AC over DC 7

[BharatNT2IE]

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.

 

[cswilson]

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.

 

[inpran]

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.

 

[MiketheEngineer]

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??"

 

[potteryshard]

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...

 

[racookpe1978]

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.