In my latest column for the Daily Telegraph I discuss the risks associated with NESO’s ambition to run the power grid without gas by the end of this year and to reduce its minimum inertia requirement. It has boasted already of running “95% carbon-free’” although this has only been possible due to a quirk of carbon accounting rules which designates the burning of wood at an industrial scale as zero carbon. That is despite the carbon dioxide emissions from wood-pellet biomass being higher than for the coal it replaced.
Whether this ambition will be possible even with the dodgy carbon accounting rules is unclear. In the summer months, gas power stations are often turned up by the system operator in order to stabilise the grid, providing important inertia to control voltage across the network. Throughout the summer, NESO has observed unexplained voltage oscillations (which I will discuss in another post – this is being held up by junk frequency data being reported to Elexon which has yet to be corrected, despite Elexon requesting re-submission), including on the day before the Iberian blackout. These oscillations can, in extreme circumstances, cause the type of grid failure that led to the loss of eleven lives on 28 April in Spain and Portugal.
Despite the risks, NESO wants to reduce inertia limits
Last month, NESO published its Frequency Control and Risk Report (“FRCR”) for 2025 in which it proposed reducing the amount of inertia it is required to hold from 120 GVA.s to 102 GVA.s. It claims such a move would save up to £96 million, a drop in the ocean of the total system balancing costs which were last week reported at £2.7 billion for 2024/25. NESO cites the use of batteries and other assets such as synchronous condensers procured through its Stability Pathfinders as providing alternative forms of inertia to conventional generators.
NESO’s claims about inertia management in the report and elsewhere don’t stack up. In the Operability and operations analysis annex to its Clean Power 2030 advice to the Government, NESO states that “we currently ensure system inertia is always above 120 GVAs.”
This chart (from the FRCR) implies that the 120 GVA.s limit is never breached during 2024, but in fact, analysis of the raw data show that in calendar year 2024 in outturn inertia was below this level in 9.6% of settlement periods, rising to 10.7% for the IFA year 2024/25 (1 April 2024 – 31 March 2025). If we compare with the actual inertia limits in place at the time of 140 GVA.s up to 28 February 2024 and 130 GVA.s from 28 February to 18 June 2024, the lower limit was breached in 15.4% of settlement periods in 2024.
While some people may take comfort saying this indicates the system can be safely run at lower levels of inertia, the fact that NESO fails to meet its obligations with such regularity is far from comforting. And the reason for this failure lies in the fact that it doesn’t actually know either in real time or afterwards, what inertia actually is or was at any time.
While NESO has invested in some real-time measurement systems, these are currently only deployed in Scotland. Elsewhere, the control room relies on estimates. So it is hardly surprising that when it calculates actual inertia after the fact it turns out to have missed the mark so often. In fact, according to statements made in the NESO Operational Transparency Forum (“OTF”) (specifically on 29 January 2025), even the outturn inertia data are estimates rather than actual measurements:
“System outturn inertia is estimated as the sum of the inertia provided by Generators, the contribution from Demand Side and the contribution from Pathfinder Units,”
– NESO, Operational Transparency Forum slides, 29 January 2025
These estimates are based on which conventional power stations are synchronised at any moment, using pre-agreed inertia values published for each generator type. The control room uses these static figures combined with real time SCADA data to model total system inertia. However, this method does not account for fast-changing conditions or inertia contributions from newer assets such as batteries, inverter-based generation such as wind and solar, or embedded generation.
While post-event analysis may offer a more accurate picture, in real time, the control room is essentially working off educated guesses that have increasingly been proven wrong. Even NESO acknowledges that its estimation tools are still being developed, and real-time inertia measurement developed by Reactive Technologies (which I wrote about back in 2017) remains limited to parts of Scotland.
Ironically, the push for gas-free operation makes it even harder for NESO to know whether the system is truly safe. With few conventional generators online, the control room loses visibility over most real-time inertia inputs — meaning the estimates they rely on become less reliable just when accuracy matters most.
Another key input into system management is demand forecasting. In the past few years, NESO’s demand forecasts have demonstrated significant errors when compared with actual outturn data – up to 4.7 GW over a half-hourly interval (on a day-ahead basis), with an average of 609 MW. NESO does not publish more granular data, but if this is the average error the day before delivery, what sort of errors is it making within-day? And what is the error over the sub-1-minute intervals which are relevant for grid stability? On 22 January NESO announced an audit of its demand forecasting, with the engineer presenting this to the OTF commenting that the forecast methodology had not been looked at in “10-20 years”!
Of course, this raises concerns about what would happen if the inertia target was reduced. Were NESO to continue to undershoot at the same level, this could put the system at real risk of failure. The financial savings that could be realised by reducing the target could be quickly swamped by the economic costs of a blackout – early estimates of the cost of the recent Iberian blackout begin at €150 million, and could rise to €450 million depending on the degree to which heavy industries were able to re-start their process in line with the restoration of power to the grid.
Official report into Iberian blackout is damning
On 16 April, Spain’s system operator, Red Eléctrica de España (“REE”), boasted that it had generated 100% of its electricity with wind, solar and hydro power, for a few minutes. Two weeks later on 28 April, a prolonged blackout sent all of Iberia dark. The outage affected parts of France and even Belgium reported issues in what was Europe’s biggest ever blackout. A week later, another blackout hit Spain’s Canary Islands.
On 28 April, at around 12:30 pm local time in Spain, just before the grid collapsed, renewable sources accounted for 78% of electricity generation on the Iberian system, with solar accounting for almost 60%. By contrast, conventional generation, such as gas and nuclear power plants, comprised only around 15% of the total generation mix.
According to Raúl Bajo Buenestado of the Baker Institute, two consecutive generation loss events occurred in southwestern Spain, likely involving large solar installations. “In just five seconds, Spain lost approximately 15 GW of capacity, equivalent to 60% of its national electricity demand. The remaining generation was insufficient to meet demand, thus triggering a cascading failure across the entire grid. Various generating units were automatically disconnected to protect infrastructure, and nuclear plants were shut down in accordance with safety protocols.”
Now, according to reporting by Reuters, the first official report by the Spanish authorities has blamed REE for miscalculating its power capacity needs on 28 April, meaning that a surge in voltage led to a massive blackout. REE did not have enough thermal power stations switched on during peak hours according to Spain’s Energy Minister Sara Aagesen in a news briefing in Madrid.
“The system did not have sufficient dynamic voltage control capacity…[REE] told us that they made their calculations and estimated that (switching on more thermal plants) was not necessary at this time. They only set it for the early hours of the day, not the central hours”
– Sara Aagesen, Spanish Energy Minister
The report describes the blackout as being “caused by an overvoltage problem with a multifactorial origin: the system had insufficient voltage control capacity, there were oscillations that conditioned the operation of the system and disconnected generation facilities, in some cases in a seemingly undue way”. There is an English version here. A longer version of the report (only in Spanish) is available and I have started to look through a translation, but ChatGPT tells me it does not add anything material. I’ll come back and update if that turns out not to be the case. The charts are from this longer report.
It described the chronology of the outage as follows:
Phase 0: Voltage instability
During the days prior to the incident there voltage oscillations and on the morning of the 28th the voltage varied more intensely than normal.
Phase1: Oscillations in the system (12:00 – 12:30)
At 12.03 pm an atypical oscillation of 0.6 Hz was recorded, which caused large voltage fluctuations for 4.42 minutes. This oscillation forced the System Operator to apply measures to cushion it, such as increasing the grid meshing – restricted by low demand – or reducing the flow of interconnection with France. All these actions dampened the oscillation, but had the side effect of increasing voltage. At 12.16 pm the same oscillation was recorded again, this time smaller, and 3 minutes later there was a further oscillation of 0.2 Hz. The System Operator applied the same measures to cushion it, which also contributed to increasing the voltage.
PHASE 2: Generation losses (12:32:57– 12:33:18)
Voltage began to rise rapidly and steadily, and numerous and progressive disconnections of generation facilities were recorded in Granada, Badajoz, Segovia, Huelva, Seville, Cáceres and other provinces.
PHASE 3: Collapse (12:33:18 – 12:33:30)
The progressive increase in voltage caused a chain reaction of overvoltage disconnections that could not be contained, as each disconnection contributed to further increases in voltages. There was also a drop in frequency that led to the loss of synchronisation with France. The interconnectors with France tripped, and the Iberian peninsular saw its power grid fall to zero.
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The report goes on the identify three main causes of the blackout:
1. Insufficient voltage control
The system showed insufficient voltage control capacity for two reasons. Firstly, on the day before the incident, the System Operator scheduled the activity of 10 synchronous power plants with the capacity to regulate voltage on the 28th. The final number of coupled synchronous power plants was the lowest since the start of the year. Secondly, several of the plants capable of regulating voltage – and specifically paid for it as they were scheduled for this purpose – did not respond adequately to the instructions of the System Operator. Some even produced reactive power – the opposite of what was required – exacerbating the problem.
2.Frequency oscillations
The oscillations – the first of which was atypical and had its origin in an installation in the Iberian Peninsula – forced the configuration of the system to be modified, making it harder to stabilise voltage. After the second oscillation, the REE demanded the availability of a plant capable of regulating voltage, but it was technically impossible to instruct them before the collapse.
3. Generation trips
Some of generators that tripped would have done so before the voltage thresholds established by the regulations were exceeded (between 380 kV and 435 kV in the transmission network) ie they failed to meet their fault ride-through obligations. Other trips occurred after the limits were exceeded, as would be expected to protect equipment.
Once the cascade started the usual grid protections were unable to stop or contain the failures. Some of the protections mechanisms, such as load shedding, may even have contributed to the overvoltage by discharging the lines even more, contributing to the rise in voltages, because they acted to compensate for the drop in generation rather than to manage the voltage.
In conclusion, there was a lack of voltage control resources, either because they were not sufficiently scheduled, or because some of the units that were scheduled did not provide the service adequately, or because of a combination of both.
The Government report contains the following recommendations:
- Increased supervision and compliance monitoring of regulations such as fault ride through
- Technical measures to increase voltage control and protections against frequency oscillations. The National Commission on Markets and Competition is currently considering reforms that will allow asynchronous installations to use power electronics to manage voltage variations
- Increase the flexibility of the electricity system. The Electricity Plan 2025-2030 will prioritise industrial consumption and an increase in storage capacity
- Continues with planned increases in interconnection with neighbouring countries
Although cybersecurity was determined not to be a factor in the blackout, the Government also proposes to accelerate the implementation of European regulations and network controls that will increase detection and monitoring systems providing a higher level of grid surveillance.
Since the incident, there has been talk that REE has increased the use of gas on the grid to support inertia, however that is only part of the story.
While the amount of gas on the Spanish grid has increased by 25% since the blackout, compared with the four weeks immediately before, overall synchronous generation was about 2.7% lower due to seasonal reductions in nuclear for maintenance, and unexplained reductions in hydro. This suggests REE is still not being as conservative with the system as the blackout might warrant, not least because the genesis of the blackout has yet to be made public, suggesting that rather than a mundane grid fault, it may well have been related to the behaviour of inverters.
Like NG ESO as it was back in 2019, REE is trying to pin the blame for the blackout on non-compliance with grid codes by generators. At the time I said this was weak: why was NG ESO not monitoring Grid Code compliance? And why did windfarms not have to be compliant beyond a certain level of capacity – historically generators did not have to fully comply with the Grid Code until they were fully commissioned, but since windfarms can have over 1 GW connected to the grid, as I think Hornsea 1 did at the time, surely this rule needs to be changed. Unlike conventional generators. Windfarms commission incrementally so beyond some agreed level of capacity they should be required to meet materially all of the obligations any other generator must meet. (Not least because Hornsea also failed to make timely REMIT notices so the market thought the CCGT at Little Barford tripped first after the lightening strike but it didn’t, Hornsea did!)
But what is striking from this initial report into the Iberian blackout is that there is still no explanation for the initial cause. Three generators didn’t just trip off at the same time for no reason – the frequency oscillations described triggered their trips. But the report is silent on the origin of those oscillations. I suspect that if there had been some mundane grid fault, this would have been made public, which leads me to suspect we’re dealing with inverter-induced oscillations, something they would be reluctant to admit. We shall have to see if more comprehensive subsequent reports uncover more detail.
Ideology not money driving NESO’s agenda
The real reason NESO wants to drop the inertia target isn’t cost but ideology – to enable it to claim zero carbon running (despite the use of wood). The burning of wood isn’t the only source of smoke and mirrors – NESO also relies on embedded generation to help out. This generation, like interconnectors, is counted as zero carbon for NESO’s carbon accounting purposes regardless of their actual fuel. Some of these units are diesel and gas engines, many of which were built in the early years of the capacity market that is costing some £3.6 billion per year.
The Spanish government’s official report into the 28 April blackout confirms that the collapse was not caused by a cyberattack or discrete fault, but by a “multifactorial” sequence triggered by a miscalculation by Red Eléctrica . Investigators found that REE failed to ensure enough synchronous generation was online to maintain voltage control, leaving the system exposed when three major generating units disconnected unexpectedly – but three units don’t all trip unexpectedly at the same time – something caused them to trip.
While voltage regulation systems failed to stabilise the system, this was a consequence of the underlying planning error: too much reliance on inverter-based generation in low-demand, low-inertia conditions. Reports of voltage overshoot – rather than collapse – strongly point to uncoordinated reactive power injection by inverters, a recognised risk in weak grids with insufficient damping.
This is uncomfortably relevant to Great Britain, where NESO is seeking to reduce the system’s minimum inertia floor from 120 GVA.s to 100 GVA.s. NESO maintains that frequency response and voltage services can compensate, but Spain’s experience suggests this assumption may be fragile. Crucially, in 2024 NESO failed to meet its minimum requirement in over 15% of settlement periods – despite this being the existing safety threshold. Some have argued this proves the system is stable even at lower inertia, but this misreads the situation: the fact that no faults occurred during those periods is not proof of resilience. Rather, it highlights a more troubling truth—NESO is already struggling to enforce its current minimum, and the measurement and forecasting of inertia remain uncertain. Under these conditions, lowering the threshold looks more like a gamble than a controlled evolution. If Spain’s blackout teaches anything, it is that when planning assumptions meet physical reality, the latter always wins.
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The question is why, when the risks have so recently been illustrated, is NESO pushing in the opposite direction? In practical terms, a few symbolic minutes of “zero carbon” or “gas-free” running mean little when the use of gas is still so critical to the power grid (in winter months we could not contemplate running without gas if we want to be able to meet demand at all times). So why take the risk, just for a brief moment of virtue signalling?
Spain tried that, and it cost eleven people their lives.
Excellent article, it’s 2 am and this situation does keep me up at night anyway, I think the UK is locked in already to a future of load sheding before the end of this decade. with all but one of our Nuclear reactors retired over the next 5 years, and my friend at Hinkley Point C stating it will not come online before 2030 (Chinese money still in, although reduced amount), a substantial amount of Gas, CCGT & Open offline as well, the future is already set in my opinion.
What are your thoughts regarding the Government roll out of so called “smart meters”, their aim is to pass the threshold of 74.5% of all homes having one by the end of the year, will the temptation of dynamic pricing be too hard to resist? 6-10am & 6-10pm unaffordable electricity for a high number of households.
Ran the figures, post shut down of the above electricity generation above, then 25%,50%,75% and 100% availabity of interconnectors, Biomass, Hydro, Gas and Nuclear, minimal wind and no solar over a two week Dunkelflaute in the Winter of 2029/30, the AI models don’t give us a chance. I tried raising the topic on last weeks BBC Question Time, I managed less than a minute, blank faces from the panel, with Fiona quick to move on without discussion, with a finger point and “man with the glasses” to change the topic.
Thanks for your work, appreciated…
It worries me as well
I asked ChatGPT about the probability of a blackout in GB by 2030. Of course, it’s not the arbiter of truth but the answer was interesting…
It’s no longer in the realms of unlikely.
Full blackout: 1-2%
Large regional blackout: 20-20%
Major system stress requiring significant TSO intervention: every 1-2 years (ie pretty certain)
My assessment is these numbers may be a bit off but the direction isn’t and it’s as good a guess as any given it’s a hard thing to assess.
NESO and Ofgem are sleepwalking into very dangerous territory
I installed a Tesla home battery last month for exactly this reason. I’m now thinking I might need two.
Kathryn, I admire your work on another insightful analysis
However, I would caution against using ChatGPT for anything at all, not even as an amusing toy. It’s admirable that you declare when, where and how you use it, but nevertheless, I think it is credibility-damaging to use it at all, frankly.
As a statistical soup of text context-prediction, it can only parrot what other websites/text/books (both non-fiction and fiction) have said in a similar context, and it cannot give a remotely useful answer to such a complex question as “what is the probability of a blackout in the UK by 2030”. (ironically the only thing ChatGPT can do for certain, is increase the risk of a blackout — by consuming an extra couple of megawatt-seconds every time you ask it a question!) – please don’t devalue your own expert work by referencing such a “bullshit machine” !
BTW, to counter the “junk frequency data” that you mention, you could perhaps corroborate it against http://mainsfrequency.uk/cgi-bin/plotdayer?actual=y&average=y&50hz=y¢re=y&grid=y&dist10=y – this data is collected independently.
Also, for a future blog, I would be interested to know how grid operators can control voltage independently of frequency – doesn’t this require automated on-load tap-changing at substations, and doesn’t that contribute to accelerated wear on supergrid transformers, potentially resulting in failure? e.g. at Heathrow earlier this year? i.e. Are renewables placing too much demand on this sort of active voltage control?
And I would implore anyone to watch James Burke’s “The Trigger Effect” (1978)
Lowering the inertia limit is a naked political ploy. Politicians and their spinners are all the time reframing negative situations to make them look positive. It’s pure propaganda. Like boasting that you supplied 100% renewable energy, but omitting to admit it was for “two minutes last Wednesday” as well as keeping shtum on the power supplied by connectors from other states and/or countries which use nuclear, gas or coal.
Exactly, It’s pretty meaningless given the amount of gas needed in winter. We’re MILES away from properly being able to run without gas. They need to stop virtue signalling and start focusing on the risks. FAFO seems apt
NESO are like children playing with matches.
As you say, ideology, no doubt, with significant government pressure.
Where is the assessment of what can be achieved against the very severe consequence of a failure? Tight rope walkers have safety nets, where is NESO’s?
A total U.K. grid collapse would not be nearly so quickly restored as Spain’s was and with all the consequent loss.
Recklessness in the extreme, and for the aim of a very expensive electrical service to the severe detriment of the U.K.’s fortunes.
We need a complete overhaul of our government and the bodies it controls, but how could this be achieved?
In business you set a budget/ target, sometimes you fail to achieve the budget/ target and the effect is limited just to you and your business, ultimately you take both the financial blow and the shame of failure nut 99.99999999% of the time, no body dies.
The very thought of reducing a critical minimum target that already cannot be met shows that this is grandstanding on a epic scale- NESO must prove a point- the system must be shown to be functional using everything but Gas & Nuclear- a example of political hubris.
We know what caused the Spanish mainland blackout, we saw how they struggled, what we do not know is just how this would play in the UK, nor do we know how the UK would cope in a similar blackout in Winter- you cannot even think about the potential death toll because that would cause you to ask whether or not this dash for Zero is worthwhile.
We need to de-politicise the whole Energy sector, there role is too provide cost efficient energy and it should be from ANY fuel source available, political point scoring is going to kill people, reliance on non fossil fuel and its inherent intermittence (with the exception of course of burning wood pellets which continues to confuse me) is going to kill people.
Nobody signed up to Net Zero, however, most people are accepting that each available energy source has a role to play and that even old fashioned Gas, Coal, Oil power generation plants can and have been made cleaner and more efficient hence less polluting- we are being forced down a road that no developed economy has ever travelled- total reliance on Renewables but without a reserve of supporting energy to back it up.
We need people of Kathryn Porter’s calibre to have serious input, but would the Net Zero crazed NESO, CCC and committed politicians want to hear the harsh truth that as it stands today, 2025, the technology is not capable of providing energy 24/7/365 without fossil fuel providing the base load.
In the words of Jim Lovell, “Houston we have a problem”.
^ this.
A pint for you sir.
“Voltage began to rise rapidly and steadily, and numerous and progressive disconnections of generation facilities”…
“The progressive increase in voltage caused a chain reaction of overvoltage disconnections that could not be contained, as each disconnection contributed to further increases in voltages”
Why does disconnection of generating capacity increase the voltage?
The problem is that solar inverters don’t have any ability to regulate voltage and reactive power: they rely on other systems on the grid for doing that. The other systems included the 11 power stations that were connected to provide those services: 4 nuclear (of which 2 at full load), 6 CCGT and 1 coal. The underloaded stations – including all the CCGT – were really operating as synchronous condensers, with very limited power output: at the time of the apagon just 861MW was CCGT. The generation mix at 12:30 was 82% renewables, 10% nuclear, 3% CCGT, 1% coal and 4% others.
Ahead of the collapse, REE had also disconnected a number of reactances (capacitor banks) at substations to try to handle the frequency oscillations: these are normally switched in to balance the overall tendency of a transmission line to be inductive (behave like a coil or transformer) to try to make the line closer to a pure resistance, which maximises the real power flow and minimises reactive power flows (flows in the wrong direction for part of the cycle when voltage and current have opposite sign when they aren’t in step). However, although disconnecting them helped to damp the oscillations, it meant that the overall voltages rose because the imbalance leads to reactive power flows, and you need a higher voltage to give both the forward flow of power and the negative flow. Actually, shortly before the collapse they switched some reactances back in to provide better voltage control on the transmission lines.
They also cut back on exports to France to try to reduce the grid flows from South to North, partly because there was limited transmission capacity in the middle of the country, and partly because the French were not happy with the wild swings in power transmission during the periods of oscillation – up to 800MW every 1.67 seconds that resulted from the instabilities.
The overvoltages led to lots of small generators tripping out between 12:32:00 and 12:32:55 – some 525MW in total of which some 317MW was less than 1MW (i.e. small solar). These either saw even higher local voltages relative to normal or had protection settings that caused them to trip out (probably because they were older installations with more sensitive settings – newer ones are required to “ride through” tougher conditions).
At 12:32:57 an (unidentified) power station in the province of Granada tripped out: it had been providing 355MW of active power and absorbing 165MVAr of reactive power to help control voltage. That led to a further trip (detail redacted) and the collapse of exports to France to zero. 1.3 second later there was a similar trip (again detail redacted) that saw France exporting to Spain, reaching as much as 895MW. The key point here is that the loss of generators providing reactive power control/voltage control meant that voltage was only going to go up, leading to more generators tripping out.
At this point we start seeing substations disconnecting transmission lines, and the grid frequency starts deviating across the country. In fact, in a couple of locations they chart the frequency rose because of local oversupply due to transmission disconnections, even as it was falling in what became a rapid cascading trip elsewhere.(see Grafico 18 in the report). The frequency collapse leads to sharp deviation with the frequency in France, and with exports from France peaking at almost 4GW (above theoretical nominal capacity), the interconnector lines trip out as the country descends into blackout. The lowest recorded frequency before the last units tripped out was 45.89Hz. On the way down, pumped storage pumping started tripping out at 49.5Hz – while this reduced demand, it also cut a vital source of inertia, but probably already the grid was destined for full blackout. Demand disconnections from 49 Hz to 48Hz were insufficient to arrest the decline.
Please stop answering may questions to Kathryn.
Neso has become a danger to our energy security.
Thank you, Kathryn,
Clear analysis as usual, and I couldn’t agree more with your conclusions: NESO is driven by ideology, not engineering knowledge.
Can anyone provide a figure please for the amount of inertia available from a thermal generating plant, such as Drax?
This analysis, essentially done by a regression, suggests Drax has an inertia constant of around 4 seconds. That is, if you multiply the capacity that is running by 4 you get the GVAs of inertia it provides.
It gives some idea of the range of values that apply across various kinds of generation.
https://arxiv.org/pdf/2210.03661#:~:text=Inertia%20values%20for%20the%20power,GVAs%20and%201.59GVAs%20respectively.
No.
Because having less than you know you need when you probably need a contingency reserve any way is simply gambling with our national energy supply to no actual purpose, to boost politicians egos by putting lives at avoidable risk .
Wholly irresponsible.
Have all the competent engineers left the NESO building?
Brian RL Catt CEng, CPhys, MBA
I find it difficult to follow why the grid voltage rose as chunks of the supply network shut down. Is there a simple layman explanation that could be included in future such reports? If the disconnections only apply to supply, not demand, how does it relate to the power (V*A) that the remaining plant have to supply? I guess I‘m showing my ignorance of how the grid system actually works!
I picked up a couple of extra points from an article in El Pais the Spanish top newspaper:
Apparently some 22% of renewables plants they looked at had problems with some of their inverters, particularly the elements that were supposed to ensure they maintained their power factors within limits, contributing to overvoltages. When the power factor is kept close to 1 there is little or no reactive power to help create overvoltage. Power factor is the cosine of the angle of a right angled triangle with the reactive power measuring the opposite side and the active power the adjacent side, so when reactive power is small, the angle is close to zero and the cosine close to 1.
It also pointed out that when the grid is connected in a more meshed fashion (as it was in response to the fluctuations in frequency) and is operating at low throughput it generates reactive power that leads to overvoltages: the lines behave more like capacitors. Hence why they disconnected reactances which are capacitors at the same time as switching in the additional links.
Re: problems with inverters: It would be interesting to know if it were the inverters of specific manufacturer(s), that were actually working as designed, i.e. bad design, or if they had failed. Was it a technical fault that the maintenance schedule wasn’t picking up, where they actually need a periodic off-line inverter test procedure?……was there a periodic maintenance test procedure?
I actually run an old car, which is supposedly unreliable, which with proper maintenance, and high component/consumable quality and a planned system of component replacement on a timescale before the MTBF (mean time before failure), it is actually incredibly reliable.
If they have components failing, then the failure mode needs to be understood and introduce a maintenance schedule to check for such failures and then introduce a replacement schedule that actually preempts such failures. The small additional cost of early replacement is worth it to maintain reliability if the cost is not excessive.
If the engineers don’t design/maintain the plane properly, you can’t blame the pilot when it crashes. Perhaps all grid control engineers are test pilots now, working with an aircraft of unknown response characteristics.
Re Iberian Peninsular Blackout : The PLL circuitry of their grid following inverters must having been working overtime ! Stronger grid forming inverters required ?
Something goes wrong and then the finger pointing starts. Whose to blame, bad design, bad decisions, or you can say that it is an opportunity to evaluate alternatives, a time to learn.
We can add to complexity to make the national grid the gold standard power supply, such that voltage is finely regulated, and so too the frequency. But if say we are mostly running computers or low voltage equipment or lights that have a wide tolerance to voltage and frequency, we can start to ask, do we need to have the grid absolutely tightly controlled, or, can we adapt by those people who need 50Hz to the nearest 0.1Hz, or 240V to the nearest 0.1V, to actually take responsibility for it themselves?
Is it possible that it would be cheaper to have the national grid as a raw power source, with fluctuations of both voltage and frequency, within broader limits, and then for those people for whom it really matters, that they have to invest to get the closely controlled, fine tuned power supply that they need.
I have just looked at a camera battery charger, power input 100-240V 50/60Hz……..it really doesn’t care. Isn’t it time we think about the power supply specification and how much it costs to run, and if it is really needed? Perhaps this is too radical for the 240V 50Hz purists, but one has to ask, if the complexity of integrating thousands of power supplies and the control becomes far too great, do we need to think of and design the power supply in a different way?
Everything in my house does not care if it’s 220V-240V, or 45-55 Hz. Is what we need actually new legislation that changes who takes responsibility for the fine control of voltage and frequency when it is actually needed, and new products and electrical equipment?
Perhaps then we wouldn’t have supply failures because of a bit of resonance within broader limits that wouldn’t happen with 10-50 centralised power generators, but becomes almost inevitable trying to synchronise 20,000,000 different power supplies.
But of course, if you don’t design power supplies with voltage control, what do you expect? Don’t you need a zener diode on each phase to ensure that the voltage cannot go above a prescribed maximum? Yes, it’ll change the waveform to quasi-sine wave (starting to go towards square wave) during the voltage excursion, but at least the voltage would never go above that set peak. Put a resistor in to control the current, to protect the zener diode, and to get rid of some of the power/energy.
Do we really need all inverters to be “grid-forming” i.e. have the ability to control both frequency and voltage?
If this gets any worse, or becomes unreliable, I’ll be going off-grid, with the mains just as a back-up power supply (battery charger), just taking off-peak electricity and my own solar power, and take full responsibility for my own voltage and frequency with several off-grid hybrid inverters in parallel to get the stable power supply of enough power output for the whole house. There are always alternatives.
Of course if every grid-tied inverter were fitted with whole cycle voltage control, then there would be no way that the voltage could exceed the prescribed voltage of a pure sinewave…….for the whole grid. Wouldn’t it then be self-regulating?…….dumb control, without any human intervention.
The extension of unregulated voltage is that one day it takes two hours to cook toast and the next day your toaster blows up… unless you pay extra or go it alone.
Not that unregulated, just broader limits, such as the 220V-240V and 45Hz to 55Hz, or 48 to 52Hz, or 49 to 51Hz, 2 minutes for toast, plus or minus a few seconds. But then you are right about process control, but then surely any process control equipment can be designed to be tuned. Or is say 230V -240V good enough? Most processes have monitoring of specific characteristics that vary from batch to batch, and adjust inputs to achieve the final product over time. If it really matters to you that your toast is done in 2 minutes on the dot…… should we pay more for your demands?
An obvious use of AI to control the time of process, based on input of national grid voltage to maintain a consistent product…….not that difficult, surely?
Many industrial processes control for temperature, and have thermostatic control, does it matter that it takes an extra 3 or 4 seconds for your perfect toast to be achieved, or your kettle to boil?………maintaining temperatures requires taking the rate of heat loss into account, so requires variable, periodic heat input. I’ve done many processes that required various fixed temperatures, and that’s all based on thermostatic control………feedback loop of temperature, from sensor, controlling electrical input for heat…….your boiler at home…..controlling gas burn on/off. There’s always a cut-in temperature and a cut out temperature, yes, the temperature wobbles between a maximum and a minimum, the temperature isn’t actually fixed. If my cooling fan blows at 1000 rpm or 1100 rpm, when it has higher settings of 2000 rpm and 3000 rpm…….I don’t care. Someone else might, but I really don’t.
If you are an industrial customer, are you on CHP already to improve the energy efficiency?…….off-grid, already setting your own voltage and frequency?
Is the National Grid killing itself and over-investing to maintain a precision that isn’t needed?
Please list the processes that require a precise voltage and frequency, or please list the processes/equipment that need precise timing control/speed. Even your tyre fitter when you get a puncture on your car has a motor to help get the old tyre off and new one on, or is it your hairdresser with the hairdryer?……do they really care? if your mains CD player goes at 49Hz or 51Hz, can you tell the difference, or are you on a solid state device with the computer chip clock speed determining the rate?
What if motor speed control is mostly controlled by chip clock speed now?
Your electric car doesn’t rely on a very long cable attached to the mains at all times.
Have a think…….which ones rely on such precision?
You can’t operate a grid like that any more than you can fly an aircraft with one wing doing 550mph while the other does 480mph. If the grid isn’t synchronised and subject to voltage/reactive power control it will soon break. See Spain. The switch to inverter based power supply for consumption devices has several effects: on the positive side it does reduce inrush currents when a motor is started up. But once the motor is running the load on the grid will be slightly higher to include the power loss in the inverter itself. It will no longer be contributing to inertia from the consumer side, so more inertia will be required on the generator side to maintain overall grid stability to the same standard. Also, it is likely that the inverter will result in increased reactive power flows, although larger ones will have power factor correction to meet grid code connection standards. There are also metering issues. You could design a heater with a power factor of zero, so the meter would register no power consumption with the current and voltage being exactly 90 degrees out of phase so half the time power flows forwards and half backwards – but the resistance heats up whichever way the power flows. Free heat: but you wouldn’t be allowed to do that. For now, LED lamps with a power factor of 0.5 are being allowed to be sold, so you get half the power for free.
If you want an unstable grid you can expect regular blackouts. There are very good reasons why we have grid codes to limit excursions in frequency, voltage and phase. Try some third world countries to see what happens if you don’t.
OK, you have to have everything synchronized, and you have to deal with the reactive power, but after that, how much leeway is there for voltage and frequency, realistically?
Is one of the problems that for an isolated inverter, it can maintain frequency when it hasn’t got enough DC input power, but the voltage drops, but without suitable design constraints (regulation failure) can easily increase voltage over the mains voltage required if they have excess DC voltage and power available, whereas with alternators, with appropriate voltage control (excitation control), the frequency is the obvious sign of insufficient/excess power input, because you are literally dealing with a rotating element that because it is driven by a rotating engine, if there isn’t enough torque, the engine slows, hence frequency drops, or if there is too much torque the engine speeds up and frequency increases.
Isn’t the problem also that the failure modes are far more severe for inverters than alternators, and that you really need many more inverters controlling a smaller %age of the power, such that when they do fail, it’s not like having the steering suddenly disconnect in your car?
For the alternators, don’t you know when excitation control fails?
If an alternator fails, isn’t the usual failure mode a loss of one of the phases?
Yes, you can damage some inverters by operating them at too high current (power output) than designed, but most grid-tied inverters are designed to run at their maximum output as an absolute limit all day long for 10-15 years.
Is the problem here that the hierarchy of disconnections needs to change? If the voltage increases, isn’t the most likely cause going to be a failing inverter, not controlling voltage output, therefore switching off solar PV or wind is going to be more important.
Isn’t the failure in Spain also an indication that each facility isn’t monitoring its own performance and operational parameters, and isn’t disconnecting automatically when things start to go wrong?
Is it also the risk of too few inverters controlling too much power individually a potential problem…….utility scale solar in particular?…..100s of MW in the Spanish case.
I found a likely explanation for the 0.2Hz oscillations which are explained in this paper:
https://www.researchgate.net/profile/John-Kabouris/publication/224179133_Low_frequency_oscillations_in_the_interconnected_system_of_Continental_Europe/links/55bf70ce08ae092e9666907e/Low-frequency-oscillations-in-the-interconnected-system-of-Continental-Europe.pdf
The paper (from 2010) notes:
In the Spanish system 6.700 MW of new CCGT capacity
has been installed with PSSs tuned for damping the global
mode 1 (0.2Hz oscillations). In addition the PSS’s (Power System Stabilisers) settings of 2.400 MW of
capacity have been fine tuned to improve the damping of this
mode. The new settings in the PSSs have been elaborated by
the manufacturers of the power plants, and the theoretical
results demonstrate considerable improvement of the damping.
So switching off the CCGT capacity to let in solar removed a key source of damping of these oscillations: I can find no admission of this in the official report.
In conclusion the paper notes:
As the system expands, the number of Global modes
increases and their damping is reduced especially in cases of
large power transfers from the border areas to the central
Europe region. Consequently the implementation of
appropriate damping measures becomes more complex.
The expected increase of wind power generation mainly at
the periphery of the system will increase such power transfers
putting new challenges in damping inter area oscillations and
needs further investigation of the oscillatory modes.
ENSTO-E report on a similar incident of oscillations in December 2016:
https://eepublicdownloads.entsoe.eu/clean-documents/SOC%20documents/Regional_Groups_Continental_Europe/2017/CE_inter-area_oscillations_Dec_1st_2016_PUBLIC_V7.pdf
In this case the trigger was apparently disconnection of a 400kV link in France while Spain was exporting. As with the recent events, they cut exports to help control the oscillations.
Clearly solar and wind are not equipped to help. Whether they aggravated the situation on April 28th is not clear. More analysis required from ENTSO-E.
An excellent report from Kathryn having much agreement. Some of the initial responses with the political dressmaking is valid. I would comment there has to be an initiating cause, notably not mentioned in the report where subsequent analysis essentially dealt with the symptoms. Attempting a non carbon grid performance is futile as there has to be a driving fault but there needs to be a driving motive for political change that only comes with disconnection. What better time than midsummer!
The most important message is to restore engineering integrity, such people do not exist in the political realm.
Derek G Birkett (retired grid control engineer)
Hello, re Smart Meters
The understanding as far as I understand the requirement for same is to allow the load to be aligned with the available supply as to form the Smart Grid, local outages no doubt are less news worthy
There is also a shortform report in English from REE
https://d1n1o4zeyfu21r.cloudfront.net/WEB_Incident_%2028A_SpanishPeninsularElectricalSystem_18june25.pdf
which is readable and gives a good overview of events preceding and upto system black. It should shock any politician in the UK about how in a matter of seconds you can lose power system and although not referenced the fact Spanish have created an over complicated distributed generation system should be warning to the perils of too much autonomous based inverter generation
It offers rather more detail than the official report, although in certain aspects it does read as an attempt to exonerate REE from culpability. For example, they don’t mention the power oscillation damping built into CCGT plant – which had it been operational would have avoided creating the dangerous grid configuration that led to overvoltages, and would have provided more, better and more reliable reactive power and voltage control, as well as the extra inertia. Just because some grid code statement claims that parts of the system ought to respond in particular ways is no guarantee that they can in reality do so. Operating at minimum plant levels is unlikely to be conducive to full provision of ancillary services. REE need to operate with the system as it is, not what they wish it to be. There also seem to be problems of divided responsibility at main grid substations. Saying that the 400kV side was OK, but not the 220kV side, which is not their responsibility is head in the sand stuff. Also, they frame many disconnections as being out of code with no opportunity for the facilities to respond. I’m sure that there is actually a range of reasons why plants and distribution/collection substations tripped out. If they are saying that tap voltages on transformers were incorrectly set that probably means there is insufficient data feed to monitor what the appropriate settings should be.
At the end of the day it appears that REE were a bit like headless chickens trying to balance a dangerously creaky system not designed to operate at high levels of renewables penetration. Lots more kit required to enhance grid stability – I’d include more storage in the South to avoid loading transmission with the feed to pumped storage in the mountains of the North, as well as synchronous condensers and STATCOMs that they mention. And try running the kit they have properly, not skimping on grid stabilisation in all forms.
Thanks to the extra detail and some sleuthing on openinfrastructuremap.org I have located the solar plant responsible for the 0.6Hz oscillations. It’s 500MWp, but limited to 391MW export, at the time of commissioning in April 2020 it was Europe’s largest covering 1,000 hectares.
https://www.iberdrola.com/conocenos/nuestra-actividad/energia-solar-fotovoltaica/planta-fotovoltaica-nunez-de-balboa
Checking it out on Google maps others seem to agree: there are several comments posted to the effect that this was the origin of the blackout.
ENTSO-E have now provided a preliminary update here
https://www.entsoe.eu/publications/blackout/28-april-2025-iberian-blackout/
It doesn’t really tell us anything new. Perhaps we will get something with more revelations later.
One thing all this makes me think of is PIO, pilot induced oscillation, where the designed damping within the system, delayed response of pilot and controls only exacerbates the swings, slow response of system to control inputs, leads to eventually crash/fail.
The information about taking 1.5 to 2.5 hours to get CCGT synchronised with the Grid, before it can be used illustrates an important point, where if the CCGT systems were designed to act as synchronous condensers when they are not actually producing power, by clutch engagement/disengagement would make the response far more rapid and maybe could have prevented the event. You cannot have a key technology/technical solution taking so long to implement, when it is needed to react faster with 100 times the rate of response.
Haven’t we just got to learn from the mistakes of others and introduce appropriate design changes, just like the reports from aircraft crashes, to steadily improve the design?
One has to ask that if the original oscillations were not reacted to, would the system have just righted itself, without any inputs from the grid controller? Many of the actions appear to have amplified the problem, where trying to control the oscillations resulted in voltage runaway.
The grid controllers are flying a very complex aircraft, hopefully the designers will give them a chance of not crashing it, by giving them the controls that either work autonomously or are effective within an appropriate timescale when called upon.
It is the job of grid controllers to anticipate their needs and ensure facilities are connected accordingly. Warm gas is fast start at 90 minutes. They were lucky to have had that option, as the plant had been running until 9 a.m.. Had the chosen other plant for overnight support they might have been looking at 4 hours to get something in the right geographic area. They failed to take precautions when the first oscillations appeared 2 hours ahead of the events, and with solar still ramping up (solar noon is after 2p.m. in SW Spain) adding to the need for voltage control. Simple N-1 planning would have allowed them to turn down the offending solar and replace with CCGT or hydro. Instead, they let it increase output from 250MW to 350MW after the first major 0.6Hz oscillations.
In the days immediately after the apagon I found a report of an REE insider view that was quite clear that there was insufficient CCGT connected with power oscillation damping. He was right: operator error. They had the tool and failed to use it.
It does make one start to worry, where if we have most generators working at 100% power output (solar/wind), how much dynamic range is available for fluctuations if a 1GW link goes down, such as an interconnector. Surely the bigger power sources must have inertia associated with them, because they present the greatest risk to the system. A 1GW interconnector that can suddenly just stop transmitting power (e.g. russian shadow fleet ship dragging anchor through the interconnector)…….what hope is there for the rest of the system to cope?
This is why I would think having the back-up as a fleet of wartsila reciprocating engines, not CCGT, with clutched links to their generators acting as synchronous condensers is the best way to get the rapid response.
I would also say ship tracking is essential and better cable armour/defence.
Or is it that the interconnector substations need battery back-up with synchronous condensers to control the rate of change of output from the interconnector, such that there is a buffer of time to enable an adequate response of other generation capacity to build up output to replace the lost interconnector?
If battery back-up is for 4-5 hours, would give CCGT plenty of time to respond, but 4-5 hours at 1GW output is a hell of a battery system, starting to get expensive.
One can only hope that like airline pilots get time on flight simulators to practice emergencies, that someone is actually intelligent enough to know all the possible risks/failure modes (FMEA – failure mode effect analysis) and is able to understand the impact of such risks, put in mitigating technology and can also train the grid controllers appropriately.
We have introduced new risks into the grid, and different characteristics than it had previously, you just hope the training and appreciation of the change of characteristics is fully understood.
The Boeing 737 Max crashes happened because of that very reason, different technology, different characteristics, with a really bad effect on controls when the system goes wrong and inadequate pilot training.
NESO’s (95%) Clean Power 2030 aims to use over a year just 5% gas for electricity generation to cover the times when the green generated electricity, interconnectors and DSR (aka rolling blackouts) is insufficient to keep the grid functioning. Is this likely to be achievable as even when no gas is needed for electricity generation, there will still be the need for thermal inertia? Or are NESO expecting to solve the inertia problem without the need for thermal inertia by 2030?
Energy integrity in my recent blog really requires some explanation. I have had two decades of shift experience as a grid control engineer, retiring at the millennium. Amongst my colleagues the consensus of views was that wind development was going nowhere. I am a chartered engineer with a degree in electrical engineering.
Security of supply is a concept not fully understood by accountants, civil servants and certainly politicians. An obvious example is with the premature decommissioning of coal resource that enables a year of coal storage at site. This failure of policy led to a hike in consumer bills that was forced by gas turbines becoming the only source of dispatchable generation available. This was compounded by the failure of policy to develop gas storage, intended but never realised, unlike France and Germany who both have considerable storage capacity available.
If there is excessive renewable generation on the Iberian grid as seems likely then a series of consequences arise. Luckily this fault arose in summer. Winter would be much different. Restoration of inertia would be an immediate priority. It has been clearly demonstrated with the UK grid that the stated levels of inertia required are essentially arbitrary, placing reliance on computer modelling that has had questionable success elsewhere.
The GB grid system has a similar generation profile as the Iberian peninsula although the latter is part of the EU grid system. The GB grid is around seventh in scale of the EU grid. A similar fault situation would then be only a matter of time before the same event happens for the UK. However restoration would take much longer. There has to be an initiating cause which has not been identified in the report on the Iberian shutdown. The Spanish fault report suggests by omission that the real problem is excessive renewable generation being operated on their grid system.
It is essential for this fact to become known. The sudden realisation of this situation demands an end to renewable policy that has already been recognised by the US. For fifteen years this direction of policy across Europe has hobbled conventional manufacture. For the UK this capability has largely disappeared making the UK reliant upon electrical manufactured imports. Not just for generation supply but also for the considerable backlog of distribution equipment as was revealed with the Heathrow incident. This is at a time when we are reliant upon power interconnection from the EU to meet peak winter demands upon the GB grid system. Even in midsummer this facility is being fully used.
The dearth of power supply makes inevitable an end to the policy of stimulating further consumer demand upon the GB grid system. EVs cannot be encouraged and air heat pumps need to be constrained in order too prevent extensive distribution refurbishment that would cripple urban transport by replacement cable excavation. Failure of supply would only leave the option of rolling power cuts through smart metering as our reliance upon continental interconnection cannot be assured.
As a final word recent scientific publications have raised genuine doubts on the whole narrative of global warming. Richard Lindzen and William Happer, both emeritus professors from the US and Professor Judith Curry. Michael Simpson and Soren Hansen from the UK with a Canadian publication.
These consequences reveal the need for engineering direction that existed post war at a time when electricity demand had doubled every seven years. This is why integrity of engineering is so important.
Hi,
I’m going to be slightly contentious and ask whether all this concern about inertia is just a short term worry. With the rise of asynchronous generation and the increasing tendancy to put most synchronous loads behind inverters, what is actually trying to pull the frequency around?
Are we heading towards a grid where the concern would be voltage collapse, not frequency collapse?
Of course, I do agree that inertia is a worry in the short term, but for how long?
“Asynchronous” sources and loads will still “pull the frequency around” (try connecting a small solar inverter to a petrol generator and see what happens!) because they source – or sink current in phase with the voltage. If you were to take a spinning motor of the permanent-magnet AC variety and place an “asynchronous” load on it (e.g. resistors across the phases) it would slow down. If you did the same with magic “negative resistors” (which you can make out of op-amps) it would speed up. It’s a simple case of conservation of Energy.
But ultimately transformers also place upper and lower bounds on frequency. Too low and the iron core will saturate, causing a massive spike in internal heating (a transformer is a ‘dead short’ to DC. Take the frequency too low and the efficiency drops off in a non-linear fashion). Too high and magnetic losses and capacitive losses increase, which also causes internal heating.
The other trouble is, one simply cannot operate an AC grid without stable frequency – if the frequency is too uncontrolled then you risk events such as “out-of-phase reclosure” where a circuit breaker closes between two parts of a grid that have drifted out of phase, resulting in a massive bang, i.e. up to double the short-circuit current and the shearing of shafts between engines and generators.
Sorry, I missed this comment.
If you were to connect a small solar inverter to a modern inverter generator then I would expect the voltage to run away (at least up to the point where the inverter hit a voltage limit) not a change in frequency or a change in the speed of the generator (other than the fact that the generator speed would probably drop because its control system would see no load). Of course whether the system operates for long enough to hit voltage limits does depend on anti-islanding protection of the inverter which might attempt to inject a slightly out of phase current to see if can move the frequency and who knows what the generator inverter would make of that.
Similarly if you overloaded that small generator you would see a voltage collapse, not a frequency change. Of course the voltage would collapse because the engine is overloaded, has slowed down and can’t deliver the power to the inverter for it to maintain the voltage but that is invisible to the load.
I’m absolutely not saying that we don’t need a stable frequency – we certainly do. What I’m saying is that we are slowly (give or take nuclear) moving to a world where the frequency isn’t related to anything rotating and consequently the grid will tend to behave differently. We will still need something akin to inertia but how it is provisioned may well end up looking very different.
I keep an eye on Siemens, because if you need any tech then they’ve usually got a solution ready to install.
https://www.siemens-energy.com/global/en/home/products-services/product-offerings/grid-forming.html
The solutions are available, but I was wondering if we could just increase the inertia with nuclear, fit a few extra flywheels to get the inertia of nuclear up and then that gives plenty of inertia when all other inertia based systems are offline. If we get to 8GW to 10GW nuclear, then all we would have to do is increase its inertia 3x and you’ve certainly replaced all inertia in summer if just on solar and wind and no CCGT.
But in that case there would be nowhere enough power handling capability for voltage control, so would have to be put on nodes or solar/wind output.
For a grid system, effectively there shouldn’t be a no load scenario under normal running, it’s always GWs, somewhere between 20 and 50 for the UK.
Some solar PV and wind could be set up in the future with battery support to provide grid stabilisation services.
If you switch to reciprocating engines for back-up away from CCGT, the cost is about 1/3rd capital investment, as CHP, which would give plenty of money to invest in other grid services.
Hi Dale – TBH I was talking about a conventional generator rather than an another inverter. Connecting two inverters without any synchronous inertia at all would result in one or both either tripping off or being damaged as you say – but with a synchronous generator e.g. a relatively large diesel say, then it is slightly more representative of a real energy system. If the load on the engine is too much, then the speed and therefore frequency will reduce (and electronic control will try to increase the throttle to compensate) – but if the load is -negative- (due to low load and the presence of the small grid-following inverter), the speed (and thus frequency) would increase. A grid-following inverter would then “follow” this new higher frequency, and ultimately it may increase uncontrollably, and if the inverter did not shut down on over-frequency, it would perhaps destroy the generator.
What I am trying to say is that in an AC power system, frequency and voltage are intrinsically linked – voltage comes from the physical speed of magnets moving past coils, and so does frequency. Mechanical inertia is the only thing preventing these from running away and destroying the entire network.
Maybe if _all_ “inverter-based resources” such as Wind, Solar, Interconnectors, and perhaps loads such as EVs and heat pumps were mandated to implement some “virtual inertia” through a combination of software and (battery) energy storage, then it might be possible to dispense with mechanical inertia, but these virtual-inertia grid-forming inverters are currently rare and very expensive, partly due to the dumping of “dumb” grid-following inverter tech by China.
Dale,
frequency is the measure of power input to a grid against the load on the grid.
It is not an arbitrary figure that can be simply manipulated. An excess of load to input cause a frequency drop and vice versa.
Do reciprocating engines provide grid inertia and if so, how does it compare with rotating turbines?
Yes they do. Diesel generators have an H value of 1-2 seconds. But that can be boosted by linking them directly to a flywheel, as was done at King Island off Tasmania. The flywheel is also there to stabilise the very gusty wind output (at least when the turbines work and the wind blows), with the diesel also helping to smooth the output through extremely flexible operation. For such a tiny grid (peak demand ~3MW) it has an amazing stable frequency due to the size of the flywheel and the ability to dump surplus power of up to 1MW into a resistor that just heats the air. A battery and small solar completes the rig, but contribute little.
The problem with any AC system is that the theoretical divergencies from stable operation can go in several directions. Voltage runaway (VR), voltage collapse(VC), frequency increase (FI), frequency drop (FD), individually or in combinations of voltage/frequency, VR+FI, VC+FI, VR+FD, and VC+FD. Which means a whole lot of technology and control to stop these.
The inertia gives you time to respond, unless you have a system such as solar/wind/battery that isn’t working at full output that can instantaneously respond, virtual inertia – off-grid, grid forming pure sinewave inverters do that …….but they are automated, as below a certain level of inertia, no human can manage the control of it.
With the ECU for a car, petrol or diesel engined, it monitors certain sensors and adjusts accordingly to keep it tuned, right amount of fuel/ignition timing for the demand. One can imagine a future with more automation in the grid control room if the back-up/CCGT gets far more distributed as CHP instead of the large output limited number of facilities.
Isn’t the problem that we’ve concentrated inverter (asynchronous generation) into large output facilities that when they go wrong, has a significant impact. One inverter controlling just 3kw output just doesn’t do the same damage as a group of inverters/one inverter controlling 300-500MW. End-user installations of smaller capacity just don’t cause the same problems.
If they had voltage measurement across the whole grid at every power generator, would they have picked up the higher voltage being caused by a failing inverter? Surely there were some abnormal voltage gradients/current flows that could have been seen?
That is absolutely correct for a grid where the power balance is managed by adjusting the rotational force applied to an alternator and where the majority of the loads draw current at a power factor that seeks to lower that frequency.
What happens if the power balance is managed by (for example) a battery system where the power output is independent of frequency and where the majority of the load is in phase?
I would suspect that such a system would (unless deliberately engineered to mimic a system based on rotating machines) behave more like a DC network than the AC grid we are familiar with today.
Maybe I’m looking too far into the future…
In a small grid system, I would think that we could eventually have no synchronous condensers, if like off-grid systems we have synchronized grid-forming inverters, with excess reserve power as high voltage DC, either as battery or solar or wind, that the inverters can automatically use to increase/decrease their power output automatically on demand, with no human control. But that would require excess DC power available all the time (the virtual inertia). With large grid systems though, are synchronous condensers essential to stop abnormal resonance?
The CCGT/wartsila back-up would be acting as battery chargers, that would have plenty of time to respond, and would not be connected directly to the grid, but only as battery chargers. This means that there would be no need for 1.5 to 2.5 hours for synchronization, and therefore their power would be available instantaneously.
I think that there is a 1GW CCGT + battery facility being installed, cost £1 Billion. One does wonder how they intend to connect it, whether they are going to have the CCGT acting as synchronized, inertia providing facility, or are they going to start to experiment with having inverter only connection to the grid, with variable output, an off-grid type system (grid-forming) that is fully automated for power output, only controlling voltage and frequency……..with the excess of DC available, with the CCGT only acting as a battery charger?
With reciprocating engines, the output can be a slightly noisy sinewave due to the instantaneous changes in rotation speed (compression and power strokes especially), which CCGT doesn’t have, but having them as only battery chargers changes the game. If we go to only grid-forming inverter connection with virtual inertia, where all back-up (fuel burning) is linked to battery charging, then a bit of AC noise from a reciprocating engine doesn’t matter at all.
For an inverter to have “virtual inertia”, it needs to have some serious battery (or supercapacitor) power, to store the energy that would otherwise be stored by a physical flywheel. And the inverter itself needs to be well overspecified in terms of power rating (it needs to be able to provide the peak power from the battery, which may be several times its continuous rating).
This all adds cost, and without regulation, nobody will bother.
But it is a lot more “extra cost” to add these features than it should be – because the crude grid-following inverters which are toxic to the grid are being dumped at below-cost by China.
Or you have solar PV not putting out maximum output and have controllable inverters, or multiple inverters in parallel that can be switched on to increase output, say only putting out 20MW of 100MW maximum output. With inertia, it is stored energy or at least an energy reservoir that is instantaneously available, that stops/slows the rate of change of frequency. Does it really matter if it is physical rotation, that everyone is used to or just an excess of available power? One can imagine in the summer with excess solar PV that a different control system could exist, if every kW of power isn’t being used. Link up a battery to the solar to allow for loss of sunlight, you’re starting to get dispatchable power.
Dispatchable power is literally having an energy reservoir, with CCGT it’s as gas. All renewables can be made dispatchable, but it does depend on the cost, and there are cheaper ways and more expensive ways.
But in the summer, what if we get to the stage that there is excess solar power available, and no CCGT is ever required for 6 months of the year, and the electricity is so cheap that it is worth the cost of storage, because the daytime price is a few pence per kwh and in winter it gets more expensive, so the storage companies can get a good return?
How would you store electricity that is the same price or lower than what you are paying for gas now?
There could be a summer price inversion with electricity costing less than gas, if only the government knew how to achieve it. Currently (ha, ha) Off-peak electricity is about 6p/kwh, and gas is 6p/kwh, what would you do to get electricity below the price of gas…….do you know how?
Electricity costs at 6p/kwh to run a car at 0.25kwh/mile, equates to 1.5p/mile, even my picanto at 45mpg, uses 1kwh/mile, 4x as much energy (4-stroke engine petrol engine is about 25% efficient)……16.8p/mile, 11x as much in cost terms……yes the government doesn’t help with fuel duty!!!
If we are spending say £30 Billion a year, £0.3 Trillion over 10 years on petrol/diesel/oil imports for petrol/diesel, how much can you spend upgrading the grid and spend every year maintaining it (if everyone moves over to electric cars), before you start wasting money?…….when the sunlight and wind input are free?……where’s the break even point, i.e. what is the maximum you can spend on the grid that means that it is always cheaper to run electric cars rather than petrol/diesel?
Spending £30 Billion on something that just goes up in smoke, is far less valuable than something that keeps providing greater energy/financial efficiency.
£30 Billion, over £400 per every person in this country man, woman and child, per annum on imports.
The UK uses a total of 50 Billion litres of petrol + Diesel per annum, very inefficiently…….about £65 – £75 Billion cost to buy at the garage.
If you are paying more for house electricity, does it matter if you are paying less for travel, the other major energy cost in your life!
When electric cars get cheaper, it’ll be a no brainer.
Currently batteries are targetting an annual revenue of around £50-80,000/MWh of capacity. Some manage a bit more. But if you restrict them to charging in the summer for discharge over the winter then they can only earn the discharge price per MWh even if charging is free (including top-up because of self-discharge over longer periods). That is never going to be anywhere near enough to justify investment, even if they charge say £500/MWh for their output.
The economics of long-term battery storage isn’t any good, but can be used throughout the summer daily for the nighttime. Charge during the day from solar and then provide stability services, virtual inertia, and output at night…..no fuel burning required for 6 months of the year. But theoretically, this will be done by the end users, storing their own solar power. Any storage tech won’t be a single event storage per annum, because there are several periods of excess solar/wind throughout the year, but cost of storage is critical to make it work economically. We will still need some sort of liquid/gaseous fuel for the depths of winter. How can I put this succinctly? Chemical batteries are useful for short-term repetitive cycling, daily or weekly, if the electricity is cheap enough. As you say, I certainly wouldn’t restrict their use to a single charge/discharge per annum, with 2,000 cycles you don’t want to have capacity that is so expensive to last for 2000 years…….it doesn’t make any economic sense. It’s using the battery storage for mostly summer overnight output, daily, then to help as one storage solution during the winter. We’re going to have to get the cheapest form of replacement for CCGT as well.
Conventional inertia energy represents just a few seconds of output of a generator (and it’s often quoted that way – seconds of full output). The typical rate of drawdown even in more dramatic grid events is only a few percent of total output, but across a portfolio of generators it can add up to significant support if one of them trips out. Its big advantage is that it is entirely automatic as a consequence of the laws of physics, whereas synthetic inertia only operates when a frequency deviation has been reliably detected, which causes a delay in activation of up to half a second. Batteries can contribute up to their full output capacity in support almost immediately after that, but such step changes can create other problems with harmonics. Mostly they are set to respond proportionately to measured frequency deviation only reaching full output/charge rate at -/+0.5Hz. Conventional generators may be able to add power output after a few seconds. Responses tend to be damped to avoid creating oscillations of over and under generation. See droop control.
And this is the important point, if you link all back-up (fuel burning) to automated battery charging only, linked to automated grid-forming inverters you can embed the back-up and distribute it widely as CHP, and almost get rid of the centralized grid control room……..autopilot for a grid system!…….who would have thought?
Once again a very incisive piece.
I am very concerned that the same thing will happen here unless policy is again driven by engineers rather than politicians and commercial interests.
The electricity system is now seen as a passive trading platform not as a complex dynamic machine with many modes of failure. Some modes have yet to be discovered by unsuspecting control room operators.
There have been several expensive and detailed reviews of these trading arrangements focused on legal/commercial aspects but no engineer led technical studies into how a future system should safely operate.
The popular and simplistic notion of a cyber attack in Spain has been ruled out but both there and here a dangerous virus has invaded the minds of all those within the various bodies involved, driving an all pervasive and superficialy persuasive narrative.
There is next to no independent thought and no analysis excepting yourself – keep up the good work.
I like the conclusion and comment “Technical measures to increase voltage control and protections against frequency oscillations. The National Commission on Markets and Competition is currently considering reforms that will allow asynchronous installations to use power electronics to manage voltage variations”.
I would have thought that it would be a no brainer that with the problem that inverters can easily cause high voltage excursions (if they cutout automatically when their voltage isn’t high enough or have the electronics designed such that they cannot provide current that they have to have voltage control. If the designers don’t understand the failure modes and put suitable controls on the equipment that could cause the problem, there really is no hope.
I can understand that the divergence of voltage caused by inverters causes a control problem with CCGT and frequency divergence/oscillation coupled to a voltage oscillation as it battles to control the excess voltage (power), but to operate a grid free of CCGT, or with very few rotating alternators, you have to have other means to control the voltage. One has to ask, has the UK invested in more synchronous condensers than Spain?
If you have no CCGT on a grid/synchronous condensers, you have to have sufficient power handling capability that can control the voltage, somewhere on the system. If it isn’t designed into the inverters (and monitored constantly, just like the frequency of the grid), it has to be somewhere else, it’s just common sense. If they didn’t invest so much in synchronous condensers, did they have clutches on all CCGT to allow them to act as synchronous condensers?
It’s almost like they’ve had two control systems fighting each other, one for inverters, where voltage control is essential (or in this case a lack of control that really was needed), and one for alternators/synchronous where voltage control is straightforward and the main system control/feedback for power demand is via frequency.
With just inverters (no synchronous alternators) providing power they can synchronise and provide 50Hz no matter the power output if you set the circuits up to time the waveform, but voltage has to be controlled and maintained by the circuitry, and if it is the only system of control because frequency has been fixed, there has to be an excess of DC power available and the circuits have to have the current handling capability for whatever demand you have. It’s just common sense if you’ve done just some basic electronics, with off-grid inverter based systems, with no physical inertia at all.
Did anyone on the Spanish grid actually check the total power handling capability of their voltage control circuits for the whole grid? Everyone goes on about inertia, frequency control, but voltage control also has to be done and the equipment has to be able to handle the power……..turn off CCGT…….oops, can’t manage the voltage!……..how does that happen?
Tim,
all very interesting but haven’t we lost the sight of that we need to produce electricity, reliably and economically?
The obvious answer is not renewable generation at all. It is all negative with no positives.
The CO2 controlling climate hypothesis is dead, it just doesn’t know it yet, but that message is slowly but surely getting through.
Nuclear and coal are sensible, saving gas for other uses, in the longer term.
It’s obvious to me that spending more money, time and effort on sources of unreliable diffuse energy is not going to be sustainable in our world. Therefore energy dense fuels such as used in nuclear power technology are our only hope of providing for the future energy requirements of billions more people.
Yesterday afternoon there was full import from all continental interconnections. At 3pm all the interconnections were at full bore with low carbon at 88%, renewable at 67%, wind 3.4GW, solar almost 10GW. All the CCGT at 2.76GW. Looks much like the suggested try out!
I would suggest to Nicholas Lewis’ blog that he should consider the loss of 582MW as an instructed de-loading. This would enable a path to a N fault for this incident.