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The Cause of a Power Systems Failure That Led to a UK Blackout

September 18, 2019 by Sam Holland

The 9th of August this year saw the worst UK blackout since 2008, with issues involving transport disruptions and almost a million British households left in the dark. Demands have since been made for reports that account for the mass power failure.

The issue started at 16:54 on the 9th of August, and the resultant power cuts were experienced in London, the South East, the Midlands, the South West, Yorkshire, the North West, Cornwall, and Wales. According to Great Britain’s gas and electric system operator, National Grid (NG) in both its interim and detailed report, the power failures were triggered by a lightning strike. 

The event saw the almost-simultaneous outage of 2 major generators (which are well over 100 miles apart): 1 that failed at 16:54 that day; the other at 16:58. The former was based in Little Barford, while the other was off-shore in Hornsea Wind Farm, and together, they disconnected 1,378 megawatts of energy from the grid (which ultimately culminated in almost 1.9 gigawatts of lost power.)

As discussed later, the seriousness of the incident led to such energy authorities as Ofgem (the Office of Gas and Electricity Markets) calling for a report from NG, in the interest of confirming whether the proper protocols were followed, and what can be learned for the future.


The Effects of the Event and its Aftermath

The effect of the power cuts led to road, rail, and air transport delays, alongside the loss of electricity in hospitals, homes, and other buildings throughout England and Wales. A similar event (the London Blackout) has not been experienced in the UK since 2003.


Power concentrations UK aerial view.

An aerial view of the United Kingdom, lit up in certain areas to signify concentrations of power. Image credit: Bigstock.


A response was made the next day by National Grid, specifically National Grid Electricity System Operator (NGESO). They cited the “rare and unusual event” as the product of the 2 failed generators and the said lightning strike.

The electricity system operator explained that the networked power generators that weren’t subject to power outages defaulted to automatically increasing their output. When this fell short, a backup system was triggered successfully—but at the expense of load-shedding (i.e. the shutting down of parts of the network to avoid a total blackout) across the UK.

(In its interim report to Ogfem on of the order of events, NGESO provided this infographic timeline.)

NGESO’s statement continued:


“[T]he system was [quickly] secured, and the electricity system operator [i.e. NGESO] gave the all-clear to the distribution network operators (DNOs) ... within 15 minutes, so that they could start to restore demand.

“All demand was reconnected by the DNOs by 17.40 [the same day]. We appreciate the disruption caused and will continue to investigate the generators involved ... to understand the lessons learned.”


Now, after a few weeks have passed since the blackout, we look at such lessons learned.


Taking Stock of the UK Power Outages

The event was unprecedented and led to chaos over much of the UK. Accordingly, here are 2 key points and their corresponding questions that must arise:

1. A UK event like this hasn’t occurred in over 15 years. Could the fairly common instance of lightning really have caused the two power outages in the first place?

2. A detailed report was demanded from Ofgem and other organisations due to the seriousness involved. What, or who, should ultimately be blamed for the major power cuts?

With these 2 questions in mind, we reach our conclusive discussion below.


An unlit street in Chester, England—one of the many cities affected by the blackout. Image credit: Flickr.


What We Now Know About the Blackout Cause

Regarding the first question above: again, NGESO itself deemed the power cuts “rare and unusual”: National Grid is used to experiencing over 1,000 lightning strikes a year on its transmission lines, and they tend to have no effect at all (as confirmed in their said interim report to Ofgem).

While in and of itself, the lightning strike should have been non-eventful, there was already a minor drop in the power of Little Barford Gas Station and Hornsea Wind Farm’s generators before the lightning even struck. This altogether led to a drop in power frequency from 50.5Hz to 48.8Hz, which was enough to trigger the said load-shedding response.

However, this is where the second question comes in: what, or who, should ultimately be blamed for the major power cuts? After all, many commentators have questioned why only a small frequency drop such as this was a default condition to warrant such wide-scale automatic load shedding in the first place.

The question of what power frequency levels should (and should not) call for autonomous power cuts was the responsibility of NGESO to determine in its detailed report (which was submitted on the 9th of September) to Ofgem—the appendices of which you can read here.


What is to Blame? 

All in all, while the drop in frequency may sound small, it was clear that the seemingly minor drop in frequency was still a significant one: NGESO has reported that one of the many lightning strikes that took place on the 9th of August occurred at the same time as the electrical fault, and National Grid is therefore “confident that a lightning strike was the cause of the fault”. 

To quote the report’s executive summary on the events that followed the lightning-triggered frequency drop:


“Initially the offshore wind farm responded as expected by injecting reactive power into the grid, thereby restoring the voltage back to nominal.

“However, ... as [Hornsea One’s] active power reduced to cope with the voltage dip and the reactive power balance in the wind farm changed, the majority of the wind turbines ... were disconnected by automatic protection systems.

“[Such a] de-load was caused by an unexpected wind farm control system response, due to an insufficiently damped electrical resonance in the subsynchronous frequency range, which was triggered by the event.”


The “unexpected wind farm control system response” suggests that the UK power cuts were ultimately a software-based one. Both National Grid and Hornsea One have since responded accordingly: the former plans to introduce updated stability software, while the latter has, to quote NGESO’s report, “stabilis[ed] the control system to withstand future grid disturbances in line with grid code and connection agreement requirements”.

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