Since, and unless electricity has properties I’m not aware of, it’s not possible for electric power plants to produce only and EXACTLY the amount of electricity being drawn at an given time, and not having enough electricity for everyone is a VERY bad thing, I’m assuming the power plants produce enough electricity to meet a predicted average need plus a little extra margin. So, if this understanding is correct, where does that little extra margin go? And what kind of margin are we talking about?
In: Engineering
> it’s not possible for electric power plants to produce only and EXACTLY the amount of electricity being drawn at an given time
Your assumption is practically incorrect. The grid only works, because it is able to produce exactly what is demanded.
Well maybe not exactly, but it’s within say 0.1% of the demanded electricity.
> I’m assuming the power plants produce enough electricity to meet a predicted average need plus a little extra margin.
Nope, the total capacity of power plants is sized to handle the maximum possible/reasonable demand you will ever see (~3x average demand), but the real time power delivered is exactly what is needed, right in that moment.
The closest thing that occurs to what you are thinking of is what dispatchers do. These are technicians who decide when a power plant needs to be turned on, or where to direct or pull power. They’re actions though are not for real time regulation or to exactly match power, they’re to instead make sure a plant or feeder line isn’t being over stressed.
> So, if this understanding is correct, where does that little extra margin go?
Your understanding is not correct, there’s no little extra margin, except for essentially rounding errors. The grid is regulated in real time. The extra margin is what accounts for the slight variations in grid frequency over time.
I think your misunderstanding is due to not understanding how you can regulate such a giant system. **You are probably thinking you would need to know exactly how much power is being demanded, then using this information, make adjustments. But you actually don’t need to know anything about how how much power is being demanded.** Instead you can target a particular metric, in this case frequency, and ensure that frequency is locked at all times. This will inherently regulate generated power to equal demanded power.
Grid power is mostly generated with synchronous machines. These are electrical generators, which are fed an external current (excitation current), which the generator will synchronize to if you apply enough torque to its rotor to keep the rotor spinning at enough speed. So the grid voltage itself, is fed into the generator to synchronize the generator’s output frequency, to the grid’s frequency.
Synchronous motors will remain locked to the input frequency of the excitation current, unless you demand more or less power than what the motor is delivering. They do this by spinning their rotors up to a speed, where each rotation of the rotor, passes a number of magnetically charged poles, that results in a matching alternating current frequency to what the input excitation current has. A mechanical speed governor may be employed, to ensure the rotor speed stays locked at the ideal speed, to preserve the synchronized relationship between input excitation current frequency, and generated current frequency.
The resulting grid frequency is the product of the average power delivery, load demand and rotational characteristics of all the generators on the grid working together.
The grid is frequency regulated, it targets a fixed frequency, 60Hz in North America. By targeting frequency, this inherently allows the electricity generated to almost exactly equal electricity demanded.
If electricity generated exceeds what is demanded, the frequency will naturally rise. Put simply, if you demand less electricity than what your generators produce, that means the relative amount of torque load applied by the generator’s windings to the generator’s rotating shaft, is reduced in relation to the torque being applied by the power source to the generator’s input shaft. With less torque load, the generator is able to more freely spin at a faster speed. With a faster speed, the frequency will rise, since the rotor will pass over those magnetic poles more quickly.
Conversely, if you demand more electricity than is supplied, the frequency will reduce, since all the generators will slow down.
If the grid over produces, all of the synchronous machines (generators) hooked up to the grid will speed up. This will cause the grid frequency to rise. Since grid frequency is regulated, a control system will apply counteracting torque (via braking or reducing power source energy output) or lower the excitation current in their synchronous machines, to slow things down, and thus lower the generated power to match the demanded power.
Every single generator gets to see the grid’s frequency, and every single generator has a regulation system targeting the same frequency. Through completely independent action, but tied to the same feedback signal (grid frequency), the entire grid can be regulated.
**TLDR: The grid is frequency regulated. Frequency regulation forces generated power to almost exactly equal demanded power. You don’t need to actually know what power in and out is to regulate! Frequency fegulation is constantly occurring in real time, every generator on the grid is synchronized to the grid, and all are targeting the same frequency, 50 Hz or 60 Hz. Every generator will make micro-adjustments to its power output (throw more or less coal into the fire, or release/apply mechanical braking), to keep that frequency at 60 Hz.**
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