AC, DC, and Power Transmission
Dave the Embalmer, Mon Mar 20 2006, 05:53PMI'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!
Re: AC, DC, and Power Transmission
Dr. Dark Current, Mon Mar 20 2006, 06:24PM
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...
Dr. Dark Current, Mon Mar 20 2006, 06:24PM
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...
Re: AC, DC, and Power Transmission
Bjørn, Mon Mar 20 2006, 07:47PM
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.
Bjørn, Mon Mar 20 2006, 07:47PM
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.
Re: AC, DC, and Power Transmission
Wolfram, Mon Mar 20 2006, 08:49PM
Wikipedia has a lot of good information on this:
Wolfram, Mon Mar 20 2006, 08:49PM
Wikipedia has a lot of good information on this:
Re: AC, DC, and Power Transmission
Dave the Embalmer, Mon Mar 20 2006, 09:12PM
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 Bjørn 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.
Dave the Embalmer, Mon Mar 20 2006, 09:12PM
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 Bjørn 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.
Re: AC, DC, and Power Transmission
mikeselectricstuff, Mon Mar 20 2006, 10:31PM
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.
mikeselectricstuff, Mon Mar 20 2006, 10:31PM
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.
Re: AC, DC, and Power Transmission
Steve Conner, Mon Mar 20 2006, 11:04PM
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.
Steve Conner, Mon Mar 20 2006, 11:04PM
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.
Re: AC, DC, and Power Transmission
Marko, Mon Mar 20 2006, 11:10PM
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?
Marko, Mon Mar 20 2006, 11:10PM
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?
Re: AC, DC, and Power Transmission
johnf, Mon Mar 20 2006, 11:54PM
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
johnf, Mon Mar 20 2006, 11:54PM
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
Re: AC, DC, and Power Transmission
Simon, Tue Mar 21 2006, 12:08AM
For example, the new transmission cable across the strait between Tasmania and mainland Australia. (Search for "Basslink".)
Simon, Tue Mar 21 2006, 12:08AM
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.
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".)
Re: AC, DC, and Power Transmission
Bjørn, Tue Mar 21 2006, 02:58AM
100kV is no problem, 500 kV is common. You just stack a number of diodes or thyristors. You can buy completely standard thyristors that take 4.5kV at 3kA.
Bjørn, Tue Mar 21 2006, 02:58AM
100kV is no problem, 500 kV is common. You just stack a number of diodes or thyristors. You can buy completely standard thyristors that take 4.5kV at 3kA.
Re: AC, DC, and Power Transmission
teravolt, Tue Mar 21 2006, 04:46AM
AC is mutch more convertable than DC there fore more easaly transmited thanks to Tesla and Westing House at the turn of the century. Also Tesla worked for Edison who was a DC man so Tesla left and invented Tesla coils and poly phase circuits and the rest was history.
teravolt, Tue Mar 21 2006, 04:46AM
AC is mutch more convertable than DC there fore more easaly transmited thanks to Tesla and Westing House at the turn of the century. Also Tesla worked for Edison who was a DC man so Tesla left and invented Tesla coils and poly phase circuits and the rest was history.
Re: AC, DC, and Power Transmission
..., Tue Mar 21 2006, 05:26AM
there was a thread almost itentical to this on the old forum (but I can't find it).
I think it was going more on why do power companies use hv instead of using more copper and a lower voltage (copper is expensive), but a silimar thing goes here. It just costs too dang much to put dc-dc converters everywhere. There is an article arround refering to some huge ass vacuum tube that was supposed to able to do the converstion, but the article seemed to have a lot of bs it it.
I think they decided that at 60hz there was not much more corona losses than with dc, so they use it. I believe another reason that it caught on soo much was that it made for much better motor designs (you didn't have to use the brushed to reverse the polarity, so they last a lot longer, and you can get bonuses like them all beign syncronised).
..., Tue Mar 21 2006, 05:26AM
there was a thread almost itentical to this on the old forum (but I can't find it).
I think it was going more on why do power companies use hv instead of using more copper and a lower voltage (copper is expensive), but a silimar thing goes here. It just costs too dang much to put dc-dc converters everywhere. There is an article arround refering to some huge ass vacuum tube that was supposed to able to do the converstion, but the article seemed to have a lot of bs it it.
I think they decided that at 60hz there was not much more corona losses than with dc, so they use it. I believe another reason that it caught on soo much was that it made for much better motor designs (you didn't have to use the brushed to reverse the polarity, so they last a lot longer, and you can get bonuses like them all beign syncronised).
Re: AC, DC, and Power Transmission
IamSmooth, Wed Mar 22 2006, 05:50PM
I believe I started the thread on the last site. I was asking the same question.
IamSmooth, Wed Mar 22 2006, 05:50PM
I believe I started the thread on the last site. I was asking the same question.
Re: AC, DC, and Power Transmission
Bjørn, Thu Mar 23 2006, 01:05AM
The reason three phase AC was used over 100 years ago is that it was the only workable solution. Today DC is more efficient but conversion is very slow since fairly short term economical goals is the deciding factor.
Bjørn, Thu Mar 23 2006, 01:05AM
The reason three phase AC was used over 100 years ago is that it was the only workable solution. Today DC is more efficient but conversion is very slow since fairly short term economical goals is the deciding factor.
Re: AC, DC, and Power Transmission
Carbon_Rod, Thu Mar 23 2006, 02:02AM
AC based systems are popular and have well documented solutions. High power DC over long distances has numerous other issues most people have simply forgotten after the 1940s.
However I have seen off-the-grid homes that run a DC primary lighting and temperature control system with an AC-Inverter for traditional equipment. It’s just cheaper to build it this way sometimes and gets more efficiency out of the system by avoiding unnecessary power conversion & conditioning.
Both DC and AC generators have their advantages under certain conditions.
What is the power source and intended use (home or commercial)?
Carbon_Rod, Thu Mar 23 2006, 02:02AM
AC based systems are popular and have well documented solutions. High power DC over long distances has numerous other issues most people have simply forgotten after the 1940s.
However I have seen off-the-grid homes that run a DC primary lighting and temperature control system with an AC-Inverter for traditional equipment. It’s just cheaper to build it this way sometimes and gets more efficiency out of the system by avoiding unnecessary power conversion & conditioning.
Both DC and AC generators have their advantages under certain conditions.
What is the power source and intended use (home or commercial)?
Re: AC, DC, and Power Transmission
FastMHz, Fri Mar 24 2006, 11:35PM
Very interesting discussion. I always thought AC was better for long distance transmission than DC...at least, what's what they taught in school when you learned about the Edison vs Westinghouse battle. Aside from that, AC is good because of transformers and cheap induction motors whereas DC motors need to be brushed...OT: I wish we in the US had 240v standard in all wall outlets though instead of 120v.
FastMHz, Fri Mar 24 2006, 11:35PM
Very interesting discussion. I always thought AC was better for long distance transmission than DC...at least, what's what they taught in school when you learned about the Edison vs Westinghouse battle. Aside from that, AC is good because of transformers and cheap induction motors whereas DC motors need to be brushed...OT: I wish we in the US had 240v standard in all wall outlets though instead of 120v.
Re: AC, DC, and Power Transmission
vasil, Sat Mar 25 2006, 06:51AM
AC is considered better because you can transform the electric parameters, cheap and easily, after your necesities, any place, anytime. AC transformers can work, in any wild conditions without much survey.
vasil, Sat Mar 25 2006, 06:51AM
AC is considered better because you can transform the electric parameters, cheap and easily, after your necesities, any place, anytime. AC transformers can work, in any wild conditions without much survey.
Re: AC, DC, and Power Transmission
Carbon_Rod, Sat Mar 25 2006, 09:17AM
“DC motors need to be brushedâ€
If you meant a generator then this assertion is not completely true. It is common to have permanent magnets rotating around/between fixed coils. DC generators are easier for windmills as rpm fluctuations will not be as much an issue to remain usable. The ones that we evaluated had almost no ripple in the DC except a minor blip. I was rather amazed by the design as I was expecting to see a classic filter or regulator doing this -- yet it was without either. Most shocking was the price tag… ouch… =]
Alternators are perhaps the more popular devices these days (especially the heavy duty automotive units with external diode blocks.) The choice is easy if ones looking at buying 1 km of cheap drainage pipe or heavy copper wire. Often people try to get the generator as close to the dwelling as possible to reduce cost and maintenance issues. As it is much easier to regulate water flow it is often better for stable performance output. These are plentiful and cheap… so often are the preferred devices for both commercial and residential use – just don’t forget to clean the silt trap once in awhile.
Carbon_Rod, Sat Mar 25 2006, 09:17AM
“DC motors need to be brushedâ€
If you meant a generator then this assertion is not completely true. It is common to have permanent magnets rotating around/between fixed coils. DC generators are easier for windmills as rpm fluctuations will not be as much an issue to remain usable. The ones that we evaluated had almost no ripple in the DC except a minor blip. I was rather amazed by the design as I was expecting to see a classic filter or regulator doing this -- yet it was without either. Most shocking was the price tag… ouch… =]
Alternators are perhaps the more popular devices these days (especially the heavy duty automotive units with external diode blocks.) The choice is easy if ones looking at buying 1 km of cheap drainage pipe or heavy copper wire. Often people try to get the generator as close to the dwelling as possible to reduce cost and maintenance issues. As it is much easier to regulate water flow it is often better for stable performance output. These are plentiful and cheap… so often are the preferred devices for both commercial and residential use – just don’t forget to clean the silt trap once in awhile.
Re: AC, DC, and Power Transmission
Ben, Sat Mar 25 2006, 09:29PM
There are point to point DC transmission lines between Canada and the US.
Ben, Sat Mar 25 2006, 09:29PM
There are point to point DC transmission lines between Canada and the US.
Re: AC, DC, and Power Transmission
HV Enthusiast, Sun Mar 26 2006, 12:58AM
Contrary to popular belief, HVDC distribution has been around for over 50 years (since 1954 commercially) and there is a enormous amount of DC distribution systems all over the world.
High voltage DC is being used all over the world. There are many advantages over DC transmission vs. AC transmission.
Some of these include:
1. DC can carry much more power than AC for a given conductor size.
2. No limits to the distance it can be transferred (opposed to AC which does have limitations)
3. Active / Reactive power is much more easily controlled without additional equipment in DC lines (AC requires a lot more overhead and equipment to do this . . . with DC it can be controlled via the switching)
4. Power flow direction can be changed easily (not so with AC)
5. Plus there is plenty of data on HVDC lines. The first commercial HVDC line was built back in 1954!!! Its been around for a long time.
HV Enthusiast, Sun Mar 26 2006, 12:58AM
wrote ...
Carbon Rod wrote
AC based systems are popular and have well documented solutions. High power DC over long distances has numerous other issues most people have simply forgotten after the 1940s.
Carbon Rod wrote
AC based systems are popular and have well documented solutions. High power DC over long distances has numerous other issues most people have simply forgotten after the 1940s.
Contrary to popular belief, HVDC distribution has been around for over 50 years (since 1954 commercially) and there is a enormous amount of DC distribution systems all over the world.
vasil wrote ...
AC is considered better because you can transform the electric parameters, cheap and easily, after your necesities, any place, anytime. AC transformers can work, in any wild conditions without much survey.
AC is considered better because you can transform the electric parameters, cheap and easily, after your necesities, any place, anytime. AC transformers can work, in any wild conditions without much survey.
High voltage DC is being used all over the world. There are many advantages over DC transmission vs. AC transmission.
Some of these include:
1. DC can carry much more power than AC for a given conductor size.
2. No limits to the distance it can be transferred (opposed to AC which does have limitations)
3. Active / Reactive power is much more easily controlled without additional equipment in DC lines (AC requires a lot more overhead and equipment to do this . . . with DC it can be controlled via the switching)
4. Power flow direction can be changed easily (not so with AC)
5. Plus there is plenty of data on HVDC lines. The first commercial HVDC line was built back in 1954!!! Its been around for a long time.
Re: AC, DC, and Power Transmission
Carbon_Rod, Sun Mar 26 2006, 08:37AM
"Contrary to popular belief, HVDC distribution has been around for over 50 years (since 1954 commercially) and there is a enormous amount of DC distribution systems all over the world."
DC distribution to regular home grids was halted a long time back in these parts. Are you saying people buy DC power from the company directly in some places? I have not heard any non-historical and non-private reference. Hmmm... This sounds neat...
Yep, people now even know why these lines hang off centre, get solar flare and capacitor damage. =]
A DC biased AC line is often undesirable as it introduces numerous other issues like transformer problems etc. -- Can get very complex indeed... IIRC one community even had a legacy Tesla power plant rusting away until the 1970's when the local hydro companies river cleanup project literally blew it out of the water.
EVR:
1.) Energy is always conserved
2.) Try it under load... And no cheats with super conductors =D
3.) Its called zero-cross switching in these parts. Its not that complex.
4.) No comment... even with a few beers I still don't know the context =P
5.) Dependent on location. We have had many unique lighting and power systems in mining operations -- some known and others are more obscure.
Dave, "Can anyone shed some light on this matter? "
Phasers, resonance, and magnetics can be tricky topics to explain -- minus the beers. Each system (dredges up a very stale argument) has their strengths and weaknesses depending on the source and application -- I assumed you meant a residential service line based on the context.
Some questions you may want to ask yourself:
a.) Power source: Wind/Water/Solar/Pole etc…
b.) Source to Service location obstacles
c.) Construction supply costs in your area
d.) Intend to sell the power back to the grid to subsidize maintenance costs
e.) Legal and environmental restrictions
Cheers,
Carbon_Rod, Sun Mar 26 2006, 08:37AM
"Contrary to popular belief, HVDC distribution has been around for over 50 years (since 1954 commercially) and there is a enormous amount of DC distribution systems all over the world."
DC distribution to regular home grids was halted a long time back in these parts. Are you saying people buy DC power from the company directly in some places? I have not heard any non-historical and non-private reference. Hmmm... This sounds neat...
Yep, people now even know why these lines hang off centre, get solar flare and capacitor damage. =]
A DC biased AC line is often undesirable as it introduces numerous other issues like transformer problems etc. -- Can get very complex indeed... IIRC one community even had a legacy Tesla power plant rusting away until the 1970's when the local hydro companies river cleanup project literally blew it out of the water.
EVR:
1.) Energy is always conserved
2.) Try it under load... And no cheats with super conductors =D
3.) Its called zero-cross switching in these parts. Its not that complex.
4.) No comment... even with a few beers I still don't know the context =P
5.) Dependent on location. We have had many unique lighting and power systems in mining operations -- some known and others are more obscure.
Dave, "Can anyone shed some light on this matter? "
Phasers, resonance, and magnetics can be tricky topics to explain -- minus the beers. Each system (dredges up a very stale argument) has their strengths and weaknesses depending on the source and application -- I assumed you meant a residential service line based on the context.
Some questions you may want to ask yourself:
a.) Power source: Wind/Water/Solar/Pole etc…
b.) Source to Service location obstacles
c.) Construction supply costs in your area
d.) Intend to sell the power back to the grid to subsidize maintenance costs
e.) Legal and environmental restrictions
Cheers,
Re: AC, DC, and Power Transmission
ragnar, Sun Mar 26 2006, 09:26AM
Surprising for most people, skin depth in copper is 8mm-9mm depending on your frequency (50Hz vs 60Hz)... not all that deep! (assuming conductivity 1.68E-8 ohm/m, permeability 1.2566E-6 uN/A^2). Plenty of high-tension powerlines are steel-cored with aluminium strands around the outside, so don't use those figures when thinking about real powerlines.
I got a fright when that hit me. I'd genuinely though that skin depth was totally insignificant at ~50Hz.
ragnar, Sun Mar 26 2006, 09:26AM
Surprising for most people, skin depth in copper is 8mm-9mm depending on your frequency (50Hz vs 60Hz)... not all that deep! (assuming conductivity 1.68E-8 ohm/m, permeability 1.2566E-6 uN/A^2). Plenty of high-tension powerlines are steel-cored with aluminium strands around the outside, so don't use those figures when thinking about real powerlines.
I got a fright when that hit me. I'd genuinely though that skin depth was totally insignificant at ~50Hz.
Re: AC, DC, and Power Transmission
Wolfram, Sun Mar 26 2006, 01:50PM
Guys, we are talking about two entirely different things here. This thread is about DC power transmission, not DC power to households. DC power transmission is common (as stated a lot of times in this thread), but it is of course made back into AC before being transformed down and distributed to households. The fact that they use semiconductors does not limit the voltage (as also stated earlier in this thread), there are infact several 1.2MV lines in use (check the Wikipedia link I posted earlier in this thread), and I remember reading about a 1.67MV DC line between USA and Canada (do not quote me on this).
I hope these misunderstandings are out of the way now.
Wolfram, Sun Mar 26 2006, 01:50PM
Guys, we are talking about two entirely different things here. This thread is about DC power transmission, not DC power to households. DC power transmission is common (as stated a lot of times in this thread), but it is of course made back into AC before being transformed down and distributed to households. The fact that they use semiconductors does not limit the voltage (as also stated earlier in this thread), there are infact several 1.2MV lines in use (check the Wikipedia link I posted earlier in this thread), and I remember reading about a 1.67MV DC line between USA and Canada (do not quote me on this).
I hope these misunderstandings are out of the way now.
Re: AC, DC, and Power Transmission
HV Enthusiast, Sun Mar 26 2006, 02:37PM
Wrong. For a given conductor size and length, equivalent resistance of a transmission line (especially over long distances as is common with high voltage power transmission) is always higher for AC vs. DC, mostly due to skin effect. So yes, for a given conductor size, DC lines can carry more power than AC lines.
Uh wrong again. Due to standing waves, reflections on the line, AC power transmission distances are limited. This is the reason DC power lines are used in areas where extremely long transmission distances are involved.
Uh nope. Not even close. In AC lines, this is mostly controlled by switching in and out capacitors on the lines, and in power substations, very large capacitors are switched in and out depending on line conditions. With DC, there is no need for any additional equipment. Control can be done through the actual switching during up and down AC/DC and DC/AC conversion.
The power grid is a very complex structure. Depending on circumstances, it is sometimes necessary to divert electrical flow from one generating plant to another, move power from this point to that point, etc.... The power grid is a very complex switching network. AC lines are typically unidirectional so this makes switching a bit more difficult. However, with DC transmission, the lines are effectively bidirectional and power flow direction can be easily switched.
Attached is a diagram of some of the larger HV DC transmission lines located around the world, some in place since the 1950's.
HV Enthusiast, Sun Mar 26 2006, 02:37PM
wrote ...
EVR:
1.) Energy is always conserved
EVR:
1.) Energy is always conserved
Wrong. For a given conductor size and length, equivalent resistance of a transmission line (especially over long distances as is common with high voltage power transmission) is always higher for AC vs. DC, mostly due to skin effect. So yes, for a given conductor size, DC lines can carry more power than AC lines.
wrote ...
2.) Try it under load... And no cheats with super conductors =D
2.) Try it under load... And no cheats with super conductors =D
Uh wrong again. Due to standing waves, reflections on the line, AC power transmission distances are limited. This is the reason DC power lines are used in areas where extremely long transmission distances are involved.
wrote ...
3.) Its called zero-cross switching in these parts. Its not that complex.
3.) Its called zero-cross switching in these parts. Its not that complex.
Uh nope. Not even close. In AC lines, this is mostly controlled by switching in and out capacitors on the lines, and in power substations, very large capacitors are switched in and out depending on line conditions. With DC, there is no need for any additional equipment. Control can be done through the actual switching during up and down AC/DC and DC/AC conversion.
wrote ...
4.) No comment... even with a few beers I still don't know the context =P
4.) No comment... even with a few beers I still don't know the context =P
The power grid is a very complex structure. Depending on circumstances, it is sometimes necessary to divert electrical flow from one generating plant to another, move power from this point to that point, etc.... The power grid is a very complex switching network. AC lines are typically unidirectional so this makes switching a bit more difficult. However, with DC transmission, the lines are effectively bidirectional and power flow direction can be easily switched.
Attached is a diagram of some of the larger HV DC transmission lines located around the world, some in place since the 1950's.
Re: AC, DC, and Power Transmission
Sulaiman, Sun Mar 26 2006, 03:01PM
What makes an ac line unidirectional ?
seems strange to me.
Sulaiman, Sun Mar 26 2006, 03:01PM
What makes an ac line unidirectional ?
seems strange to me.
Re: AC, DC, and Power Transmission
Carbon_Rod, Sun Mar 26 2006, 09:39PM
Actually I had asked this question a few years back on the old 4hv for insight into this very problem of syncing AC generators to the service lines. search old.4hv.org
I am getting bored with the “dead parrot†rhetorical arguments about apples and oranges...
I can’t tell who is more off topic in this thread now. lol
But if this thread is still alive in a few days I may drop by again.
Good luck,
Carbon_Rod, Sun Mar 26 2006, 09:39PM
Actually I had asked this question a few years back on the old 4hv for insight into this very problem of syncing AC generators to the service lines. search old.4hv.org
I am getting bored with the “dead parrot†rhetorical arguments about apples and oranges...
I can’t tell who is more off topic in this thread now. lolBut if this thread is still alive in a few days I may drop by again.
Good luck,
Re: AC, DC, and Power Transmission
mikeselectricstuff, Sun Mar 26 2006, 10:24PM
mikeselectricstuff, Sun Mar 26 2006, 10:24PM
Sulaiman wrote ...
What makes an ac line unidirectional ?
seems strange to me.
It makes perfect sense if you read all the stuff the makers of exotic speaker cables claim... (and can stop yourself laughing...) What makes an ac line unidirectional ?
seems strange to me.
Re: AC, DC, and Power Transmission
HV Enthusiast, Mon Mar 27 2006, 12:37AM
Bidirectionality is not a function of the actual wire. Its a function of the entire power system architecture.
HV Enthusiast, Mon Mar 27 2006, 12:37AM
Bidirectionality is not a function of the actual wire. Its a function of the entire power system architecture.
Re: AC, DC, and Power Transmission
Sulaiman, Mon Mar 27 2006, 08:29AM
OK, but what makes a line (or part of the system) uni-directional?
AFAIK wires, transformers, capacitors etc. don't know about directionl,
voltage, current, power, phase etc. etc. meters don't care.
I beliave here in UK (where the country/grid is a small part of a wavelength long)
power stations get into synch. with the grid
connect, and increase power output.
I've always thought of an electrical grid like water and pipes.
So what makes a high voltage ac line/part of a national grid uni-directional?
Sulaiman, Mon Mar 27 2006, 08:29AM
OK, but what makes a line (or part of the system) uni-directional?
AFAIK wires, transformers, capacitors etc. don't know about directionl,
voltage, current, power, phase etc. etc. meters don't care.
I beliave here in UK (where the country/grid is a small part of a wavelength long)
power stations get into synch. with the grid
connect, and increase power output.
I've always thought of an electrical grid like water and pipes.
So what makes a high voltage ac line/part of a national grid uni-directional?
Re: AC, DC, and Power Transmission
Quantum Singularity, Wed May 31 2006, 03:25AM
I know this thread is a bit old but since it was brought up again I thought I'd take a stab at the above question. Like EVR said a transmission network is actually pretty complicated. Although a line is bidirectional and not uniderectional per-say, when your dealing with AC there are bad things that happen when you try to take several hundred megawatts going in one direction and lets say, turn it around to going the other direction in like 1-cycle. Although I dont have all the technical reasons on hand, that is one of the issues that happened in the north eastern U.S. and into canada a couple years ago. Due to a cascading transmission like failue, other interconnecting transmission lines which were pushing power one way had to turn around and go the opposite way. This occurance will cause all sorts of protective relays on lines and generators to open. Hence a large part of the north east U.S. went out black. Obviously there were alot of other factors at play in that particular scenario, but that 'turn-around surge' really caused some problems. I dont know if thats what EVR was getting at but thats about all I can think because otherwise most transmission lines are bidirectional. I am however not sure what the difference with a DC line would be in the above scenario?
Quantum Singularity, Wed May 31 2006, 03:25AM
I know this thread is a bit old but since it was brought up again I thought I'd take a stab at the above question. Like EVR said a transmission network is actually pretty complicated. Although a line is bidirectional and not uniderectional per-say, when your dealing with AC there are bad things that happen when you try to take several hundred megawatts going in one direction and lets say, turn it around to going the other direction in like 1-cycle. Although I dont have all the technical reasons on hand, that is one of the issues that happened in the north eastern U.S. and into canada a couple years ago. Due to a cascading transmission like failue, other interconnecting transmission lines which were pushing power one way had to turn around and go the opposite way. This occurance will cause all sorts of protective relays on lines and generators to open. Hence a large part of the north east U.S. went out black. Obviously there were alot of other factors at play in that particular scenario, but that 'turn-around surge' really caused some problems. I dont know if thats what EVR was getting at but thats about all I can think because otherwise most transmission lines are bidirectional. I am however not sure what the difference with a DC line would be in the above scenario?
Re: AC, DC, and Power Transmission
mike0t4ever, Sat Jun 03 2006, 06:56PM
in our power systems analysis text book the way they calculate power flow was using a [ABCD] constants (it's essentially the same as for microwave frequency equipment)
one of the results that was also weird the first time the prof persented at lecture, was that with AC lines it becomes possible for the voltage at the receiving end to be higher than at the sending end depending on how much current is drawn by the recieving load.
the equations are a little messy and i'm in no mood to plagiarize the book right now
mike0t4ever, Sat Jun 03 2006, 06:56PM
Sulaiman wrote ...
OK, but what makes a line (or part of the system) uni-directional?
AFAIK wires, transformers, capacitors etc. don't know about directionl,
voltage, current, power, phase etc. etc. meters don't care.
.....
So what makes a high voltage ac line/part of a national grid uni-directional?
OK, but what makes a line (or part of the system) uni-directional?
AFAIK wires, transformers, capacitors etc. don't know about directionl,
voltage, current, power, phase etc. etc. meters don't care.
.....
So what makes a high voltage ac line/part of a national grid uni-directional?
in our power systems analysis text book the way they calculate power flow was using a [ABCD] constants (it's essentially the same as for microwave frequency equipment)
one of the results that was also weird the first time the prof persented at lecture, was that with AC lines it becomes possible for the voltage at the receiving end to be higher than at the sending end depending on how much current is drawn by the recieving load.
the equations are a little messy and i'm in no mood to plagiarize the book right now
Re: AC, DC, and Power Transmission
Quantum Singularity, Tue Jun 06 2006, 12:28AM
I beleive what you are refering to is the Ferranti Rise. I cannot remember formulas off hand but it has to do with the Mvar loading on the line, and if it is very lightly loaded or open ended, the line will act like a 'shunt capacitor' and the voltage will rise. The highest voltage is seen at the open end of the line. On transmission lines shunt caps are used for var support and help boost the voltage. With an excessive amount of var support and not very much load, the voltage will be high.
Quantum Singularity, Tue Jun 06 2006, 12:28AM
...with AC lines it becomes possible for the voltage at the receiving end to be higher than at the sending end depending on how much current is drawn by the recieving load.
I beleive what you are refering to is the Ferranti Rise. I cannot remember formulas off hand but it has to do with the Mvar loading on the line, and if it is very lightly loaded or open ended, the line will act like a 'shunt capacitor' and the voltage will rise. The highest voltage is seen at the open end of the line. On transmission lines shunt caps are used for var support and help boost the voltage. With an excessive amount of var support and not very much load, the voltage will be high.
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