Most power is transmitted by overhead ____ lines, with ____ being used for special purposes.

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Overhead AC lines carry the bulk of transmitted power because alternating current can be easily transformed to very high voltages for long-distance transmission and then stepped down again for distribution. Pushing power at high voltages means lower current for the same power, which reduces I^2R losses and makes the system more efficient. AC also fits naturally with the way generators and motors are designed and with standard voltages used throughout the grid.

Direct current is reserved for special cases. High-voltage direct current transmission is advantageous for very long distances or undersea/underground cables, and for interconnecting grids that do not operate at the same frequency. DC has lower losses in some long-haul scenarios and allows precise control of power flow, but it requires converter stations at both ends to convert AC to DC and back, which adds cost and complexity. That’s why DC is used for those specific applications rather than general overhead transmission.