How does current flow in a transmission line?

We know that current flows when there is voltage difference, but in power systems the voltage is same at both ends. So how does the current flows in a transmission line?

The best way to think about this is to picture the distributed model of a transmission line. Each capacitor is charged as the signal moves down the transmission line. I like to think of the charge like an ice tray on an angle that you are filling from one side.

DC current will flow straight down the transmission line.

You should be careful to note that in this context you seem to not be talking about a signal transmission line, which is what Joe answered about. Such a transmission line is specifically not a lumped system.

You seem to be talking about "transmission lines" such as transfer power between generating stations and the customers miles away. That's a totally different issue.

The basic answer to your question is that the voltage is NOT the same at both ends. A power transmission line might be labeled as "400 kV", and that will be roughly true. But, these transmission lines have non-zero resistance (except some very specialized superconducting ones). They will therefore develop some voltage accross them when current flows. At this level of detail, the transmission line is just a resistor and Ohm's law applies.

High voltage is used for such transmission lines because that reduces the current for the same power. That's desirable since the losses are proportional to the square of the current. And there ARE losses due to the resistance of the transmission lines, and they do get warm. It's a issue the power companies are well aware of and carefully trade off with other things, like the cost of thicker cable, other losses that start to increase at very high voltages, etc.

ok,
thank you for the answer. but is it related to the angle between current and voltage? or depends on the load angle of the alternator?

You are overthinking this. At 60 Hz, even 100 miles of high voltage transmission line is a small fraction of a wavelength and you can think of it largely as a resistor with some inductance in series. Angle between the current and voltage is more a issue between the generator at one end and the load at the other.

Transmission lines mostly use three-phase alternating current (AC), although single phase AC is sometimes used in railway electrification systems. High-voltage direct-current (HVDC) technology is used only for very long distances (typically greater than 400 miles, or 600 km); submarine power cables (typically longer than 30 miles, or 50 km); or for connecting two AC networks that are not synchronize

The voltage is only really the same at both ends when there is no current which is almost never the case in a transmition line. The line is laced with capacitors to compencate for power lag making it even harder to see and as you know the voltage is bumped up so high as to make the current seem tiny. If you think about what happens in a brownout where there is a really huge power drain you can see the voltage is not truely the same at both ends.

ok, thanks

ok,thanks

Current does not flow, charge flows.

Current is charge flow. Therefore saying "current flow" is the same as saying "charge flow flow."

That's just pedantic silliness. Get over youself.

Common usage defines the language. Yes, the charges flow. But in english at least it's also fine to say the current flows. What verb would you use to describe what the current is doing otherwise, "exists"?

The same thing holds true for a river. It is the water that flows and you can say "the water is flowing south". But it's also perfectly fine to say "the river flows south", or "the current is flowing south". If you want to be so pedantic you should object to the first too because the whole river isn't actually flowing, only the water in it. The river stays in the same place.

Words can have multiple meanings and accepted usages.

Charge flows. Current is.