DC vs AC for long-distance power transmission?
(What is the threshold distance? as a function of voltage? and what are the losses? Which countries do this?)
(I'm an MSEE, so you can assume knowledge of two-ports and transmission lines if you want, or some links.
But please include a paraphrase explanation with typical numbers, which everyone can understand)
Answer:
DC is used for long distance transmission because the equivalent AC line would be beyond its steady state stability or dynamic stability limits.
Recall that power transfer across an AC transmission line is approximately:
P = |V1| * |V2| * sin (delta) / X, where
V1 & V2 are the sending & receiving end voltages
delta is the phase angle difference between V1 & V2, and
X is the line impedance (Z is almost all reactive on a transmission line, so X is a close approximation.
Since impedance is essentially a function of line length, and the practical upper limit of voltage is 345 kV, 500 kV, or 765 kV, depending on the utility, there are definite upper limits to line lengths for AC transmission. (When I get back to my office midweek I will look up the power transfer numbers based on line length & voltage.)
There are ways to reduce the effective series impedance (X) of an AC transmission line, by using series capacitors, to increase power flow across the line, but series caps introduce other complications. In some instances, the line can be operated closer to its inherent stability limits by using active power electronic devices near each end (an SVC or STATCOM). Ultimately, though, the decision to use DC transmission is clear when doing bulk power transmission over very long distances.
The skin effect does have some impact on actual power losses, when considering DC vs. AC, but these are only a minor consideration.
I think it is because AC current travels more on the outside of the wires, while DC uses the whole wire more evenly. So ... AC sees, effectively, less copper ==> more resistance ==> more power loss.
See "skin effect" in this link.
Also, AC couples to the ground, and over 100's of kilometers, there is some radiation into space (quarter-wave about 1500 km at 50 Hz).
Higher voltages are better transmitted over long distances and it was easier to step up AC using transformers. But the skin effect cause higher losses in AC transmission. So, if we can step up DC to high voltage, DC transmission is better. The longer the distances, the more is the saving.
Most Long distance Transmission is Very high voltage AC, 75,000V and up. There was some talk if using 500,000V DC some years back but I don't know if they followed through with it. For real fun read up on the work Nicoli Tesla did, which is all of the electric we use today. Edison was a noisy nobody.
The higher the A/C voltage you are transmitting the lower the losses. Again this is why the US transmits voltage at very high voltage levels and then lowers the voltage as it gets closer to residential houses. Alot of countries use 220 voltage outlets because as I said earlier higher voltages means less line lose. The United States as a whole would SAVE energy if we switched from 110 outlet voltage to 220 outlet voltage. Technically it is Amps which create line loss not voltage. Watts=Amps * Volts, so to have the same watts of power you can double the volts and cut the amps in half. Cutting the amps in half cuts line loss. Hope this helps.
As you have pointed out, DC transmission lines can be cost effective if the AC/DC and DC/AC conversion costs are less than the incremental line losses of an AC transmission line.
I recall hearing that the break-even point was around 400 miles. Obviously this will vary depending on the load.
Southern California has a HVDC converter station that ties to a 846 miles long ±500 kV DC transmission line starting in Celilio, Washington. The line travels though Oregon, Nevada, and California. The converter station was build in 1970 with a rating of 1400 MW at ±400 kV. Since that time line upgrades and converter station expansion and enhancements have been made. The present ratings are:
Power transmitted: 3100 MW
DC Voltage: ±500 kV
DC Current: 3100 A
In 1985 the line was upgraded to ±500 kV and
The major components at the converter station are:
DC Filter - Prevents unwanted harmonics from passing onto the DC line.
Smooting Reactor - Smooths the DC waveform after conversion.
Tyrristor/Mercury Arc Valves - Convert AC to DC and vice versa. (Made by ABB)
Controls - The converter firing controls command the thyristors to turn on and off for the DC conversion process.
Converter Transformer - Acts as a buffer between the DC and AC system.
AC Filters - Prevent unwanted harmonics from passing onto the AC system.
The longest HVDC link in the world is currenlty the Inga-Shaba 1700 km 600 MW link connecting the Inga Dam to the Shaba copper mine, in Africa.
The links below provide more details on the converter stations and the evolution of HVDC transmission lines.
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