Short answers, pulled from the story.
The Grand Coulee Dam has a 6809 megawatt nameplate generation capacity. Six circuits of five-hundred kilovolt lines carry this entire power output from the hydroelectric plant in Washington State.
Lucien Gaulard and John Dixon Gibbs built an early transformer called the secondary generator in 1881. This invention preceded the first long distance alternating current line which stretched one mile for the 1884 International Exhibition of Electricity in Turin, Italy.
Efficient long-distance transmission requires high voltages to reduce losses produced by strong currents. Transformers change voltage levels for both transmission and distribution phases to maintain efficiency across vast distances.
Submarine connections up to one hundred kilometers in length connect electricity grids between islands like Great Britain and continental Europe. These submarine HVDC systems interconnect grids without the limitations imposed by cable capacitance on alternating current systems.
The US Northeast faced blackouts in 1965, 1977, 2003, and other regions experienced failures in 1996 and 2011. Transmission companies determine maximum reliable capacity of each line typically less than physical or thermal limits to prevent such events.