Why Is AC-Current More Efficient Over Long Distances ?

[Magluvin]

AC isn't more efficient to transfer than DC.  DC is.

AC suffers inductive and capacitive losses as well as skin-effect and proximity-effect.
AC is easier to transform up and down, though, but that has nothing to do with distance.

Skin effect is more at high freq, not at 60 or 50hz, I believe.

Havnt tried it yet but say if we have to run an extension cord 100ft from a 120v AC outlet to run a 300w halogen work light.  If we replaced the extension cord with 2 separate wires that were say 3ft apart for the 100ft, would there be any change in power at the light in these 2 situations?  Lets exclude the 3rd ground wire for this idea.

I am thinking that when the 2 wires are close together, even twisted pair as in the extension cord, would transfer the power to the light better than the separated ones, and the mutual induction of each wire to the other compliment current flow, where the separated wires would have to deal with their own self induction possibly reducing the output to the light in comparison.

Mags

[tagor]

https://en.wikipedia.org/wiki/HVDC_Cross-Channel

2000 MW system (1986)Because the first installation did not meet increasing requirements, it was replaced in 1975–1986 by a new HVDC system with a maximum transmission rating of 2,000 MW between France and Great Britain, for which two new converter stations were built in Sellindge (UK) and in Bonningues-lès-Calais (Les Mandarins station), near Calais, (France). Unlike most HVDC schemes, where the two converter stations are built by the same manufacturer, the two converter stations of the 2,000 MW scheme were built by different manufacturers (although both have subsequently become part of the same parent company, Alstom). The Sellindge converter station was built by GEC^[3] and the Les Mandarins converter station was built by CGE Alsthom.
This HVDC-link is 73 kilometres (45 mi) long in route, with 70 kilometres (43 mi) between the two ends. The undersea section consists of eight 46 kilometres (29 mi) long 270 kV submarine cables, laid between Folkestone (UK) and Sangatte (France), arranged as two fully independent 1,000 MW Bipoles, each operated at a DC voltage of ±270 kV. Cables are laid in pairs in four trenches so that the magnetic fields generated by the two conductors are largely cancelled. The landside parts of the link consist of 8 cables with lengths of 18.5 kilometres (11.5 mi) in England, and 6.35 kilometres (3.95 mi) in France.^[4]
In common with the 1961 scheme, there is no provision to permit neutral current to flow through the sea. Although each station includes an earth electrode, this is used only to provide a neutral reference, and only one of the two electrodes is connected at a given time so that there can be no current flow between them.
The system was built with solid-state semiconductor thyristor valves from the outset. Initially these were air-cooled and used analogue control systems but in 2011 and 2012 respectively, the thyristor valves of Bipole 1 and Bipole 2 were replaced by more modern water-cooled thyristor valves and digital control systems supplied by Alstom.^[5]
This system remains the world's largest-capacity submarine cable HVDC system.[citation needed]
In November 2016 during Storm Angus a ship dragging an anchor cut four of the eight cable components, reducing capacity by 50%.^[6] It is expected that repairs will be completed by February 2017

[Doug1]

Skin effect is more at high freq, not at 60 or 50hz, I believe.

Havnt tried it yet but say if we have to run an extension cord 100ft from a 120v AC outlet to run a 300w halogen work light.  If we replaced the extension cord with 2 separate wires that were say 3ft apart for the 100ft, would there be any change in power at the light in these 2 situations?  Lets exclude the 3rd ground wire for this idea.

I am thinking that when the 2 wires are close together, even twisted pair as in the extension cord, would transfer the power to the light better than the separated ones, and the mutual induction of each wire to the other compliment current flow, where the separated wires would have to deal with their own self induction possibly reducing the output to the light in comparison.

Mags

Look up ,up at the transmission lines. See any twisted pairs? See any twisted pairs going from HV high towers to the substations? You have to also realize there will be some greater measure of magnetic interference with other conductors in a non cancelled condition which might not be too healthy.

[mscoffman]

These above items reminds me of when they first started putting Celluar Phone antennas on active high voltage AC power distribution towers at least around here.
That has to be as tricky as heck because the dimensional control of the cell radio feed has to do duplexing (transmitting and receiving nearby frequencies at the same time)
means they have to run signals through copper pipe like conductors. If lightning triggered power discharges into the RF radio equipment I doubt it would last very
long. Also even the small continuous static discharges have got to be radio noisy. I have to salute the designers who first thought they could do this and the technicians
that work on this type of equipment.

[Jeg]

Also Leedskalnin has said something related to this.

"You have been wondering why alternating currents can run so far away from their generators. One reason is between every time the currents start and stop there is no pressure in the wire so the magnets from the air run in the wire and when the run starts there already are magnets in the wire which do not have to come from the generator, so the power line itself is a small generator which assists the big generator to furnish the magnets for the currents to run with. I have a generator that generates currents on a small scale from the air without using any magnets around it."