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The executive chairman of renewable developer Anesco has criticised the electricity system operator (ESO) for failing to properly prepare for the foreseeable loss of hundreds of megawatts of embedded generation during last month’s blackout.
Steve Shine was commenting on the ESO’s final report to Ofgem which the regulator published earlier this week.
The event was triggered by a lightning strike that caused a fault on the transmission network. This disturbance activated the loss-of-mains protections for 500MW of distributed generation – roughly 150MW with vector shift relays and 350MW with rate of change of frequency (RoCoF) relays.
At the time, the ESO had enough frequency response available to cover the largest single potential loss on the system – 1000MW.
The report says the loss of embedded generation with vector shift protection is “expected for a lightning strike on a transmission line”.
But Shine said this begs the question: “If National Grid were aware that a 1000MW loss could lead to a further 500MW of generation being lost, why were they not securing 1500MW?”
He claimed if the ESO had been able to cover this additional loss then the blackout may have been avoided.
The ESO’s report explains that it only covers potential losses from generators with vector shift protection “where it considers it appropriate to do so.”
“The real question then becomes, why did National Grid not consider it appropriate to secure for vector shift on this occasion,” said Shine, “and how often do National Grid not consider it appropriate to secure the system against this event?”
A spokesperson for National Grid Electricity System Operator said: “Regardless of the 500MW loss of embedded generation, the combined loss of Little Barford and Hornsea was greater than 1000MW – the amount of reserve the ESO was holding on 9 August as per the Security and Quality of Supply Standards.”
They continued: “Embedded generation could only be tripped off by a direct strike on the physical National Grid electricity transmission system, causing a vector shift in voltage, and leading to embedded generation disconnecting itself to protect itself. This would be independent and coincidental from any issues with conventional generation plants, which may cause them to disconnect themselves from the transmission system.
“As per our reports to Ofgem, this was therefore an extremely rare combination of circumstances. The loss of 500MW of embedded generation may have exaggerated the rate of frequency change but was coincidental and not causal to the power cut – which would have happened anyway with the synchronous loss of Little Barford and Hornsea.”
The issue of nuisance tripping by distributed generators with overly sensitive loss-of-mains protections is already being addressed through an industry-led initiative. As part of its report, the ESO has called for a review of the programme’s timetable.
Ofgem recently approved the last in a series of modifications to the Distribution Code requiring embedded generators to replace vector shift protections and alter the settings for RoCoF relays.
Anesco has developed more than 100MW of battery storage – a key source of frequency response – and is planning to grow the portfolio to 380MW by 2020.
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