I think something like this wins that category:

https://en.wikipedia.org/wiki/Eastern_Interconnection

It's strange to count this as single machine. But if we go this way, the north Chinese interconnection and the continental Europe interconnection are bigger (https://en.wikipedia.org/wiki/Wide_area_synchronous_grid).

I suppose it's partly a semantic question that hinges on what you count as a single "machine" and what's a system or a network.