National Grid interim report into the power cut - what does it tell us?
Simon Gallagher
Electrification Expert | Managing Director at UK Networks Services | CEng | FIET | FEI | MBA
Ofgem today published the interim report from National Grid Electricity System Operator into the power cuts last week caused by a significant frequency excursion on the transmission network. My thoughts on what happened can be found here.
The report is an interesting read, and while it adds to our understanding, it raises as many questions as it answers.
Summary
The report can basically be summarised by “a lightning strike on an overhead 400 kV transmission circuit caused it to trip but reclose successfully after 20 seconds as designed. This also caused Little Barford CCGT and Hornsey Windfarm to trip, removing 1,378 MW of generation from the system. NGESO only procure enough frequency response services to secure against a loss of 1,000 MW therefore the system frequency fell rapidly, but was successfully recovered by the Grid Code enforced Distribution Network Operator (DNO) Low Frequency Demand Disconnection (LFDD) scheme, by disconnecting 5% of the UK loads at the time, about 1,000 MW.”
Some Facts
Before discussing what does the above mean, here are some facts I think are important:
- The weather was normal for this time of year
- Load was normal for this time of year (32 GW)
- 500 MW of embedded generation disconnected, which the report refers to as ‘small’
- Lightning strikes to the transmission system are common
- NGESO was holding 1,023 MW of frequency response
What happened - Generators
The NGESO interim report tells us that a lightning strike was the root cause of the event. However, it is not clear how a lightning strike (a common occurring on the transmission system) caused the tripping of generation.
Considering Hornsea, immediately after the lightning strike the site de-loaded from 799 to 62 MW (2 modules tripped, 1 remained in service). NGESO state that the voltage fluctuation (as a result of the lightning strike) caused ‘automatic protective systems’ to activate. Subsequently these systems have been ‘fine-tuned’ and the wind farm is now ‘operating robustly’. Given no other large generator tripped, one possible conclusion is there was a protection setting issue revealed by the event.
At Little Barford, within seconds of the trip at Hornsea, the 224 MW steam turbine tripped. 1 minute later the associated gas turbine (now presumably operating in open cycle mode) was tripped as a result of high steam pressure in the steam bypass. The second turbine was then tripped manually by the power station staff, due to high steam pressures. What is not clear is if this was a reaction to what was going on in the first gas turbine and steam turbine or a separate issue with the second set. In any case, RWE state that the initial trip was caused by ‘discrepancy between three speed signals’. I am not sure what the means but it would appear to be coincidental to the Hornsea trip caused by the voltage disturbance from the lightning strike.
What Happened – Frequency
NGESO had enough frequency response services held in reserve to respond to maximum in-feed loss of 1,000 MW. NGESO are clear in their report that because this event resulted in a combined loss of generation over this amount, the system was never going to be able to contain the frequency without resorting to extreme measures – the DNO LFDD systems. However the actual picture is a little more complex, as illustrated by the frequency trace below, adapted from the graph within the NGESO report (it is interesting the NGESO refused to provide me with the frequency second by second data when I requested it last week, telling me that all data is being reviewed!):
With the initial trips at Hornsea and Little Barford at 16:52, the cumulative lost transmission connected generation was 981 MW, which was within the ‘maximum infeed loss’ that NGESO had secured for. It is not clear therefore why the frequency fell rapidly all the way down to 49.1 Hz, which then tripped 500 MW of embedded generation. Given that between the first trip at 16:52:33 and the subsequent trip at Little Barford at 16:53:31 (58 seconds) the generation lost was 981 MW, it would appear NGESO was outside the ‘unacceptable frequency condition’, a potential statutory breach.
By 16:54, a total of 1,378 MW of transmission connected generation was gone, as well as 500 MW of embedded generation, causing the frequency to fall to 48.8 Hz causing the DNO LFDD relays (stage 1) to operate, disconnecting 1,000 MW of DNO MW of load, allowing the system frequency to recover. We all know the rest of the story – National Rail badly affected, millions of customers off supply.
Conclusions
NGESO secured against a single loss of 1,000 MW – but suffered a loss of 1,878, this is the cause of the frequency collapse. It is interesting that Ofgem approved a change to the ‘National Electricity Transmission System Security and Quality of Supply Standard’ in December 2014. Had this change not have not been made, it is likely NGESO would have had to carry additional frequency reserve – at a cost to all consumers. Given Ofgem are now investigating this event, is there a conflict of interest here?
NGESO calculated that the largest transmission infeed loss was 1,000 MW, as well as 500 MW of embedded generation. The frequency reserve carried is calculated as the maximum of the two – not a combination of the two. Given that the frequency drop triggered by a loss of 981 MW triggered the disconnection of this 500 MW of embedded generation, this now seems not the best approach.
The report makes clear that wind and solar variability was not a factor – but does not discuss the issue of limited inertia on the system. Much more work is required in this area to understand the true levels of frequency response it really needed – especially fast acing services like batteries.
More questions than answers…….
Head of Department, Energy
4ycontinued due to post length limit Further, it seems likely the deloading of Hornsea initiated the inertia event, hence why it so important to know what inertia largest loss was being considered. Had the lightning strike taken off all 3 little Barford sets instantaneously via a double circuit fault, their inertia would have been lost and the frequency drop would have been much deeper. In fact there appears to be many more onerous potential faults that would be secured events under SQSS. It is very likely that the disturbance from the lightning strike on a main circuit plus the allowable wobble from the loss of generation compounded to appear to give the appearance of a bigger inertia event than the generation loss on its own. It seems clear that the system was being run insecure. There are multiple reports of simultaneous trippings and frequency instability, all warning signs that the grid is being run in a very brittle state. So, the big question, why are ESO not recognising the brittleness and taking appropriate action? Not long for the final report to go to OFGEM, so lets see.
Head of Department, Energy
4yA few more thoughts that may be worth sharing. Page 3 ‘The ESO was keeping 1000MW of automatic backup.... to cover the loss of the single biggest generator on the grid” Page 17 “The ESO must also consider the inertia of the system...” What is not stated is that SQSS requires a double circuit fault to be secured. Double circuit faults can take of generators simultaneously and instantaneously. It is also not clear what size of instantaneous loss was being covered for inertia, just that it was considered. Inertia is a big issue for grid control. NG routinely pull back generation to limit this loss to below 1000MW as it is the cost effective solution. The frequency graph show Little Barford ST and the transformer loadings occurring at the same time stamp of 16:52:34, no points of seconds for these time stamps. We must assume therefore that an inertia event had already occurred less than 0.5 seconds after the lightning strike. In which case everything else is irrelevant and NG are very lucky the overall impact was so little.
Electricity Operations and Energy Modelling Specialist at Electricity Efficiency
4yAs regards the embedded Gen trips it is possible that microgeneration Inverters have been shipped with tight Frequency trip settings. As the Germans and Italians found and had to retrofit 35GW of kit!!!
Head of Department, Energy
4yA very nice report that shows that the grid had adequate reserve and all the disruption is down to generators falling of when they should have rode the disruption without issue. Another way to read this is an admission that the UK grid is routinely run insecure. A lightening strike on a line causing the instantaneous trip of a generator is a secured event. Had the lightening strike that started this stuck a few miles north at Little Barford, all three generators would have come off simultaneously, not over nearly 1.5 minutes The quoted SQSS states that for a secured event the frequency should recover to 49.5 within 60 seconds. Some 58 seconds after the initial fault the frequency was well below 49.5 and falling when the second Little Barford generator came off. The SQSS was therefore breeched for a loss of 981MW plus the 500MW of Loss of Mains. The second little Barford set did trigger the low frequency demand tripping relays (1134MW loss). The 500MW loss of embedded due to Loss of Mains protection is a big issue. With a lightening strike, there is always fairly widespread voltage depressions which will cause this loss. As stated above a lightening strike taking off a major generator is a single credible event; and is therefore secured. There are several generators and interconnections capable of generating 1000MW as a single credible loss. The report states that the 500MW embedded generation loss for 'Loss of Mains' was 'in line with ESO forecasts' It is therefore clear that a lightening strike causing the loss of 1000MW of large generator plus 500MW of embedded is expected and must therefore be a secured event- it can not be considered as extreme. What questions come out of this? For me: Why was the reserve inadequate to stop a secured event going outside of statutory limits? As frequency drop and inertia are related, this leads onto was the inertia high enough? With weather risks, active lightening and 30% wind generation why had ESO not exercised their prerogative to increase reserve above the minimum? Does OFGEM follow National Grids advice too closely when agreeing the response levels and security risks? After 30 years working in the UK and European TSO / ESO /TO industry, MGC Engineering and Consultancy was established to bring independent expertise to these debates.
Happy Days on Islay
4y3.3% spinning reserve on our national grid is not very much, is this a cost saving decision or trying to reduce carbon emissions.