Comment by toast0

The US runs three major grids: East, West, and Texas. There's interconnections, but capacity is limited. Sounds like the UK is similar here with the bottleneck between Scotland and southern England.

Within the US grids, there's really subgrids with interconnection and bottlenecks, too, but those interior bottlenecks aren't brought up as often as say overnight wind production in Texas being over local demand as well as interconnect capacity.