https://www.aemo.com.au/Electricity/National-Electricity-Market-NEM/Security-and-reliability/-/media/80004354B88B4FEF96EAB673DDC99820.ashxAFAIK the generators of large (hundreds of megawatts per generator unit) power plants are steered using an atomic clock to make sure that the frequency is stable long term. It is not like these generators are freerunning. The large power plants form a low impedance to which inverters can synchronise. With the large power plants gone at some point, there needs to be a way to keep the grid synchronised to an atomic clock. You can't have a large number of independent inverters pump power into the grid at any phase / frequency they like.
From the above PDF, the frequency's normal range control band is +/- 0.3%. Back when people wore watches, the watch crystals are accurate to 20ppm. Atomic clock not required. Over a short timescale, big power plants
are free-running. But they have such large inertia that load changes take seconds to have an impact. Over a long term (several days) you can try and keep time with an external reference but it's absolutely not necessary in a world where nobody uses synchronous clocks any more.
Grid-tie inverters cannot just "willy nilly" export at whatever phase they want, the circulating currents of an inverter trying to do that would let magic smoke out. Therefore, because they stay synchronised, they cannot "drift" away from the grid. The grid as a whole can drift, but individual generators have to stay synchronised.
Grid-tie inverters synchronise with the existing waveform, and then start exporting in-phase (by locally raising the voltage ever so slightly, thus managing to push current back out). If voltage goes too high, they back off (Volt-Watt control).
Modern grid-tie inverters also have Volt-Var mode. This absorbs or generates reactive power to limit voltage excursions. It also produces a leading or lagging effect on the grid, that is to change the frequency by a tiny amount. The inertia of the rest of the system limits the impact.
The next step is to have some kind of frequency-reactive control. AEMO already has FCAS which will turn generators on or off, or loads on or off, if the frequency goes outside bounds. We would need something similar in distributed generation to ramp up generation when frequency is low, and ramp it down when frequency is high. Then load-shedding sort of disruptions can be avoided.
As others have said, it's just a control system issue. Very easily solvable (especially since it's someone else's problem
![Laughing :-DD](https://www.eevblog.com/forum/Smileys/default/smiley_laughing.gif)
). There will likely be economic incentives for EVs to export to the grid when the grid needs energy.
Zero mass/inertia might not be needed, but something has to define the base frequency and voltage.
Yes, then you need a grid
forming inverter. A single inverter can do what it likes (if it has the capability). As soon as you have more than 1, they need to synchronise.
What stops a drift? If each inverter runs at its own defined frequency and voltage then are there any circulating currents with a tiny discrepancy with the grid?
As above, you cannot be substantially out of phase because the circulating currents will make a rather exciting light and noise show. But yes, on a smaller scale you circulate a lot of current trying to fight the grid.
I suppose the ultimate test is to run a synchronous clock off the grid, if it stays dead accurate then it is stable.
Doesn't really matter. The grid can wander up and down in frequency all the time and still be stable. The point is the rate at which it wanders, and you won't pick that up with clock-watching.
What if a sudden load appears, lightning strike or similar, do the inverters just skip over the transients and just carry on?
An inverter "just" skips a direct strike in the same way a human "just" keeps playing golf after being hit.
Depends on the size of the disturbance. Small blips will sail through, bigger blips will cause an inverter to disconnect, and it might cause catastrophic failure of the nearest few inverters.
The grid is described as an infinite bus bar, nothing that occurs overloads it or upsets it.
By whom? Very simplified view, and that's useful for some analogies but important to know the limitations of such a simplistic understanding.