Electrical AC vs DC over distance

[Hendog]

I'm trying to understand how it is that AC has less loss vs DC when transmitted over long distances. I've Googled it and found some statements such as this one:

"When A.C. is supplied at higher voltages in long-distance transmission, the line losses are small compared to a D.C. transmission."

It's not clear whether they are making the comparison at the same voltages for each. I would think the resistance of the line would be the same for any current regardless of the voltage being AC or DC, no? The only thing I can think of that would make the difference is that AC, for lack of a better term, rides over the surface of the conductor.

I know it's easier to step up and step down the voltages via transformers in AC, but besides that, all things being equal, let's say at a given voltage of 120 volts AC and also DC, does AC actually travel more efficiently over the same line than does DC? Or is it that the advantage is only achieved at higher voltages, say 70,000 Volts?

I'm missing something here. Maybe the answer requires a level of math beyond my grasp, but I'm hoping the brain trust here will have the answer(s) in laymans terms I would understand.

[oldE]

Just to complicate things for you further, long distance transmission on new construction is Ultra High Voltage DC.

Best
Les

[911 Rod]

There is a very interesting documentary on Netflix (or YouTube maybe) about Tesla and Edison that might answer some of your questions.

[masraum]

Originally Posted by 911 Rod

There is a very interesting documentary on Netflix (or YouTube maybe) about Tesla and Edison that might answer some of your questions.

That sounds interesting. I'll have to check that out!

[masraum]

Good question, hendog, I'm looking forward to the answers. I've heard/read similar before and never looked into it further, but I am curious as well.

[kolo]

If a tesla is stolen....
Does it become an edison?

[Hendog]

Originally Posted by 911 Rod

There is a very interesting documentary on Netflix (or YouTube maybe) about Tesla and Edison that might answer some of your questions.

I will definately tune into to that. Thanks!

Originally Posted by kolo

If a tesla is stolen....
Does it become an edison?

No, it becomes a Pursuit...

[aschen]

I think you got it, AC just allows for much more convenient stepping up and stepping down I don't think there is a fundamental gain in transmission efficiency from AC alone

[masraum]

Originally Posted by aschen

I think you got it, AC just allows for much more convenient stepping up and stepping down I don't think there is a fundamental gain in transmission efficiency from AC alone

That occurred to me while reading the OP, that it may not be the actual transmission that's easier/better with AC, but the fact that the Voltage change is easier that makes the overall process easier with AC.

[LWJ]

I have s buddy that works for BPA. He geeks out pretty hard on this. My understanding is that AC does travel better with less loss.

However. There is a real big DC line from the Columbia River to California. He told me the reason but I forgot.

[Pazuzu]

AC is easier to make (it naturally comes out of rotating generators)
AC is easier to use for most of our large appliances (motors like AC)
AC goes to 0 power every cycle, which means that the heat produced on a line (lost power) is sqrt(2) less than the lost heat from DC
AC changes directions each cycle, so the magnetic field formed collapses and changes direction each cycle, so it tends to average towards zero, in how it affects actual objects (like your brain)
It's easy and pretty efficient to go from AC to DC, it's very lossy going from DC to AC
AC can be stepped up to very high voltage/very low current for thin wires and massive power transfer easier than DC can be stepped up.

There's probably some aspect of self-inductance that makes one better than the other, but I don't care enough to figure that out. I think that AC actually gets worse when the self-inductance is calculated, since the magnetic field is changing. DC systems have no inductance, the magnetic fields are static.

At very high voltage, with large wires spread well apart, over long distances, both types of electricity are pretty much equal.

[Hendog]

Originally Posted by Pazuzu

AC is easier to make (it naturally comes out of rotating generators)
AC is easier to use for most of our large appliances (motors like AC)
AC goes to 0 power every cycle, which means that the heat produced on a line (lost power) is sqrt(2) less than the lost heat from DC
AC changes directions each cycle, so the magnetic field formed collapses and changes direction each cycle, so it tends to average towards zero, in how it affects actual objects (like your brain)
It's easy and pretty efficient to go from AC to DC, it's very lossy going from DC to AC
AC can be stepped up to very high voltage/very low current for thin wires and massive power transfer easier than DC can be stepped up.

There's probably some aspect of self-inductance that makes one better than the other, but I don't care enough to figure that out. I think that AC actually gets worse when the self-inductance is calculated, since the magnetic field is changing. DC systems have no inductance, the magnetic fields are static.

At very high voltage, with large wires spread well apart, over long distances, both types of electricity are pretty much equal.

Yep, I would agree...not that it matters: whether I agree or not would not change the reality of physics.

So line losses should be the same whether AC or DC is my understanding. It's really all the other aspects of generating, stepping up/down, conversion which incur losses. AC is easier to generate and manipulate wheras DC requires more "involvement" which in turn inherently incurs loss.

What got me here is a discussion with a junior colleague at work. He told me he ran AC instead of DC to some device at their cabin "because the line losses were greater with DC over 14 Ga wire". I said the losses would be the same but he likes to argue with me so he can "prove" me wrong. He said he did "the math" I tend not to engage otherwise he gets frustrated and angry so I let him "win". Ya, sure, whatever.

[3rd_gear_Ted]

By using HV AC transmission lines the conductor size can be smaller and save $$$.
AC Voltage losses also can be managed with other electrical components like voltage stabilization capacitors.
Only God and Electricians can make light

[Arizona_928]

The transmission of electric power over long distances can be done using either alternating current (AC) or direct current (DC). However, AC is generally preferred for long-distance transmission due to several key factors that reduce power loss:

Voltage Transformation: AC can be easily transformed to higher voltages and back to lower voltages using transformers. Higher voltage transmission reduces current for the same power, and lower current reduces resistive losses (power loss due to resistance in the wires). This makes AC transmission more efficient over long distances.
Skin Effect: AC experiences what is known as the skin effect, where alternating current tends to flow near the outer surface of the conductor. This effect increases the effective cross-sectional area of the conductor for AC, reducing resistance and hence power loss. However, this effect is more pronounced at higher frequencies and can increase losses if not properly managed.
Reactance and Phase Shift Management: AC systems can manage reactance (inductive and capacitive effects in the transmission line) and phase shifts between current and voltage. This management, through the use of capacitors and inductors, optimizes power flow and minimizes losses.

However, it's important to note that with advancements in technology, High Voltage Direct Current (HVDC) transmission is becoming more viable and is being used for specific applications where it can be more efficient than AC. HVDC is particularly advantageous for very long distances (typically hundreds of kilometers), underwater and underground cables, and for connecting asynchronous grids. HVDC minimizes resistive losses and eliminates the reactance and phase shift issues inherent in AC transmission.

[Rusty Heap]

Side note as I was an Electrical Engineer at Boeing on all the commercial airplanes.

Plane runs on both AC and DC.

AC runs at 400hz instead of 60hz. This allow more RMS value and lets you uses much smaller transformers on board.

DC bus on the aircraft is 28 VDC, and uses Lipo Batteries on new generation 777 and 787 programs

[David]

I work in the electric power generation industry but my knowledge of electrical engineering is limited to two courses I barely understood. What I do know is the longer the insulators you see attaching the wires to the tower, the higher the voltage and the farther you should stay away. Anything over 6’ long is really bad stuff!

[Hendog]

Originally Posted by 911 Rod

There is a very interesting documentary on Netflix (or YouTube maybe) about Tesla and Edison that might answer some of your questions.

The Current War on Netflix. A movie with Benedict Cumberbatch; runs 1:42. We watched it last night and enjoyed it.

It seems like AC is certainly more convenient to manage given the stepping up/down of voltages. Not a clear answer but I think that's the main reason AC is more wide spread.

Originally Posted by oldE

Just to complicate things for you further, long distance transmission on new construction is Ultra High Voltage DC.

Best
Les

I will have to look up the advances of Ultra High DC.

[GH85Carrera]

I listened to AC/DC for 8 hours and many hundreds of miles in distance one road trip. I just listened to my entire collection of their CDs.

[Hendog]

I'm now into the rabbit hole. OLD E suggested reading up on Ultra Hight Voltage DC. Here's a link to an article from an industry magazine https://www.powermag.com/benefits-of-high-voltage-direct-current-transmission-systems/

These are excerpts from that article:

Typical utility-scale power plants generate alternating current (AC) electricity, and most electrical loads run on AC power. Thus, the majority of transmission lines carrying power around the world are of the AC type. However, there are instances when high-voltage direct current (HVDC) transmission systems offer significant benefits.

“One big advantage to HVDC is the efficiency of power transmission over long distances,” George Culbertson, vice president of power delivery markets for HDR, told POWER. “If the transmission line route is longer than about 300 miles, DC is a better option because AC lines have more line losses than DC for bulk power transfer.”

The challenge, however, is that to transmit via HVDC, two converter stations are needed. First, the AC power must be converted to DC to begin the transmission process, and then when it gets to the desired tie-in destination, the DC power must be converted back to AC to be utilized on the grid.

In 1997, ABB commissioned the world’s first HVDC demonstration project using voltage source converters (VSCs).

VSC technology was further improved when Siemens introduced a modular multilevel converter (MMC). The Trans Bay Cable project, which runs between San Francisco and Pittsburg, California, was completed in 2010, using Siemens’ HVDC Plus system.

“HVDC is the technology of choice for reliably and efficiently transmitting large amounts of power over long distances with minimal losses…

“A DC line can deliver comparable amounts—or even higher amounts—of power using only two sets of conductors as opposed to three, so the towers don’t have to be quite as large resulting in much less installed cost on the transmission part of it. You can also run longer DC lines underground.

I'm digging up the answers to my own question, but hey I'm learning something which means "I ain't dead yet"

[scottmandue]

The lizard people that rule the world dislike DC (it gives them headaches) thus most high power lines are AC