OT? Long Distance High Voltage Electricty Transmission

railfancwb · Aug 31, 2006

Before I left on vacation there was a thread about high voltage long distance electrical transmission. I presume it is gone now, at least for awhile, since "General" is temporary.

Anyway, someone said that real high voltage was now being converted from AC to DC, transmitted across country, and converted back to AC at the other end.

I semi-watched high voltage long distance transmission lines while driving around out west, and all I noticed were three or six wire plus ground (ground being that top light weight wire).

So my question is...would DC transmission need three wires? I know a six wire line might be three two wire DC circuits...

Thanks, Charles

cruzinonline · Aug 31, 2006

The only strand I know of is from Delta Utah (Intermountain Power) to Los Angelos Ca. The plant is operated (maybe even owned) by the LA Water Department. It's the only plant I know of that transmits DC. As far as the number of lines, "perhaps" multiple lines (in multiples of 2) would be required for the volume of power.

USMCPOP · Aug 31, 2006

http://www.answers.com/topic/high-voltage-direct-current

Evan · Aug 31, 2006

Hydro Quebec has a 1200 km long high voltage DC line (450 kvdc) from James bay to Nicolet substation in Quebec. BC Hydro has a DC undersea cable from the mainland to Vancouver Island. There are also six DC interties in the US. The longest is the Pacific intertie from Oregon to Southern Ca. It is 1362 km long at 400kvdc.

(ground being that top light weight wire).

That wire is for lightning protection.

Orrin · Aug 31, 2006

Whenever you see a transmission line it will most likely be three phase AC, thus three conductors.

The DC lines are few and far between. When you do see them, they'll have two conductors.

Here in the Pacific Northwest we have the line that runs from Celilo, Oregon, (near The Dalles) to Sylmar, California.

I've heard of one DC inter-tie that begins and ends in the same switchyard. It facilitates interconnection between two separate AC systems without having to synchronize them.

When I worked at the Fort Peck Dam in Montana, we had two separate AC systems terminating in our switchyards, the Eastern US and the Western US.

Shortly before I arrived there attempts were made to synchronize between the two systems. It never worked. It seemed to take forever to synchronize the systems. Then, when they were and as soon as the circuit breakers were closed at Fort Peck and Stegall, Nebraska, the breakers would pop right back open, again.

The two systems were far too massive and enormous power swings would overload the circuit breakers. A DC intertie doesn't care if the systems are synchronized, or not.

Regards,

Orrin

aboard_epsilon · Aug 31, 2006

I thought it was Tesla who brought in AC so that power could be sent over long distances ...

and Edison could not make DC travel that far ...without problems ..
now your all saying they are reverting back again. :confused:

All the best.mark

rayman · Aug 31, 2006

Looking on a map where I work that shows power plants and distribution lines, the DC lines in USA are:

Coal Creek plant in ND to central WI

Young Plant to Superior, WI

Reading, MA up into Quebec and off the map to the north

The Dalles Plant in north central Oregon down to north of the L.A. CA

High Desert plant southwest of Barstow, CA to west central Utah through Las Vagas

And in Canada, Winnipeg to north off the map.

jmp · Aug 31, 2006

Check out the book "Networks Of Power" by Thomas P. Hughes. subtitled Electrification in western society, 1880-1930.

Evan · Aug 31, 2006

Mark, the difference is the voltage. Losses are a product of the current. The higher the voltage the lower the current for a given amount of power. The problem Edison faced was he had no way of converting low voltage DC to high voltage for transmission and back down to low voltage again. With the development of high power high voltage thyristors that has changed and it is now economical and relatively easy to do the conversion as well as converting from DC to AC to DC etc.

macona · Aug 31, 2006

aboard_epsilon: DC is inefficent at low voltages bit good at high and the problem is that in the past it has been easy to get HV DC but there was no real way to get it back to AC. Now with newer tech they can do it. AC at high voltages has a skin effect where most of the current only passes through the outer portions of the conductor. Thats why most buss bars in substations and power plants are actually hollow tubing. I think oild is sometimes pumped down the center for cooling, but I am not sure.

440roadrunner · Aug 31, 2006

"AC at high voltages has a skin effect where most of the current only passes through the outer portions of the conductor."

That is absolutely NOT true. AC skin effect is a function of FREQUENCY not VOLTAGE The AC skin effect at the low freq used in power lines is negligible.

Evan · Aug 31, 2006

That is absolutely NOT true. AC skin effect is a function of FREQUENCY not VOLTAGE The AC skin effect at the low freq used in power lines is negligible.

What you say is not so. However, it isn't the voltage that matters, it's the frequency. When dealing with high power transmission the skin effect at 60 hz matters a lot.

In a copper wire, the skin depth at various frequencies is shown below.
frequency δ
60 Hz 8.57 mm
10 kHz 0.66 mm
100 kHz 0.21 mm
1 MHz 66 µm
10 MHz 21 µm

In Engineering Electromagnetics, Hayt points out that in a power station a bus bar for alternating current at 60 Hz much more than 1/3rd of an inch (8 mm) thick is wasteful of copper, and in practice bus bars for heavy AC current are rarely more than 1/2 inch (12 mm) thick except for mechanical reasons. A thin film of silver deposited on glass is an excellent conductor at microwave frequencies.

http://en.wikipedia.org/wiki/Skin_effect

chuckey · Aug 31, 2006

In the UK we use cheap off-peak nuclear generated power from France, this is transmitted via a DC link. The next time you are over here you will detect a faint smell of garlic when you turn on the lights!
frank

lathehand · Aug 31, 2006

I live in the San Francisco Bay Area and every time I drove east I looked for the DC intertie: I thought it would be in the Central Valley. I was visiting friends in Nevada and we got the motorcycles out and rode the power line roads. At one stop I looked at the high voltage circuit and I'll be damned, it had only two wires. The Pacific Intertie runs through western Nevada.

Carl

jim rozen · Aug 31, 2006

So, I take it that for a 60~ power transmission
line that has conductors larger than about 20mm
in diameter, they make them out of tubing.

How do they put the holes in the center of
those long wire???

And who's in charge of QC to be sure the
hole goes all the way from one end to the
other????

Jim

aboard_epsilon · Aug 31, 2006

Thanks guys

Is high voltage dc just as likely to kill you .

would it tend to jump a spark further than AC ...if you were under them lines on a wet day.

or don't it have a ground

ps if it wasnt as dangerous would it not be easy to steal it ..by putting a coil buried underneath of them conductors ....is the magnetic field greater than ac.

all the best...mark

Evan · Aug 31, 2006

So, I take it that for a 60~ power transmission line that has conductors larger than about 20mm in diameter, they make them out of tubing.

Nope. The 500kv line that runs through here is multiple cables for each phase. They are arranged at the corners of a cube with X spreaders to hold them in place/apart. That gives 12 "wires" for the three phases, 4 lines per phase. Also, the wires do in effect have a "hole" in them. The wires are multistrand electrical aluminum wrapped around a steel cable core. They have a pretty large surface area.

Evan · Aug 31, 2006

Mark,

A dc field won't induce a current in a coil. The field has to be time varying.

As for deadliness, DC is more dangerous as it clamps muscles without letting up.

As for arcing, free air is a diode when ionized. Negative potentials will arc much easier than positive potentials. The difference to ionize free air is about 1000 volts easier for negative to positive than positive to negative. The voltage it takes depends on the potential difference and the gap as well as how concentrated the potential is. It's much easier to ionize air from a point than a large area.

440roadrunner · Aug 31, 2006

"As for deadliness, DC is more dangerous as it clamps muscles without letting up"

NOT TRUE

It is AC that causes your muscles to "clamp" at some voltages.

DC almost always causes your muscles to violently "kick"

440roadrunner · Aug 31, 2006

Evan quoted a skin effect depth:

"In a copper wire, the skin depth at various frequencies is shown below.
frequency δ
60 Hz 8.57 mm"

First , that figure is the DEPTH, not the diameter. What that means, is that a conductor can be at least TWICE that figure in diameter before skin effect becomes even somewhat more than negligible.

Furthermore, (and you may know this) skin effect is NOT calculated based on the ENTIRE DIAMETER of a bundle of stranded conductors, but ON EACH INDIVIDUAL STRAND, at least for the huge conductors on a HV transmission line. For smaller, fine wound stranded wire, the individual strands start to "act" more like solid conductor wire

This means that you could have a bundle of stranded cable, each strand of which was plus or minus some 18mm FOR EACH STRAND, with who knows how many of each strand twisted into the complete cable, and the skin effect would be negligible.

This is why there is such a silly arguement over high power speaker wire. If you are using, say 6 gauge speaker wire in your kilowatt speaker system, as long as the wire is stranded copper, then even at the 25Kilo-hertz that "you wish you could hear" the skin effect on that cable would be meaningless.

(All these ads about silver plated speaker wire is just hype.)

OT? Long Distance High Voltage Electricty Transmission

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