Ofgem, the UK’s electricity industry regulator, has received a detailed report from the TSO (National Grid Electricity System Operator) into the causes of power cuts on 9 August that left more than a million people without power, some for several hours, and disrupted the rail network for more than 24 hours.
NGESO says that Following comprehensive internal analysis together with input and analysis from key stakeholders the details of the events leading up to, during and following the loss of supply are now more fully understood.
Its report sets out conclusions, lessons learned and changes implemented by the ESO, and makes recommendations for consideration by government, Ofgem and industry.
Immediately before 4:52 pm on 9 August Great Britain’s electricity system was operating normally. There was heavy rain and lightning storms around the transmission network north of London. Overall, demand for the day was forecast to be normal for the time of year. Around 30% of the generation was from wind, 30% from gas, 20% from nuclear and 10% from interconnectors.
A lightning strike occurred on a transmission circuit (the Eaton Socon – Wymondley Main) at 4.52 pm. The protection systems operated correctly and cleared the lightning in under 0.1 seconds. The line then returned to normal operation after about 20 seconds. The voltage profile of the network immediately after the fault was within standards and was not a factor in subsequent impacts.
Coincident with the lightning strike, there was a loss of about 150 MW of small embedded generation connected to the distribution network, owing to vector shift protection. Loss of embedded generation through vector shift protection is expected for a lightning strike on a transmission line.
However, immediately following the lightning strike, Hornsea offshore windfarm reduced its energy supply to the grid by 737 MW and Little Barford power station’s steam turbine tripped, reducing its energy supply to the grid by 244 MW. This generation would not be expected to trip off or de-load in response to a lightning strike. This therefore appears to represent an extremely rare and unexpected event.
Back up response
The cumulative loss of 1131MW of generation caused a rapid fall in frequency, which in turn caused a further 350 MW of embedded generation to disconnect from the system under rate of change of frequency (RoCoF) protection.
The TSO was keeping automatic ‘backup’ power response at that time to cater for the loss of the largest in-feed at 1000MW – the level required under the regulatory approved Security and Quality of Supply Standards (SQSS).
However, the total generation lost from vector shift protection, the two transmission connected generators and subsequently rate of change of frequency protection was 1481MW, and as such was above what was secured for under the SQSS. This meant that the frequency fell very quickly and went outside the normal range of 50.5Hz – 49.5Hz to a level of 49.1Hz.
All the ‘backup’ power and tools the ESO normally uses and had available to manage the frequency were used (this included 472 MW of battery storage) to stop the frequency fall (at 49.1Hz) and were recovering it towards 50Hz.
However, just as the frequency began to recover (and reach 49.2Hz) there was a further trip of a gas turbine of 210 MW at Little Barford power station (due to high pressure in the steam bypass system following the failure of a bypass valve to operate correctly). This brought the cumulative loss of generation to 1691MW.
All of the available backup power had already been deployed and the cumulative scale of generation loss meant that the frequency then fell to a level (48.8Hz) where secondary backup systems acted automatically to disconnect approximately 5% of demand (the Low Frequency Demand Disconnection, LFDD, scheme). This enabled the recovery of the frequency and ensured the safety and integrity of the network. (Note that following the LFDD schemes being triggered, the second gas turbine at Little Barford tripped at 187 MW meaning the total loss of generation was 1878 MW).
Conclusions and recommendations ?
The NGESO analysis has identified the following areas where lessons can be learned: ?
• Communication processes and protocols, in particular during the first hour, should be reviewed to support timely and effective communication in any future event; ?
• The list of facilities connected to the LFDD scheme should be reviewed to ensure no critical infrastructure or services are inadvertently placed at undue risk of disconnection; ?
• The settings on the internal protection systems on electric trains should be reviewed to ensure they can continue to operate through ‘normal’ disturbances on the electricity system. ?