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LAN_403

HV Enthusiast

Tue May 30 2006, 02:40PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

One of the biggest reasons DC lines are used for high voltage transmission vs. AC lines is that AC high voltage transmission lines are limited in the length of transmission they can run. There are no limits to the length of DC transmission lines. This is why the longest high voltage transmission lines are DC based, not AC.

Quantum Singularity

Wed May 31 2006, 02:51AM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

FWIW, it just so happens that I work for a major power company (AEP) and although I do not work with the transmission group (I am in the generation side of things) I did have to go through a bit of studying on the transmission side to to get my NERC certification. I am not familiar with other areas but the U.S. is divided into 3 interconnects, the Western Interconnect, the Eastern Interconnect, and ERCOT which is Texas. Texas formed their own sort of island and did not want to join the other 2 interconnects. Here is were the DC lines come in. Texas also didnt want to be bound under the burden of maintaining frequency in synch with the rest of the U.S. and the only way to do that is to use DC which obviously has no frequency or synch component. Like nik282000 pointed out. To do this every Tie Line to ERCOT goes through and AC/DC converter onto a DC tie line, over the border, then goes through a DC/AC converter back to 60Hz. There is a lot of loss this way, and thats the price they pay for not having to synch up with thier neighbors. By the sounds of it, DC lines are used in other countries and might have other reasons, but as far as I know the only ones here are the links to Texas and dont really have anything to do with efficiency. There could be more in the U.S. but those are the only ones 'of interest' that have come up in anything I have seen here. I might have some researching to do...

cbfull

Sat Jun 03 2006, 08:23PM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

Referring to the original question, power companies do reduce resistive losses by using higher voltages. If you look at the equation P=E^2/R, for a given resistance you can minimize power loss by maximizing voltage, to the point where the current is not determined by I=E/R. Once the current reaches the maximum possible value based on Ohms law, you are basically using the transmission lines as heaters. If you can keep the current below that value (i.e. current limiting), there is a gain in efficiency.

For example, if you could transmit power at say 1 million volts, the amount of current that would flow based on demand would be very, very small. Looking at P=I^2*R, you can see that there is very little power being lost as heat when the current is kept small.

My point is that transmitting high voltage DC would be straightforward, but AC would introduce more heat because of the zero crossing voltage. The power lost as heat would be least at the peak voltage, and highest once the voltage drops into the I=E/R range. That is the very nature of a sine wave voltage.

Those of you who have built your own inverters using transistors/MOSFETs know that the transistor gets hotter when the base voltage is somewhere between 0 and peak. We try to keep the base(gate) voltages as square as possible, and it is solely for the purpose of heat management. If we could control the bases with sine waves and not get any heating, that's what we would most likely do. Square waves are incredibly noisy from an electromagnetic standpoint (I think I said that right). A perfect example of this is the self-tuning flyback circuit. The bases of the transistors are fed with a signal coming from the feedback windings, and they tend to be somewhat sinusoidal, depending on a number of factors. The transistors are prone to overheating, but designing a circuit that feeds a square wave to the transistors and still maintain phasing is not an easy task. That's why ICs like the 555 are popular.

I am not an electrical engineer so I do not pretend that this is factual, but I think it is true for the most part. Feel free to correct me.

Steve Conner

Sat Jun 03 2006, 10:43PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Everything ... said is right. 1A RMS of AC causes the same I2R losses as 1A of DC, by definition. So you don't win anything on current.

Where you win is on the corona losses and insulation budget. The power transmitted in an AC circuit is Vrms*Irms (assuming unity power factor) but corona loss, and the size of insulators needed, both depend on the peak voltage which is ~1.4 times the RMS. With DC, the power transmitted is V*I, and the peak voltage for insulation purposes is just V, so it's 1.4 times better. Comparing DC to three-phase AC complicates that a bit, but DC still works out better.

Of course, that "betterness" has to be set against the cost of the inverters and rectifiers (and the cost of the losses in them) so the net effect is that HVDC is economical for very long transmission lines.

There are also certain situations, that other posters mentioned, where HVDC does nice things that you just can't do any other way. Pumping power between grids of different frequencies is one. Long undersea or underground cables, that have such high capacitance that AC would really struggle to get through, are another great application.

DC links are usually made bidirectional (ie, either end can be a rectifier or an inverter) because it doesn't really add much cost or complexity, and it's handy to be able to transmit power in either direction. AC lines can do that naturally, but protection equipment is sometimes set up to prevent it for strategic reasons.

Here in the UK we have a 2000MW HVDC link with France

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