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Lately, I’ve been reading about the many advantages of HVDC transmission systems for long distance transmission, undersea links, and others. The historical reason of why AC was picked over DC was mostly due to the invention of the transformer, which allowed easy manipulation of AC voltage enabling high voltage transmission across long distances.

However, after the invention of mercury valves, thyristors, IGBTs and all these components that have made DC transmission feasible, I’ve been thinking that if we had a purely DC network we could get rid of all AC/DC rectifiers that we find in our electronic appliances. This could improve energy efficiency greatly and save tons of money in resources.

If we had a chance to start over, could a DC-based transmission system be the better1 choice, or would AC still come on top?

1: with better, I mean more energy efficient.

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    \$\begingroup\$ There's no clear answer to this, which makes it a "non-constructive" question. However, I'd point out that to achieve galvanic isolation (which would still be required, even in a DC-DC converter) requires transmitting an AC signal across a magnetic or capacitive interface. \$\endgroup\$ – Dave Tweed May 1 '13 at 18:54
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    \$\begingroup\$ You'd still need voltage converters; replacing AC/DC rectifiers with DC/DC converters isn't a noticable improvement. Now, there might be benefits to house-level low-voltage DC (12 or 24V?) \$\endgroup\$ – pjc50 May 1 '13 at 18:58
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    \$\begingroup\$ Judging by upvotes, the community seems to like the question. But before answering, please remember, "We expect answers to be supported by facts, references, or specific expertise," and not "debate, arguments, or extended discussion." \$\endgroup\$ – The Photon May 1 '13 at 19:01
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    \$\begingroup\$ The question clearly states a criteria - efficiency. One could entertain the alternative interpretation of cost efficiency rather than energy efficiency but given the name of the site the first would be the default assumption. The body of the question alludes to both, so good answers might weigh the differences. \$\endgroup\$ – Chris Stratton May 1 '13 at 20:42
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    \$\begingroup\$ @ChrisStratton, we should also try to answer the question in the text, "If we had a chance to start over, ... would AC still come on top?" --- which is probably more about cost efficiency than power efficiency. \$\endgroup\$ – The Photon May 1 '13 at 20:49
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A Bit of History

The suggestions behind this topic go against what a lot of electrical engineers have been taught since their first circuits course - that AC is better for power transmission. After all, in the "war of currents" in the late 1800's, it was Tesla who helped Westinghouse fight for AC, eventually defeating Edison's dreams of a DC empire.

The primary advantage of using AC over DC at this time was efficiency. It became increasingly easy to transform one AC voltage to another, especially when compared to the cost, difficulty, and inefficiency of converting one DC voltage to another at that time. According to Joule's First Law, the amount of power wasted as heat in the transmission lines is proportional to the current squared. Considering the transmission lines have a known, (basically) fixed resistance, then for a transmission of the same amount of power, much more is wasted in a low-voltage, high-current transmission than a high-voltage, low-current transmission. As stated, it was very impractical to convert DC voltages to a high enough level to over come the line loss when compared to the relative ease of transforming AC voltages.

As a side note, many places never fully switched from the original DC transmission systems to AC until the mid 20th century.

You can read all about the history here.

Enter Modern Electrical Design

This is not to say AC doesn't have it's own problems. The skin effect is one example of AC being less efficient than DC, but it still doesn't compensate for the above mentioned line losses. Another issue is that of corona discharge occurring at high transmission voltage levels. Over long distances, AC power also has stability issues. This IEEE article sites a few different distances, noting that the line reactance can be compensated for in distances up to 600 - 700 miles.

With modern implementations of mercury arc valves, thyristors, and IGBTs, and efficient means of DC voltage conversion, HVDC transmission is not only possible, but overcomes many of the problems faced with HVAC transmission. The overall transmission distance is much greater, and the mentioned AC effects are overcome. In addition, the cost associated with HVDC is less than HVAC, once a distance threshold has been crossed. This cost differential is discussed in detail in this paper which includes a breakdown of power substation cost as well. The cost is also discussed in the link provided by Jake in his answer.

The fact is that the current electrical infrastructure is based around AC power transmission. The vast majority of modern technology requires this type of power for proper operation, and had AC never been used I doubt we would have many of the technilogical "advancements" we known and love. Theoretically, using HVDC alone could prove to be more efficient, but to compensate for the difference in cost, a hybrid HVAC/HVDC system is the best solution, at least in this moment in human development.

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  • \$\begingroup\$ So, if I understand correctly, the easyness of use of AC at the time helped electricity to flourish during the 20th century. However, HVDC seems to be better choice today once above the break-even distance from a cost perspective. Disgregarding cost contraints, does HVDC have any techincal shotcommings when compared to current AC systems though? \$\endgroup\$ – Oniros May 1 '13 at 21:23
  • \$\begingroup\$ @Oniros - Exactly. Asking what might we do if we could do it all over again is a pointless question because our present ideology and decision making processes are based on history. If that history were to change, so would our line of thinking. A better question would be how can we improve upon the current electrical grid design to improve both cost and energy efficiency, given our past and present technology. \$\endgroup\$ – Kurt E. Clothier May 1 '13 at 21:27
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    \$\begingroup\$ @Oniros, I don't know enough to comment on the shortcomings of the current HVDC system. I would imagine there is something about it in one of the links presented in my answer. Specifically, the "disadvantages" section of this wiki page: en.wikipedia.org/wiki/High-voltage_direct_current \$\endgroup\$ – Kurt E. Clothier May 1 '13 at 21:37
  • \$\begingroup\$ @Oniros this is a good answer, but you accepted it to quickly. Just unaccept it and wait a few days, and you might get even better answers! Kurt: no offense meant. \$\endgroup\$ – Keelan May 2 '13 at 10:49
  • \$\begingroup\$ @CamilStaps - Of course... that is how this place works! But to be fair, my answer was accepted after the question was closed. So it's safe to bet Oniros didn't expect any additional activity. \$\endgroup\$ – Kurt E. Clothier May 2 '13 at 21:51
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With the cost of copper and other useful metals rising, and the environmental issues with insulating oils used in them, high power AC transformers are becoming very expensive, compared to their solid state equivalents. It's not only the cost of the windings, but the large metal cases, and transportation and installation costs associated with such large transformers.

A switch to high voltage DC transmission would probably be a wash in terms of efficiency, however it could be lower cost if we didn't already have our current infrastructure in place. Transformers would still be needed, but rather than low frequency power transformers we would couple semiconductor DC-DC converters with high frequency power transformers which can be much smaller (thus cheaper) for the same amount of power conversion capacity.

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  • \$\begingroup\$ "environmental issues with insulating oils used in them..." - These days you can get vegetable-based insulating oil which is non-toxic and biodegradable, so this is less of an issue than it used to be. As a bonus, the vegetable oil is also less easily flammable than mineral oil, so transformers are less likely to explode into fireballs under fault conditions. \$\endgroup\$ – Li-aung Yip Sep 29 '13 at 6:15

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