Inertia is how well a system responds to a change in frequency.

For example, if a large amount of loads switch off, the system frequency decreases. In response to this, synchronous generators will produce more power to bring the frequency back to normal. This is as a result of the big spinning mass in the generator that wants to maintain its rate of rotation.

Synthetic inertia involves configuring control loops in the battery inverters to cause them to have a similar response to a synchronous generator. The bonus is that you can tune this control as much as you like depending on grid conditions.

> Synthetic inertia involves configuring control loops in the battery inverters to cause them to have a similar response to a synchronous generator.

So you get a clock, monitor the grid, calculate the current that would be drawn from a short-circuited dynamo right now, and get the inverter to generate that current, all within a few microseconds, and without setting the battery on fire? I'm impressed.

Mostly.

https://www.nrel.gov/news/features/2020/renewables-rescue-st...

https://www.nrel.gov/docs/fy19osti/73207.pdf

Just to add on to this, here's a good talk that goes through the basics of VSGs (Virtual Synchronous Generators) and how they work.

https://youtu.be/2BHbUPDxwJc?t=668

Is explained in the article:

Conventional Generators have enormous energy stored in their rotating parts, and the grid ties all these generators together as if they were on the same shaft. This maintains the phase and frequency of the grids electricity by providing "synchronous momentum".

However this synchronous momentum is lost when the mechanical generators are switched off. And so giant batteries and inverters are used instead to provide this critical function.

To put it another way, the giant batteries are not used to power the grid, but instead used to lower the AC impedance of the grid, eg provide synthetic inertia.

This is pure speculation, but I wonder if they are using GPS time signals to synchronize all of the inverters perfectly in frequency and phase. Or maybe they just have them resist changes in frequency like an actual spinning flywheel would, and let phase drift.

In Europe the grid is controlled by the operators, inverters that are grid tied follow the operator commands to increase frequency or decrease frequency. The phase is kept synchronous with UTC.

Do inverters communicate with the grid operator, or do they just monitor the frequency they see on the mains? And how do they all agree on what UTC is with sufficient accuracy? At 50 Hz, a 1% deviation is only 200 uS.

Modern solar inverters communicate with the grid operators, less modern ones follow the grid frequency. The grid is allowed to deviate from UTC by a few seconds, in some edge cases it ramps up to minutes, but it eventually comes back in sync with UTC. Good enough for kitchen clocks, simple timers and the likes.