Gridwatch’s Leo Smith On Renewable’s Instability

By Paul Homewood

  h/t Douglas Brodie

 

Leo Smith, who runs the Gridwatch website left this comment on WUWT the other day, following up on John Constable’s article here.

Being an engineer, Leo knows his stuff inside out, and his comment is worth reposting in full:

 

This is a good article but from the comments it seems that some of the important detail has not been fleshed out enough.
First of all let me say something about solar power in the UK.There is quite a lot of it, but none of it is metered centrally by the grid, and, as far as I know none of it is under central control. If you look at the demand curve graphs you will see that there is a big midday notch. I have been monitoring, via my website, Gridwatch, (https://gridwatch.org.uk) the same data as is used here, since 2011. There never was a midday notch until solar power came along, and this ‘notch’ is actually the effect of millions of domestic solar panels and a few commercial solar farms kicking in and feeding local demand. As far as the central monitoring is concerned this represents a fall in ‘seen’ demand and so ‘the notch’. A couple of years back Sheffield University, in a zeal for all things renewable, starting publishing their estimate of total GB solar power, based I think on sampling a few sites who agreed to share their output figures. I now incorporate that data with suitable caveats on the same website and database.

This solar power is absolutely ‘wild’ on the grid. As far as domestic panels go, there is absolutely no way to control the output centrally at all. I am less sure about the few commercial solar farms as to what their contractual arrangements are and whether (like wind) they can be ‘constrained off the grid’.

Wind farms are similarly wilful in their output, although both solar and wind are reasonably predictable to within a couple of GW several hours ahead, and a couple of GW is something the grid should be able to cope with – typically, if there is a massive and sudden shortfall if a power station trips out, the pumped hydro at Dinorwig and the small hydro sets get an alarm and can be up to power in seconds. So long as they can ‘hold the fort’ for half an hour or so without running out of water, that is enough to get the gas sets up and running. Similarly a sudden drop in demand is catered for by the inverse – hydro stops and then gas sets are shut down.

Contrary to popular belief not much ‘spinning reserve’ is kept online. I suspect that what actually happens is that the hydro plants are used to modulate very small and rapid fluctuations in demand over the timescale of minutes.

This however is not the instability which is the matter of concern here, what is a matter of concern is the grid frequency – nominally 50Hz± not a lot. Traditionally this was held to 50Hz exactly on average over the day so that synchronous electric clocks kept proper time, but minor fluctuation was tolerated. And a major part of that stability was inherent in the design of the generating plant – massive turbines and rotors in the alternators held to 3000rpm (for 3 pole designs) represent a surprising amount of stored kinetic energy. More than the average battery installation for sure, and in the case of – say – lightning striking a main grid link and shorting it – this is what supplies the overload current until the arc self extinguishes. Then of course we have electric trains which are constantly stopping and starting with power flows to and from the grid (most these days push braking energy back into the grid) .

This is all well and good until – because renewables have priority on the grid – there is hardly any conventional generation on the grid at all. Peak solar and a bit of wind is capable of putting over 16GW onto the grid and typically when Europe has a surplus in summer, we will be importing another 4GW via the undersea high voltage DC links. So that’s 20GW of ‘unconventional’ generation on a grid whose maximum total demand is in the middle of the day scarcely 30GW. Typically there will be a 3GW of wood burners at Drax plus about 6-8GW on nuclear power on the grid. And all the coal, gas and hydro shut down.

And this is where the problems start to get nasty. All those renewables, and the DC power under the sea, feeds the grid via inverters. Electronic circuits that chop the DC and shove it through high frequency transformers to generate an approximate 3 phase sinusoidal waveform that is fed to the grid via chokes and capacitors to improve the waveform . And this has to be in phase with the grid and locked to its frequency.
How do they know what the frequency is? They monitor it of course.

And what happens if the frequency, now hardly controlled by spinning turbines, drops due to a temporary overload?

THEY SHUT DOWN AND DISCONNECT THEMSELVES!

And instead of 2GW lost from the grid, because a reactor has tripped, that’s 20GW, and a total cascading blackout across the entire country.
That is the problem that is concerning engineers, myself included.

You may recall that during last summer’s blackout, arguably the biggest problem was how embedded renewable generation suddenly dropped off, as the system frequency fell.

It would appear from John Constable’s analysis of the official investigations that this sudden loss of generation was never fully explained.

What we do know though is that problems of this nature will become progressively worse as more and more renewable energy comes on board.