand imbalance is considered to exist when RTE generates one or more messages about insufficient offers on the balancing mechanism (alerts or degraded modes) to encourage participants to submit additional offers.
There were far more tight situations on the balancing mechanism in 2020 than in past years. The heath crisis was one of the main reasons, given its impact on consumption, which was difficult for actors to predict in the face of unprecedented and repeated health measures, and on generation, particularly with nuclear capacity availability being far below average between June and September.
The number of instances that required upward rebalancing (when power consumption exceeded generation) rose to 163 half-days, more than twice the 2019 level. These situations can be caused by a wide range of factors relating to the availability of generation capacity (number of plants in maintenance and extensions thereof, reduced fuel use during the summer to prepare for the winter, social movements, unscheduled unavailability, environmental constraints).
Instances requiring downward rebalancing also doubled versus 2019, to 28 half-days (see box below, For a better understanding). Most of these situations occurred during the first lockdown, when downward flexibility was required.
Tight situations due to excess supply
It is easy to imagine tight situations arising when electricity demand exceeds supply, but there can also be instances where the transmission system must contend with surplus supply because consumption is low (decline in activity due to holidays, weekends or bank holidays). Such situations occur more often in summer, when less power is used, but also on weekends in the spring, particularly when wind and solar output are high in Europe and demand is low.
At these times, France may need to export large quantities of energy to avoid ending up with too much supply. When the limits of export capacity are reached, RTE may request that generating units reduce or suspend their production temporarily. Because priority is given to must-run renewable sources, fossil-fired thermal and nuclear generation in particular may decrease substantially.
Instances of surplus generation have been increasing in recent years, notably because of the continued development of renewable energy sources (and their lack of participation in market mechanisms that would allow them to be modulated) and the rise in minimum production requirements for nuclear reactors (technical constraints declared by plant operators).