cable sizing / panel distance

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Re: cable sizing / panel distance

Postby Gordon-Loomberah » Sat Jul 12, 2014 12:51 pm

Cherokee Solar wrote:The 2 x L - I believe - takes into account that there are two wires (positive and negative) so no need to double the cable length.


Yes that's correct, there is resistance, and the associated current dependent volt drop, in both wires.

There's a vote for mppt regulators...


But not a large enough majority for you? ;)

I think there is slightly more loss in AC than DC due to hysteresis losses, which is probably a fair part of the reason that really long cable runs such as that under Bass Straight and North to South NZ are DC.
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Re: cable sizing / panel distance

Postby Cherokee Solar » Sat Jul 12, 2014 4:13 pm

Gordon-Loomberah wrote:But not a large enough majority for you? ;)


Very funny! :lol:

It all comes down to how much do you want to spend. At what point do I ditch the existing system and change regulators. Plus I just I've spent so long mucking around with them, that the regulators just work now.
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Re: cable sizing / panel distance

Postby davidg » Sat Jul 12, 2014 11:02 pm

Gordon-Loomberah wrote:which is probably a fair part of the reason that really long cable runs such as that under Bass Straight and North to South NZ are DC.

It's due to an effect called "skin effect" which can become an issue in very high voltage AC transmission lines. DC does not suffer the effect at all. An excellent reason why DC is used for long undersea runs and the fact you only need a single cable, you don't have to run in sync at either end etc, the fact is for that type of run the losses are much, much lower, than trying to do it with AC.

For those that don't know what it is here's an explanation - http://en.wikipedia.org/wiki/Skin_effect 8-)
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Re: cable sizing / panel distance

Postby Helipos » Sun Jul 13, 2014 2:50 pm

Skin effect is more a problem for RF cables where you have a rapidly expanding and collapsing magnetic field.

For underground cables losses thanks to the capacitance is a problem for large runs (ie Bass Link)
For overhead transmission the line can appear capacitive or inductive depending on how much current is flowing through it. This can be easily rectified with capacitor or inductors being switched in to the system.

DC Allows each grid to act reasonably independent so they don't have to be synchronised and are able to push power a certain direction through the link with little regard to the AC voltages at each end.
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Re: cable sizing / panel distance

Postby davidg » Sun Jul 13, 2014 5:51 pm

Helipos wrote:Skin effect is more a problem for RF cables where you have a rapidly expanding and collapsing magnetic field.

It is certainly a problem with AC high voltage transmission lines.

From Wikipedia
Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor, and decreases with greater depths in the conductor. The electric current flows mainly at the "skin" of the conductor, between the outer surface and a level called the skin depth. The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor. The skin effect is due to opposing eddy currents induced by the changing magnetic field resulting from the alternating current.

The rest is at http://en.wikipedia.org/wiki/Skin_effect

At higher frequencies the effect increases all true though ;)
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Re: cable sizing / panel distance

Postby dinkum » Sun Jul 13, 2014 8:55 pm

mmmm ! ,so i'm guessing ,
dinkum wrote: " an amp is an amp is an amp "
is an amp..it's quite interesting how a simple question can spark a debate , [ but all good ]..maybe tesla was right when he was vying for dc grid against edisons ac grid....
Cherokee Solar wrote: that the regulators just work now.
why fix it it, if it ain't broke..i like the "kiss " principle , but i have to change , due to copper being so expensive these days... thanks all...dennis.
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Skin Effect

Postby Tracker » Mon Jul 14, 2014 8:10 am

..
I had understood from my earliest lessons, that skin effect applied to high frequencies..
This is likely why Optus street cable has a copper plated steel core..
I would not have thought that 50Hz would have any skin effect, and then i think of the typical multistrand coonductor and wonder where any skin effect might take place ... at the skin of the strand or the skin of the bundle..
IMHO skin effect should not be considered at the voltages and frequencies we are talking about..
..
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Re: cable sizing / panel distance

Postby Helipos » Tue Jul 15, 2014 9:13 am

Skin effect will still be there, but its resulting impact will be minimal.

http://www.siemens.com/press/pool/de/events/2012/energy/2012-07-wismar/factsheet-hvdc-e.pdf

A quote from the link above
Cable links longer than approximately 80 km are only possible with HVDC transmission. That’s because for underground or submarine cables, hardly any electricity is delivered when AC lines are 80 km or longer (The cable capacities absorb the usable electricity). For an HVDC transmission link using submarine cable, such as the one planned between Scotland and England at 600 kV and 2,200 MW, there will be an energy loss of less than 3% in total (including cable and converter losses).
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Re: cable sizing / panel distance

Postby Helipos » Tue Jul 15, 2014 6:02 pm

Continuing from my previous post.

I'd read that to mean the PF will get soo terrible it would need an uneconomical about of reactance added to keep the PF close to 1.
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Re: cable sizing / panel distance

Postby Tracker » Wed Jul 16, 2014 8:40 am

Helipos wrote:.......I'd read that to mean the PF will get soo terrible it would need an uneconomical about of reactance added to keep the PF close to 1.


Would that explain the odd device that I have seen on some country poles .. something that looks like a transformer, a canister with insulator connectors, with links to the main conductors but no outer output.. :?:
..
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