Wire and Cable Technology Blog

I think this is a sensible thing to to as DC can be carried over further distances.

I think this makes great sense, in a world where you don't actually attempt to use the power that you are transmitting.

Of course in the real world, although we have some pretty efficient DC to DC conversion technologies, I don't think they scale well to megawatt levels. The I^2 R losses of the line is the same for AC or DC so we still need to transmit power long distances at hundreds of thousands of volts and the last time I checked I didn't see any power transistors rated at 1,000,000 volts. Nor is it difficult to see that semiconductor geometries do not provide a route there.

Maybe we can build vacuum tubes to handle that kind of voltage but that's going to take a large envelope. Certainly impressive, but come on now, did they really think this one through?

Sir,

Re. power transistors rated at 1,000,000 volts: I recall driving past the southern terminus of the PCI (Pacific Coast Intertie) in the early 1960's. It looked like a huge switchgear yard enclosed on the sides and top with chain link fencing. I believe they were using thyristors for the conversion of the 1.1MV DC back to AC. I never heard what the power levels were, but I wouldn't be surprised if it was well over 500MW.

--JMM

Well perhaps I am uninformed. I know they make some really big thyristors but I had never heard of any that would handle hundreds of thousands of volts.

Anybody got a part number or a data sheet?

The best I could turn up in a search was a Misubishi part that would withstand 12KV and while that is most impressive, it is a far cry from several hundred KV but I guess you could stack 40 or so. The power isn't an issue, many of the parts would switch megawatt loads but the standoff voltage would seem to be problematic. Maybe it isn't an issue, but I am curious about that part.

Rcapper,

I did some further searching and suggest the following link:

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

I found errors in my posting. There are two DC lines, operating at ±500kV-DC. Apparently the ones I remembered seeing in the 1960's were neither of these. The first inverter installation at Sylmar outside of LA was with mercury valves, but has since been replaced with thyristor towers from ABB. Total power capacity of the two inverter plants is 3.1GW. My reference to 1.1MV was based on a speech I scanned in the Congressional Record, which is close to the ±500kV.

--JMM

Thanks for the information. Clearly I was unaware but I'm not surprised at all. We (as a race) seem to be pretty resourceful when there's a buck to be had. But that is indeed an impressive system!

Friends,

Excellent approach. Very high voltage DC transmission lines have been used for decades in the USA. Consider the Pacific Coast Intertie that connects sources and users in British Columbia, Washington, Oregon, and California. In 2001 it had three 500kV AC lines and one 1,100kV DC line.

With DC lines, in addition to the reduced inductive losses, you greatly simplify the protective relaying schemes because there is no worry about frequency synchronization. Thus, sudden changes in loads would not cause the cascading of generating plants dropping off line because they could not maintain the proper frequency. This would also make additions and changes to the system very easy to do.

In addition, fears of harm to plants or humans would be eliminated because there is no varying electromagnetic field. (I will not get involved in any debate about the reality of such fears. The DC approach puts the entire topic into the "no-problem" category.)

It seems to fulfill the engineer's desired K.I.S.S.

-JMM

I believe this is probably a practical approach for transmission of power. One question though, If we ever do get to superconducting electrical power system, what will be the impact? I'm not all that familiar with the impacts of magnetic fields on such systems. Thoughts anyone?

I was a little surprised by this - I thought it had been proven that AC power distribution is surperior to DC power distribution....

I fail to understand the reasoning behind logn distance distribution using DC.

http://cityroom.blogs.nytimes.com/2007/11/14/off-goes-the-power-current-started-by-thomas-edison/

http://www.knobblegrud.com/blog/20071005ADCpowergrid.html

http://science.slashdot.org/article.pl?sid=07/11/16/225213

Guest,

For distribution, AC is better because you can easily convert from one voltage to another with a transformer. The distances involved are relatively short, so any additional energy losses on the lines is more than recovered by cost savings on the equipment. Usually there are many points where power is being taken from the system.

For transmission, you usually have very few sources and very few destinations. You also often have a large distance between them (which can be over 2000kM). Now your line losses are more significant. All lines have resistive losses calculated with I²R. However, for AC lines you also have capacitive and inductive losses because the electric and magnetic fields are changing sinusoidally at 50 or 60 Hz (depending on your location). If the lines are well separated from each other and the surrounding terrain, these losses are lower because they are proportional to the distance. An equivalent circuit for an AC transmission line includes capacitance between lines and for line to ground, as well as inductive coupling between lines and between any line and nearby ferromagnetic materials. For DC lines, since the magnetic and electric fields are constant, these capacitive and inductive losses are negligible.

Finally, within the conductor,the magnetic field in an AC circuit forces the current to mostly flow on (or near) the surface of the conductor. This is the "skin effect", and is significant when figuring the current-carrying capacity of large AC conductors. For DC conductors, since the current can flow equally well through all parts of the conductor its DC resistance is lower, so your I²R losses will also be lower.

Look in the NEC tables for conductor properties. For 1000 MCM copper, R_DC= 0.013Ω/1000ft, R_AC=0.015Ω/1000ft, and Z_AC=0.032Ω/1000ft. For the more commonly used aluminum these values are 0.021, 0.023, and 0.039 respectively. As the size goes up, the AC resistance and reactance values will diverge from the DC resistance even more.

When dealing with superconductivity, I believe that the resistance drops to essentially zero, but I suspect the X_L and X_C reactances won't. Therefore, I would expect the same savings to occur with DC versus AC on super-conductors. However, the higher cost of maintaining them at a temperature for superconductivity would mean that these would most likely be used for shorter runs, particularly where they are buried or otherwise spaced closely together. Again, when buried or spaced closely together the losses for AC are greater per foot of conductor (compared to DC), so overall distance for cost-effectiveness can be significantly less.

Finally, consider the proper length needed for a simple dipole radio antenna (you feed power into its middle). For it, L(ft) = 468/F(frequency in MHz). At 60 Hz, L = 468*10⁶/60 = 7.8*10⁶ feet or about 1480 miles. This is uncomfortably close to the lengths of some long-distance transmission lines. Therefore to prevent 100% losses through 60 Hz RF radiation, they either have to interrupt these lines with substations spaced much less than 1400 miles apart or run them on DC.

--JMM

Post 11 by imueller is an excellent example of a resoponse from a knowledgeable person - it is lucid - and more importantly right to the point of the question.

AC inductive losses are the majority loss for very long XM lines. DC lines don't have this, and have similar corona (or capacitive) losses, since that's based mostly on voltage.

I went to college (as it happens, on the west coast of the US) with a guy who ran the economic case for DC power transimission lines. At that time, the cost - including the conversion stations - balanced out at about 700 mi. The Pacific Intertie's line losses were about 19Ω, if memory serves me right. I never ran the calcs for the AC line losses myself, since Bob generally knew his stuff.

We also run DC interties to connect the western US power grid with the Eastern, to prevent reflections (the width of the US is just close enough to a 1/4 wave @ 60Hz). I'll wager the DC intertie near Gotland in Sweden is to isolate the Swedish grid from the Danish/European. Ecuador in 2002 had a country wide power outage for 24 hours because of a problem originating in Columbia.

I also visited the Sylmar facility in the mid 80's. At that time, most of the phases were using mercury arc valves as thyristors (mercury "splash" was the conductive path), but new ABB solid state converters were carrying the current for C phase. IGBTs were barely known then, so it must have been made up of many stages of high voltage SCRs. The operating voltage back then was 800kV, they've bumped it a few times since then. Wikipedia says the DC voltage is only 500kV, but I distinctly recall looking right at the DC voltage gauge that said 0.8 MV, and I know I heard from people who worked at PG&E it had been bumped to 1MV or higher.

JMUELLER has the right answer, with a few modification.

I know of a DC transmission between the danish coast and sweden by DC for the simple reason, that no insulation could withstand the peak voltages generated by the AC contemplated. It was early 70s and the DC was about 2Mvolts. So, by both of our examples it is neither new nor novel for wind power. Why would it be? Is wind generated power distinguishable from any other on a transmission line?!?

The one modification I would do to the transmission line presentation is the following. A TL line is still the same, even if it is fed by DC. Any transient elicits TL response, no matter what, and all the consequences. In the 60s Siemens (big in power generation and transmission) developed coax power transmission lines insulated by distilled water of about 1-2metres diameter. It was a good idea, high power capability, but transient response was terrible due the high capacitive loading by the water's high dielectric constant of about 80+.

The typical public electric utility moves glacial when it becomes to improvements unless it has to.There has been a recent study done by AEP(american electric power) on an eleven mile section.This somewhat new type of high voltage power line that instead of using a steel core they have opted for a composite carbon fiber core that is stronger lighter in weight and allows more aluminum to be wrapped around it. This allows for less line loss, less sag over 30 percent less fossil fuel to be used. The amount ampacity can be increased.Saves a huge amount of CO2 the greenhouse gas everyone worries about

The name of this co. is Composite Technology based in the U.S. in Irvine Calif. the unbelievable part of this is that the chinese who are trans.power starved have been buying this wire in many modes by the thousands of miles with a contract to buy more.

Here is a link to the co. and the study done by AEP

http://www.compositetechcorp.com/Press%20Releases%20-%20PDF/Pritchard%20Capital%20Partners%20Preso%20v13%20final.pdf

The report of the study has just been released so I expect this new wire will change things my hope is it turns some people in the industry on their head. I also hope that all of the turbines that service electricty to the grid starts using this just for the fact that less line loss will make wind a great investment for the future. Composite Technology was just featured on CNBC Friday.

One of the key issues for the future is right of way (ROW) and use of HVDC addresses that as its footprint is much reduced compared to AC transmission. As it becomes more difficult to expand ROW, due to environmental concerns, converting an existing AC line to DC to convey significantly more power just makes good sense. A review of Google hits using "HVDC" will show the interesting attributes of HVDC. Another advantage of a continent wide grid is a balancing out of peak loads in different time zones.

MB