Maximum lenght of a overhead line

[keesem]

Hi,
I'm trying to define the maximum length of overhead lines based on it's characteristics (R,X,max current, max power) and voltage level (high voltage).
So basically, I want to know what's the maximal length of 10, 20, 35, 110, 220, 400 kV line that is acceptable due to voltage drops and other factors.

I found on Wikipedia this: "Very approximately, the allowable product of line length and maximum load is proportional to the square of the system voltage" - (

http://en.wikipedia.org/wiki/Electric_power_transmission

- Capacity heading).
So, if I have a 110kV line with a max load of 122 MVA, it turns that the "allowable" lenght is 99.18. but wikipedia doesn't mention if it's miles or kilometers.

Is this formula applicable for my issue? Is it correct? Is it for miles of kilometers?
Is there another way to calculate this?

Thanks in advance,
keesem

 

[waross]

When the voltage drop exceeds the range of the On load Tap changer to correct the voltage you may consider that either the line is too long or is too heavily loaded. Some designers may allow a few percent over load for short term peak loading.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cranky108]

We don't use those voltages, but we do have some long spans over some canyons. The limiting factor is the suport structurers, and the spacing between the conductors due to the winds. We also use 1 meter (3 feet) diameter airplane balls on the lines, because of some medical pilots fly through those canyons.

I supose like some river crossings I have seen, you could use spacer arms, or arms supported by steel wires.

The company I work for once would use pull down structures in the valleys to control the spacing and spans. They were wood structures weighed with concerte to hold the lines down. They also had some seasonal pull down structures.

 

[wroggent]

Line loadability limits:
1.)thermal (short lines)
2.)voltage drop
3.)power transfer (long lines)

Real power delivered is given roughly by:
P=VrVs/X*sin(d)
Vr=Receiving voltage
Vs=Sending voltage
X=line reactance
d=phase angle across the line

Max practical angle is about 35 degrees.

 

[cranky108]

Maybe you should not be using wiki for engineering information.

 

[stevenal]

I believe he is talking about line length, not span length.

 

[wareagle]

The Wiki article mentions line losses in a transmission line. Losses in watts = I² x R. For a fixed length of line the
resistance R is fixed. For a fixed resistance, the heat losses dissipated is = V² x R. Therefore is you are using selected conductor size and a given acceptable loss, the selection is governed by V².

 

[bacon4life]

If you use a formula described as "Very approximate," kilometers and miles are practically the same thing. The formula is a handy rule of thumb when comparing two different power line voltages. It doesn't allow you to calculate a specific distance, much less a distance down to 0.01 miles. In practical terms, the selection of voltage class depends on the existing infrastructure nearby just as much as the calculated "optimum" voltage.

Surge Impedance Loading (SIL) is another conceptual way to discuss the capacity of transmission lines as described in this paper:

http://arproducts.org/pdf/ehv_trans...increase_in_surge_impedance_loading_level.pdf

 

[cranky108]

I have been told that in practicle terms a line is limited to about 1 mile per kV. You can play with this in many cases, but between voltage drop, loadability, and protection, that's about the limit.

 

[collies99]

If you include reactor banks/capacitor banks, SVC, synchronous condensers then this exercise may be all for naught.

 

[collies99]

Also like to add series capacitors.

 

[stevenal]

Or rectifiers.