# Transmisison Line Shunt Capacitance

1. Nov 2, 2016

### yabb dabba do

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

2. Nov 2, 2016

### Staff: Mentor

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.

3. Nov 2, 2016

### yabb dabba do

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

4. Nov 2, 2016

### Staff: Mentor

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.

5. Nov 3, 2016

### jaus tail

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

6. Nov 3, 2016

### Staff: Mentor

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.

7. Nov 6, 2016

### tech99

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.