How to estimate active-power transmission capacity based on voltage levels and line lengths

I know voltage levels and line lengths of high-voltage transmission lines and I am looking for a rough estimation of the maximum active-power transmission capacity during normal operation. By normal operation I mean power levels that can be sustained over long durations. I understand that many factors are impacting this and that a 1-to-1 map from voltage to active power level is hard to give. But I am looking for rough estimations only, and I am happy with any upper-bound, lower-bound, or range estimations.

An example would be: out of my window, I can see a transmission line and all I know about it is that it's a single-circuit 220 kV line. In which range will the active power capacity of this line be?

• Well, thermal aspects, given that the line is already rated for the voltage you're planning to operate on, are pretty much the only limiting factor I can think of. How "large" is "this estimation is rather large"? – Marcus Müller Apr 3 '20 at 10:49
• Usually transmission line voltage drop is the design criterion rather heating. – Transistor Apr 3 '20 at 10:50
• 1 GW springs to mind as a rough value. – Andy aka Apr 3 '20 at 10:56
• @MarcusMüller if the thermal limit is the only limit in your opinion that's an answer I would say. Is it possible to estimate the thermal limit from voltage level? – Tim Tröndle Apr 3 '20 at 11:01
• No, thermal limit will be related to $I^2R$ where R is the cable resistance. Line voltage is not a factor. – Transistor Apr 3 '20 at 11:03

1 Answer

You should be able to work it out from line resistance and current flow. Voltage is almost irrelevant. Find the cross sectional area of the cables used and assume that designers do not want to raise their temperature by more than X degrees over ambient in free air. You need to look up thermal dissipation in free air

• Voltage is almost irrelevant to the thermal limit, yes, but having established the max current that way, voltage is very relevant to the overall power transmission capacity. – user_1818839 Apr 3 '20 at 11:05
• I do not know line resistance and current flow of these lines, unfortunately. Should I understand your answer as: "it is not possible"? – Tim Tröndle Apr 3 '20 at 11:44
• "Find the cross sectional area of the cables used" I'll get my slide caliper out. – Oldfart Apr 3 '20 at 12:13
• @Oldfart Well, we have this thing called "google" these days – Dirk Bruere Apr 3 '20 at 16:16
• @TimTröndle it is always possible to estimate... If you don't know specific values for the line you are dealing with, then find out for similar lines. Searching "220kV conductor section" yields this caledonian-cables.co.uk/products/hv/220kv.shtml which gives a range of cross sectional areas and resistances which may help refine that guess. – user_1818839 Apr 4 '20 at 16:52