Low voltage 12V relays driven by a microcontroller plus transistor
need a flyback diode to suppress an inductive voltage spike when they
are switched off.
There's a whole realm of difference between disconnecting a primary inductance circuit AND disconnecting a secondary load.
Do ordinary power transformers (say, at a distribution substation)
need some sort of similar flyback protection in case a large fraction
of their load is suddenly shed
No, because the primary winding is connected to the AC distribution network and that distribution voltage can be regarded as having a very, very low impedance hence, any occurance of the load being dropped on the secondary side will result in absorption of transformer stored energy directly into the AC grid supply. Sure, there might be a little current surge but that will be "clamped" by the very low impedance of the grid supply.
I would imagine that a resulting voltage surge for those who remain
connected to the grid would be very undesirable.
Not at all, the voltage fluctuation due to the energy release from the transformer will be measured as a few volts. The distribution voltage supply will soak it up like blotting paper.
What about lower current transformers in "power" household
applications?
It makes no difference; scale is not an issue.
In practice is there a good way to estimate the magnitude of the spike
If you know the distribution voltage source impedance then you can calculate the disturbance (a few volts) based on knowing the leakage inductance of the transformer and the peak current being taken at the moment the load is shed.
I think another scenario might help. If the transformer primary were disconnected suddenly (circuit breaker etc.) then there could be a huge voltage surge across the contact breaker's contacts and that, can cause significant damage to the CB if it isn't protected sufficiently. This would be equivalent to using a flyback diode on a DC circuit.
Footnote
All AC circuits these days are protected against indirect lightning strikes and these protection devices (in buildings and with line transformers) will clamp voltage surges. So, even if there were some kind of load shedding surge, there are grid components that "clamp" to prevent excessive voltages arriving on the plugs of appliances. You can, these days rely on surges protection devices for limiting internal household and office supplies to a peak spike of about 1500 volts.
