EngineerHub.orgBefore 16:52:33 BST
Normal system operation
The Great Britain power system operates close to 50 Hz while ESO secures frequency response for a 1,000 MW infeed loss.
- Frequency
- 50.00 Hz
- Infeed loss
- 0 MW
- LFDD demand
- 0 MW
01 // Interactive regional replay
Select a stage or run the event.
Great Britain Transmission System StatusPublished regional boundaries, event locations and a stage-dependent educational transmission backbone.
Regional geometry: published source · transmission topology and stage states: educational.
Current stageNormal system operation
Normal operation
Regional status
Normal
Stressed
Critical / LFDD
Demand disconnected
Restoring
Where
What changed
System effect
Response
System consequence
Measured scaleCumulative infeed loss
Stage 1 of 9Event progression
System indicators
FrequencyControlled
Deviation0.00 Hz
Minimum reached50.00 Hz
System stateSynchronous and energised
Rolling operator event console
2019 Great Britain power outage timeline
A static summary of the same nine stages used by the interactive regional replay.
16:52:33.490–.564 BST
Lightning fault is cleared correctly
A phase-to-earth fault occurs on the Eaton Socon–Wymondley 400 kV circuit. Transmission protection clears both ends within 74 milliseconds.
- Frequency
- 49.97 Hz
- Infeed loss
- 150 MW
- LFDD demand
- 0 MW
16:52:33–34 BST
Major and embedded generation losses reach 1,131 MW
Hornsea One de-loads by 737 MW, the Little Barford steam turbine trips by 244 MW and approximately 150 MW of embedded generation trips on vector-shift protection.
- Frequency
- 49.55 Hz
- Infeed loss
- 1,131 MW
- LFDD demand
- 0 MW
16:52:34–16:53:18 BST
RoCoF losses deepen the deficit, then response arrests it
A further estimated 350 MW of embedded generation trips on RoCoF protection. Cumulative infeed loss reaches 1,481 MW before response arrests the first decline at 49.1 Hz.
- Frequency
- 49.20 Hz
- Infeed loss
- 1,481 MW
- LFDD demand
- 0 MW
16:53:31 BST
Little Barford GT1A trips and frequency falls again
Little Barford GT1A disconnects 210 MW while all available frequency response is being delivered.
- Frequency
- 49.05 Hz
- Infeed loss
- 1,691 MW
- LFDD demand
- 0 MW
16:53:49.398 BST
Frequency reaches 48.8 Hz and LFDD operates
Stage-one Low Frequency Demand Disconnection automatically removes 931 MW of demand.
- Frequency
- 48.80 Hz
- Infeed loss
- 1,691 MW
- LFDD demand
- 931 MW
16:53:58 BST
Little Barford GT1B trips after LFDD
Little Barford GT1B disconnects a further 187 MW, taking cumulative infeed loss to 1,878 MW.
- Frequency
- 48.95 Hz
- Infeed loss
- 1,878 MW
- LFDD demand
- 931 MW
16:54:20–16:57:15 BST
Response and control-room actions restore frequency
More than 1,000 MW of response and a further 1,240 MW of ESO control-room actions stabilise the system.
- Frequency
- 50.00 Hz
- Infeed loss
- 1,878 MW
- LFDD demand
- 931 MW
16:58–17:37 BST
Disconnected demand is restored
ESO progressively authorises DNOs to reconnect demand. All DNOs confirm restoration complete by 17:37.
- Frequency
- 50.00 Hz
- Infeed loss
- 1,878 MW
- LFDD demand
- 0 MW
02 // System frequency
Source-based milestones with educational interpolation.
50.00 HzGreat Britain system frequency
ConditionControlled
Deviation0.00 Hz
Minimum shown50.00 Hz
System stateSynchronous and energised
Model boundary: source-based event milestones are connected by educational interpolation. The Great Britain system remained synchronised and energised throughout the disturbance.
03 // Cause-and-effect chain
The complete event in five engineering steps.
01 / FAULTLightning creates a short circuit that transmission protection clears in 74 ms.
02 / INFEED LOSSHornsea One, Little Barford and embedded generation disconnect unexpectedly.
03 / RESPONSE EXHAUSTEDResponse arrests the first decline, but another Little Barford trip removes remaining margin.
04 / LFDDFrequency reaches 48.8 Hz and 931 MW of demand is automatically disconnected.
05 / RECOVERYResponse, control-room actions and staged demand restoration return the system to normal.
04 // Event figures
Official milestones with explicitly labelled interpolation.
Cumulative infeed loss and LFDD demand
Cumulative infeed lossDemand disconnected
The final cumulative infeed loss was 1,878 MW; LFDD disconnected 931 MW of demand.
View infeed-loss and LFDD data
| Stage | Cumulative infeed loss (MW) | Demand disconnected (MW) |
|---|---|---|
| 1. Before 16:52:33 BST | 0 | 0 |
| 2. 16:52:33.490–.564 BST | 150 | 0 |
| 3. 16:52:33–34 BST | 1,131 | 0 |
| 4. 16:52:34–16:53:18 BST | 1,481 | 0 |
| 5. 16:53:31 BST | 1,691 | 0 |
| 6. 16:53:49.398 BST | 1,691 | 931 |
| 7. 16:53:58 BST | 1,878 | 931 |
| 8. 16:54:20–16:57:15 BST | 1,878 | 931 |
| 9. 16:58–17:37 BST | 1,878 | 0 |
Frequency transient
System frequency48.8 Hz LFDD trigger
Intermediate points are educational interpolation between official milestones.
View frequency data
| Stage | Frequency (Hz) | Condition |
|---|---|---|
| 1. Before 16:52:33 BST | 50.00 | Near nominal |
| 2. 16:52:33.490–.564 BST | 49.97 | Near nominal |
| 3. 16:52:33–34 BST | 49.55 | Low |
| 4. 16:52:34–16:53:18 BST | 49.20 | Low |
| 5. 16:53:31 BST | 49.05 | Emergency |
| 6. 16:53:49.398 BST | 48.80 | LFDD threshold |
| 7. 16:53:58 BST | 48.95 | Emergency |
| 8. 16:54:20–16:57:15 BST | 50.00 | Near nominal |
| 9. 16:58–17:37 BST | 50.00 | Near nominal |
Response and restoration actions
Frequency responseControl-room actionsRestoration progress ×12.4
The restoration series is scaled for display. Response and control actions are shown as reported milestones, not a dynamic dispatch trace.
View response and control-action data
| Stage | Frequency response shown (MW) | Control-room actions (MW) | Demand restoration (%) |
|---|---|---|---|
| 1. Before 16:52:33 BST | 0 | 0 | 0 |
| 2. 16:52:33.490–.564 BST | 0 | 0 | 0 |
| 3. 16:52:33–34 BST | 0 | 0 | 0 |
| 4. 16:52:34–16:53:18 BST | 900 | 0 | 0 |
| 5. 16:53:31 BST | 1,000 | 0 | 0 |
| 6. 16:53:49.398 BST | 1,000 | 0 | 0 |
| 7. 16:53:58 BST | 1,000 | 0 | 0 |
| 8. 16:54:20–16:57:15 BST | 1,000 | 1,240 | 15 |
| 9. 16:58–17:37 BST | 1,000 | 1,240 | 100 |
Broad regions by status
NormalStressedCriticalDemand disconnectedRestoring
Regional status is an educational geographic illustration, not an official DNO-by-DNO outage chronology.
View regional status data
| Stage | Normal | Stressed | Critical | Offline | Restoring |
|---|---|---|---|---|---|
| 1. Before 16:52:33 BST | 5 | 0 | 0 | 0 | 0 |
| 2. 16:52:33.490–.564 BST | 3 | 2 | 0 | 0 | 0 |
| 3. 16:52:33–34 BST | 0 | 3 | 2 | 0 | 0 |
| 4. 16:52:34–16:53:18 BST | 0 | 2 | 3 | 0 | 0 |
| 5. 16:53:31 BST | 0 | 0 | 5 | 0 | 0 |
| 6. 16:53:49.398 BST | 0 | 4 | 0 | 1 | 0 |
| 7. 16:53:58 BST | 0 | 4 | 1 | 0 | 0 |
| 8. 16:54:20–16:57:15 BST | 4 | 0 | 0 | 0 | 1 |
| 9. 16:58–17:37 BST | 5 | 0 | 0 | 0 | 0 |
05 // Event explanation
The transmission fault was cleared correctly. The widespread outage developed because several unexpected generation losses occurred almost simultaneously with the voltage disturbance and were followed by additional trips.
Why the first frequency decline stopped
Frequency response delivered at least 650 MW within about eleven seconds and 900 MW by 16:53:04. This arrested the first decline at 49.1 Hz and supported a recovery to 49.2 Hz.
Why frequency fell a second time
Little Barford GT1A then disconnected 210 MW while all available response was already being delivered. The system therefore had insufficient remaining headroom to absorb the new loss without emergency demand disconnection.
Why the map uses broad regions
Official reports provide national frequency, plant and DNO information, but not a continuous administrative-region animation. The map therefore uses five broad educational regions to show stress, LFDD and restoration without claiming an official geographic sequence.
Why transport disruption lasted longer
Electrical supplies were restored quickly, but some rail equipment and trains required manual restart or operational recovery. The page therefore distinguishes power-system restoration from dependent-infrastructure recovery.
07 // Event interpretation
What the event demonstrates about frequency security and dependent infrastructure.
08 // How the barriers performed
A concise view of prevention, protection and recovery barriers.
09 // Engineering background
Open each topic to review the principles illustrated by the event.
When connected demand exceeds generation, rotating machines initially supply the deficit from stored kinetic energy and slow down. System frequency therefore falls until response increases generation or demand is reduced.
Generators are expected to remain connected through specified short voltage disturbances. The transmission fault was cleared quickly, but the coincident plant responses produced large infeed losses.
These loss-of-mains methods detect changes in frequency behaviour or voltage phase. In 2019 they disconnected substantial distribution-connected generation during a transmission-system event.
LFDD disconnects selected demand at defined low-frequency thresholds. Removing 931 MW of demand helped arrest the 48.8 Hz decline and preserve a synchronised system.
Reconnecting too much demand at once can create a new imbalance. ESO therefore authorised DNO restoration progressively after frequency and reserve levels were secure.
10 // Key engineering lessons
Four transferable lessons for modern low-carbon power systems.
11 // Frequently asked questions
Short answers to common questions about the 9 August 2019 event.
The lightning fault initiated the sequence, but transmission protection cleared it correctly. The widespread loss of supply followed the near-simultaneous loss of major and embedded generation.
The ESO technical report records 1,878 MW of cumulative infeed loss: 737 MW at Hornsea One, 641 MW across Little Barford units and an estimated 500 MW of embedded generation.
Frequency fell to 48.8 Hz, triggering stage-one Low Frequency Demand Disconnection.
Low Frequency Demand Disconnection automatically removes selected demand when frequency reaches emergency thresholds, helping rebalance generation and demand.
DNO relays disconnected 931 MW of demand, affecting approximately 1.1 million customers.
No. The system remained synchronised. LFDD and frequency response arrested the decline and avoided a much more severe system collapse.
Some trains and rail assets shut down or required manual intervention to restart. Operational recovery therefore continued after electricity supplies had been restored.
No. The event chronology and national values follow official reports. Stage colouring is an educational illustration applied to eleven geographic regions through five broader operating-status groups.
12 // Related case studies
Compare this frequency event with other EngineerHub disturbance replays.
13 // Sources and model limitations
Primary reports and the boundaries of this educational reconstruction.
National Grid ESO: Technical Report on the events of 9 August 2019.
Ofgem: Investigation into the 9 August 2019 power outage.
UK Government / E3C: GB power system disruption on 9 August 2019: Final report.
Office of Rail and Road: Report following railway power disruption on 9 August 2019.
Map geometry: Great Britain regional boundaries are adapted from the Wikimedia Commons NUTS 1 statistical regions of the United Kingdom map by Dr Greg and Nilfanion. It contains Ordnance Survey data © Crown copyright and database right 2011 and is used under CC BY-SA 3.0. Muted neighbouring coastline context is derived from GSHHG, released under the GNU LGPL. Transmission corridors and stage colouring are educational.
The map uses geographic regional boundaries but remains a teaching backbone rather than a complete transmission or DNO topology. Event times, plant losses, frequency milestones, LFDD quantity and restoration milestones follow official reports. Intermediate frequency points, regional status and corridor loading are educational interpolation.