Thread by @OtherProfGreen

This report will gives very interesting account for engineers looking to understand what happened. I'll try to summarise. The sequence is more complex and subtle than was evident from public-domain information on the day itself, 9th Aug. Initial cause was a lightening strike ...

https://twitter.com/ng_eso/status/1163696228770766849

Lightning affects power lines many times a year. Large currents are seen, circuit breakers open to disconnect the line, in 70 ms in this case. Those circuit breakers re-close automatically, in this case after 20 s and stay closed if the "fault" has gone & lighting will have gone

And hundreds, perhaps thousands of times year that is all. There are parallel circuits to the one that disconnects (the system has built in redundancy) and no generators or customers are affected. But 9th Aug was different ...

The disturbance to the voltage caused by the lightning and line outage then caused three things to occur at essentially the same time (within 0.5 s). Hornsea wind farm saw the voltage perturbation, attempted to help corrected it and then some internal anomaly was detected ...

which caused two modules to abruptly disconnected with loss of 737 MW of power infeed. The Little Barford power station was connected directly to the line affected by lightning. It has two gas-turbine generator and a steam-turbine generator (it is combined-cycle) ...

Its steam-turbine had anomalous speed measurement data and shut down losing 244 MW. The voltage disturbance will have involved and magnitude and an angle shift. It looks like about 500 MW of distributed generation (small wind, solar etc.) interpreted this as "loss of mains" ...

and disconnected. That's a loss of about 1480 MW so frequency started dropping as remaining generators slowed down while still supporting 33,500 MW of demand. It dropped to 49.1 Hz. While it was dropping 650 MW of "response" stepped up. That was a mixture of ...

of part-loaded generators, batteries & demand response operating within 10s. The batteries will have been very fast and undoubtedly helped a lot. Over the next 20s a further 350 MW of response arrives to a total of 1,000 MW which is what National Grid ESO was holding to cover ..

For outage of largest generator running that day. The mix of response is interesting: 200 MW gen; 450 MW battery and 350 MW demand. That is very different from 10 years ago when it would all have been generation (part-loaded gas or coal). So far, so good, no demand disconnection

only contracted (voluntary) demand response. Then the bad news. With steam unit shut down, the gas units at Little Barford can't operated. The first gas unit trips after about 1 minute losing a further 244 MW. We're down 1,691 MW with the reserve of 1,000 already deployed.F

falls further to 48.8 Hz which is where the second line of defence is pre-programmed to come in. It's called low-frequency demand disconnect, LFDD. About 1.1 MW customers were disconnected. More would have gone at 48.7 Hz but fortunately frequency started to recover.

Over the next 4 minutes NGESO control room called up 1240 MW of further action (secondary reserve) and got the frequency back to 50 Hz and so could instructed distribution network operators to start reconnecting customers. Somewhere in there second gas unit at LBAR tripped too.

Reconnection of disconnected customer took 15 to 50 minutes.

So, what do we make of all this?
1) It is very rare to see large generator disconnected in response to lightning strike and/or line outage ...

2) So losing two to the same event is exceptionally rare and in that light the system responded well.
3) The last time two generators were lost in quick succession was 28/5/2008 with half a million customers lost to LFDD so it is rare
4) LFDD is very painful for those affected .

but 5% suffered so that 95% stayed connected to a functioning grid.
5) There will be technical lessons learned at LBAR and Hornsea with perhaps implication for other generators.
6) Rate at which frequency changed, 0.16 Hz/s, was high in historic terms but ...

to be expected with our new low-inertia grid (wind turbine and PV don't naturally contribute to the spinning mass of the generator fleet so frequency moves faster when supply-demand out of balance)
7) The mix of response was healthy - batteries and demand response were quick.

8) Could this be prevented? If NG ESO had been holding 2,000 MW of reserve not 1,000 MW then no LFDD would have been needed. NGESO spends close to £300M/year contracting response/reserve. Spread across 25 million customers that's a £12/year component of your bill. Double it?

Perhaps we should have a public discussion of whether we want to protect our selves from outages of 2 large generators, or 3 or 4 and what we would be prepared to see in our bills to have that.

9) Could more be done to refine "loss of mains" protection to avoid losing distributed generation. This is already underway anyway. Lots of techy information on websites e.g. energynetworks.org/assets/files/A…

10) Is wind a problem? Well first off, UK has halved carbon emissions from electricity from 150 million tonnes per year in 2012 to under 70 last year and wind was a big part of that. But yes there are problems, challenges, to address as we add more wind.

We need to address further reduction in inertia (seen over second), covering short-term fluctuation and forecast errors (or hours), periods of low wind speed (days) and seasonal variations (more demand on dark winter evenings).

This is why I and thousands of other engineers are in the game; we love solving tricky problems that have important benefits to society. This is what we're all talking about.

8) Could this be prevented? If NG ESO had been holding 2,000 MW of reserve not 1,000 MW then no LFDD would have been needed. NGESO spends close to £300M/year contracting response/reserve. Spread across 25 million customers that's a £12/year component of your bill. Double it?

Perhaps we should have a public discussion of whether we want to protect our selves from outages of 2 large generators, or 3 or 4 and what we would be prepared to see in our bills to have that.

9) Could more be done to refine "loss of mains" protection to avoid losing distributed generation. This is already underway anyway. Lots of techy information on websites e.g. energynetworks.org/assets/files/A…

10) Is wind a problem? Well first off, UK has halved carbon emissions from electricity from 150 million tonnes per year in 2012 to under 70 last year and wind was a big part of that. But yes there are problems, challenges, to address as we add more wind.

We need to address further reduction in inertia (seen over second), covering short-term fluctuation and forecast errors (or hours), periods of low wind speed (days) and seasonal variations (more demand on dark winter evenings).

This is why I and thousands of other engineers are in the game; we love solving tricky problems that have important benefits to society. This is what we're all talking about.

11) Were NG careful over who got disconnected? It is actually the local distribution network operators (DNOs) like UKPN, WPD, SPEN who implement LFDD in their local networks. The industry as whole is working to make this more geographically refined. But that doesn't seem to be

the problem. Ipswich hospital wasn't disconnected but its own protection system operated. The traction supply to Thameslink wasn't lost but their own trains shut down when they detected frequency of less than 49 Hz and then drivers could reboot them. Two tractions supplies ...

to DC lines were lost because of their own protection operating not demand disconnect. There is more investigation to be done here on what demand was force off and what shut itself down and how we arrange this better in future.
That said disconnecting 1.1 million customers is ..

painful for them and is a lot resort that the industry must strive to reduce to very rare occasions.

A version of this thread that might be easier to read (and with some typos corrected) is now on the @EnergyFuturesIC blog.

https://twitter.com/EnergyFuturesIC/status/1163775277455073280?s=20