# Question On AC DC voltage Hystersis Loss

Discussion in 'General Electronics Chat' started by hitmen, Oct 10, 2014.

1. ### hitmen Thread Starter Active Member

Sep 21, 2008
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DC voltage is not used in transmission because the I2R loss is high on transmission lines.
So AC voltage is used.
Now, alternating current is defined as electrons flowing in different directions.
Isnt a lot of power dissipated when electrons are forced to flow in another direction?
I dont understand.
Conversely, in DC voltage, electrons only flow in one direction.

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Of course DC is used for high power electricity transmission. AC is not exclusively used.

3. ### hitmen Thread Starter Active Member

Sep 21, 2008
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Hi you are not answering my question

4. ### JDT Well-Known Member

Feb 12, 2009
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The I2R loss is the same for AC and DC. This is the power loss due to the resistance of the wires.

With AC the inductance of the wires and the capacitance between the wires and the wires and ground have additional effects. These are "reactive" effects and don't dissipate power (although they may cause additional power losses elsewhere in the system if care is not taken).

At high AC frequency, other effects can cause an increase in the effective resistance of the wire. But not at low frequencies (50 - 60 Hz).

No extra power is dissipated just because the direction of the current is reversed.

Actually, DC is used for long-distance high voltage power transmission especially for under-sea cables.

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Sorry - I know I'm a pain in the **** . Unfortunately this is another factual error in this thread - or perhaps simply an oversight. There is "skin effect" in AC current conduction which is not present in DC conduction. The I^2R [Joule heating] losses in conductors are somewhat higher with AC than with DC for the same conductor gauge and equivalent heating current.

Last edited: Oct 10, 2014
6. ### Papabravo Expert

Feb 24, 2006
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The "trick" in AC power transmission is to raise the voltage while simultaneously reducing the current. A transformer will do this with AC, but not DC.

7. ### JDT Well-Known Member

Feb 12, 2009
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I did mention this:-
But I thought I would not get too technical at this stage!

8. ### alfacliff Well-Known Member

Dec 13, 2013
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skin effect is not applicable at 50 to 60 hz.
likewise unless when switching on and off, hystresis is not applicable to dc.

9. ### profbuxton Member

Feb 21, 2014
233
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DC can and is used for power transmission but usually at very high voltages and quite long distances. There are some high volt dc lines used in some overseas countries ( Sweden or one of those countries). There are some technical reasons for using DC for these applications including lower losses particularly capacitive charging losses. Also can be used to connect systems which do not need to be synchronised.
The voltage is very high so the current can be low for the same power delivery. The method is to rectify high volt AC to DC and then convert it back AC at the receiving station. Only modern semiconductors have allowed this approach effectively. China has an 800KV line 2000km long (google HVDC power).
AC lines have reactance (inductance & capacitance) which requires generating stations allow capacity for "reactive power" IE VARS which does no useful work but requires cable capacity in the system for this "reactive power " to flow.
As noted, "skin effect" only comes into "play" at radio frequencies and above and is generally ignored at power line frequencies.

10. ### crutschow Expert

Mar 14, 2008
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3,244
Another important factor is that, since the peak voltage for AC is higher than the RMS voltage, for a given maximum allowable insulator voltage, you can transmit 1.4 times more power with DC as compared to AC.

As far as the reversal of the electron motion in AC, the electron's mass is so minute and its motion is so small, that the energy required to reverse there motion is intestinal.

Last edited: Oct 11, 2014
11. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Skin Effect is significant in certain situations at power line frequencies. Very high ampacity power cables are designed with the intent of reducing skin effect.

12. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Hopefully not an afore to unknown medical condition.

13. ### profbuxton Member

Feb 21, 2014
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Gentlemen, I stand corrected. I have researched and find that "skin effect" does need to be taken into account at power frequencies particularly for high current transmission lines and in transmission sub stations.

14. ### crutschow Expert

Mar 14, 2008
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How true. Of course I meant infinitesimal.