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AC, DC, and Power Transmission

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LAN_403

HV Enthusiast

Sun Mar 26 2006, 12:58AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

wrote ...

Carbon Rod wrote
AC based systems are popular and have well documented solutions. High power DC over long distances has numerous other issues most people have simply forgotten after the 1940s.

Contrary to popular belief, HVDC distribution has been around for over 50 years (since 1954 commercially) and there is a enormous amount of DC distribution systems all over the world.

vasil wrote ...

AC is considered better because you can transform the electric parameters, cheap and easily, after your necesities, any place, anytime. AC transformers can work, in any wild conditions without much survey.

High voltage DC is being used all over the world. There are many advantages over DC transmission vs. AC transmission.
Some of these include:

1. DC can carry much more power than AC for a given conductor size.

2. No limits to the distance it can be transferred (opposed to AC which does have limitations)

3. Active / Reactive power is much more easily controlled without additional equipment in DC lines (AC requires a lot more overhead and equipment to do this . . . with DC it can be controlled via the switching)

4. Power flow direction can be changed easily (not so with AC)

5. Plus there is plenty of data on HVDC lines. The first commercial HVDC line was built back in 1954!!! Its been around for a long time.

Carbon_Rod

Sun Mar 26 2006, 08:37AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

"Contrary to popular belief, HVDC distribution has been around for over 50 years (since 1954 commercially) and there is a enormous amount of DC distribution systems all over the world."

DC distribution to regular home grids was halted a long time back in these parts. Are you saying people buy DC power from the company directly in some places? I have not heard any non-historical and non-private reference. Hmmm... This sounds neat...

Yep, people now even know why these lines hang off centre, get solar flare and capacitor damage. =]

A DC biased AC line is often undesirable as it introduces numerous other issues like transformer problems etc. -- Can get very complex indeed... IIRC one community even had a legacy Tesla power plant rusting away until the 1970's when the local hydro companies river cleanup project literally blew it out of the water.

EVR:
1.) Energy is always conserved
2.) Try it under load... And no cheats with super conductors =D
3.) Its called zero-cross switching in these parts. Its not that complex.
4.) No comment... even with a few beers I still don't know the context =P
5.) Dependent on location. We have had many unique lighting and power systems in mining operations -- some known and others are more obscure.

Dave, "Can anyone shed some light on this matter? "
Phasers, resonance, and magnetics can be tricky topics to explain -- minus the beers. Each system (dredges up a very stale argument) has their strengths and weaknesses depending on the source and application -- I assumed you meant a residential service line based on the context.

Some questions you may want to ask yourself:
a.) Power source: Wind/Water/Solar/Pole etcâ€¦
b.) Source to Service location obstacles
c.) Construction supply costs in your area
d.) Intend to sell the power back to the grid to subsidize maintenance costs
e.) Legal and environmental restrictions

Cheers,

ragnar

Sun Mar 26 2006, 09:26AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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Posts: 1425

Surprising for most people, skin depth in copper is 8mm-9mm depending on your frequency (50Hz vs 60Hz)... not all that deep! (assuming conductivity 1.68E-8 ohm/m, permeability 1.2566E-6 uN/A^2). Plenty of high-tension powerlines are steel-cored with aluminium strands around the outside, so don't use those figures when thinking about real powerlines.

I got a fright when that hit me. I'd genuinely though that skin depth was totally insignificant at ~50Hz.

Wolfram

Sun Mar 26 2006, 01:50PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Guys, we are talking about two entirely different things here. This thread is about DC power transmission, not DC power to households. DC power transmission is common (as stated a lot of times in this thread), but it is of course made back into AC before being transformed down and distributed to households. The fact that they use semiconductors does not limit the voltage (as also stated earlier in this thread), there are infact several 1.2MV lines in use (check the Wikipedia link I posted earlier in this thread), and I remember reading about a 1.67MV DC line between USA and Canada (do not quote me on this).

I hope these misunderstandings are out of the way now.

HV Enthusiast

Sun Mar 26 2006, 02:37PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

wrote ...

EVR:
1.) Energy is always conserved

Wrong. For a given conductor size and length, equivalent resistance of a transmission line (especially over long distances as is common with high voltage power transmission) is always higher for AC vs. DC, mostly due to skin effect. So yes, for a given conductor size, DC lines can carry more power than AC lines.

wrote ...

2.) Try it under load... And no cheats with super conductors =D

Uh wrong again. Due to standing waves, reflections on the line, AC power transmission distances are limited. This is the reason DC power lines are used in areas where extremely long transmission distances are involved.

wrote ...

3.) Its called zero-cross switching in these parts. Its not that complex.

Uh nope. Not even close. In AC lines, this is mostly controlled by switching in and out capacitors on the lines, and in power substations, very large capacitors are switched in and out depending on line conditions. With DC, there is no need for any additional equipment. Control can be done through the actual switching during up and down AC/DC and DC/AC conversion.

wrote ...

4.) No comment... even with a few beers I still don't know the context =P

The power grid is a very complex structure. Depending on circumstances, it is sometimes necessary to divert electrical flow from one generating plant to another, move power from this point to that point, etc.... The power grid is a very complex switching network. AC lines are typically unidirectional so this makes switching a bit more difficult. However, with DC transmission, the lines are effectively bidirectional and power flow direction can be easily switched.

Attached is a diagram of some of the larger HV DC transmission lines located around the world, some in place since the 1950's.

Sulaiman

Sun Mar 26 2006, 03:01PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

What makes an ac line unidirectional ?
seems strange to me.

Carbon_Rod

Sun Mar 26 2006, 09:39PM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

Actually I had asked this question a few years back on the old 4hv for insight into this very problem of syncing AC generators to the service lines. search old.4hv.org

I am getting bored with the â€œdead parrotâ€ rhetorical arguments about apples and oranges...

I canâ€™t tell who is more off topic in this thread now. lol

But if this thread is still alive in a few days I may drop by again.

Good luck,

mikeselectricstuff

Sun Mar 26 2006, 10:24PM

Registered Member #311 Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253

Sulaiman wrote ...

What makes an ac line unidirectional ?
seems strange to me.

It makes perfect sense if you read all the stuff the makers of exotic speaker cables claim... (and can stop yourself laughing...)

HV Enthusiast

Mon Mar 27 2006, 12:37AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Bidirectionality is not a function of the actual wire. Its a function of the entire power system architecture.

Sulaiman

Mon Mar 27 2006, 08:29AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

OK, but what makes a line (or part of the system) uni-directional?
AFAIK wires, transformers, capacitors etc. don't know about directionl,
voltage, current, power, phase etc. etc. meters don't care.

I beliave here in UK (where the country/grid is a small part of a wavelength long)
power stations get into synch. with the grid
connect, and increase power output.
I've always thought of an electrical grid like water and pipes.

So what makes a high voltage ac line/part of a national grid uni-directional?

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