Transmitting power over long distances what is better AC or DC?

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I found this answer to a related question. The part of the answer that's confusing me is:

Transmitting DC power over a long distance is inefficient. Thus AC supply is a far more efficient to transmit power.

According to Siemens it's quite the opposite:

Whenever power has to be transmitted over long distances, DC transmission is the most economical solution compared to high-voltage AC.

Also, from Wikipedia

HVDC transmission losses are quoted as less than 3% per 1,000 km, which are 30 to 40% less than with AC lines, at the same voltage levels.

Is the posted answer correct?

- - EDIT - -

Chris H made a very important observation (see his comment below): The context of the post I mentioned was of low voltage whereas I was blindly thinking of high voltage. Indeed I learnt loads by the answers and comments. Thanks.

Arraval

Posted 2018-03-07T12:05:36.320

Reputation: 133

18"Yes!" <Edison> "No!" <Tesla>Carl Witthoft 2018-03-07T16:19:53.500

5Note that your first quote is from an answer in the context of low voltage DC. That means high currents, and high resistive losses.Chris H 2018-03-08T11:57:23.653

7The reason why the traditional lore is that transmitting DC is less efficient is because in the olden days, it was very hard to convert DC between voltages without significant power loss. For this reason, early DC transmission lines tend to be very low voltage (by comparison to other methods of transmission). Look on the railways for example - in the UK, the older DC third rail system is 750V DC, whereas the newer overhead AC system is 25,000V AC. The idea being that the 750V would almost directly power train motors, whereas 25,000V would be stepped down with a transformer.Muzer 2018-03-08T13:58:56.893

1(the physics reason behind low-voltage transmission being inefficient is because V=IR, so the current will drop with a higher voltage for the same load, but P=I^2R, so the power lost due to the resistance of the wire will be significantly less because current has dropped).Muzer 2018-03-08T14:03:42.063

Answers

34

It is more efficient to transmit DC using about the same infrastructure. This is because of several effects:

  1. Skin effect experienced with AC. There is no skin effect with DC.

  2. Higher voltage allowed with DC for the same transmission lines. The lines have to withstand the peak voltage. With AC, that is 1.4 times higher than the RMS. With DC, the RMS and peak voltages are the same. However, the power transmitted is the current times the RMS, not peak, voltage.

  3. No radiation loss with DC. Long transmission lines act as antennas and do radiate some power. That can only happen with AC.

  4. No induction losses. The changing magnetic field around a wire carrying AC current causes induced voltage and current in nearby conductors. In effect, the transmission line is the primary of a transformer, and conductors near it are secondaries. With DC current, the magnetic field doesn't change and therefore doesn't transfer power.

Another advantage of DC is that it doesn't require synchronization between grids. Two AC grids need to be phase-synced to be connected together. This gets tricky when the distances are large enough to be significant fractions of a cycle.

The flip side is that AC is easier to convert between voltages. Converting DC back to AC to dump it onto the local grid at the receiving end is not a trivial process. It takes a large plant to do this, which means significant expense. That expense is only worth it if the transmission distance is long enough so that the efficiency savings outweigh the cost of the DC-AC conversion plant over its lifetime.

Here is a example of what it takes to convert high voltage DC back to AC:

DC power from large dams in Quebec enters at top right. This plant converts that to AC and dumps the power onto a large regional AC transmission line in Ayer Massachusetts at 42.5702N 71.5242W.

The expense of building and operating this plant is worth it due to the significant power savings of transmitting DC instead of AC. Synchronization was also a factor in using DC.

Olin Lathrop

Posted 2018-03-07T12:05:36.320

Reputation: 9 935

1Thanks a lot. The explanation is very enlightening. Just to wrap up and maybe for others out there with the same confusion: The comment about DC being less efficient than AC (in the answer I linked) is in overall terms not correct?Arraval 2018-03-07T13:34:06.507

2You linked to a lot of things, none of which I followed. Questions and answers need to be self-contained. In your case, there was enough of a question here to answer it here, so I did.Olin Lathrop 2018-03-07T13:36:15.563

I find it a bit odd that you start the answer with skin effect, because that isn't particularly relevant at 50/60 Hz. (On second thought, it is probably more relevant than I intuitively though because of the $\sqrt.$ dependency and the thickness of power lines, but still...)leftaroundabout 2018-03-07T13:55:00.630

The added picture is interesting as an example, but to the untrained eye it looks a lot like any number of AC substations that are scattered around.hazzey 2018-03-07T15:39:31.950

@hazzey: The point was only to show the size of the infrastructure needed to convert DC to AC at these voltages and power levels. You can follow the lat/lon coordinates I gave if you want to delve into this more yourself. Google Earth has more detail than what I show.Olin Lathrop 2018-03-07T18:07:29.720

1@leftroubdabout: 9.38mm at 50Hz for copper, according to WikipediaPlasmaHH 2018-03-07T19:48:15.170

Are the effects of the capacitance significant enough to consider?PlasmaHH 2018-03-07T19:49:13.537

1We also run a one million vdc line from Oregon to San Diego--California consumes a huge amount of the power from our dams in the pacific northwest (Although this is currently changing a little due to solar production in California)Bill K 2018-03-07T19:55:34.680

3re: "AC easier to convert between voltages" Worth a historical note in Edison v Tesla days this was even more of a driver, in fact we might say it was practically impossible for Edison to step up/down his DC. It took some decades before high voltage DC transmission was even feasable.agentp 2018-03-07T22:59:57.027

1AC also has the advantage that you can phase shift the incoming signal to export more or less power, just using passive componentsFerrybig 2018-03-08T08:00:35.450

@agentp Rotary converters are as old as power generation. Rotary HVDC was done as early as 1889, merely 5 years after first AC line.Agent_L 2018-03-08T13:32:06.717

Isn't point 3 and point 4 the same thing, physically speaking? That thing about "causes induced voltage" is exactly what antennas are made to do, after all.Arthur 2018-03-08T14:41:12.370

@Art: Point 3 is about radiation. That would happen in free space with nothing around the lines. Point 4 is about the transformer effect, which requires nearby stuff to act as the secondary.Olin Lathrop 2018-03-08T15:16:51.663

@OlinLathrop Isn't the transformer effect basically just other conductors nearby picking up the "antenna signal" coming from the primary? I can't really see how it's fundamentally different.Arthur 2018-03-08T15:19:44.623

@Art: No. Tranformers work only on the magnetic field. Radiation requires the magnetic and electric fields working together in a organized way to propagate energy thru space. It takes roughly a wavelength for the E and B fields to organize into forever-propagating radiation. Transformers work on distances much smaller than that. Something absorbing radiation outside the near field doesn't load the antenna more. Power taken from a transformer secondary loads the primary. Transformers and antennas are different.Olin Lathrop 2018-03-08T15:29:49.710

@Ferrybig Can't HVDC converters also be adjusted to affect the power flow? You would have problems on a DC grid where you don't have control of all converters, yes.immibis 2018-03-10T03:02:57.937

37

I actually worked on HVDC schemes, back in the mid-to-late 90s. Olin Lathrop's answer is partially right, but not quite. I'll try not to repeat too much of his answer, but I'll clear up a few things.

The losses for AC primarily come down to the inductance of the cable. This creates reactance for AC power transmission. A common misconception (repeated by Olin) is that this is due to transferring power to things around it. It isn't - a coil of wire halfway between here and the Magellanic Cloud will have precisely the same reactance and cause precisely the same electrical effects sat on your desk. For this reason, it's called self-inductance, and the self-inductance of a long transmission cable is really significant.

The cable does not lose any significant power from inductive coupling with other metalwork - this is the other half of that common misconception. The effectiveness of inductive coupling is a function of the AC frequency and the distance between the cables. For AC transmission at 50/60Hz, the frequency is so low that inductive coupling at any kind of distance is utterly ineffective; and unless you want to get electrocuted, those distances have to be several metres apart. This just does not happen to any measurable extent.

(Edited to add one thing I forgot) For cables running underwater, there are also very high cable capacitances due to their construction. This is a different source of reactive losses, but is significant in the same way. These may be the dominant cause of losses in underwater cables.

Skin effect does cause higher resistance for AC power transmission, as Olin says. In practise though, the need for flexible cables makes this less of an issue. A single cable thick enough to transmit significant power would generally be too inflexible and unwieldy to hang from a pylon, so transmission cables are assembled from a bundle of wires held apart with spacers. We'd need to do this anyway, whether we were using DC or AC. The result of this though is to put the wires within the skin effect zone for the bundle. Clearly there is engineering involved in this, and there will still be some losses, but by this happy coincidence we can make sure they're a lot lower.

Buried and submarine cables are a single thick cable, of course, so in principle they could still get bitten by the skin effect. Heavy-duty cable construction though will generally use a strong central core which provides structural integrity for the cable, with other connectors wound onto that core. Again, we can use that to our advantage to reduce skin effect in AC, and even HVDC cables will be built the same way.

The big win in power transmission though is eliminating reactive losses.

As Olin says, there is also an issue with joining two power grids together, because they will never be exactly the same frequency and phase. Clever use of filters in the mid-20th century did allow connection of grids, but designing these was as much art as science, and they were inherently inefficient. Once you've got your power transmitted in DC though, you can reconstruct AC with the exact same frequency and phase as the destination grid, and avoid the problem.

Not only that, but it is much more efficient to convert from AC to DC and back to AC again, instead of trying to use filters to compensate for phase and frequency. Grids these days are commonly joined with back-to-back schemes. These are essentially both halves of an HVDC link next to each other, with an enormous busbar between the two instead of kilometres of transmission cable.

Graham

Posted 2018-03-07T12:05:36.320

Reputation: 471

+1 for the details and clarification provided. I can't upvote it because of my few reputation points. Neither I can comment on the original post (the one that sparked my question). It would be great if someone can post a comment [there] (https://engineering.stackexchange.com/a/295/15211) because I think it's misleading. Many thanks.

Arraval 2018-03-07T14:58:03.013

One could probably paraphrase the problems with AC as "at a scale of thousands of miles, 60Hz AC behaves just as counterintuitive as the waveguides-antennas-coax-and-gold-plating-black-magic RF world"rackandboneman 2018-03-07T15:22:50.477

@Arraval Added a comment on that reply to clarify.Graham 2018-03-07T15:23:20.423

Inductance is undesirable, but by itself doesn't cause losses.Olin Lathrop 2018-03-07T18:14:04.177

I agree that external inductive and capacitive effects are minimal, but they do exist. I remember the story of a farmer who got free power due to a wire fence run along a power line. Radiation is again small, but significant enough that lines are twisted at regular intervals.Olin Lathrop 2018-03-07T18:17:46.747

@OlinLathrop: could the farmer have been along a single wire ground return? I would imagine a fence going along a SWGR system would have fairly decent coupling to it.user1512321 2018-03-08T19:00:18.650

4

They're talking about complexity and costing (\$\$\$\$\$)

The people saying "DC is less efficient" are using the word "efficiency" to talk about design factors like complexity of conversion hardware, and more critically, its cost.

If we have a Santa Claus machine that can pop out DC/DC converters as cheap and reliable as comparable transformers, then DC wins. (on skin effect alone). However out in the practical world, once your boots are laced up and lineman's gloves are on, you run into a few other hitches.

  • In AC, the speed of light creates phasing issues as loads move around - particularly a problem on electric railways, which is why they like ultra-low frequencies like 25 Hz or 16-2/3 Hz. This problem goes away with DC.
  • You can't increase current. Current is limited by wire heating, and wire heating is already based on RMS of AC.
  • Most of the installed base of transmission and distribution towers are made for 3-phase "delta", so they have 3 conductors. It's hard to use all 3 wires effectively in DC, so DC will reduce the effective capacity of these lines significantly by wasting a wire. How much? DC carries the same as single-phase AC, and 3-wire 3-phase carries sqrt(3) (1.732) times as much. Ouch.
  • You could well increase voltage. AC lines are insulated for peak voltage [peak = RMS * sqrt(2)] so you could hypothetically up DC voltage to that. However...
  • Once DC power strikes an arc, it is very difficult to extinguish because it never stops (unlike AC, where every zero-crossing gives the arc a chance to extinguish). This may be addressable with arc-fault detection. AC lines already have reclosers that will auto-reconnect after a trip; a DC recloser could retry after as little as a few milliseconds, replicating the effect of the AC zero crossing.

Harper

Posted 2018-03-07T12:05:36.320

Reputation: 315

Railway AC systems used to be e.g. 16.6 Hz because early AC motors were inefficient at higher frequencies (but transformers are large and heavy at low frequencies, leading to a trade-off). Newer systems generally use the same frequency as the ordinary electricity distribution grid, which modern power electronics handle just fine. Speed-of-light delays are a non-issue; the overhead wire is broken into phase-separated sections every several dozen kilometers anyway.Henning Makholm 2018-03-10T18:11:59.467

@HenningMakholm but the distribution isn't.Harper 2018-03-10T18:22:28.047

0

All else being equal DC transmission is more efficient than AC transmission at the same nominal voltage due to the elimination of reactive losses.

However all else is rarely equal.

  1. At a given voltage DC is far more prone to sustaining arcs than AC.
  2. It is only relatively recently that we have developed the ability to convert between DC voltages with reasonable cost and efficiency. At high power levels it is still more expensive and less efficient than transformers.

The result is that DC systems have tended to operate at lower voltages than AC systems and this is what has got DC it's reputation for being inefficient.

Voltage has a massive effect on cost and/or efficiency of transmission. If you halve the voltage then to maintain the same level of resistive losses you have to quadruple the size of the conductors. Alternatively you have four-times the losses for the same size of conductors.

The exception to this is very high power point to point power transmission over long distances, over undersea cables or between non-synchronised grids. In these cases the costs and hazards involved with converting the AC used on the grid to high voltage DC become more justified.

Peter Green

Posted 2018-03-07T12:05:36.320

Reputation: 121

0

There's some article from ABB about losses and investment cost: Economic and environmental advantages

Engineer Inverse

Posted 2018-03-07T12:05:36.320

Reputation: 1

2This is a comment not an answer. Web links without any detail here are of little value to this site because web links can become non functional.Fred 2018-03-09T07:51:12.610