On the evening of 23 June, Great Britain came closer to the edge than on any other night this summer.
On NESO's official numbers, national de-rated margin fell to 4.1 GW — the tightest evening of the summer. Frequency sat below 49.8 Hz for 48 minutes between 18:51 and 20:03, bottoming at 49.656 Hz. At 20:01 NESO requested Emergency Assistance on an interconnector.
A word on that margin figure, since it recurs below. De-rated margin is NESO's own calculation, resting on NESO's assumptions about how much of each plant type will actually turn up. It is the best public measure of how much headroom the system had, and it is not an observation — it is an estimate, published by the operator whose performance it describes. Not everyone accepts it as the last word, and 23 June in particular has been the subject of public argument about how close Britain really came. The frequency trace and the metered flows in this article are measurements; the margin numbers are NESO's.
And at 19:00, while all t…