[xhkkffbf]

Why do we care about keeping frequency stable when more and more products simply put the power through a rectifier to turn it to DC? The lamps all have LED "bulbs". The computers have power bricks.

Do we really care if the compressors in the air conditioners and the refrigerators turn at a slightly different rate? Okay, maybe a rapid shift in frequency could be damaging, but a slight drift sounds okay mechanically.

Or am I wrong?

[PeterisP]

This flywheel is exactly about compensating rapid changes in production/consumption so that they do not cause rapid shifts in frequency which can cause disruptions to the whole grid.

The household electric devices or the long term frequency drift are not relevant there, it's about keeping the huge turbines everywhere else running at a stable rate. The other way to achieve rapid compensation for frequency drop is to disconnect consumers, i.e. automatically triggered blackouts.

[tankenmate]

A change in frequency generally changes the efficiency of voltage regulators / voltage converters; over a very large network this can exhibit behaviour that looks a lot like negative resistance, this can induce runaway voltage conditions.

[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!

[rcxdude]

The frequency of the grid is a strong indicator of the balance between supply and demand. If it has drifted off by even a small amount the system is close to a runaway collapse (modern switch-mode power supplies are actually quite bad from this point of view: when the voltage drops their effective power draw actually increases, in comparison to a lightbulb or traditional motor). And when the system collapses in an uncontrolled way it can quite easily cause severe damage to generating equipment which will take it offline for a long time. So there's a lot of stuff which goes into cutting out load and/or generators before bad stuff happens, and it tends to err on the side of caution, because a 1 hour blackout is far better than a significant amount of generation capacity being offline for months.

[dredmorbius]

Inputting energy to the grid must occur at grid frequency. You want that stable. And there are still applications which themselves rely on grid frequency for timming and synch. Change or vary freq and you'll violate lots of assumptions.