The flow of rivers naturally varies from season to season, but this does not apply to the water that powers Hydro-Québec’s turbines. The immense masses of water held back by the dams serve precisely this purpose: they are intended to form a “cushion” that allows production to continue even at low water levels (the time of year, towards the end of summer, when the rivers are at their lowest level achieve) or to continue long periods of drought despite low water levels. And conversely, we can also reduce production when demand is low and store more water behind dams for future use. Strictly speaking, it is not a “battery”, but it still allows energy to be stored.
The same goes for wind turbines: When it is very windy, the state-owned company can reduce the amount of water flowing through its dams, thereby slightly raising the level of its reservoirs.
However, Maxime Nadeau, director of grid operations at Hydro-Québec, points out that it cannot actually produce excess electricity and be “lost.” Production must constantly be based on demand, and very precisely, because as soon as the two are decoupled a little too much, this can have unfortunate consequences for all Hydro customers. Let’s see why.
Two types of currents
There are essentially two types of electric current: “direct current,” in which the electric charges always go in the same direction; and “alternating current,” in which electrons instead move back and forth in wires, changing direction several times per second. The Hydro-Québec network, like most major power grids in the world, runs on alternating current. In North America, the grids all have the same frequency, 60 hertz (Hz), meaning the electrons move back and forth 60 times per second – in Europe it’s closer to 50 Hz.
If more electricity is produced in such networks than is needed to cover current demand, this leads to an increase in frequency. It’s a bit like stepping on the accelerator of a car: it delivers more energy to the engine, which starts running faster.
“For most electronic devices equipped with their power rails, frequency changes are not a real problem,” says Mr. Nadeau. However, devices with motors are designed to operate at a specific frequency. So if they are not exactly 60 Hz, their behavior will not be optimal and if we move too far from 60 Hz there is a risk of breakage. (…) Motors in industry can be very sensitive to this, and factories have protection systems that switch off if the frequency fluctuates too much. »
Three levers
It is therefore important to adapt production to demand. Hydro-Québec even has a team that does just that: the Network Operation Center, where Mr. Nadeau works. And there are three ways to ensure the frequency stays at 60Hz.
The first, explains Mr. Nadeau, takes place at the production sites themselves. In each dam there are several “turbine-generator groups” where the turbine produces electricity and the generator converts it to alternating current at 60 Hz. “The number of groups is not the same in all power plants,” says Mr. Nadeau. For example, there are 16 in LG2, eight in Manic-2 and only two in Sainte-Marguerite 3. (…) But each group is equipped with a device called a speed controller, whose main function is to keep the frequency at 60 Hz.”
A second “lever” available to Hydro, and undoubtedly the most important, is to predict electricity needs every minute and continuously adjust production. “We start with demand forecasts for the next day,” explains Mr. Nadeau, “and we will check with production to see what is available – because there can always be groups that need to be maintained.” There are also the forecasts for wind energy and the Exchanges that we can have with the networks around us. And then we know whether we have everything we need to meet demand or whether we need to make further adjustments. »
“We still have very good precision,” he continues. Demand forecasts for the next day are created minute by minute, and this data is updated every five minutes. We then monitor demand in real time, whether by adjusting the groups’ production [turbine-alternateur] that have already been started, by starting those that have been stopped, or by stopping those that are no longer needed. »
However, it is impossible to adapt production to demand perfectly and to the exact second. For this reason, Hydro has a third tool to keep the frequency constant. “If the balance is not immediately perfect, there may be small fluctuations in frequency,” admits Mr. Nadeau. And to control them, we have a system called “grid frequency adjustment”, which constantly measures the grid frequency and automatically increases or reduces production depending on current needs. Not all turbine generator groups are connected to this system, but there are 95 of them [sur un total de 332] Who are. These are very subtle adjustments. For example, with a variation of 0.1 Hz, production is adjusted by a few hundred megawatts (MW). »
Considering that Hydro’s plants have a total capacity of more than 137,000 MW, it is clear that the system of forecasts and “manual” adjustments still allows a high level of precision, even if it is not always perfect.
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