A software update that could have kept Hornsea One operational through the freak incidents that lead to last August’s huge power cut - potentially avoiding it altogether - was just days away from installation, it has emerged.
Orsted’s world-leading offshore wind farm was in the process of demonstrating compliance as a new generator when lightning struck a transmission cable in Cambridgeshire, sparking a chain of events that led to one million people losing power.
It left trains incapacitated across the rail network, while four hospitals had to revert to emergency back-up systems and Newcastle Airport was forced to close. It emerged on Friday the company has agreed to pay £4.5 million into Ofgem’s redress fund as part of a £10.5 million industry contribution following the publication of the official reports.
Set to be fully commissioned early this year, the Grimsby-operated 1,218MW farm was knocked out alongside RWE’s gas-fired plant at Little Barford on the early evening of August 9. Together they exceeded the back-up available, leading to strategic disconnections to manage the loss across the grid.
Hornsea, then entering the final two months of the installation campaign, had two of the three 400MW units that feed from the giant arrays off the Humber approaches generating at capacity due to the windy conditions, with the third unit – in which turbines were still being installed from the pre-assembly port of Hull – permitted to export 20 per cent.
It has been common practice for the farms to be ramped up as they are built out, with the clean electricity contributing to the huge increase in renewables and subsequent reduction of coal as a fuel source from the system, while money earned helps cover some of the ongoing construction costs.
When the lightning strike struck, it caused a voltage fluctuation across the transmission network, which prompted a response from Hornsea One in an attempt to correct it. Initially aiding, the control system then “responded unexpectedly” which ultimately led to Hornsea One offshore voltage being depressed to a point where protection systems were activated on the two units.
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This rapidly de-loaded the wind farm’s overall output from 799MW to 62MW. Had it stayed on the grid may have been able to cope with the loss of Little Barford.
A report into the huge power cut by the Energy Emergencies Executive Committee, , tells how the manufacturer of the offshore assets, Siemens Gamesa, had identified that this unexpected control system response could be mitigated by implementing a software update it had already prepared as a performance enhancement of the control system, thereby improving damping of system oscillations – which can affect system voltages. The reports said oscillations are a normal inherent feature of electrical power systems and not problematic when adequately damped.
Such oscillations were expected to occur when Hornsea One reached 1,200 MW capacity later in the construction phase, predicted by software modelling when responding to voltage fluctuations.
The report said: “A control system software update was prepared by the manufacturer to address this and was due to be installed on August 14, well in advance of Hornsea One reaching 1,200 MW capacity. Following the events of August 9, the manufacturer identified that the unexpected response from the control system was caused by the same root cause that would lead to the oscillations mentioned earlier. Hence, on August 10 the manufacturer and Ørsted installed the software update, thereby ensuring that should the same or worse voltage fluctuation occur on the transmission system, Hornsea One will not de-load.”
Generation at Little Barford was lost in three stages over a 90 second period. The first turbine trip was due to a discrepancy between speed readings of the turbine shaft; the second to an excessive build-up of steam pressure in the pipework; and the third to a manual shut down by the plant operator due to steam safety valves opening from continued build-up of steam pressure.
The initial cause behind the discrepancy between speed readings remains unknown due to data limitations, however since the outage, RWE has undertaken a review into this sequence of events and applied a number of fixes, including identifying measurement system tolerances and adjusting associated sensitivities to increase resilience and maintain safety; balancing steam flows between the two boilers that feed the steam turbine; and verifying the pressure capability overhead of the system and allowing use of this margin to achieve greater resilience.
“As a result, if a similar event was to occur again RWE is confident that Little Barford Power Station would not suffer a trip of any of its generators,” the report said.
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