# Designing a DC transmission line

#### togahockey15

1. The problem statement, all variables and given/known data
Design a dc transmission line that can transmit 225 MW of electricity 185 km with
only a 2.0% loss. The wires are to be made of aluminum and the voltage is 660 kV.

2. Relevant equations

P= IV = I^2 * R
R= p(L/A)
Power loss = Pf - Pi

Attempt at a solution
I worked through the equations above and found that the cross-sectional area of the wire, if it were a straight wire, would be equal to 1.27*10^-4 square meters. However, I have a bad feeling that this question might be asking for an answer in terms of step-up and step-down transformers, which is a topic I could definitely need some assistance with. Could anyone help explain this to me? My textbook wasn't too helpful... Thanks alot!

#### chroot

Staff Emeritus
Gold Member
1) It's asking for a DC transmission line, while transformers are AC devices.
2) It does not mention the any step-down voltages.
3) If you're only given four numbers, you're likely to only use equations with four or fewer numbers. Certainly, any realistic problem involving transformers will have many more important numbers than this problem provides.

You're definitely doing the right thing by just considering the I^2 R loss.

- Warren

#### togahockey15

Hey Warren,
This is probably a dumb question, but what exactly do they mean by transmission line, then?

-Nick

#### chroot

Staff Emeritus
Gold Member
In this case, a piece of wire. :)

- Warren

#### Gear300

Transformers are part of an AC system so you don't need to worry about them. You more or less listed the equations you need. Since they've already provided you with a set voltage source, all you really need to consider for this design is the transmission line.

#### togahockey15

So would the design simply be how long the wire is and what its cross-sectional area should be?

#### mgb_phys

Homework Helper
So would the design simply be how long the wire is and what its cross-sectional area should be?
Yes (and some extra letters because you can't have a reply < 4characters)

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