AC vs DC Transmission Lines

technoextreme

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technoextreme
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Just wanted to make sure that the assertion that AC is better than DC at long distance transmission is wrong. DC is better at transmission but the technology didn't exist at the time to actually step the voltage up. I inadvertently started a huge debate about this and I just wanted to make sure that I am correct. Common sense would dictate that if places like New York were installing high voltage DC transmission lines then there is something to use DC as opposed to AC. The physics dictate that you wouldn't loose energy due to the dielectric looses. Also, is DC equipment more explosive (?) than AC equipment? I've heard about transformers blowing up but I am completely unfamiliar with large scale DC equipment. I know DC is safer than AC for humans and in fact in the United States the AC is the best possible frequency to kill someone with.
 
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From what I learned, AC is easier to produce, it can go farther with less help, and it is easier to convert AC to DC.
 
technoextreme said:
Just wanted to make sure that the assertion that AC is better than DC at long distance transmission is wrong.
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Using DC transmission, you would not have to worry about losses due to e.g. power factor, or skin effect.

Disadvantages that come to mind are the obvious issues as far as stepping up/down of the voltage (that may be less of a problem today with the advancements in high power electronics.)

Then I would assume there are considerations with regards to the directional nature of currents and electric fields causing the transfer of charged particles (e.g. ions), resulting in e.g. corrosion - or simply the accumulation of (somewhat conductive) gunk on insulators.

I have no idea as far as the magnitude of these effects in practical applications, though.
 
DC is very handy for long distance power transmission because it’s more efficient, but AC is easier to distribute and slightly easier to produce because there are fewer points of failure (no brushes, etc).
 
There was a thread on this forum a couple of years ago on this subject (maybe someone with better forum search skills than I can locate it), or at least it touched heavily on long distance high power DC transmission lines used in Europe and/or Britain and even as submarine cables.

IIRC, there was lots of good references and even photos of the solid state conversion (SCR/transistor) used where they joined the AC grids at each end. Seems one big advantage was not having to synchronize the joined systems.

Cheers,

Dave
 
Well, since upgrading the national grid is a big deal right now, perhaps we will see DC run to local substations for conversion to AC. That would probably make some difference, at least.
 
Fronzel said:
From what I learned, AC is easier to produce, it can go farther with less help, and it is easier to convert AC to DC.
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Yeah I know that. I haven't kept up enough with DC conversion to know what is what. It seems like it is useful for underwater applications.
 
Then there’s the matter of what it’s being used for. In the late 1800s to early 1900s electricity was mostly used for lighting and simple motors. AC motors are more reliable and efficient than DC motors.
 
TheDaver said:
Then there’s the matter of what it’s being used for. In the late 1800s to early 1900s electricity was mostly used for lighting and simple motors. AC motors are more reliable and efficient than DC motors.
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Generally, DC motors are more efficient, especially PM DC motors.
 
DC transmission offers the ability to transmit bulk quantities of electricity without reactive losses and thus voltage drop at the receiving end. For this reason it has often been used on the very highest and longest transmission lines, where the cost of installing reactive compensation to maintain the voltage would be prohibitive.

Underwater lines are, for obvious reasons, made from insulated cables rather than exposed metal. Power cable is naturally capacitive, that is, it generates reactive power. Lengthy cables would, if AC, generate so much reactive power that there would be little capacity remaining on the cable for the desired active power.

As CaveDave says, DC transmission lines free transmission system operators from having to synchronise their systems. It is highly doubtful, for example, that the French and British grids could be coupled stably together via only the single dual 1000 MW link that currently connects them.

DC is particularly controllable -- the convertors act as a massive load at one end of the link, and an equally massive generator at the other. With the changing conditions that grid operators are facing, such as more volatile renewable-based generation markets, DC transmission is looking increasingly appealing to large system operators. My company is exploring the impact of restringing major UK transmission lines as DC.
 
technoextreme said:
I thought that was backwards. I know Ben Burch has the answer. AC motors are more efficient at least if you want to use them in electric cars.
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High power motors with advanced technology, where the speed and load change, are most efficient when driven with variable freq, variable voltage AC. High power fixed speed AC 60hz motors are also quite efficient. The most common AC induction motors (.1 to 2 hp) aren't all that efficient compared to similar power brush commutated PM DC motors. It really depends on usage. Those in servo control apps such as golf carts, robotics, and such have traditionally used DC PM motors since they are both efficient and easily controlled. This is however changing and the efficiency crossover point continues to drop.

I think virtually all electric car designs (50-200 hp) are using high end variable freq/voltage electronic drives for high efficiency. They are quite different animals than the ubiquitous AC motors common around the house. But they too will eventually change.
 
BillC said:
As CaveDave says, DC transmission lines free transmission system operators from having to synchronise their systems.
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This is likely to be a major point for the future of power grids. AC is fine when you've got relatively few large power stations generating power, but when you start throwing in hundreds of small wind turbines, solar cells, wave machines and so on the management requirements go through the roof. It may well turn out to be cheaper to switch to DC rather than try to adapt existing grids to cope with renewable power.
 
technoextreme said:
I know DC is safer than AC for humans and in fact in the United States the AC is the best possible frequency to kill someone with.
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This was a myth started by Thomas Edison because felt threatened by the work of Nikola Tesla and tried to discredit everything he did, even to the point of publicly electrocuting animals with AC current to demonstrate that, "See? AC is dangerous!" :oldroll:
 
Psi Baba said:
This was a myth started by Thomas Edison because felt threatened by the work of Nikola Tesla and tried to discredit everything he did, even to the point of publicly electrocuting animals with AC current to demonstrate that, "See? AC is dangerous!" :oldroll:
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It's not a myth. The impedance of the human body is lowest at around 60Hz.

The let-go threshold for 60Hz AC is about 15mA. For DC it's about 75mA.

At 60Hz AC, 60mA through the heart can induce ventricular fibrillation. For DC, 300-500mA is required.
 
Psi Baba said:
This was a myth started by Thomas Edison because felt threatened by the work of Nikola Tesla and tried to discredit everything he did, even to the point of publicly electrocuting animals with AC current to demonstrate that, "See? AC is dangerous!" :oldroll:
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Its a myth that happens to be true. :D Edison was full of ******** when he said AC is more dangerous but it doesn't discount the fact that 60Hz AC is more dangerous. The let go current goes up as the frequency goes up though.
 
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Given enough current, either one will kill you just as dead. Saying that DC is "safer" than AC is a bit like saying that TNT is safer than Dynamite. It depends on how you define "safe" I guess.
 
The DC is safer than AC assertion goes all the way back to Edison. He held public electrocutions of animals to support it. DC does require much larger conductors for a given current, as it tends to travel through the center and AC travels more over the surface. You can shrink an AC conductor by increasing surface area(stranded vs. solid), but not a DC conductor.

The biggest reason why DC distribution died out is that there was a copper shortage at the time. The conductors were huge, mains distribution used 3" diameter solid copper. It had to be buried beneath the streets. The individual lines that fed into residences were around 1" diameter. Using that much copper today would be insanely expensive.

EDIT: I've been looking for photos of the hardware that was used, and haven't had much luck. Some years back the magazine Engineering In Miniature did a series on it, and printed some amazing photos of the grid's installation in New York and the junction boxes used to feed the house.
 
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wackyvorlon said:
DC does require much larger conductors for a given current, as it tends to travel through the center and AC travels more over the surface.
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The so called "skin effect" is not allegedly very important for typical household wires with AC currect at 60Hz. The skin effect only works for very thick wires or for high frequency AC. At least according to this page: Electricity Misconceptions - Electric Charges only flow on the surfaces of wires? Wrong.

PS. The full index page of electricity misconceptions: http://amasci.com/miscon/elect.html
 
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http://www.answers.com/topic/skin-effect

DC had to be sent long distances at the voltage used by the end user, since it cannot be stepped down by transformers. AC can be sent at high voltage to the neighborhood transformer at lower current, then transformed to lower voltage (220) at high current. This means that the long lines can be thinner than they would be for DC, since the current is lower, and resistive losses are less.
 
rsaavedra said:
The so called "skin effect" is not allegedly very important for typical household wires with AC currect at 60Hz. The skin effect only works for very thick wires or for high frequency AC. At least according to this page:
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Wrong.:) Namely because of the faulty premise that AC power lines only operate at 60Hz. That inductive kick that you get from turning on a vacuum is actually a much higher frequency.
 
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rsaavedra said:
I'm not wrong, the page does says so, which is what I claimed. The page might be wrong though :)
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Actually, the page isn't technically wrong. It just engaged in the same bad science generializations that it is railing against. For every system that exisists you can have two different states you can measure its reaction from. One reaction is fleeting which is called a transient while the other is steady state. I don't think there is any gaurentee that the transient will be a higher frequency.
Given enough current, either one will kill you just as dead. Saying that DC is "safer" than AC is a bit like saying that TNT is safer than Dynamite. It depends on how you define "safe" I guess.
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Errrr.... If I remember the phsyiological differences correctly you'd have to be a moron/suicidal to electrocute yourself with DC electricity. Depending on what source you read there is no current from which you can release your grip upon.
The DC is safer than AC assertion goes all the way back to Edison. He held public electrocutions of animals to support it.
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Would you stop bringing up Edison in this discussion. It has nothing to do with Edison. Animals? Try humans.
 
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wackyvorlon said:
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DC does require much larger conductors for a given current, as it tends to travel through the center and AC travels more over the surface.
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This is complete and utter rubbish. The impedance of a piece of copper wire goes up with frequency. I.e. the losses in it are lowest at DC (= 0Hz AC) and rise with increasing frequency.

http://en.wikipedia.org/wiki/High-voltage_direct_current

Advantages of HVDC over AC transmission

Reducing line cost. HVDC needs fewer conductors as there is no need to support multiple phases. Also, thinner conductors can be used since HVDC does not suffer from the skin effect
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You can shrink an AC conductor by increasing surface area(stranded vs. solid), but not a DC conductor.
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How do conductors know if they're AC or DC?

The biggest reason why DC distribution died out is that there was a copper shortage at the time. The conductors were huge, mains distribution used 3" diameter solid copper. It had to be buried beneath the streets. The individual lines that fed into residences were around 1" diameter. Using that much copper today would be insanely expensive.

<snip>
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This is just wrong. AC won because it can be stepped-up and down relatively easily using transformers.

ETA: ...which allows thinner conductors to be used because the losses in a conductor are proportional to the square of the current flowing in it. Thus it is more efficient over long distances to step the voltage up / current down at the generator side and the voltage down / current up at sub-stations.
 
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Ivor the Engineer said:
This is complete and utter rubbish. The impedance of a piece of copper wire goes up with frequency. I.e. the losses in it are lowest at DC (= 0Hz AC) and rise with increasing frequency.
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You are speaking of losses due to inductive reactance. That is completely different from what I am speaking of. I was addressing skin effect, which influences current carrying capacity of the conductor.
 
wackyvorlon said:
You are speaking of losses due to inductive reactance. That is completely different from what I am speaking of. I was addressing skin effect, which influences current carrying capacity of the conductor.
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http://en.wikipedia.org/wiki/Skin_effect

The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the "skin" of the conductor. The skin effect causes the effective resistance of the conductor to increase with the frequency of the current. Skin effect is due to eddy currents set up by the AC current.
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wackyvorlon said:
You are speaking of losses due to inductive reactance. That is completely different from what I am speaking of. I was addressing skin effect, which influences current carrying capacity of the conductor.
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No, he is speaking of the skin effect. AC can *only* travel near the surface of a wire, while DC can use the entire thing. The depth into a conductor current can travel is inversely proportional to it’s frequency, which is why DC can use the entire conductor. IIRC at 60Hz skin depth is ~1/4 of an inch.
 
lomiller said:
No, he is speaking of the skin effect. AC can *only* travel near the surface of a wire, while DC can use the entire thing. The depth into a conductor current can travel is inversely proportional to it’s frequency, which is why DC can use the entire conductor. IIRC at 60Hz skin depth is ~1/4 of an inch.
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Correct. Also, Ivor is right. Silver plating is often used on copper tubing or bars for tank circuits (high power output tuned circuits). The sliver plating reduces the effective "resistance" due to skin effect, and as a result, maintains a higher Q.

ETA: BTW, silver is the very best electrical conductor. Copper is a close second.
 
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technoextreme said:
Wrong.:) Namely because of the faulty premise that AC power lines only operate at 60Hz. That inductive kick that you get from turning on a vacuum is actually a much higher frequency.
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Why would that matter? Transients are short and don't have that much power in them, and the frequencies won't be that much higher than 60 Hz.

technoextreme said:
Errrr.... If I remember the phsyiological differences correctly you'd have to be a moron/suicidal to electrocute yourself with DC electricity. Depending on what source you read there is no current from which you can release your grip upon.
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Now that's just silly. If DC were able to paralyze regardless of the current, then you'd see people frozen in place by batteries all the time. Breadboards would require a spotter, and Taser wouldn't have to keep stepping up the voltage.

Ivor the Engineer said:
How do conductors know if they're AC or DC?
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He presumably means Litz wire, which is obviously an AC conductor because you wouldn't bother using it for DC.
 
Dilb said:
Why would that matter? Transients are short and don't have that much power in them, and the frequencies won't be that much higher than 60 Hz.
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Transients from switching a vacuum cleaner do have a lot of power in them. Perhaps you meant energy? When the transient power is integrated over time the component due to transients is indeed quite small. There are a few devices, such as laptop power supplies and cfls that generate continuing harmonics, some with fairly high power peaks but they are not a significant contributer to the overall load on electric mains so the 60hz model remains pretty good. There is some concern and increasing regulation of these devices to control power factor due to harmonics over and above the traditional problem of phase shift power factor reduction.
 
There is DC underwater lines linking denmark with norway and sweden. It uses one wire only with ground for return.
The neccecary electronics for stepping the voltage up/downhave historicaly been too expensive/not invented.
AC is also easier on the breakers and lightswiches as any sparks stop at zeropoint on the sinecurve. DC breakers needs a sturdier construction and larger gaps.

AC is more dangerous than the corresponding DC voltage due to the sinecurve, the peak voltages are higher and can press more milliamps through a human.
 
Toke said:
<snip>

AC is more dangerous than the corresponding DC voltage due to the sinecurve, the peak voltages are higher and can press more milliamps through a human.
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True, but if you refer to the numbers in my previous post on the relative danger of AC and DC current:

Ivor said:
It's not a myth. The impedance of the human body is lowest at around 60Hz.

The let-go threshold for 60Hz AC is about 15mA. For DC it's about 75mA.

At 60Hz AC, 60mA through the heart can induce ventricular fibrillation. For DC, 300-500mA is required.
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you'll notice than the fatal DC current is about 5 times the fatal 60Hz AC current.

I.e. 60Hz AC is more dangerous than DC at the same current.
 
Another advantage of DC transmission is that for a given level of insulation and current rating, DC can transmit more power, since it runs at the peak voltage continuously. Insulators for AC have to be rated at peak, not RMS voltage.

Obviously, this is an especially big advantage for underwater cables, where the insulation is a significant expense. But it helps aboveground systems as well.

- Dr. Trintignant
 
Ivor the Engineer said:
True, but if you refer to the numbers in my previous post on the relative danger of AC and DC current:



you'll notice than the fatal DC current is about 5 times the fatal 60Hz AC current.

I.e. 60Hz AC is more dangerous than DC at the same current.
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I did not notice that one, it fits quite well with the 30mA treshold of the sumcurrenttransformer/earthleakbreaker installed in household fuseboxes.
The grounding for the installation must have a impedance of less than 1667 ohm for a maximum of 50V on the casing of your frigde/stove/laundrymachine.
 
Cuddles said:
This is likely to be a major point for the future of power grids. AC is fine when you've got relatively few large power stations generating power, but when you start throwing in hundreds of small wind turbines, solar cells, wave machines and so on the management requirements go through the roof. It may well turn out to be cheaper to switch to DC rather than try to adapt existing grids to cope with renewable power.
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"Visibility" of dispersed renewable power is of particular concern to system operators, particularly when that power is of a volatile nature. It also has the unfortunate tendency to be sited in remote areas, which creates additional transmission problems, not least in obtaining wayleaves for route construction.

Any 'switch' to DC is however likely to be very limited in scope: we're talking here of restringing a handful of major transmission lines, rather than outright replacement of the high-voltage system. I very much doubt that we would see in the UK more than half-a-dozen internal DC links before 2020, for example.

On an aside, given suitable interest, I could possibly arrange a group visit to the Electricity National Control Centre coincident with TAM London. It wouldn't be a long visit: after 20 mins we would have exhausted the sights, but it would be en route for anyone interested in going on to Stonehenge, for example.
 
Ivor the Engineer said:
This is complete and utter rubbish. The impedance of a piece of copper wire goes up with frequency. I.e. the losses in it are lowest at DC (= 0Hz AC) and rise with increasing frequency.
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Ah, come on, guys. The reason isn't impedance or skin effect. The reason they needed such huge conductors coming out of the plant was because they distributed the electricity at the final user's voltage, because they couldn't step it up or down with transformers, just as Olowkow said above. If they used 200 volts in the customer's house, it had to leave the plant at two hundred volts, and the size of the conductor needed to handle, say, 10MW (50,000 amps) is huge indeed. I don't think the assertion that they needed inch thick conductors at the average customer site is correct, though.

Skin effect at 60Hz is so minor that it can be effectively forgotten. A conductor of AWG 0 size (the size of 150 amp house service conductors; 53sqmm) or smaller will show negligible skin effect at 60 Hz. A conductor 1" in diameter shows an increased impedance to 60Hz of 10%. The skin effect is proportional to the frequency and inversely proportional to the diameter of the conductor, and also varies with the conductor material.
 
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Toke said:
There is DC underwater lines linking denmark with norway and sweden. It uses one wire only with ground for return.
The neccecary electronics for stepping the voltage up/downhave historicaly been too expensive/not invented.
AC is also easier on the breakers and lightswiches as any sparks stop at zeropoint on the sinecurve. DC breakers needs a sturdier construction and larger gaps.

AC is more dangerous than the corresponding DC voltage due to the sinecurve, the peak voltages are higher and can press more milliamps through a human.
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One point that seems to have been forgotten, but raised partially here, is that AC arcs tend to self-extinguish as the voltage drops to zero. There may well be some folks around that can remember sparks burning up from faulty light fittings run on DC.
 
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