Little Barford power station near St Neots has been carrying out further tests to establish exactly what its role was in a massive power cut which left 1.1 million customers without a supply and thousands of local commuters hit by the closure of King’s Cross railway station.
Power regulator Ofgem published a technical report on Tuesday, drawn up by the National Grid Electricity System Operator (ESO), into the incident on August 9 which began with lightning strikes on a transmission circuit from Eaton Socon to a site near Stevenage.
The Government is also reviewing the actions of the ESO, which says there are areas where lessons can be learned, and will report its initial findings to the secretary of state.
RWE, which runs the Little Barford power station, said it had begun further tests this month during a scheduled power outage and would be working with equipment manufacturers to compare results with those from manufacturing and commissioning tests.
It said that the investigations so far had shown that the uninterrupted power supply (UPS) systems had functioned correctly.
RWE said it believed that the UPS changed over to a battery back-up system as a result of a "system disturbance" and the reason for this was still to be determined and was subject to further investigation.
"Based on our recent experience at Little Barford we are confident that the UPS functions as designed and we are confident in its ability to demonstrate this," the RWE account said.
It said a comprehensive investigation into the steam turbine tripping was still taking place.
"Upon initiation of the steam turbine trip the gas turbines went into bypass mode of operation, which is the normal response to allow sustained operation," RWE said. "For reasons presently unknown, a high-pressure excursion occurred on GT1A (one of the turbines), resulting in an automated trip. Given the prevailing conditions, all systems functioned as expected. GT1B was manually tripped shortly after as a result of excessive steam pressure."
The ESO report said a lightning strike on the Eaton Socon circuit, one of the three in the area of the line at the Stevenage end, happened at 4.52pm and was cleared in less than a second - but this coincided with the tripping of the Little Barford power station and a drop in power from an offshore windfarm.
The "extremely rare" event, the first in more than a decade, sparked an emergency power cut system after reserves dropped below a critical level, which led to 1.1 million customers being without electricity for up to 45 minutes.
But the cut led to knock-on effects elsewhere, including the closure of King's Cross railway station and the shutting down of 60 trains in the region, half of which could not be restarted without an engineer being called out, and to disruption at Ipswich hospital and Newcastle airport.
The report said nearly 13,000 lightning strikes were recorded that day, with some on the electricity circuit which did not cause major problems.
Twenty-three trains had to be evacuated and thousands of other rail passengers had their journeys delayed with 371 trains cancelled, 220 part-cancelled and 873 delayed.
Train operator Govia Thameslink Railways said around 60 of its class 700 and 717 locomotives underwent a protective shutdown because of the power drop but only half could be restarted by their driver.
GTR said the cut should not have caused a permanent lockout on half the trains and manufacturer Siemens was developing a software patch to enable drivers to restart the engines as well as investigating ways of enabling them to operate on lower power.
The ESO said areas where lessons could be learned included better communications, a review of critical services potentially affected by the emergency power cut system and settings on trains being changed so they could continue to operate on lower power.
It also called for a review of power supply security standards, including whether it was appropriate to provide for a higher level of resilience in the system; assessing whether it would be appropriate to establish standards for critical infrastructure, such as hospitals and transport, covering conditions on electricity supply their internal systems should cater for and to review the timescales for delivering the accelerated loss of mains change programme to reduce the risk inadvertent disconnection of embedded generation.