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AC or DC

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tomas632

New Member
Hi Guys,
I was discussing with a friend why AC is used for power transmission over DC and apart from the fact that power electronics for DC voltage stepping up/down was high cost at the time the "grid" was established we did not know any other reasons. Does AC have higher losses than DC, if so why? also does the stray inductance/capacitance in the transmission lines adjust the power factor of the voltage/current on the AC line?
Hope you can help
 

BrownOut

Banned
Does AC have higher losses than DC, if so why?

No, the losses are the same. AC can be easily transformed to higher voltages for long line transimssions.

also does the stray inductance/capacitance in the transmission lines adjust the power factor of the voltage/current on the AC line?

No, power factor correction is required when the end user's systems cause the voltage and current to be out of phase. The correction mechanisms is installed at the customer's site.
 

tomas632

New Member
how are the losses the same? does skin effect not make any difference? also does the increased heating in the dc conductor not have any negative effect?
 

Dr_Doggy

Well-Known Member
not much skin on power lines(and skin effect is for higher frequencies,ie..current flows for long enough time to penetrate through the body) , and air has no magnetic resistance(little), heat in dc causes more wires to melt.......

losses are less in AC across the wire(from plant) since wire is tuned to resonance, and less heat waste, but there is loss going from AC-DC, but still worth it in the long run
 

crutschow

Well-Known Member
Most Helpful Member
DC gets hot fast compared to the same current in AC
There is no significant difference in heat or power loss between AC and DC for the same wire and the same RMS current at power line frequencies (I²R is I²R).

The reason AC is used over DC is the ease at which voltages can be converted by a transformer. Thus the high voltage long distance lines can carry high power at a relatively low current. This is then stepped down by a transformer to a lower voltage, higher current, (typically a few thousand volts) so it can be more easily and safely be carried by the power lines in the city. This is then finally converted to the 120/240V household power by the local transformer near the residences.

The exception to this are some very high voltage DC lines that carry high power for long distances. DC has no reactive losses and, for a given peak voltage, carries more power (peak voltage is the limiting factor in high voltage lines). But it must be converted to lower AC voltages at the end of the line by elaborate high voltage inverters. Imagine the design of an solid-state inverter that can convert DC in the neighborhood of 400,000 volts to a lower voltage.
 
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KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Recently went to a talk where inverter technology (power plant size) on the order of 12 MW from solar power can be used to adjust the power factor at the point of insertion to the grid. The inverter can create current at 0 volts if it has to to adjust the power factor.

It was kind of neat to see the role of nuclear, natural gas, wind, hydro, coal and wind on the grid.
Nuclear and coal is somewhat hard to instantaneously throttle back. Natural gas is easy to control generation. Wind you have to use it, otherwise speed may get out of control.

3 phase power is extremely efficient for rotary motion. True two phase power did exist, but has largely been depreciated. A friend of mine has a 1 HP motor/compressor from 1914 which used true 2 phase power.

Power losses are still I^2*R where R is the resistance of the wire. There are superconducting transmission lines:

**broken link removed**

DC and AC systems were used for power transmission in it's infancy.
 
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