Question

Why does USA use DC instead of AC

Answer 1

The largest losses in long distance electrical power transmission come from energy lost in the resistance of the power line.
If P is the power transmitted, and R is the resistance of the line:
P=IV
P=IV
Ploss=I^2R=P^2R/V^2

If P is fixed by community demand, then you can reduce lost power dramatically by increasing the transmission voltage. As a result, all long-distance power transmission, AC or DC, is done at high voltage.

The advantage of AC has always been that it is easy to change the voltage up and down with a transformer; DC requires more equipment and some losses to convert.

That being said, transferring AC power between separate grids requires making sure the phase of the power transmitted matches from the two grids (so that the power from the two grids doesn't cancel or ring), which is difficult and expensive. This is not a problem for DC, so DC lines are used in cases such as where power is transferred from another grid to increase the capacity of an existing grid, or between countries that use different frequency power.

Capacitance between the AC phases (usually 3 phases are transmitted at once over a line) or between the line and the surrounding soil or water causes losses that are not a problem with DC. Therefore, undersea high voltage lines tend to be DC.

Overall line loss is also lower per 1,000 km, so very long distance transmission lines sometimes use DC.

To add HVDC (high voltage direct current)is more economically feasible at longer distances- which is important considering the costs of the extra rectification/inversion equipment:

• DC does not suffer from the skin effect- a characteristic of AC that causes more current in the conductor to flow along its periphery than along its core.This increases the effective resistance of the conductor, increasing the line losses and requiring you to use thicker conductors (copper is expensive).
• DC lines can carry more current per conductor. The power delivered through AC lines is determined the RMS (root mean square) of the peak line voltage. The peak line voltage is about 41% greater than the RMS voltage, while the power delivered through DC lines is determined by the peak voltage itself (which is constant).

Also, you don't have to deal with grid or alternator synchronization issues (frequency, voltage or phase order/phase angle) between grids connected through HVDC links- making a cascading blackout less likely on those fronts.

These are the reason why USA use DC current