Cable Selection for Wind Farms

I did. And consulted my instructor but was told that I am missing something. Tried some more, searched the internet as well.....but couldn't find any specific requirment for Wind farms. So finally turned to CR4.

If someone can just point me in the right direction, I'll take it further.

OK. British Standard 7671.

err.... What exactly is your problem. Any cable sizing is to be based on design voltage insulation withstand (earthed or unearthed) & current withstand (load and fault) subject to applying the required correction factors. Permissible voltage drop limits, operating environment etc will also need to be considered. The local code should be the guide.

I was asked to find that what difficulties shall be faced in terms of cables in the following situation :

Multiple wind turbine-generators are connected together via cables at 33kV voltage level. What will be the problems in terms of cables when the farm is on no-load and when it is running on, let's say, 80% load.

I tried to solve this by going to the basics of cable sizing for wind farms specifically....but no success yet.

Why would there be anything for wind farms specifically?

Current is current, voltage is voltage, and ambient is ambient regardless.

Hint: if the plant is on no-load, then there will be no current.

when the system is on no-load current, only dielectric losses will be there.

So what? What has that got to do with cable sizing?

Assuming wind farm locations require long cable lengths for interconnection suggest you consider:

- Voltage rise due to Ferranti effect which may overduty equipment or prevent downstream paralleling due to voltage difference.

- Capacitive load on generating equipment due to cable charging current which may, depending on qty / size of units on line, cause operation outside reactive capability limits.

you might want to start out with the amount of power you expect to generate at your peak capacity and work from there

I'm assuming that this is a "mind exercise" for some class on alternate energy based upon the apparent simplicity and lack of pertinent data in the question, so my answer is "Very carefully".

btw- Old Tex is right, it's the voltage rise across the cables that can cause a lot of problems when handbook engineers just look at the first order effects like current carrying capacity but are totally clueless about the subtleties of what they're doing.

Wind farms are full of operational difficulties because nobody has spent the money necessary to have a distributed control system manage all those microgenerators and their constantly varying output.

Just curious, what is the maximum over voltage estimated in the system during load throw or extreme operating conditions. Transient over voltages within the limits of cable voltage rating should not cause insulation failure. If however the voltage regulating systems of wind units are so bad, then it is better to select the next higher voltage level of cable insulation. (till such time the desk top designers can come up with better design of voltage regulating equipments).

Instead of connected them to each other connect each to the common grid & synchronise.

Follow the local cable sizing standard. Usually a single size is used for all the daisy-chained WTGs. Rise in voltage may be looked into, however it depends on the length of the feeder and cable charging and can be mitigated by choosing equipment rated for higher voltages. Another point to consider is the cable impedance/ charging. It may lead to difficulty in injecting/absorbing the reactive power to/from the grid. So too high impedance or too high charging are both bad.Be mindful of the grid code. Load flow study would reveal this issue if it is present.

Apart from the many sound advice about distance, capacitive effect, impedance etc may I add that the following factors that should be considered: The current capacity (Iz) i.e the current carrying capacity of the cable in the situation where it is installed; **The design current (Ib) i.e the actual current to be carried by the cable and that would determine the voltage drop in the cable (voltage loss) on reaching the destination; The voltage drop per meter length of the cable, (Check IEE regulations), which I think, would be influenced by (Ib). The correction factor (Ca) for the ambient temperature; the correction factor for grouping(Cg), (if applicable) and the correction factor (Ci)for thermal insulation. You must know (Iz). Iz = Tabulated current (It) x Ca x Cg x Ci (Amps). I guess that the cables would be sheated. So in sizing of the cable these are some of the factors to consider. Iz must be greater than Ib ( Iz >> Ib.

NEC 2011, article 694 applies up to 100 KW