OT Electrical question

typically 600 KV

Two factors come into play to make them more efficient. YOu don't' have the voltage drop from the inductive component of the Impedance, so you have lower I2R loss, but the big one is, you also lose most of the shunt losses from the capacitance of the line to ground (and the other phases).

Why, the HVDC transmission line fairy leaves them if you've been good, of course. :)

jk

Quite right. I had it backwards. If there were an inductance effect across the line, the drop would go down with frequency. (too many double negatives). There wouldn't be an inductance effect across the line however as far as I understand. So any inductance effect in series with the line would have the same effect as any capacitance effect across the lines - which both cause the power loss to increase with frequency.

Cool.

Hi jk

If this guy has 100 feet of # 12 wire drawing 10 amps at 24 volts, how much difference would there be for AC compared with DC?

Jerry

jk sez:

Blatently wrong ! Everyone knows AC transmission falls off because of skin effect. Skin effect is that amount of conductive surface left over when RF radiates from a conductor. Duh !

Bob (six munths ago I cunt spel .....) Sw>

IIRC, it's noticable on thick (like an inch or two diameter) conductors even at 60Hz.

Best regards, Spehro Pefhany

Damned near impossible - but the conversion is easy when you convert the DC to AC, step it up, and then convert back. During the big Power Wars at the beginning Edison didn't have the modern electronics technology to pull this off, so the Westinghouse AC system won big.

But where this hits in landscape lighting is that the effective resistance of the wire is a bit lower with the electrons doing a "Conga Line" on AC and only having to sway back and forth to push the power along. With DC, each of those little electrons have to do a marathon run the entire length of the wire, and the circuit resistance is more of an issue. Not much of a difference, but it's there.

The only reason they do the HVDC transmission line is so they can easily connect between different regional power grids that are often not in phase with each other - doing the AC/DC conversion at each end means it doesn't matter, one region could be running at 400Hz and the other at 50Hz, and the HVDC intertie would work the same.

The generators are held in lock-step through the region by the grid interconnections. If one region is at 59.998 and the other at 60.002, and they're out of phase when they try connecting the conventional intertie, you are going to get a big flash and bang show...

And they do adjust the frequencies in the wee hours of the morning - when there are load surges through the day the frequency droops a slight bit, and they crank it up to get a synchronous clock back into exact step over the 24-hour period.

They have a little counter that has to get to 5,184,000 at the end of the day, give or take a bit.

-->--

The resistance is identical AC or DC, but the impedance is higher on AC. No way around it, simple physics.

It A reason, but not the only reason. Beyond a certain distance, it is more efficient.

Not really lock step, which implies in phase, as well as in sync.

Nope, it is a continuous, ongoing thing.

jk

Sphero, belaboring the obvious, sez:

"> IIRC, it's noticable on thick (like an inch or two diameter)

Hey! I already covered that when I mentioned 60 Hz RF.

Bob (knows 60 Hz radiates, too ...) Sw>

Hi jk

How much higher is the impedance of a 100 foot long length of # 12 copper wire with 60 Hz, compared with DC? I think this is where Don Foreman made his big mistake.

Jerry

It is irrelevant what the distance is, whatthe current is, or what the voltage is.

relevant data is Conductor material WIre Size Frequency WHat type of wireway it is in, and configuration of circuit(s)

Assuming a circuit that returns along the same path, magnetic wire way, #12 uncoated copper wire, STRANDED DC resistances ohms/kilometer = 6.50, Impedance Ohms/km .223 AC resistance Ohm/km at 60 Hz 6.6

jk

Hi jk

Well, I didnt actually think there would be even 1/10th of an ohm difference between 60 Hz and DC. But, the glibness of your post to correct Don Foreman where you begin with " Uh, no it doesn't." caught my attention. I am sure Don is well aware of the methods that can be used to account for loss due to frequency, but he probably figured that would have confused what could be a practical answer, like the one he gave.

I do realize that *I* learned something.

Jerry

Naw Jerry, you gotta be kidding !! Don Foreman doesn't make no stinking mistakes ! BTW, that Z would be measured (if it could be measured) in uu Ohms.

Hi Bob

I was wondering what would be the difference in voltage drop for AC compared with DC when the load is a conjugate match for the inductive part of the load seen by the "generator". Maybe you can figure that out.

Jerry

Bullshit. There are a number of long distance HVDC transmission lines that run in the +250 KV range with no "Substations". Transmitting AC over the same distances have higher radiated losses as the wire becomes a 60 Hz antenna, instead of a transmission line.

From the early days of power generation: Since DC wouldn't allow the use of transformers, the power plants had to be closer to the loads. It was broken into small areas with feeds back to the power plant where each group was individually adjusted to meet the current load. Also, light bulbs were graded and sold by voltage, depending where you were along that DC feed.

AC could be stepped up, fed through a HV transmission line and dropped back to the needed voltage anywhere along the service area with a fairly constant voltage.

A cheap transformer in a flimsy metal box will make noise when the power is applied

Cheap transformers do hum, all the time. He might have a blown fuse in the LV side of the transformer. If he had a shorted turn, it would blow the fuse, or trip the circuit breaker the transformer is connected to.

OK, but low-voltage yard lighting wire does not typically run for kilometers thru magnetic raceway.

In Jerry's example, resistance is 0.374 ohms, inductive reactance (for insulated wires spaced 0.2" apart) is .0063 ohms so Z = ( | R+jX | ) = .37405 ohms. I'd call that difference negligable.

References:

(Head hanging abashedly) I failed to take into account the kilometers of yard lighting magnetic raceway.

OK, Don, What would the difference in the DC compared with DC resitance be if the load is purely resistive as seen by the generator? I'm trying to ask, ?would "resonating" that series inductance make the AC losses closer to the DC losses?

Jerry