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expert reaction to Ofgem launching investigation into power cuts, and initial report from the National Grid Electricity System Operator (NGESO)

Ofgem has launched an investigation into the national grid power outage in early August 2019.


James Robottom, Energy Lead, IET (the Institute of Engineering and Technology), said:

“We welcome National Grid’s interim report into the UK-wide power outage on the 9 August 2019.  However, while it is encouraging to see their response, the report does not get to the heart of the problems caused, as they run much wider than the National Grid.  This incident raises serious questions about the resilience of the UK’s energy and related infrastructure.

“The Institution of Engineering and Technology (IET), along with the Energy Systems Catapult (ESC), has been highlighting for some time the lack of whole systems thinking within the power network.  This incident highlights the desperate need for greater coordination across what are increasingly complex and interrelated energy, transport and communications systems.

“The impact of this lack of whole systems thinking was highlighted by the large degree of disruption faced by rail passengers from what was a comparatively minor electrical incident, stranded because signalling systems and trains were not able to restart once power supplies had been returned to normal.  This lack of understanding about impacts across other sectors and on individuals also meant hospitals, residents and business were all impacted.

“The electricity system has changed a great deal since the existing governance was put in place in the 1980s and it is no longer fit for purpose.  We now have a highly complex system of which the National Grid is only one part, interconnected physically and through data and information flows to many other systems (such as the Network Rail system and subsystems within it).  This complexity makes current silo-based thinking increasingly risky as society’s dependence on electricity continues to increase, for example through the internet and data, electric vehicles, heating and cooling, and dependency within the home, especially for the vulnerable.

“The technical governance of these complex systems requires a step change from today, it needs to be holistic, agile, flexible and embracing of the full range of system participants.  Without that we can expect to see more unexpected consequences across the whole system, as well as a failure to seize the benefits whole systems cooperation can bring, not just for major events on the National Grid, but also for the much more numerous power cuts experienced locally every day.

“The IET/ESC’s Future Power Systems Architecture (FPSA) Programme has been exploring these issues in considerable depth.  It has concluded that to ensure that new and old technology, business models, security, resilience, redundancy and cross sector cooperation can be effective in these situations our governance structures have to be modernised, before technical and commercial solutions can deliver the value society now expects.

“The IET wrote to the Secretary of State, Greg Clarke, on 10 April 2019, highlighting that the current system was not fit for purpose to transform the energy sector and will be writing to Andrea Leadsom shortly to reiterate these issues on behalf of the profession.”


Prof Tim Green, Co-director of the Energy Futures Laboratory, Imperial College London, said:

“The report gives us a very interesting account for engineers looking to understand what happened on that day almost two weeks ago.  The sequence is more complex and subtle than was evident on the day itself.

“The initial cause was a lightning strike.  Lightning affects power lines many times a year and is routinely dealt with. This one struck near or on the overhead line connecting substations at Eaton Socon (near St. Neots) and Wymondley (near Hitchin).  Large currents were seen in the substations, and circuit breakers opened to disconnect one of the two lines on that route (in this case it only took 70 milliseconds).  Those circuit breakers re-closed automatically, after about 20 seconds in this case, and stayed closed because the “fault” had gone, that is, the large current caused by the lightning strike had ceased.  They would have re-opened and locked open if a permanent fault such as a damaged line had been the cause. 

And hundreds, perhaps thousands of times year that is the end of the story.  There are parallel circuits to the one that disconnects, present as part of the grid’s built-in redundancy, and no generators or customers are affected.

“But it was different that day…

“The disturbance to the voltage caused by the lightning and the line outage then appears to have led to three things to occurring at essentially the same time (within half a second of each other).  First Hornsea offshore wind farm saw the voltage changes and attempted to help correct them.  But then an internal anomaly was detected, which caused two modules to abruptly disconnect taking 737 MW of that wind generation off the grid.

“The second issue was that the lightning strike disturbed the Little Barford power station; it is connected to Eaton Socon substation at one end of the affected line.  It is a combined cycle plant, with two gas-turbine generators and a steam-turbine generator (the steam is raised from the heat in the exhausts of the two gas-turbines).  The disturbance lead to anomalous speed measurement data at the steam-turbine and so it automatically shut-down taking an additional 244 MW offline.

“This voltage disturbance rippled out across the system resulting in sudden shift in angle of the voltage (the starting point of the AC voltage cycle measured against a reference point).  That matters because “distributed generators” like small wind and solar plant use this as one way to detect “loss of mains” and about 500 MW of these small generators detect this as problem and disconnected creating the third loss.  Now with a total loss of about 1,480 MW of generation, the frequency started dropping as remaining generators slowed down while still supporting 33,500 MW of demand.  It dropped quickly to 49.1 Hz.

“However, while it was dropping the first line of defence, the cavalry if you like, was coming to the rescue in the form of 650 MW of “response” power.  That was a mixture of part-loaded generators, batteries and demand-response (voluntary reduction of load from some customers, mostly commercial and industrial customers) that National Grid Electricity System Operator (NGESO) had contracted to be in place.  The batteries will have been very fast and undoubtedly helped a lot.

“This was not the only backup and within 20 seconds a further 350 MW of response arrives.  This brings the total backup deployed to 1,000 MW which is what NGESO was holding to cover and outage of the largest generator running that day (sometimes it holds more, 1,300 MW, if bigger units are running).  The mix of response is very interesting too, 200 MW of generation; 450 MW batteries and 350 MW demand-response.  Only 10 years ago it would have been very different, it would all have been generation (part-loaded gas- or coal-fuelled generators).

“So far the system was working as it should, no demand disconnection, only contracted (voluntary) demand-response.  The system was still short of generation but a 480 MW gap is not insurmountable, the frequency had stabilised and further actions, secondary reserve, would be called up to restore the frequency to its normal operating range of 49.2 – 50.2 Hz.  But then there was some bad news and a series of unfortunate events.

“With the steam unit shut down, the gas units at Little Barford can’t operate.  The first gas unit tripped about 1 minute after the original event losing a further 244 MW.  We’re now down 1,691 MW with a reserve of 1,000 MW already deployed.

“At this point frequency falls again, now to 48.8 Hz, which is where the second line of defence is pre-programmed to come in.  It’s called low-frequency demand disconnect (LFDD).  This disconnects about 5% of demand, it was 1.1 million customers on this day. More would have been disconnected at 48.7 Hz but fortunately frequency started to recover.

“Over the next 4 minutes NGESO control room called up 1,240 MW of further action (secondary reserve) and got the frequency back to 50 Hz.  During this process the second gas unit at Little Barford tripped too but the effect was minor.  After a further ten minutes, when NGESO were confident the recovery was secure, they instructed distribution network operators to start reconnecting customers.

“I think it is worth noting that the time between the lightning strike and the starting to reconnect consumers was about 15 minutes and all customers were reconnected 35 minutes after that.  The detailed reasons why the railways were so badly affected by these events is outside the remit of this report but is an issue that needs investigating.  The report does say that the railway systems were not generally part of the LFDD disconnection but they reacted to the perturbation of the grid frequency and voltage.  This did cause serious and longer-term impacts on the public and lessons will need to be learnt.

“So, what do we make of all this?

“1. It is very rare to see large power stations disconnected in response to lightning strike and/or line outage.  Losing two to the same event is exceptionally rare and in that light the system responded well.

“2. The last time two generators were lost in quick succession was May 2008 with half a million customers lost to LFDD, so we can say it is rare.

“3. LFDD is very painful for those affected but 5% suffered so that 95% stayed connected to a functioning grid.

“4. There will be technical lessons learned by the owners of Little Barford power station and Hornsea wind farm and there will perhaps be implications for other generators.

“5. The rate at which frequency changed, 0.16 Hz/s, was high in historic terms but to be expected with our new low-inertia grid (wind turbines and solar panel don’t naturally contribute to the spinning mass of the generator fleet so frequency moves faster when supply and demand are out of balance).

“6. The mix of response was healthy – the contracted batteries and demand-response were quick to act and that is why they are used in low-inertia grids.

“7. Could this be prevented?  If NGESO had been holding 2,000 MW of reserve not 1,000 MW then no LFDD would have been needed.  NGESO spends close to £300M/year contracting response/reserve.  Spread across 25 million customers that’s a £12/year component of your bill.  Should we double it?  Perhaps we should have a public discussion of whether we want to protect ourselves from outages of 2 large generators, or 3 or 4 and what we would be prepared to see in our bills to have that provision.

“8. Could more be done to refine “loss of mains” protection to avoid losing distributed generation.  Actually, this is already underway anyway.  There is lots of technical information available online.

“9. Is wind a problem?  Well first off, UK has halved its carbon emissions from electricity from 150 million tonnes per year in 2012 to under 70 last year and wind was a big part of that.  But yes, there are challenges to address as we add more wind and decarbonise more deeply.  We need to address further reduction in inertia (seen over seconds), covering short-term fluctuation and forecast errors (over an hour), periods of low wind speed (days) and seasonal variations (more demand on dark winter evenings).  These challenges are acknowledged across the industry and are being addressed.

“Our grid system is part way through a major transition because it is in the vanguard of reducing to zero the carbon emissions of our whole country.  Clearly, that transition must be done in way that means that power cuts are very rare events.  This is why I, and thousands of other engineers, are in the game; we love solving tricky problems that have important benefits to society.  This is what we’re all talking about.”


Miles Seaman, Member of Energy Centre Board, Institution of Chemical Engineers, said:

“It is an inherent nature of complex energy distribution systems that multiple uncorrelated disruptive events can cause a system failure which only very extensive and costly redundancy could prevent.  I trust that this will not turn into a blame game causing unnecessary expenditure.”


Prof Keith Bell, Professor of electrical engineering at the University of Strathclyde, and a co-Director of the UK Energy Research Centre (UKERC), said:

“The electricity system must have a continuous balance between the generation of power and its use.  If there is too much generation, the system’s speed – its frequency, which should be around 50 Hz – increases.  If there is too little, it falls.  On the evening of Friday August 9th, combined losses of generation within a short space of time caused the system’s frequency to drop significantly leading to disconnection of some electrical load.

“One notable thing in the description of what happened on August 9th is the potential role of the lightning strike on an overhead line.  There is an interesting choice of words in National Grid’s report: the losses of power from Hornsea and Little Barford are associated with that line fault.

“Faults on the transmission network due to lightning strikes occur typically tens of times a year.  Provided network protection operates correctly, no generation should be lost from the system as a consequence of such faults.  Normally, that is exactly what happens.  According to National Grid ESO, the line protection did operate correctly to isolate a fault just north of London at just after 1652 on August 9th.  One main question now is exactly why protection or control equipment at the two generators responded as they did and caused the loss of generation.

“Hornsea wind farm is still quite new with the third of three stages still under development.  It is my understanding that the wind farm is operating under what is called an ‘Initial Operational Notification’.  This means that it is allowed to connect to the network and export power onto it but National Grid ESO has not yet fully verified that the wind farm complies with all the requirements on generators written into the Grid Code.

“Little Barford started operation and received its ‘Final Operational Notification’ around 20 years ago.  It is not yet clear why a nearby transmission network fault should have caused the steam turbine there to trip.

“Other issues relate to what was happening on the distribution networks.

“It seems that the transmission network fault caused some generation to be lost from the distribution networks.  This is due to a known issue with the type of protection (‘vector shift’) used on some small scale generation.  However, it added up to what National Grid ESO estimates to be around 500 MW.  This plus the 737 MW lost from Hornsea and the 241 MNW initially lost from Little Barford, a total of 1481 MW, was enough to start a rapid fall in system frequency.  This, in turn, may have triggered the loss of further generation from the distribution network due to the operation of some different protection, in this case sensitive to high rates of change of frequency, again a known issue.  The net result was a ‘loss of infeed’ within a short period of time that was much greater than the fast reserves – ‘frequency response’ – that National Grid ESO typically schedules.

“A minute after the loss of the Little Barford steam turbine, one of the two gas turbines there tripped automatically.  This took system frequency down to 48.8 Hz, the threshold at which defensive ‘low frequency demand disconnection’ (LFDD) operates automatically on the distribution networks.  This has the effect of disconnecting demand in order restore a balance between generation and demand in order to avoid a more severe system collapse.

“LFDD succeeded in nudging system frequency back up again and prevented a worse outcome.  Only the first of nine thresholds of LFDD was triggered, intended to disconnect 5% of demand.  However, according to National Grid ESO’s report, this disconnected 8 supplies to Network Rail signalling.  This leaves the following two questions as yet unanswered:

“1. Would it have been possible for the Distribution Network Operators (DNOs) to have implemented LFDD in such a way as to have avoided disconnecting Network Rail supplies?

“2. What level of resilience against losses of power supplies do Network Rail facilities have?

“In addition, it is not yet clear if the operation of LFDD succeeded in reducing demand for power from the transmission network by as much as 5% though, if it was less than that, it appears to have been enough.  There also appear to have been variations between the DNO areas in how much demand reduction was delivered.

“A further issue is that equipment on a particular type of train is reported to have tripped and taken significant time to be re-started.

“The virtually coincident loss of so much transmission connected generation was a rare event.  If, as it appears, the loss of power was triggered by a transmission network fault, it should not have happened.  It is still to be determined exactly why equipment at Hornsea and Little Barford responded as it did.  However, equipment failures can and do happen, and it is possible though rare for more than one failure to occur simultaneously.

“The event shows the need for and value of system defence measures.  However, because it cannot be guaranteed never to trigger them, they need to be implemented in a cost-effective way that minimises the impact on critical supplies.  Operators of those critical supplies should, in any case, take steps to ensure sufficient resilience.

“The loss of distribution connected generation appears to have compounded the losses of power from the transmission system.  The industry had already agreed to take steps to change the protection settings of such generation.  However, according to a presentation by the Energy Networks Association in April, it will involve around 50,000 installations and the programme of work to make the changes will take 3 years.”




Declared interests

Prof Tim Green: “Tim Green is the co-director of Energy Future Lab, Imperial’s cross-disciplinary energy institute with responsibility for facilitating interdisciplinary research and postgraduate education in energy and in coordinating its dissemination.  He is also Deputy Head of the Department of Electrical and Electronic Engineering.  His personal research focuses on the analysis and technology to support the development of a low carbon electricity supply network that is able to accommodate variable renewable sources and new widespread electric vehicle charging while still delivering a cost effective and very reliable service. His research specialisation is in power electronics for use in power systems.”

Prof Keith Bell: “I hold the Scottish Power Chair in Smart Grids at the University of Strathclyde.  Scottish Power sponsors the Chair, not the individual academic.  I speak as an independent academic.  I have had various research projects over the years with the electricity companies, but only on the condition that results are published and, from my point of view, with the aim helping to improve engineering practice.  I have also done work with Government and Ofgem.  As a Chartered Engineer I have to uphold certain principles of honesty and ethics. F or more on my background, work and associations, see”

None others received.

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