Orsted has traced the recent outage at its Hornsea One offshore wind farm to a problem with the control software.
The loss was one of the catalysts for a major blackout in August that left many commuters stranded during the Friday evening rush hour. The company says it has since updated the software to prevent the issue from occurring again.
The findings are contained within the appendices to National Grid Electricity System Operator’s (ESO) final report on the incident, which was submitted to Ofgem on Friday and published by the regulator earlier today (10 September). It includes a section submitted by Orsted explaining how and why the outage at Hornsea One occurred.
According to the submission, a fault on the power line between the Eaton Socon and Wymondley substations due to a lightning strike at one of the pylons, caused a large drop in voltage at Hornsea One.
In response, the control system for the windfarm increased its injection of reactive power to the grid to bring the voltage back up.
When the voltage reached and then surpassed the 400kV target, the control system reduced the injection of reactive power to stop the overshoot. It then continued oscillating back and forth, raising and lowering reactive power output as it sought to settle the voltage at the right level.
At one point, a fall in voltage coincided with a reduction in reactive power (and hence an increase in active power), causing a surge in current that triggered the windfarm’s protection system.
“Prior to the event, each stage of Hornsea One had been successfully modelled and physically tested in line with all grid code requirements,” the submission explains.
“While the potential for oscillations had been considered, there had been no reason to suggest that Hornsea One would have responded to a fault on the grid in the way that it evidently did.
“To the best of Orsted’s knowledge this is the first time such oscillations have led to a de-load of a windfarm.”
The main body of the ESO’s report says these large oscillations in active and reactive power were “unexpected” and “should not have occurred”.
The configuration of the network assets at Hornsea One was a “contributory factor” as it created “a weak internal network environment”.
The appendices to the report also include an update from RWE’s investigation into the cause of a series of outages at its Little Barford combined-cycle gas turbine plant, the last of which triggered the low frequency demand disconnection procedure that cut power supplies to more than a million customers.
The company has confirmed that the first outage, affecting the plant’s steam turbine, was caused by a discrepancy in speed signals following the aforementioned fault on the transmission network.
The gas turbines began operating on bypass mode but shortly afterwards one of them automatically tripped due to a “high-pressure incursion”, the cause of which is still unknown.
RWE then manually disconnected the remaining gas turbine following a build up of excessive steam pressure.
As part of its report to Ofgem, National Grid Electricity System Operator has called for review of security of supply standards determine whether it is necessary to provide “higher levels of resilience”.