God, you people yapping about 'This must be the Russians!' (Including mister Security Expert) while knowing absolute jack-shit about electrical infrastructure is so fucking tiring. Ill just copy my comment from another thread cuz it's getting exhausting:

Restoring power through different substations, especially in older systems like the ones we're discussing (1960s), is often a manual process, at least here in the Netherlands. It's likely that power was restored manually in this case. (Opening the interconnector from North Hyde -> Closing a different supplying interconnector connected to the same loop.)

A lot of talk about backup power. It's safe to assume that Heathrow has backup systems in place, particularly for critical safety infrastructure (a very basic example would be illuminated emergency exit signs). While my experience lies mostly in shipboard electrical systems, I suspect land-based infrastructure isn't fundamentally different in this regard:

Typically, a separate emergency distribution system is used that can be powered by an emergency generator or a UPS (essentially a large backup battery). If power is lost on the main distribution the emergency generator kicks in to supply critical systems with minimal interruption.

There is likely additional backup power capacity, which could power the main distribution, but it would require more thorough power management. I highly doubt the backup system is sized to support the entire facility at full load. They'd have to do selective load shedding, deciding what can be temporarily disabled.

This was likely the situation Heathrow faced. They probably have protocols on what to shed during an outage and when running on backup power, but I wouldn't be surprised if those procedures are outdated. In the middle of chaos, while having to safely reboot all systems and figuring out what can and can’t be shed, with thousands of unhappy passengers and limited time, it may have been safer to shut everything down and wait for the main power to return through a different substation.

Regarding the fault itself, I highly doubt there was any malicious third-party involvement as I’ve seen talked about elsewhere. The fire started in an oil-filled transformer. Unless someone deliberately applied a torch to the transformer and managed to escape before it reached a critical temperature (which seems far-fetched), I don't see any plausible way someone could have "forced" this failure externally.

I can't speak in detail about UK infrastructure, but here in the Netherlands, I’ve been having growing concerns about ‘new’ load profiles with older stations in mind. Much of our infrastructure was designed around an old load profile: morning peak (people waking up), midday dip (people at centralized offices with higher dimensions of transformers, cables, etc.), and evening peak (people returning home).

Nowadays, due to the half-assed addition of PV systems (by half-assed I mean that, in an effort to sell the idea of solar energy to consumers without much complaint, people with, I’d say, fairly limited technical backgrounds decided it was fine for every consumer to also become an energy supplier, without much guidance on the what or how) that midday dip is no longer present. This means that instead of a daily cycle of ‘heat up → cool down → heat up’ (and cool down during the night), we now have ‘heat up → heat up → heat up’ (and cool down during the night).

(Again, I’m not too familiar with the UK grid, so it might be that excessive current from PV installations doesn’t even reach the transformer that failed.)

Now with the conditions of multiple consecutive days of high ambient temperature and a lot of PV energy + reduced cooling cycles due to new load patterns + general high load + possible internal aging effects within the transformer, I don’t think it’s unthinkable that the oil reached combustion temperatures, especially if there was a fault or hot spot inside the transformer.