Why dont we use dc supply line for transmitting commercial electricity?

Question

Why dont we use dc supply line for transmitting commercial electricity?

Answer 1

Joe is correct high voltage DC is much more efficient than AC. From wikipedia ( http://en.wikipedia.org/wiki/Hvdc )

"Advantages of HVDC over AC transmission

The advantage of HVDC is the ability to transmit large amounts of power over long distances with lower capital costs and with lower losses than AC. Depending on voltage level and construction details, losses are quoted as about 3% per 1000 km. High-voltage direct current transmission allows efficient use of energy sources remote from load centers..."

Other advantage is that there is no phase sync of generators needed that have caused some major large scale power outages. See http://en.wikipedia.org/wiki/Electrical_...

The reason that it is not widely used in electrical grid systems is that it would cost a lot to replace the existing HVAC grid infrastructure. Hopefully sometime in the distant future we will have a reliable HVDC electrical grid system.

Answer 2

AC current is more efficient to transmit over long distances since there are fewer losses involved in transmitting AC current

Answer 3

http://science.howstuffworks.com/electri...

Answer 4

it is used extensively in russia, and it is becoming more common because most wind and solar stations are using it. read about it here: http://en.wikipedia.org/wiki/High-voltag...

it is a great efficancy improvement over AC. also allot of wind stations are in the ocean. transmitting AC underwater gives very high losses.

DC is THE BEST for power transmition. end of argument. when the original power grids were being built, the technology needed to switch large amounts of DC didnt exist. AC power lines are nothing but really long antennas transmitting their power to the ground (grounding effect). that is why they put high voltage power lines so far above the ground.

DC works great in transformers. my truck does it everyday. the only requirement for electricity to pass through a transformer is a changing magnetic field. every time my points in my distributor fire a spark plug the power surges and DC passes through a transformer. for large scale DC power distribution (like the lines that run int canada to power our missile defense systems) a giant flash bulb is used. when it flashes, the magnetic field changes and power flows.

to answer your question it is because the power systems are out of date. originally DC power grids like the ones tesla designed for edison would require a station about every mile and would use wires as thick as your arms.

here is a pic of the national power grid: http://encarta.msn.com/media_701509077/t...

Answer 5

The simple answer is: line losses.  Remember, W = E*I = I^2*R, so lower I and higher E means less I^2R loss at a given power. So more power can be sent down the wire than with lower voltage, or smaller wire used. This saves costs.

But not only do you need to step up the voltage, you need to step it back down too. With AC this is simple to do using transformers, which have no moving parts, are very efficient, last a long time and are fairly cheap.

With DC this step-up and step-down process is much more complicated and less efficient - motor-generator sets, inverters and so forth, all which have shorter life than transformers.

Also bear in mind they were figuring all this stuff out in the late 1800's so they didn't have the same kind of electronics we have now, but these differences are still largely true.

That said, it's not only theoretically possible to send high-voltage DC, it also has advantages of its own: less capacitive coupling (dielectric) loss, less reactive I^2R loss, no need for synchronization and so forth. See the link below for more details.

Answer 6

Actually we do use DC for power transmission.  A long time ago I toured a facility in minnesota that received DC power from North Dakota.  

If you want a long and fascinating answer about how AC got to become the standard instead of DC, look up the Edison/Westinghouse electricity debate.  It was a battle of wits and politics.  Very interesting.

Answer 7

AC allows simple voltage transformation (up or down) using only a transformer.  DC requires complex equipment (motor generators or inverters) to change voltage.  To deliver electricity over a distance it's voltage must be increased for efficiency.  The voltage must then be reduced for final delivery for safety.  DC is sometimes used for very high voltage, high power transmission where the losses and cost associated with the conversion equipment are made up for by the higher voltage possible using DC and the lack of reactive impedance in the lines.