As far as I know that’s usually a split-phase system: You get 110V between either phase and neutral, and 220V between the phases.
In a three-phase system (like the ones used in most of Europe), that effect isn’t as pronounced since you usually have at most 120 degrees phase difference instead of 180 and the voltage between two phases is less than double, so less appliances make use of that (I think some stoves do).
> As far as I know that’s usually a split-phase system: You get 110V between either phase and neutral, and 220V between the phases.
We Brazilians tend to call it "110V", but it's often a three-phase system with 127V between phase and neutral and 220V between phases.
Or sometimes it's 115V from phase to neutral and 230V from phase to phase (probably a split-phase system like you said). And in that case, us Brazilians would still call the lower voltage "110V", and the higher voltage "220V".
You can't trust a Brazilian when they say the voltage is "110V" or "220V". These numbers are only an approximation, which might have been correct for some places a century or two ago.
A Brazilian would say "110V" for all these lower voltages between 115V and 127V, and would say "220V" for all these voltages between 208V and 230V.
At least the frequency is always 60Hz (unlike the rest of South America, which uses 50Hz; we have to use HVDC links when importing or exporting electricity to the neighboring countries).
Using HVDC to interconnect async grids is very common.
This presentation is a nice deep dive on the situation in Europe: we are even building HVDC “corridors” embedded inside some of the sync grids. Pricey gear but valuable too!
HVDC corridors make sense because it's functionally impossible to build new overhead transmission lines anywhere, so operators are forced to bury the transmission lines. But buried AC transmission lines have terrible loss and are super expensive, buried HVDC lines are slightly less expensive (but require expensive terminal stations), and have much lower losses.
> it's functionally impossible to build new overhead transmission lines anywhere
This isn't really true, new overhead lines are being built all over the place. It definitely depends where they're being built and there is definitely strong opposition from councils in e.g. some parts of Germany, but see for example in the Netherlands there are definitely new overhead lines being built: https://www.tennet.eu/nl/projecten/provincies/groningen/eems...
That being said it's true that it's much more challenging than it used to be to get planning permission.
> we are even building HVDC “corridors” embedded inside some of the sync grids
We also have that here in Brazil; the Xingu-Estreito and Xingu-Terminal Rio HVDC links run in parallel with the high-voltage AC links. On the other hand, the Madeira HVDC link (https://en.wikipedia.org/wiki/Rio_Madeira_HVDC_system) is AFAIK normally disconnected from the AC grid on the generation end (it has a direct connection, probably used to start up the power plants, but normally uses a back-to-back HVDC converter instead of it).
My local power company (in the USA) is known for running slightly higher voltage than the spec. In the past I have seen 127V - and wrote a letter complaining about it. Which probably got me on their list of annoying customers...
Currently it's at 121.8V - either they have finally adjusted the tap on the neighborhood transformer or there's a heavy load at the moment (hot water heaters running from people showering this morning?)
According to ANSI C84.1 (which I believe is the relevant US specification), there is a tolerance of +/- 5%, which would put 127V just barely out of spec.
I could imagine that utilities calculate some average base load and adjust transformers accordingly to accommodate the expected voltage drop, given that 110/120V is relatively low and drops can be significant?
Obviously not an expert, but I wonder if they run a higher voltage in order to give them some margin from frequency dropping too far below 60 Hz (caused by generators slowing down) during high load periods?
Just for comparison other countries that claim "230v" have that last one - 220-240v between the phase you get and neutral and 440v between phases if you get 3-phase
Europe also just doesn't need split-phase systems. A regular outlet will provide 230V @ 16A, or about 3.7kW. That's enough for virtually all household appliances, and single-phase 32A circuits for 7kW stoves are quite common too.
Households these days often do have a 3-phase hookup, but it's quite rare to have 3-phase equipment. Maybe a stove, and with the energy transition larger solar setups, electric car chargers, and large heat pumps. But most of the time that 3-phase hookup is just treated as three separate 1-phase sections.
In a three-phase system (like the ones used in most of Europe), that effect isn’t as pronounced since you usually have at most 120 degrees phase difference instead of 180 and the voltage between two phases is less than double, so less appliances make use of that (I think some stoves do).