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How do we balance the grid in Belgium? What is the role of R3?

As no technical solution exist (yet) to store large quantities of electricity at a reasonable cost, the grid has to find ways to get balanced.

The 1st way is to ask to each electricity supplier to buy all the energy its client will need and to ask all producers to make sure all their production will be used. They have to do that every day for the next day. And they can continue to buy and sell their shortage or surplus in real time during the day. If at any given time a producer is producing less than expected or the clients of a supplier are consuming more, these actors will be exposed to imbalance prices. They will pay penalties. But paying penalties does not help consumer to have electricity in their homes. The balancing is not only about money it must also be done physically.

So on top of balancing financial penalties, the transportation grid operator is buying capacity reserves and paying to have spare capacity always available “just in case”. These reserves of different types will be used when different “imbalance” events happen.

Example of the grid balance in France


The capacity reserves can be very fast (real time) or slower. We call fast reserve “R1” of automatic frequency restoration. It has to react within 500 milliseconds(= 0.5 scd) when a tension on the grid occurs (in the form of a small deviation from the 50hz frequency norm) and can last only for a few minutes. It is symmetrical. This means consuming more (or injecting less) when too much energy is on the grid (frequency above 50hz) or injecting more (or consuming less) when not enough energy is there. R1 is typically provided by traditional power plants that can run faster or slower when needed and more and more by batteries unloading/reloading for short term fluctuations. R1 is activated nearly all the time in one way or the other. R1 is very valuable and not that easy to provide because it means that the power plant participating must not run at its full technical capacity.


For larger fluctuations, bigger reserve are needed. These are called R2. They are activated several times per day when R1 alone is not enough to automatically recover the frequency. R2 can react typically within 2min. R2 is like R1, quite difficult to provide given the very short reaction time. Today in Belgium it is provided by power plants running just below full capacity keeping  a limited “hot” reserve capacity “just in case” at the disposal of transport grid operator. R2 is activated a few times per day.


If the imbalance is larger and long lasting,  R2 will not be enough. The grid operator will start activating slower reserves. R3 reserves have to be available within 15 minutes. We call this manual frequency restoration reserves. R3 is activated several times per week.  Large power plants and pump storage hydro do participate to R3 also with the spare capacity they do not need. And given the fact that R3 must be available in 15min more sources of flexibility can be used such as the flexibility provided by industrial processes and some diesel back up generators. .

In Belgium NextFlex valorizes most of the flexibility of its clients on the R3 mechanism. Every month the transportation grid operator organizes auction to buy its R3 reserves. And every month NextFlex proposes the flex of its clients to the auction in competition with the power plants and flexibility proposed by other actors. The grid operators chooses the most economical propositions. Once the auction is closed we know how much flex could be activated. And we make sure the R3 capacity we sold to the grid operator will be there within less than 15minutes at any time during the delivery month.

Details of reserves for the grid in Belgium

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