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Comment by guerby | Hacker News Reader

Inertia is always pictured as a "good thing", my (very limited and probably wrong) understanding is that when production drops abruptly because of a failure inertia of rotation based generators serves as a temporary battery to cover for the imbalance.

But what happens when demand drops abruptly in an inertia heavy grid?

Eg if some evil AI turn off plenty of IoT controlled electrical appliance countrywide at exactly the same time.

https://www.nrel.gov/news/program/2020/inertia-and-the-power...

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Primary frequency reserve kicks in to automatically throttle back dynamic sources. Typical throttle rate is 100% throttle response per 5% frequency deviation. Inertia (both real and virtual) just slows down the system response enough that PFR will be effective. Just about every steam plant, combined-cycle plant, and gas-turbine plant participates in PFR. There have been proposals for inverter-connected systems to participate as well, but I don't know how common it is in practice.

Once upon a time the rated load-step response for a sub-grid in the US was a step-off transient of a nuclear power site going off-line.

Thanks for your reply.

Is the Hornsdale Power Reserve "participating" in this mechanism?

https://en.wikipedia.org/wiki/Hornsdale_Power_Reserve

Since it's a Lithium battery I assume it's "inverter-connected".

At that scale, they may be doing more interesting things, like participating in whatever passes for a spot market, or even time-of-day arbitrage.

There is considerable variation region-to-region, but a common pattern is for the spot market to clear every 15 minutes to provide secondary frequency reserve, with a day-ahead market that clears 24 hours in advance, and a wide range of futures markets that build on the day-ahead market.

I am not sure that is a good analogy.

In abstract terms you can think of the inertia as the time constant of a linear ODE. For the same perturbation of the state, the larger the inertia the slowest the variation.

In a grid with low inertia changes in demand (wether increase or decrease demand) are very sudden and thus difficult to control.

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