need some more details.
Do you currently have your 240vac turbine connected to a 3 phase transformer? and if so are there any issues?
FYI: its not as simple as derating a transformer because the frequency is lower, but lets ignore that for now.
The simplest cheapest and most efficient way to cut the turbine voltage in half or in third would be to use an auto transformer, but you lose the voltage isolation of course.
If you only need to cut it in half, this is as simple as finding the mid point of the primary coils on the transfomer you got now. (provided its connected star)
When you turn a traditional transformer into an auto tranformer (remove the secondary or don't use it)
you can put a tap as low as one third on the primary coil and get the entire rated power out. (3 times the current and 1/3rd voltage) before the temperature rise of the entire transformer matches what it was before.
in reality you can't do this because the bottom of the primary can't conduct the heat away fast enough, but if its oil cooled you can.. depending on how its wound.
a 1: 2.5 ratio is more appropriate.
putting the tap at the mid point results in no increased temperature rise, and you can even increase the load 41% before you match the losses of the original transformer.
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back to the mppt converter.
it probably has a rather flat efficiency curve, and keeping the voltage high reduces other losses, so that's the best thing to do.
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derating the transformer
for small to medium sized transformers
raw power density is proportional to the frequency, and the flux density squared.
core losses are proportional to the frequency, and flux density according to the manufacture charts.
End result is that at 1/3rd the frequency, you're going to have a transformer that is about 3 times the size for matched efficiency.
if you don't care about keeping the efficiency as high as it was, and magnetizing current low, you can increase the flux about 10-15% what is typical for 60hz machines, and that will drop it down to about 2.5 times the size.
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so if you're intending to use this 1 KVA three phase transformer you got now, and its meant for 60hz 240vac and your turbine is 20 hz 240 vac,
you're going to need to remove the secondary and fill the entire space full of the same wire the primary is made from
i figure you could probably fit enough wire in there.
maybe drop the gauge one (say its wound with 17 awg, use 18)
and before you did that tap it at appropriate locations then rectify.
because there's no secondary, you can still get about 0.6 to 0.8 Kva out (depending on exactly how much copper you could get in there) before temp rise gets out of hand
-Joe
