A large scale frequency drop after the near-simultaneous loss of two generating plant was the cause of last night’s power outages, as National Grid ESO cut demand in some areas to protect the system.
Acccording to Energyst, New Power’s sister publication, RWE’s 740MW Little Barford gas-fired power station tripped first, followed by parts of Ørsted’s Hornsea offshore wind farm (story here). As a result frequency dropped from 50Hz to 48.9hz and the system automatically tried to protect itself and restore frequency by disconnecting areas and National Grid reduced demand to bring power back up. That led to outages across the country. The system operator said all areas were restored by 18:30, though transport networks and commuters were feeling the effects for the rest of the evening.
The SO has to maintain system frequency but that is made more difficult because generating plant in some cases automatically disconnect to protect themselves when the fequency moves above or below a ‘tolerance’ band around 50Hz. Disconnecting plant also put pressure in the frequency and it is possible to have cascades of dosconnection (it’s not clearthat was a factor in yesterday’s interruptions). In the past large thermal plant helped the system ‘ride through’ these events as they rely on heavy rotating equipment that have mechanical inertia. Increasingly that will be replaced by fast response from batteries, or controlled action from the demand side – National Grid has said that it wants to be able to operate the system without thermal plant when they are not otherwise required (ie there is sufficient low-carbon generation to meet demand) by 2025.
In addition to added new types of response and inertia, action is being taken to ensure that plant does not trip off unnecessarily in response to frequency changes, limiting cascade failures. In most cases automatic disconnection is more sensitive than in needed to protect the plan. This may be relatively simple – adjusting set points – but it requires action at thousands of plants. In fact, Ofgem agreed new rules on this in the last week. New plants connected in future shoud be less prone to disconnect. Meanwhile, research and development on managing and maintaining inertia is under way.
As Energyst notes, last night’s event will also cause large users to re-examine their power systems. Users will have uninterruptible power systems (UPS) in place to manage short-term grid faults, providing at least power for ‘mission critical’ services.