Short answers, pulled from the story.
Electric power distribution is the final stage in delivering electricity from the power grid to individual consumers. Transmission carries electricity at very high voltages, from 44 kV to 765 kV, over long distances from generating stations; distribution substations step that voltage down to medium voltage (2 to 33 kV) and then to the low voltages used in homes and businesses.
In the United States, residential customers typically receive electricity at 120/240 volts, delivered via a split-phase system. The 120 volt circuits power lighting and most wall outlets, while 240 volt circuits supply high-wattage appliances such as ovens and electric car chargers.
Japan's split frequency dates to the 1890s, when Tokyo's local providers imported 50 Hz generators from Germany while Osaka's providers purchased 60 Hz generators from the United States. The two regional grids grew independently and were never unified. Today Eastern Japan runs on 50 Hz and Western Japan on 60 Hz, with four HVDC converter stations capable of moving up to 1.2 GW between the two grids.
Alternating current won because transformers could step AC up to high voltages for long-distance transmission and back down near customers, giving it far lower transmission costs than DC. Edison's DC system, which ran at 110 volts from generation to end use, required generating plants to be within about 1.5 miles of customers to avoid prohibitively thick copper cables. Edison's own company switched to AC in 1892.
A single-wire earth return system uses one wire to carry current and the ground itself as the return path, minimizing the infrastructure needed to reach remote areas. SWER systems are used in New Zealand, Australia, Saskatchewan in Canada, and South Africa to electrify locations that are too distant for conventional two-wire distribution.
Traditional distribution systems only delivered power from transmission networks to customers. Modern systems integrate distributed generation resources such as solar and wind power at the distribution level, making lines two-way. Balancing supply and demand on these networks, sometimes called microgrids, requires tools including battery storage, market signals, and optimization methods such as particle swarm optimization and genetic algorithms.