The interim report by NESO has been published. A further final report is expected in June.
https://www.neso.energy/publications/north-hyde-review
Page 17 has the detail on how HAL restored their internal power network.
Heathrow Airport restoration
5.30 At 23:49 on Thursday 20 March, power was lost to Heathrow Substation A, meaning power was lost to some airport terminals and some shared systems required for overall airport operations. Heathrow Airport’s airfield ring generators automatically started to maintain
supplies to the runways and essential safety systems, maintaining the ability to land aircraft safely.
5.31 At 01:11 on Friday 21 March, Heathrow Airport Limited took the decision to close Heathrow Airport
due to the disruption caused to operationally critical systems following the power outage, and
at 02:14 issued a media statement confirming closure until 23:59 later that day.
5.32 From 02:00 on Friday 21 March, Heathrow Airport Limited’s Airport Control Engineer began to
re-energise terminals and other impacted systems, which had lost supplies from Heathrow Substation A (having been fed from North Hyde 66kV substation). Heathrow Airport Limited’s engineers used interconnections between Heathrow Airport’s intake substations.
5.33 All of Heathrow Airport’s primary 33kV substations connected to Heathrow Substation A were re-energised by 06:25, following which Heathrow Airport Limited’s engineers re-energised the wider network of around 300 low voltage substations. By 10:56 it was confirmed that power
had been restored to all of Heathrow Airport’s terminals and re-energisation of the wider
Heathrow Airport Limited network was completed by 14:23. Once power had been restored,
there was a period of safety checking to allow all parties operating the airport to access
their systems and to ensure safety critical systems were fully operational prior to passengers
arriving at the airport.
5.34 From the afternoon of Friday 21 March, Heathrow Airport Limited continued to discuss with
SSEN Distribution options for switching back to supplies from North Hyde 66kV substation but by 22:30, ultimately decided to remain using the new configuration utilising its two other intake substations.
5.35 Heathrow Airport reopened for some repositioning and repatriation flights on Friday 21 March, and for business-as-usual operation on Saturday 22 March
So in summary,
- HAL has generators for safety critical stuff to permit aircraft to continue to take off an land and that worked. But it doesn't have the capacity to run all the terminals and wider campus.
- Core HAL high voltage power network was manually reconfigured to work around the loss of North Hyde with the HV network around the HAL campus restored around 6am.
- The lower voltage network which distributes power ending up with 230V / 380V supplies around the HAL campus was fully re energised by 2.30pm. All the terminals had some power by 11am, but maybe not all power. Safely checks continued into the afternoon by various parties responsible for those systems/services.
Armchair observation - if there was more auto-changeover switching available on the high voltage network on the HAL campus then power
could have automatically diverted upon the loss of North Hyde to attempt to take power from the remaining two HV substations,
and if this had been in place then power might not have been lost at all, or perhaps a flicker of the lights and some control systems might need a reset due to a blip in the power.
But implementing auto-changeover is probably easier said than done, there's probably a lot of legacy in the basics of the HV power network on the HAL network and would probably be an expensive multi year project to implement. All the more difficult in a highly regulated and safety critical environment, with many constraints, not least the need to keep the lights on.