I was eager to know why AC current has a smaller power loss than DC over long distances. It depends on the fact that electrons aren't really going anywhere or it depends on other factors? Is there any physical explenation for this phenomenom?
4
-
5$\begingroup$ "I was eager to know why AC current has a smaller power loss than DC over long distances". From what source did you acquire this understanding? $\endgroup$– Bob DCommented Dec 11, 2021 at 16:19
-
8$\begingroup$ Does this answer your question? Why do we use AC for long distance transmission? $\endgroup$– John RennieCommented Dec 11, 2021 at 16:36
-
1$\begingroup$ I've removed a number of comments that should have been posted as answers, and discussion about them. $\endgroup$– rob ♦Commented Dec 11, 2021 at 17:14
-
$\begingroup$ High voltage power lines - clarification of energy loss $\endgroup$– FarcherCommented Dec 12, 2021 at 7:59
Add a comment
|
3 Answers
$\begingroup$
$\endgroup$
2
That is not true. If anything, AC losses are higher due to the skin effect.
The thing is that you want to transmit power. The Ohmic losses in the wire occur only due to the current:
$$P_\text{loss} = I^2 \cdot R$$
where $R$ is the transmission line resistance.
If you want to transmit 1000 W, which case is better: (i) 100 V and 10 A, or (ii) 1000 V and 1 A? The current of 1 A produces only 1% losses compared to that of 10 A.
Back in the days when electricity was being introduced, there was something called battle of currents between proponents of AC systems (Nikola Tesla) and DC systems (Thomas Edison). Edison even publicly killed an elephant* to prove AC is dangerous! The AC eventualy won and the main reason was that transformer does not work with DC. This means that in AC systems you could easily transform electricity to higher voltage (10 kV or more) and lower current for transmission, and then transform back to lower voltage (110 V or 220 V) for distribution.
Today we have equipment to also transform DC voltages and currents (power converters) which was enabled by advent of semiconductors such as diodes, transistors etc. These devices are massively used today for battery charging and other purposes. But there is not much prospect of DC replacing AC for distribution, it is simply not worth it. Some propose that DC replaces AC in buildings and houses which makes sense since most of the equipment runs on DC. Imagine that you do not have to use chargers for smartphones, television etc., but you plug them directly to the wall socket. Recent research have shown that running DC instead of AC in houses could increase efficiency by 10%.
However, in the high-voltage transmission, the HVDC is already being used for quite some time now. As I have already pointed out, resistance for AC is even higher due to the skin effect in the wire. But there are some problems associated with DC and that is circuit protection. Imagine you have to break the current flow in the wire. Due to the finite inductance of the wire, the best moment to break the current is when its value is zero. This is how modern contactors work in AC systems, since AC current crosses zero 100 (50 Hz) or 120 (60 Hz) times a second. But DC current never crosses zero, and mechanically breaking such a current would cause huge arcs which burns and destroys contacts. This is the same arc you could sometimes see the moment you turn off the lights. Higher the current, more energy needs to get dissipated through the arc.
*The story about Edison publicly killing the elephant Topsy is well established and something I have never questioned. However, the user zeldredge posted a link in the comments which claims this story is wrong. Without any further discussion, here is the link: http://edison.rutgers.edu/topsy.htm.
answered Dec 11, 2021 at 15:47
-
1$\begingroup$ Good answer, but your point about circuit breaking is dubious. For regular operations, you'd use the semiconductor-based DC-DC converters themselves for controlling the current without anything arcing. For mechanical backup purposes that's not used too often, arcing is manageable, and indeed HVAC systems also have circuit breakers that produce massive arcs but are designed to survive this. $\endgroup$ Commented Dec 12, 2021 at 0:12
-
1$\begingroup$ @leftaroundabout You cannot use power converters for circuit protection. Not that long ago I designed a DC microgrid and there was a problem that circuit breakers and contactors were not available for DC systems. Today you can buy DC circuit breakers but they are much more expensive than regular AC circuit breakers, probably because contacts must be significantly reinforced. $\endgroup$ Commented Dec 12, 2021 at 10:19
$\begingroup$
$\endgroup$
1
I was eager to know why AC current has a smaller power loss than DC over long distances
There is little difference between power losses for rms ac current at power distribution frequencies (50 or 60 Hz depending on country) and dc current of the same magnitude (accept for some skin effect loss for ac)..
What you are probably referring to is AC current reduces power transmission losses over long distances because it can be transmitted at high voltages so that currents can be low, reducing $I^2R$ heating losses, and then voltages reduced by transformers for higher currents over short distances at commercial and residential locations. DC voltages cannot be increased or reduced by transformers
Hope this helps.
-
1$\begingroup$ because it can be transmitted at high voltages so that currents can be low - That sentence would be true if you used past tense, but it's false in present tense. Efficient DC-DC converters exist, so transformers aren't the only option anymore. (Also, typo: accept/except) $\endgroup$ Commented Dec 12, 2021 at 2:50
$\begingroup$
$\endgroup$
Given that now dc to dc convertors are efficient and can be produced reasonably easily the possibility of very high voltage dc transmission has become an economically proposition. The factor which becomes important at high voltages is insulation of the conductors carry the electrical power. For overhead power lines that is the air and the insulators supporting the cables. With ac the swing in the voltage is much higher than the dc voltage carting the same amount of power. So a $\rm 1MV$ rms ac power line has a voltage swing of $\pm \rm 1.4 MV$ and so with an ac power line the insulation has be able to withstand voltages which are 1.4 times than dc power lines. You might imagine this is a constraint for high voltage power lines.
