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AC over DC
7

AC over DC

AC over DC

(OP)
Could any one be kind enough to tell me why AC is more powerful than D.C. I did not find any convincing argument over Google.I wanted to hear from experts. I am taking baby steps.

RE: AC over DC

what do you mean by "more powerful"?

=====================================
(2B)+(2B)'  ?

RE: AC over DC

and is this specific to motors? if not I suggest another forum

=====================================
(2B)+(2B)'  ?

RE: AC over DC

(OP)
Sorry for being vague. I was watching the "War of currents" yesterday, and they were talking about How D.C needed stations every 2 mile how AC could be transmitted over 100 or miles ( Step Up Step Down) etc etc. I wanted to know the science behind the AC signal. I was always fascinated but failed to understand the electrical properties. I just want to know, the mathematics behind why DC, is inferior for power distribution than AC.  

RE: AC over DC

If DC is inferior for power distribution, then why are the highest capacity bulk transmission lines DC?

It all has to do with the cost of changing voltages.  Easy with AC, difficult with DC.

RE: AC over DC

3
That is closer to religion than scientific opinion. In fact, LONG distance power transmission lines are actually DC in many cases. The truth is, both have their place in the world, each with  advantages and disdvatages.

The reality is, the "war" between Tesla / Westinghouse and Edison was more about a battle of personalities than about specific technologies. Edison proved the value of having electricity as a simple and readily available energy source for common households, but thought that it was his alone to control and distribute at his will and profit. When Tesla and Westinghouse came up with a simpler way to distribute it more widely and NOT use the infrastructure plans that Edison had already been contemplating, he appears to have become bitter and vindictive rather than embrace it as am improvement, which he (or rather his legacy company GE) ultimately did. DC is still superior in some aspects and applications to AC, and vice versa. It's just not black and white, but "shades of grey" makes for dull reading and boring documentaries.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: AC over DC

Edison vs Tesla (Westinghouse).  Start reading.

RE: AC over DC

(OP)
Thank you for the good schooling you all.

So i never understood electricity or the whole science. Now could you tell me how i can understand any books that will help me. Where do you people suggest i start. I am taking baby steps again.I want to learn.

Thanks
Bharat Chandra Penumutchu

RE: AC over DC

This forum is really for practicing Electrical Engineers, so don't be surprised if you get bumped off by the admins. But before that happens, hopefully you will see this. It's a link to a set of free on-line courses offered by Siemens. Start at the very beginning, the one that says "Basic Electricity", because that one starts with DC (simpler to understand) and works you up to AC, and explains why. After that you can choose to go further if you want to, but to be honest it starts to get fairly industrially oriented after that. Still, I have not found anything better for free.

http://sea.siemens.com/step/default.html

Good luck.

By the way if you really are a "Computer" guy as you seem to have registered as, there is a companion forum to this one called Tek-Tips.com that you might find interesting.

http://www.tek-tips.com/

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: AC over DC

The history of electrical systems is quite interesting.  Many are surprised to find the tiny towns in the middle of nowhere had electricity before larg cities.  Mining operations were the early aaptors of this technology.  The first high tension line was in Teluride CO.  If it didn't work they figured they could turn it into a ski lift!

RE: AC over DC

(OP)
I am sure, i know it is annoying for asking generic questions. Actually. I will be very careful in what i post, see that Siemens website what what i was talking about. Actually i am into WiMAX and Networking . But along the way i also want to understand nature. Because Electrical Engineering is like simply playing with Nature, so i do want just want to know,that they are simply using OFDMA for WiMAX,i want to understand in depth why and how they use it. I want to understand in depth.

RE: AC over DC

Pick up a basic electrical book - something Electricity for Dummies and go from there.  (Not really sure that book exists - but you get my drift)

Any/all types of electricity I find quite fascinating - from static to lighting to AC/DC.

By the way I think most electric chairs are DC.  For a given voltage - it will kill you much faster...

RE: AC over DC

There are some basic facts relative to the argument of AC versus DC that need to be recalled.

When Edison built the Pearl Street station, the main load was street lights.  DC generators delivered 110 V.  At some point, you had to add another generator (dynamo), but you still couldn't deliver 110 V much beyond a clustered area.

Westinghouse followed in a different part of New York City with his AC system.  Whether the voltage was 110 V AC or DC wasn't the issue.  As with Edison's system, you couldn't go very far with low voltage delivery - whether it be AC or DC.  Tesla's development of the transformer enabled the AC system to have an unfair advantage at that time because you could step up a delivery voltage and be able to transport the power much greater distances.  The DC system didn't have any means of doing this.

I won't get into the high voltage transmission area except to note that the argument basically compares the costs of the lines and terminal equipment to see what distance does the DC system become justifiable over long distance AC.  In the past, it had been in the neighborhood of 300 miles or so, that is, above 300 miles, the DC system will be the lower cost.  Today, I'm sure the breakeven distance has come down.

To say that one is more powerful than the other is a naive argument since both long distance HVAC and HVDC rely on an AC transformer.

Maybe these fundamental points with help clarify things for BharatNT2IE.

RE: AC over DC

A lot of engineering is about doing more with less.  Wire is expensive.  With bigger wire you have less loss.  Power is volts times amps.  If you transmit electricity at higher voltage the current is lower for the same amount of power.  AC transformers easily change voltage to a higher value.  This made sending power long distances cheaper.  Early on there was no way to do this cheaply with DC.  As demands on the grid have increased they have raised the voltage on transmission lines.  This has now reached the practical limit.  With the first transmission line in Teluride CO they learned something interesting.  The wire was so small in diameter that the wire glowed at night.  They had to increase the wire diameter to stop it from glowing.  The actual peak voltage of AC is much higher than the stated RMS value. Transmitting DC at the equivelent Peak AC voltage allows more power to be sent on existing lines. It is now becoming practical to have DC to AC converters on transmission lines.  That show may have implied that AC was the end all answer.  That is not true.  Which is used is based on practical considerations.

RE: AC over DC

(OP)
Thank you very very much.
 

RE: AC over DC

I think it's important to point out to the OP that AC power transmission at High Voltage [using transformers] made generating electricity at Niagra Falls ["Free Hydro-Power"]and transmitting the power to New York City practical [reasonable power losses].

It's also important to stress that three phase power and the induction motor really cannot be separated from AC Power.  All invented by Tesla.

RE: AC over DC

magoo2:

Edisons first station at Pearl St. used the 3-wire system (his invention) that is 110V+, neutral, 110V-, giving 220V across the outer conductors. Still in use today as AC: 120-0-120V giving 240VAC.

To get the 220VDC, two dynamos were connected in series, with the center point grounded. There was also a 5-wire system +250, +125. neutral, -125, -250, last used in San Francisco, CA, Who invented this is unknown. No longer exists.

BTW: before Edison invented his famous bulb, there were already arc-lighting systems in wide use, The arc lights were on a series DC circuit, running at constant current and variable voltage, which could go up to well over 500 V.
Not a thing for the bedroom or indoor use, only outdoor.

rasevskii

 

RE: AC over DC

I maintained series incandescent street lamps in the late sixties. Some circuits could hit about 8000 Volts if they went open circuit.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: AC over DC

OperaHouse:

I don't believe you are correct as to why high-voltage DC transmission is sometimes used. The energy dissipated in a conductor over a cycle of AC is the same as the energy dissipated in DC over the same time period for the same power transmitted. With AC, the power dissipation is sometimes higher, sometimes lower, over the cycle, but it averages the saem, and the thermal time constant of the conductor is far greater than an electrical cycle, so there is no effective difference.

Rather, the problem with long-distance, high-voltage AC transmission is due to AC's greatest strength: the ease of inductive coupling. AC transmission lines couple inductively to the ground, transferring energy into the ground, producing parasitic losses. The amount of these losses is very low over short distances provided proper precautions are taken -- sufficient height off the ground given the voltage, etc. But over very long distances -- hundreds of kilometers -- they can add up to be come very significant. The problem with underwater cables is much worse.

In these cases, it can make sense to shift to DC transmission, balancing the increased losses due to the conversion from AC to DC, then DC to AC, against the reduced losses along the way.

RE: AC over DC

Perhaps my point wasn't clear. Say the line has 1 Meg V AC at 50A.  That 50A line loss is the same for DC.  However at 1.4 Meg V DC you have sent a lot more power down the line using same equipment.

RE: AC over DC

But if the resistive losses are the same at 1MV DC and 1MVrms AC, then you would need larger conductors to manage 1.4MV DC and the same current.

RE: AC over DC

Cswilson,
Nope to carry same current, you would require conductor with larger diameter for AC because of skin effect. This is totally absent in DC.

RE: AC over DC

OK - all you EE rocket scientists - how does a guy step off a helicopter and work on a 13,200 V power line with out getting killed,

Yeah - I know birds don't die either - and supposedly it is because they are not "grounded".

The real question is - "Would you do it??"

RE: AC over DC

DC transmission lines can carry more power on a given conductor not because of current-related losses, but instead because of voltage.  The peak voltage of an AC line is 1.414 times the nominal voltage.  The peak voltage of a DC line is the nominal voltage.  The voltage that a line can be operated at is due to insulation and clearance issues of its construction; for a given construction an AC line can be operated at nominal voltage .707 of the allowable voltage of the DC line.  Same current and losses (except that small percentage due to skin effect) but multiplied by a higher voltage.

That said, the rationale for DC links more often relates to system stability than to power transport capability.  To send power along an AC line, the source phase angle has to be different than the receiving phase angle, just like winding up a spring.  When the load suddenly lessens, the phase angle needs to snap back to nearer the source phase angle.  Just like a suddenly released spring ocsillating and shaking, the phase angles at the source and receiving ends (which are connected in parallel with other springs with different time contstants) oscillate.  If that oscillation exceeds a sharply defined amount, the motors and generators slip out of synchronism, and high instantaneous currents trip out circuit elements.  The system falls apart, and it gets cold and dark.

As for the birds and the linemen (there is an unexpectedly congruent phrase...)  there will be a current flow between the energized line and the incoming body.  A bird is a very small capacitive body, and the current needed to bring the uncharged bird up to line potential is probably barely perceptable.  A helicopter is a much larger capcitive body, and connection to the transmission line is made with a clamp probe bonded to the aircraft as the current (and driving voltage) is much larger.

I would suggest that this is the reason that birds are much less frequently seen roosting on transmission lines (hundred of kV) than on distribution lines. (tens of kV)  I would expect that a bird landing on a distribution line can feel a barely perceptable snap, but that landing on a transmission line probably stings a bit.  I would also expect that the birds can perceive the voltage field, and it would be evident much further from the t-line, cluing them to land elsewhere.  Helicopter pilots and linemen are harder to train...

RE: AC over DC

Well, birds are not paid a great deal of money to land on very high voltage lines -> so any discomfort discourages them sufficiently.  Linesmen and helicopter pilots, on the other hand, are paid a great deal .....  8<)    

"I won't get into the high voltage transmission area except to note that the argument basically compares the costs of the lines and terminal equipment to see what distance does the DC system become justifiable over long distance AC.  In the past, it had been in the neighborhood of 300 miles or so, that is, above 300 miles, the DC system will be the lower cost.  Today, I'm sure the breakeven distance has come down."


More seriously, there are two advantages to DC transmissions between grids, and over great distances - but only if that transmission is between two distinct points.

AC HV power to DC Very High Volt power conversion is very, very expensive for the facility and the converters; and those conversion losses reduce efficiency by a measurable amount at each end.    

A DC-DC VHV long-distance line can take advantage of the fact that the return circuit can be through the ground itself sometimes - so you need to build only half of the copper to make a complete circuit.  IF - big IF there! - there is only one circuit being built.   If you start tapping off large amounts of DC VHV power in the middle of that single-conductor long-distance DC line, then you start creating significant problems in balancing the current in that single-conductor DC (plus) line.   Build a two conductor long distance DC line to allow tapping off power in the middle, and your copper and construction costs double, plus you have to build new DC-AC  conversion facilities at every tap point, plus the same AC-AC transformer yard and switchyard to get the "new" AC power out from the DC-AC converters.    

Using a DC-DC high-voltage connection between grids is a good idea - very valuable in fact, since two side-by-side HV AC grids  will not be at the same matching frequency exactly.  So converting AC-DC at each end of the grid-grid link avoids the need to synchronize two entire grids to share power, or to blackstart the dead AC grid from either end.   You just have practical limits on how much power you can get through the HV AC lines to the grid-grid junction, and across the DC-DC line, and through the AC-DC//DC-AC converters.  

For all the above - chiefly expense never recognized by the renewable energy enviro's and their politicians, you can seldom afford to build multiple DC-AC convertor facilities just on a whim in the middle of your DC-DC cross-country links.  If you have a long distance HV AC-AC line, you can tap into that HV AC line to get more power from  a small town's generator or a new single gas turbine plant in the middle of the area, or can "tap out" power from that HV AC line to a new town, new commercial plant or customer, etc.  All if takes is a conventional transformer yard and switches.   Relatively low cost.  

So, very long distance VHV DC-DC can be economical.  Under some conditions and for some lengths.  Short distance VHV DC-DC links can be economical - sometimes even essential, regardless of their costs - under some conditions.  Very long distance HV AC-AC lines are almost always economical under almost all circumstances up to certain lengths across most terrains.  Medium and short distance, medium and low voltage DC lines are very, very seldom economically justified for more than a few thousand yards - which is what Westinghouse proved 120 years ago.   

Medium distance and short distance HV and VHV AC-AC lines are economical under all medium and short distances to actually connect the current grid to all current users from all current power plants of all sizes.  

Replacing the current grid is stupid - under all conditions at all times for all lengths - except those envisioned by the enviro'ed politicians.

 

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