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AC vs. DC Powerlines and the Electrical Grid

powerlinesEver since the earliest examples of long distance electric power transmission, overhead transmission lines have been the preferred method for transporting large amounts of electric power.   As the length of any conventional transmission line increases, both the energy transfer capacity of the line and the efficiency of energy transfer decrease.  The main ways to fight this are to increase the transmission line voltage, and/or to increase wire diameter.  At the time that power grids linking major cities were first built, there was no convenient way to change the voltage of DC power, whereas the transformer made that relatively easy for AC power.  That is why AC power won the “war of the currents” in the 1880’s. Up until 1956, only AC power could be readily changed from one voltage to another (via transformers, which only work for AC power).  In 1956, ABB built the first high voltage DC (HVDC) transmission line in the West (which is still in service, though it has since been upgraded) between Gotland Island and the Swedish mainland, via a subsea cable. The Soviet Union had built an HVDC line earlier than that, between Moscow to Kashira, which was based on technology taken from the Germans after WWII. These early projects were based on mercury arc valves. Since then, HVDC has evolved a lot, and is now the best way to transmit large amounts of power great distances.

There are different trade-offs for AC versus DC power transmission.  Voltage can readily be taken up to about 765,000 volts (765 kV) for an AC powerline (this is the current maximum AC voltage in the US) but beyond that, power dissipation through dielectric loss becomes significant.  (Dielectric losses are caused when dipoles in matter align with a changing local electric field.  As the polar structures turn to follow the field, the movement causes local heating.  This is the basis of microwave ovens. The dielectric loss during transmission is equal to the total heat that is generated in materials around the powerlines due to induced motions of electric dipoles.)  At high voltage, non-resistive power dissipation via dielectric losses (for AC only) and/or through corona discharge (for both AC and DC) becomes severe.  Voltage for DC overhead powerlines can be taken up to higher voltage than the maximum practical AC voltage; at present the worldwide maximum is ±800 kV for HVDC lines. Note that the way that voltage is reported for AC vs. DC powerlines is different; a ±800 kV DC powerline has 1600 kV conductor to conductor (800 kV conductor to ground), whereas AC voltage refers to the conductor to conductor root mean square, or “rms” voltage; roughly speaking AC rms voltage is comparable to the line-to-line voltage in DC in terms of transmission capacity. In effect, HVDC voltage can go about twice as high as HVAC voltage, which explains most of the advantage of overhead HVDC lines compared to overhead HVAC lines.

Wire diameter is limited for AC transmission lines due to the “skin effect” that prevents an AC current from penetrating to the center of a large wire, whereas a DC line can be arbitrarily thick.  At 60 Hz, the skin effect becomes significant for wires greater in diameter than about an inch. Because of the skin effect in part, multiple wires arranged in a circular pattern and separated by polymer spacers are often used in high capacity high voltage AC transmission lines. Thus, overhead HVDC powerlines can transport significantly more power for greater distances than AC lines, for two main reasons: the effective voltage can be higher, and the wires can be bigger. But DC lines were not developed initially to be capable of higher voltage, nor to be able to move more power than AC lines, but rather to make it possible to put high capacity power lines underground (for security) or under the ocean (to bring power to islands initially).

To understand why undergrounding HVDC lines for great distances is feasible, while undergrounding HVAC lines for more than about 40 miles is not, it is necessary to consider the capacitance of air-insulated overhead lines versus cables, which are typically surrounded by polymer insulation and soil. Capacitance is a property of every electrical circuit, not just capacitors (which are designed deliberately for high capacitance). A wire suspended in air has much less capacitance (by about a factor of 50-100) compared to a cable, in which the wire is surrounded both by polymeric insulation and soil. The capacitance limits how fast the voltage responds at the far end of a power line when voltage is applied at the near end. Capacitance has only a small transient effect on a DC power transmission line, delaying the voltage rise at the far end of the line by milliseconds at most when voltage is applied at the near end. When capacitance of an AC line is too high though, it has a quite dramatic effect; this is the case because at 60 Hz, the voltage reverses 120 times per second (8.33 milliseconds for per reversal); each time this happens, the “line capacitor” needs to be charged up before any power can flow through the line. The much higher capacitance of a cable (especially one that is located underground or undersea) means that this limiting line capacitance is reached for a much shorter cable (50 to 100 times shorter) than an overhead line. Thus at most short bits of an AC power transmission line can be placed underground, whereas there is no problem in terms of power flow with putting a DC power line underground.

Another important property that differentiates AC from DC power lines is that for an AC line, the line power must be synchronized with the local AC grid at both ends of the line, whereas DC power can bridge between two different synchronized AC grids that are not synchronized with each other. For this reason, DC power lines are often referred to as “asynchronous links” by power engineers. Examples where this is important involve power links between the Quebec AC grid and the Eastern US grid; between the Eastern and Western US grids; between the Texas grid (ERCOT) and everywhere else; and between the incompatible 50 Hz and 60 Hz regions in Japan.

Nearly all of the above factors would seem to favor DC over AC transmission, so why are most transmission lines, and virtually all power distribution lines AC? Simply put: transformers (which change voltage of electrical power) and circuit breakers are dramatically less expensive for AC than for DC power. At the time that the first long transmission lines were built by Westinghouse between Niagara Falls and New York City, there was no such thing as a DC/DC transformer, and that hard technical limitation persisted for a hundred years (which is why we have a strictly AC grid). Today, electronic DC/DC voltage transformers are found on every computer motherboard, and can be built for high voltage, high power conversion as well…but these devices are a lot more expensive at present than conventional transformers. However, being electronic devices, these DC/DC transformers have been on a steeply declining cost curve for some time now, and it is probable that they will in the future reach cost parity with conventional AC/AC transformers. This could mean we will have a DC grid in 100 years or so, but don’t hold your breath. Meanwhile DC circuit breakers are also a huge problem, especially at high power levels above one megawatt (MW). ABB recently announced a breakthrough on HVDC circuit breakers that they say will allow HVDC circuit breakers up to one gigawatt (GW; equal to 1000 MW; still well below what will be needed to implement a supergrid). ABB has not published either a cost for their new breaker (which I estimate will be about 100 times as high as comparable AC circuit breakers), nor on-state power loss figures (which I estimate will be ~0.25% of transmitted power). We still have a way to go to having a DC circuit breaker that is capable of enabling a supergrid, in spite of ABB’s efforts to convince us otherwise. This is a problem I have been working on; I call my solution a Ballistic Breaker™.

Roger Faulkner's picture

Thank Roger for the Post!

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I K's picture
I K on Apr 3, 2013 12:26 am GMT

A super grid probably does not need more than 1-2GW capacity lines. You would not really want a 10+GW line as a fault will cause chaos in most grids Far better to have 10 x 1GW as simultaneous faults will be very rare to impossible.

Roger Faulkner's picture
Roger Faulkner on Apr 3, 2013 12:58 am GMT

I disagree; in the US we have ~1000 GW of total capacity. In order to create a continental scale market, that means having the capability of shifting ~200 GW coast-to-coast. At 2 GW that implies 100 new lines. Even given the current state of the art overhead lines (7 GW 800kV DC lines) that implies 29 coast-to-coast lines.

I am well aware this is out of the box thinking. See my post on HVDC loops for my answer on the redundancy question (which always comes up). I am heading towards making my case for elpipes, but I’m trying too fairly consider all the options….the same process that led me to realize that elpipes are a uniquely well suited basis for a supergrid.

Michael Berndtson's picture
Michael Berndtson on Apr 3, 2013 11:16 pm GMT

As an invetor of something as operationally critical as a HVDC breaker or switch, wouldn’t it be easier for you to work with an ABB or Siemans or whomever to go beyond proof of concept or demonstration? Given that power outages can cost in the $ billions in damages, one would need some multi-national corporation size liability coverage to be taken seriously. Anyway, this is interesting stuff.  Here’s more from Technology Review, “Edison’s Revenge: The Rise of DC Power:”


Roger Faulkner's picture
Roger Faulkner on Apr 4, 2013 3:29 am GMT


I have approached ABB and Siemens, of course. I actually commented on ABB’s recent announcement on HVDC circuit breakers (which you cited) here:

I have a history with ABB, having pitched elpipes to ABB Technology Ventures in 2010; I needed to get 3 top decision makers on board, and I got two out of three. This blog post tells that story:

My pitch to ABB was my last major attempt to get elpipes funded. When I failed at that, my partner quit, and it was a low point for me. I was well aware at that time that HVDC circuit breakers were another key missing piece of the puzzle but I had not worked on them. The idea for the Ballistic Breaker came to me in a dream in October 2010, and I gradually shifted my focus onto this new idea; I quickly realized that (unlike elpipes), Ballistic Breakers have many applications at lower voltage than will be necessary for the supergrid, all the way from low voltage DC in electric vehicles and data centers, through MVDC and MVAC applications including big motors, wind turbines, and microgrids as well as AC distribution substations, all the way up to HVAC and HVDC. The Ballistic Breaker is the only type of breaker I know that is applicable across the entire power spectrum, both AC and DC, from low voltage to very high voltage. However, I cannot possibly address all of that, so I’m focusing on medium voltage DC (MVDC) breakers at around one MW, where my technology has a big advantage over the incumbent options (arc chute breakers, L-C oscillatory breakers, and power electronic switches). I try very hard to keep my entrepreneurial attention on this narrow niche, but my heart is really in enabling technology for the supergrid, which I see as my life’s mission.

My PCT patent application on Ballistic Breakers is going to be published by WIPO (World Intellectual Property Organization) in a matter of days, so I have posted it for anyone who wants to take a look here:

My invention elpipes is much further along in the patent process, having been filed in six national phase patent applications now. This blog post links to the PCT patent application on elpipes:

Both elpipes and Ballistic Breakers are novel inventions built out of conventional components; neither requires any fundamentally new technology nor materials. And both are very timely inventions which I sincerely believe are critical enablers for a supergrid.

Michael Berndtson's picture
Michael Berndtson on Apr 4, 2013 1:13 pm GMT

Neat stuff. Power transmission infrastructure I’m guessing is pretty much dominated by the Swiss and Germans, with the Swedes given a bone after the ABB merger. The US still dominates much of instrumentation and controls with Honeywell, Rockwell (Allen Bradley) and Johnson. You may want to back end it through end users of DC power systems ranging from batteries to renewable power to electrical components and generation like Emerson and GE. Or even EV developers and manufactures. Nonetheless good luck dealing with the Swiss or god forbid the Swedes ;). Eventhough China is starting to go DC with transmission – it won’t be easy keeping your ideas yours.

I’m a big fan of DC conversion since most of the stuff in our houses will be powered or controlled with DC. So all this convesion from DC to AC to DC to AC to DC is nonsense and wasteful.

Alan Rominger's picture
Alan Rominger on Apr 8, 2013 8:12 pm GMT

I’ve seen advocates for infrastructure investment in the US argue for burying power lines.  For a case in point, see:

The argument goes that burried power lines are better for infrastructure security.  We all know that after every major hurricane, snow storm, etc the power company has to scramble around to fix the downed lines.  But this reason is distinct from the economic argument for long distance transmission.

You mentioned that over 40 miles for HVAC is impractical, but is this a function of the voltage?  Can local distribution through burried AC lines then be practial?  As far as I know, no one has advocated for DC distribution up to the substation or household level, and that would likely be unworkable anyway.  Does DC have any place in distribution, or is the discussion limited to large interconnects for the forseable future?  If not, would there always be some intermediate level voltages that are too local to be DC, but too high voltage to bury as AC, so then they would be impossible to bury with current technology?

Roger Faulkner's picture
Roger Faulkner on Apr 9, 2013 3:34 am GMT

All good questions. I do think the capacitance problem would limit distribution lines just the same as transmission lines; however, this is not a practical limitation, since the distribution grid (from substation to consumer) is only rarely greater than 40 miles long, and that would only be in very rural areas. In cities there are many substations, and lines from the substation to the consumer are rarely longer than about 10 miles.

The bit about infrastructure security can go both ways, because overhead lines are generally easier to repair than underground lines. I hear from industry insiders that (at least from an engineering point of view) the fact that underground lines typically take 40 times as long to fix as an overhead line (no exaggeration!) is a major reason that utility engineers are leary of underground transmission lines. Incidentally, rapid repairability is a key feature of elpipes, which are unique among power lines in that they have a train-like mobility within their conduits, which are essentially similar to a gas pipeline (which re-risks the cost of installatrion, because we know very well how to build gas pipelines).

As to the possibility of DC distribution: it is happening now on board the most modern cruise ships, in data centers, at remote mine sites, and in some other microgrids as well. The growth trajectory of DC power distribution is quite high, but starting from a small base; ABB is the clear market leader. I do expect there will be examples of DC power distribution in some communities with high penetration of solar power in the near future. DC circuit breakers remain a problem for DC power distribution; ABB has developed solutions based on IGCT power electronic breakers, and I myself have been working on a more economical approach I call Ballistic Breakers.

Evgen Dev's picture
Evgen Dev on May 8, 2013 10:10 pm GMT
Good question Rodger
Перевод текстов и веб-страниц
вопрос Rodger

Да проблема скин эфекта в высоком переменном напряжении всегда
присутствовала, для этого увеличивали диаметр проводника в виде трубы,
но осталась проблема экранирования излучения и потерь.
Но в постоянном токе принято считать, максимальная плотность тока
сосредоточена внутри проводника, что в этом случае выводит на передний
план проблему организации сверхпродящего проводника( уже есть модели с
эфектом при -80С)

Для AlanR
В россии подземные сети высокого напряжения, с самого начала строили для
безопасности промышленных обьектов на случай войны особенно ядерной, а
уже потом для улучшения инфраструктуры энергоснабжения.
Инфраструктура изначально проектировалась централизовано, с
унифицированными параметрами генерирующих мощностей и линиями передачи.
Проблемы несинхронности АС 3 фазы выравниваются Ц- балластами и
реакторами, запретом несинхронного включения нагрузки.

Вопрос дотаций, скорее политический чем экономический
Никто не захочет упускать рычаги управления(нефть, газ…)

Да и технологии пока очень дороги и не фективны.
Хотя начали появляться солнечные системы с КПД более 30% и это большой
Неудобно что не везде на земном шаре есть достаточно солнца

Введите текст или адрес веб-сайта либо переведите документ.


Yes problem skin effect it in a high alternating voltage is always present, this increases the diameter of the conductor in the form of a tube, but there was the problem of shielding and radiation losses.
But DC is considered, the maximum current density is concentrated within a conductor, in this case brings to the fore the problem of organizing sverhprodyaschego conductor (already have a model with effect it at-80C)

For AlanR
In Russian underground high-voltage grid, from the outset, for the safety of industrial facilities in the event of war, particularly nuclear, and only then to improve the energy infrastructure.
Infrastructure originally designed centralized, with standardized parameters of generating capacity and transmission lines.
The problems of non-synchronization AC 3-phase C-aligned ballasts and reactors ban non-synchronous switching loads.

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