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.
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.
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.
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/