Transmisison Line Shunt Capacitance

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Discussion Overview

The discussion centers around the presence of shunt capacitance in transmission lines compared to normal circuits. Participants explore the implications of high voltages, line lengths, and the nature of transmission lines in both power and communication contexts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question whether the term "transmission line" refers specifically to power transmission lines, noting that these lines operate at high voltages and can be very long, which contributes to significant stray capacitance.
  • Others clarify that the term also applies to communication lines and emphasize the need to distinguish between power and communication transmission lines.
  • It is suggested that transmission line models are applicable as long as the line length is long relative to the wavelength, although this is contested by some who argue that shunt capacitance is included in power grid simulations for lines longer than about one kilometer, independent of wavelength.
  • One participant raises a question about the relationship between voltage and capacitance, pointing out that higher voltage should imply lower capacitance according to the formula C = Q/V, but others counter that higher voltages can lead to increased interaction with the ground, affecting charge (Q).
  • A participant asserts that every wire in every circuit has some capacitance, including between conductors and their surroundings.

Areas of Agreement / Disagreement

Participants express differing views on the significance of shunt capacitance in transmission lines versus normal circuits, with no consensus reached on the implications of voltage and capacitance relationships or the applicability of transmission line models.

Contextual Notes

Some claims depend on specific definitions of transmission lines, and there are unresolved questions regarding the impact of voltage on capacitance in practical scenarios.

yabb dabba do
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Why is there a shunt capacitance in a transmission line, but not in a normal circuit (non-transmission line).?
 
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yabb dabba do said:
Why is there a shunt capacitance in a transmission line, but not in a normal circuit (non-transmission line).?

Do you mean a power transmission line? If yes, it is because power lines have very high voltages, and their length can be hundreds or even a thousand miles. So called stray capacitance is quite significant in those conditions.

The phrase transmission line also applies to communications. They should not be confused with power transmission lines.
 
anorlunda said:
Do you mean a power transmission line? If yes, it is because power lines have very high voltages, and their length can be hundreds or even a thousand miles. So called stray capacitance is quite significant in those conditions.

The phrase transmission line also applies to communications. They should not be confused with power transmission lines.

I think the same transmission line models are used as long as the length of the transmission line is quite long relative to wave length
 
yabb dabba do said:
I think the same transmission line models are used as long as the length of the transmission line is quite long relative to wave length
In power grid simulations, we include shunt capacitance in all lines longer than about one kilometer. It has nothing to do with wavelength.

Three phase, three wire, power lines are very different than single phase, two wire lines.
 
anorlunda said:
Do you mean a power transmission line? If yes, it is because power lines have very high voltages.

But formula for capacitance is C = Q/V.
So higher voltage, lower capacitance... ?
 
jaus tail said:
But formula for capacitance is C = Q/V.
So higher voltage, lower capacitance... ?

Higher voltages interact more with the ground, so Q is not constant.

You have to be careful with formulas in real life. Like designing a higher voltage power line. Many things other than V will change.
 
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yabb dabba do said:
Why is there a shunt capacitance in a transmission line, but not in a normal circuit (non-transmission line).?
Every wire in every circuit has capacitance. It exists, for instance, between the conductors of a telephone line, and also between every conductor and the rest of the Universe.
 

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