AC, DC, and Power Transmission / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

AC, DC, and Power Transmission

1 2 3 4 next

Move Thread

LAN_403

Dave the Embalmer

Mon Mar 20 2006, 05:53PM

Registered Member #332 Joined: Mon Mar 20 2006, 05:41PM
Location:
Posts: 2

I've been googling this extensively for a couple days now, but I've yet to yield any decent resuelts =(.

I was under the impression that, for long-range power transmission, AC or DC would work equally well. The only problem with DC is the difficulty in stepping it up or down, but that is irrelevent in this day and age when solid state switchers could take over what transfomers do for AC.

Its of course not nearly that simple, the thing that matters is cost, and I know nothing on the costs/reliability/etc. differences between using tried and true transfomers, and state-of-the-art solid state switching.

However, purely in the context of power lost from point A to point B, it seems to me that whether AC or DC is used is meaningless. 120V DC traveling down a line should be just the same as 120V AC in terms of losses, right? (Or should it be 170V DC? Not sure).

I was told that there is a very large difference, and there is another form of loss associated with DC. I heard something about DC needing to be far higher voltage than AC for transmission (this makes no sense to me), as well as 'repeater stations' needed periodically (whatever that means). Another person said that AC got around some of the loss one would get with DC, but he isn't sure why or what, but is positive that there's something like that going on.

Personally, it sounds like a load of horse pucky to me. Ohms law is Ohms law, and a power line with x ohms of resistance will cause just as much losses if the current is AC OR DC. Can anyone shed some light on this matter? The two people I heard this from I generally consider to be my betters when it comes to electrical theory.

Thanks!

Dr. Dark Current

Mon Mar 20 2006, 06:24PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

The generators in power plants spin at line frequency - they generate AC current. Why rectify it? Also the long-distance power lines are very high voltage, no such solid state switches exist.
The solid state approach is also way too unreliable, expensive...

Bjørn

Mon Mar 20 2006, 07:47PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Power transmission is actually fairly complicated. It is not as simple as getting energy from point A to point B. The power is transferred through a grid where regulation becomes very complex. Semiconductors for very large currents and very large voltages are expensive but if you can increase efficiency by 1% it will add up to a lot of money in a year.

DC is more efficient because AC has problems with corona discharge, capacitance, inductance and the skin effect. Distributing AC is technically simpler and needs less investments in hardware.

If the line is very long or has special requirements then DC can be a cheaper investment because DC lines are simpler to construct and may offset the cost of the expensive conversion stages.

Wolfram

Mon Mar 20 2006, 08:49PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Wikipedia has a lot of good information on this:

Dave the Embalmer

Mon Mar 20 2006, 09:12PM

Registered Member #332 Joined: Mon Mar 20 2006, 05:41PM
Location:
Posts: 2

Hi, thanks for the quick replies!

I would like to make myself clearer on a couple of things though:

I'm not suggesting DC would be better for power distribution, and clearly the cost of switching it is expensive when its even possible.

All I'm really asking is, ignoring every thing else, you were sending power over 10 miles of cable or whatever, would losses be indestinguishable between AC and DC (or maybe even very slightly in DC's favor from what BjÃ¸rn wrote). I'm curious to make sure that my understanding of theory is sound, and that my two friends must have misunderstood something, or that they're right, and I'm completely missing part of the puzzle.

Thanks again for the help.

mikeselectricstuff

Mon Mar 20 2006, 10:31PM

Registered Member #311 Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253

Another issue is that synchronisation over long lines becomes difficult - feeds are usually combined to match demand as required, and so must be exactly in phase - with DC, the DC-AC converter can easily be synchronised to the local AC distribution network.
For a simple point-to-point long-distance line, AC would suffer more losses dur to corona and capacitance. The optimum solution depends on the balance between the cost of the losses and the cost of the AC-DC-AC conversion.

Steve Conner

Mon Mar 20 2006, 11:04PM

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

Dave The Embalmer: You are right, the I2R losses in a wire will be the same for equal currents no matter if it is AC or DC. (assuming you go by the RMS value of the AC current.)

When you start taking into account multi-phase systems it all gets more complex. There is a table in my Newnes Electrical Pocket Book that shows what weight of copper you need for the various transmission systems, for a given constant percentage loss and peak voltage to ground. 2-wire DC is arbitrarily set at 100 and the power factor of the AC systems is assumed to be 1.

2-wire DC: 100
3-wire DC: 31.25
Single phase 2-wire AC: 200
Single phase 3-wire AC: 62.5
Two phase 4-wire AC: 200
Three phase AC without neutral: 50
Three phase AC with half-size neutral: 58.3

So 3-wire DC is actually more efficient in theory than any AC system. AC is handicapped because the peak voltage is 1.4 times the RMS. The RMS determines how much power you can carry and the peak determines how big insulators you need.

Hence, In this age of cheap large SCRs and IGBTs, they do actually use high voltage DC for some power lines, with a rectifier at one end and an inverter at the other.

Marko

Mon Mar 20 2006, 11:10PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hence, In this age of cheap large SCRs and IGBTs, they do actually use high voltage DC for some power lines, with a rectifier at one end and an inverter at the other.

hm but what can oyu do with etc. 100kV HVDC system, its ''little'' too much for semicondctors,
maybe some horrendous tubes could be used.
I always wondered how can such DC voltage be used at destination any way and safely?

johnf

Mon Mar 20 2006, 11:54PM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

AC power transmission over long distances reaches a point where no power gets to the other end of the line due to voltage and current being totally out of phase ie reactive power loss. with the AC grid even after relativly short distances synchonous capacitors are used to correct the reactive loss so that the real component of the power can in fact be used. The power conversion for DC links used to be done with ignitrons but now all systems are solid state using huge series stacks of hockey pack SCR's / IGBTs. Here vin NZ power is transmitted between the two main islands at plus 375,000 volta and minus 375,000 volts at a constant 1100 amps ie a continuous 825 megawatts and all rectification and inversion is done solid state. each end of the link is the same so power can flow either way with transmission distance of about 450 miles loop resistance of the cable = nine ohms so they loose 11 megawatts in I squared R. I understand that rectification and inversion run at better than 99% efficiency

Simon

Tue Mar 21 2006, 12:08AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

Steve Conner wrote ...

Hence, In this age of cheap large SCRs and IGBTs, they do actually use high voltage DC for some power lines, with a rectifier at one end and an inverter at the other.

For example, the new transmission cable across the strait between Tasmania and mainland Australia. (Search for "Basslink".)

1 2 3 4 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob