In addition to the reactive losses Crafter_Man mentioned, AC systems suffer from the skin effect, which causes the current to concentrate more towards the edge of the conducter, rather than being uniformly distributed. DC doesn’t suffer from this loss. Although you don’t normally associate skin effect with frequencies as low as 60 Hz (the losses are neglible unless you’ve got some thick wire or buss bar), it is present. I think certain types of stranded wires are used to help minimize the effect, also.
For large conductors the skin loss actually is significant at 60Hz - the “skin depth” at 60Hz is about 1/3 inch. For 500 MCM wire (rated at about 380 amps in conduit), the 60Hz resistance is 1.8% higher than the DC resistance. 2000 MCM wire has a 24% higher resistance at 60Hz than DC, which might explain why you don’t see it too often
ACSR (aluminum conductor, steel reinforced) wire is common- this is aluminum wire with a steel center. Since not much current is carried in the center of the wire (due to the skin effect), steel is used there to increase strength.
Hollow conductors are sometimes used in high-voltage transmission lines. This reduces skin effect losses, and the larger diameter reduces the corona voltage. The wire dissipates heat faster with a larger surface area, so you can pump more current into, too.
Well it’s been a long time since Electronics class but I do remember a little something about increasing efficency of AC with something called three phase.
In the real world three phase is the most commom way AC is recieved into your house. Thats why there are three wires. Each phase is 120 degrees out of phase with the other two.
That is one reason light bulbs last as long as they do. It smoothes the “AC hum” Actually you are looking at something like 180cycles (oops) hertz.The eye cannot detect that in an incandecent light.
Actually the skin effect does come into effect(can I say that?)with AC high tention lines. They are copper coated steel,hense high tention.
A house wired for DC would lack many of the little luxuries that we have today. Rechargable anything comes to mind first. Although it is possible to set up a lower voltage with DC you would have to set up a voltage divider and would lose most of the 120v to ground.
The electric company transmits the power as three-phase, but all the appliances in your house run on the voltage across two of them, which is single-phase. This includes light bulbs. Some heavy-duty industrial machinery uses three-phase, but you don’t have any of these in your house. Common voltage for three-phase is 208 volts.
then he wrote:
The “tension” in “high tension lines” is just another word for “voltage.” It’s not that the wires are stretched really tight, though they probably are. I think “tension” is how the French say “voltage,” but I don’t know why you often see high-voltage lines called that over here.
If DC is transmitted to your house (or sub-station) at high voltage/low-current, you’re stuck with low current. High voltage can be reduced to a usable level but, unlike AC, this doesn’t increase the amount of current available.
With the right type of DC step-up/step-down circuitry, the amount of current can increase. It’s possible to make a “DC transformer” with power electronics. The challenge is making it as low-loss and cheap as a transformer.
I told you it was a long time ago:O