First things first I am not an electrical engineer at all so please bear that in mind when you reply/comment.

I am looking at the following problem, I have a power line which connects a windfarm to a consumer.

I make the assumption that the consumer will always be able to consume the power provided by the windfarm.

I assume that the windfarm actual power generation capacity varies between zero and a maximum that is exceeding the capacity of the power line.

My question is: How can I determine the maximum amount of power that can transit through the power line from the windfarm to the consumer before curtailment in required ? or the transmission line upgraded.

So far I have used the following:

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where I(Amps) is the maximum current that the line can handle before melting. Vphase is the voltage of the line (lets assume a transmission rated at 400 kv). and pf the power factor of say 0.8.

Using this equation and assuming I is 1000 amps, Vphase 400 kv and pf 0.8, I find that the maximum power that can transit through the line is 692.8 MVA or 554.2 MW.

If that is true (which I suspect it isn't), am I correct in thinking that if the wind blows hard and the windfarm could produce over 554.2 MW then all that excess power capacity would have to be curtailed ?

  • \$\begingroup\$ If the power line not very short, you have to deal with transmission line effects. When using underground cables, their capacitance limits the useful maximum length to about 80 km. \$\endgroup\$
    – Uwe
    Feb 20, 2019 at 17:30
  • \$\begingroup\$ This really depends on distance between wind farm and load and the conductors chosen. \$\endgroup\$ Feb 20, 2019 at 21:10
  • \$\begingroup\$ Can these losses be estimated simply ? e.g. using an equation which takes the distance between the power source and the load and the initial power amount ? For example, something that tells me that if I have a windfarm producing 250 MW and that the consumer is at the other end of a 100 km line he will only receive 200 MW. \$\endgroup\$
    – Sorade
    Feb 21, 2019 at 15:46

1 Answer 1


Your calculations look correct.

Several factors can affect the maximum load:

  • Maximum allowable voltage drop.
  • Transformer rating at each end.
  • Switchgear and protection device ratings.
  • Heating effect in the cable.

All of these have to be assessed and the first limit you hit determines the power limit.

I notice that you are specifying a 0.8 PF operating condition. Bringing this closer to 1.0 PF will result in lower currents with resultant decrease in voltage drop. This may require addition of power-factor correction equipment by your users. The traditional method of encouraging good power-factor is to charge a low power factor financial penalty for anything below 0.95 PF.

  • \$\begingroup\$ Thanks for your answer. I wouldn't know how to assess the 4 factors you mentioned. It seems to me that when I look at a transmission network map the line voltage is a given. I assume the maximum current through the cable is a function of the cable itself (insulation, surface area of the copper core etc ... ), and I'm not quite sure what the PF is but I think it's to do with phase losses, and serves to convert from MVA to MW. Do you think my approach is correct in terms of using PF, I and V as my known variables ? \$\endgroup\$
    – Sorade
    Feb 20, 2019 at 15:16

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