[jnsaff2]

The grid frequency is a measure of energy balance in the grid. So anyone can measure anywhere in the frequency domain whether there is a surplus or deficit of generation. This is due to the fact that the grid itself can't store any energy, it has to be balanced all the time.

Imagine the massive spinning generators as a big mass that slow down just a little bit when you switch on a light and then that generator has to add more steam (or open hydro valve or whatever).

So anyone with an accurate enough measuring device can exactly monitor the state of the grid. We use this device [0] for example.

There are generally frequency containment reserves (FCR) that consist of different ways of generation and have their different reaction times, power and energy capacities.

Hydro for example can react in about 15 seconds, battery inverters in milliseconds. Gas turbines in minutes, coal fired plants in hours.

You can also shed energy by switching off loads (Demand side response).

The system operator is responsible for grid balancing in the short term, they have direct facilities under their control and they have contracts with generators and consumers. And there are markets to bid your generation and flexibility.

The markets in the Nordics for example are:

- FCR-N (Frequency containment Reserve - Normal operations)

- between 49.90 - 49.99 and 50.01 - 50.10 (reaction time up to 20s)

- FCR-D (Disturbance) - between 49.7-49.90 and 50.10 - 50.30 (reaction time up to 2 seconds IIRC)

- FFR (Fast Frequency response) - below 49.7 - reaction time 0.6s IIRC

Once a day you bid your capacity for the next 24h (for each hour) and then you measure the grid frequency yourself and when you detect a deviation you activate your response. You get paid for availability and activation separately. There is a ton of qualification and logging you need to do to be able to participate but the activation message is the grid frequency itself, no further communication needed.

Outside frequency regulation there is energy markets where generation and consumption is agreed 24h ahead.

[0] https://www.gobmaier.de/

[bennofs]

> Imagine the massive spinning generators as a big mass that slow down just a little bit when you switch on a light and then that generator has to add more steam (or open hydro valve or whatever).

I have always wondered, would that still hold true if the grid was fully solar-based? There would be no rotating mass in that case.

[netflixandkill]

Yes, although a significant issue with that is there is no spinning mass to "borrow" inertia from, so while modern inverters at solar sites are good at shaping output phases, they have very little capability to absorb significant frequency deviations.

Grid scale battery systems are often used for voltage or frequency stability as opposed to deep discharging as generation offsets, although that will change eventually if batteries get better enough or really cheap LNG stops being a thing.

[jnsaff2]

Yes indeed, inverter based generators (solar and batteries) don't have any inertia. But as long as the grid is still AC the frequency will be the same and the inverters would need to compensate internally. Especially with solar where a cloud going over a solar farm can easily knock off a few MW in seconds the volatility of the grid will increase and need for storage (batteries, hydro, etc) alongside Demand Side Response is getting much bigger.

[brandmeyer]

It does hold true for some systems. Wind turbine generator controls can provide virtual inertia over very short time scales (a second or few) by exchanging energy with the turbine rotor. You can also provide primary frequency reserve in the negative direction (load step-off) using exactly the same ramp control that steam plants use. Typical ramp rate is 100% of rated power for a 5% change in frequency.

However, in order to provide primary frequency reserve in the other direction, you do need additional local storage. You don't need very much. Just 10% of rated power for 15 minutes gets you to very deep renewable penetration.

The trouble isn't with the technology, its with the economics. Once you set a sufficiently high price for frequency support and primary frequency reserve, suppliers will show up.