[URSpider94]

In the electrical grid, frequency is proportional to voltage. When the frequency drops, it means that the supplied voltage is also falling. Likewise, if you go over-frequency, then the voltage will rise.

Traditionally, the inertia of thousands of tons of spinning generator turbines across the system provided this inertia, but with more and more nonlinear sources (and also nonlinear loads), that balance is disappearing.

[jillesvangurp]

Batteries are also becoming popular for the same reason. The more batteries you have the easier it is to deal with fluctuations in supply and demand as you can switch them on and off in milliseconds and they can serve to both supply GW to the grid or absorb it from the grid.

Flywheels are useful because they store a lot of energy and don't require a lot of energy to keep them spinning (i.e. topped up with energy). Simply connecting them to a generator can be done (relatively) quickly and allows them to supply power for a relatively long period of time. It's basically a mechanical battery.

Both have the advantage that they are cheaper to operate than a typical peaker plant, which is increasingly the role of remaining coal plants that are otherwise too expensive by orders of magnitudes to operate continuously. Switching those on is a last resort for energy companies. The more battery they have, the less need they have for those. And the less they get utilized, the more expensive they are to keep around. Gas plants are better but they take a long time to turn off and on again and doing that is also not cheap.

Prices have actually turned negative a couple of times in e.g. the UK in cases where the power companies were literally paying people to use their excess power just so they could avoid having to turn off plants that are expensive to turn back on. Basically, grid storage capacity allows electricity companies to smooth out peaks in demand and supply and respond extremely rapidly by either soaking up or supplying many GW.

[M2Ys4U]

Also worth noting:

We haven't even burned coal for the purposes of providing power[0] since the 11th of May. That's the longest period without coal being burned since 1882.

[0] Sort of. A couple of coal plants underwent maintenance and had to be fired up and provide power to fully test them. We had a total of around 21 hours of coal plants supplying energy and we're now at day 18 of a coal-free run, and prior to the test we had almost 68 days!