Frequency response and its role in grid stability
Ofgem has released the findings of its investigation into the events of 9 August 2019 — a day which saw the UK suffering its largest power outage in a decade.
While the report provides further clarity regarding the cause of the blackout, the event continues to raise questions over the stability of the grid and how best to ensure long-term energy security. In particular, the role that faster frequency response products play as more and more renewable generation comes online.
Frequency response and the power cut
The sequence of events that led to the blackout were triggered when lightning hit a 400kV overhead transmission line in Cambridgeshire.
In quick succession, the following losses in generation occurred:
- Little Barford — an RWE gas-fired power station connected to the transmission network, generating 641MW
- Hornsea-1 — an offshore wind farm connected to the transmission network, generating 737MW
- 200MW embedded generation unexpectedly shutting down at 49Hz
- Loss of mains protectionwhich disconnected between 500–600MW of embedded generation.
Whilst the grid currently spends between £10–15m a month holding over 1GW in response capacity — sufficient to cover the largest infeed on the electricity system — this was not enough to cover the loss of two generators and the embedded generation. The total being more than 1.8GW.
The Low Frequency Demand Disconnection (LFDD) that this level of drop in frequency triggered resulted in a further 550MW loss of embedded generation, which exacerbated the issue and may have impacted the non-delivery of 12% of the held response volume.
Ofgem has confirmed that Hornsea One Ltd and RWE have each agreed to pay a fine of £4.5 million for failing to remain connected after the lightning strike. UKPN is also to hand over £1.5 million in relation to a technical breach.
However, no fine has been given to SP Distribution who disconnected 22MW of demand in Scotland, due to an incorrect setting on SP Distribution’s Low Frequency Demand Disconnection (LFDD) equipment.
Equally, no fines have been given to the response volumes that did not deliver.
Energy security and renewable generation
With the UK committed to achieving net zero greenhouse gas emissions by 2050 and National Gird aiming to run the electricity system on 100% renewable energy for longer periods by 2025, ensuring the resilience and security of the evolving energy network remains vital.
On the day of the blackout, 40% of the generation mix was being provided by renewable sources — which is fantastic. However, the decreasing volume of ‘synchronous generation’ being provided by traditional coal results in less inertia on the system.
The inertia associated with traditional power stations has been important for helping the grid maintain a steady frequency. More renewable power sources result in less inertia, which makes management of frequency on a near real-time basis increasingly more important.
This need has already been recognised by National Grid who announced an inertia tender for April 2020. In the future, wind and solar farms could also be incentivised to provide their own ‘synthetic’ inertia.
Frequency response could provide an answer, however current products that require response times of 10 and 30 seconds, do not provide enough resilience for the system. Which is why new faster frequency response products are being introduced that will have a response time between 0.5 and 2 seconds.
While Ofgem’s investigation has confirmed the importance of frequency response for stabilising the grid, the events surrounding the blackout also raise some key questions. For example, how will National Grid ensure such services are being provided as agreed? And, what will the penalties be for those not delivering?
The grid needs greater volumes of response and the move towards faster-acting frequency products is a positive step. Such technology will be vital as more and more renewables are added to the generation mix and there is less inertia in the system. So too are systems which support this transition and the energy network of the future. Such as Upside Platform, which can utilise storage from 100,000s of distributed devices in real time, regardless of their type, size, location or scale.
What is clear is that frequency response has a crucial role to play, both in supporting grid stability and the country’s transition towards a renewable energy network. But there are questions yet to be answered, when it comes to how such services will be controlled and managed.
Data sourced from Gridwatch
Vector shift disconnects the generator from the system safely when it detects a fault and Rate of change of frequency (RoCoF) disconnects generators from the system if the RoCoF is greater than 0.125Hz/s.