why low current ,high voltage transmission is possible only with AC?

Answer 1

It is possible. However, I think the gist of your question is why do we use high voltage AC transmission. This is an interesting subject. More than 100 years, Edison and Westinghouse were in a bitter battle. Edison wanted DC power to homes and Westinghouse wanted AC.

Technically, DC isn't the way to go. Here's why. The main reason power is lost in transmission lines is the I squared R loss. So the small resistance of the wire is multiplied by the current SQUARED and that's the loss in watts. So to get, say, 100 Watts somewhere, the voltage times the current at the destination has to be 100 (I'm simplifying by assuming the current and voltage are RMS). If the wires were perfect (they aren't) you could do any of the following:

1000V @ .1A
100V @ 1A
10V @ 10A
1V @ 100A

Which would have the lowest I squared R loss? Obviously the 1000V, right? But you don't really want high voltages in peoples house where they are going to shock themselves. Sp an AC distribution system has very high voltages on the longest lines, pretty high voltages on long lines, and low voltage going the short run from the pole to your house. This is easy to do with a transformer. There is some loss in a transformer, but not much.

Transformers don't work with DC. Today we have the technology to convert DC voltages pretty efficently (switching regulators) but it is not as cheap as a transformer, and would have been totally impractical even 50 years ago. So with a DC system you have to pick as high a voltage as you can stand and pipe it into people's homes and "eat" the I squared R loss.

I'd like to tell you Westinghouse won the day because he was right. But in fact, the big factor was the state of NY (I think it was NY) executed a man with DC current, and reports were he jerked and smouldered and all sorts of bad things. The public concluded that DC was "dangerous" -- AC would have killed him about the same, but public opinion being what it is... that execution pretty much "killed" DC distribution in the US.

Answer 2

One thing that we should know is that power cannot be created nor destroyed, and since power (P) is Voltage(V) * current (I).
increasing one leads to a decrease in the other so that the product will remain constant.
During transmission, power loss is minimized by transmitting at a very high voltage (remember this reduces Current ) and so since power loss in the transmission lines I^2* R ( current squared times Resistance), will be considerably low.
This Advantage is possible because of the AC currents only, because transformers can be used to change the voltages, through induction.
This is not possible with DC because it does not vary at all

Answer 3

The power loss in transmission line is directly proportional to the square of the current. So the lower the transmission current the lesser will be the power loss. For a given power transmission voltage is inversely proportional to the current as Power = Voltage*Current. For a fixed power the higher the voltage the lower the current. However what you have learned is that the current through a particular component is proportional to the voltage in accordance with Ohm's law, Current is proportional to Voltage. In this case the power must not remain fixed or constant.

Answer 4

Advantages of high voltage transmission
Early electric power distribution schemes used direct-current generators located near the customer's loads. As electric power became more widespread, the distances between loads and generating plant increased. Since the flow of current through the distribution wires resulted in a voltage drop, it became difficult to regulate the voltage at the distribution circuit extremities.

Higher voltages reduce the transmission power loss or reduce the cost of conductors when transmitting a given quantity of power since a smaller current is required. Conductor cost is roughly proportional to the current carried, and conductor loss is roughly proportional to the square of the current, so higher transmission voltages improve the efficiency of transmission.

Low voltage is convenient for customer loads such as lamps and motors. The principal advantage of AC is that it allows the use of transformers to change the voltage at which power is used. Manipulation of DC voltages is considerably more complex, and has only become economically feasible more recently in history, along with the development and advance of high power semiconductor devices-Thyristors, IGBTs, MOSFETS, GTOs, etc.

With the development of efficient AC machines, such as the induction motor, AC transmission and utilization became the norm (see War of Currents).

Answer 5

It is also possible with DC

"HVDC
High voltage DC (HVDC) is used to transmit large amounts of power over long distances or for interconnections between asynchronous grids. When electrical energy is required to be transmitted over very long distances, it can be more economical to transmit using direct current instead of alternating current. For a long transmission line, the value of the smaller losses, and reduced construction cost of a DC line, can offset the additional cost of converter stations at each end of the line. Also, at high AC voltages significant amounts of energy are lost due to corona discharge, the capacitance between phases or, in the case of buried cables, between phases and the soil or water in which the cable is buried. Since the power flow through an HVDC link is directly controllable, HVDC links are sometimes used within a grid to stabilize the grid against control problems with the AC energy flow. One prominent example of such a transmission line is the Pacific Intertie located in the Western United States."

Answer 6

DC is difficult to put on a transmission grid because of IR losses. AC can be stepped up using transformers, negating or minimizing the effects of IR loss. That's the simple answer. The design of transmission systems is a real art.

Answer 7

AC is used because it is more efficient in power transmission, DC causes the wires to heat up. Also, DC is harder to compensate for line loses over long distances.

Answer 8

In conversion of a.c. into low current , high voltage via transformer the energy loss , mainly in the form of heat , is negligible while in case of d.c. the heat loss in the conversion via rheostat is very high enough , because transformer doesn't works on d.c. !!

Answer 9

Since there is no easy way to step up or down DC electricity.
Alternating current allows transformers to work the way they do.

Answer 10

because very smart people say so - so just accept it and move on to better discoveries