Obvious question on transformer calc

[Spark2be]

Calculating the voltage to be able to calculate the full load current of a 850kva 32kv/400v 3 phase transformer with 8% reactance:

400 x 8
100 = 32v

can some one explain this? It won't bloody sink in until I can see why I am 1. Multiplying the voltage by the reactance and 2. Dividing it by 100?? I know it will be simple to you guys...

thanks.

 

[Knobhead]

Post the full question. Reactance is normally used in fault current calculations.

 

[mankman2012]

yes pls send post the full question or the jon you are working on which is why you need to use this formular. Thanks

 

[Spark2be]

An 750kva 11kv/400v 3 phase transformer has a reactance of 5%. If a short circuit of negligible impedance occurs on the secondary while rated voltage is applied to the primary calculate the short circuit mva and the short circuit current.

I can remember and calculate the steps but what I am trying to do is understand each step and know why I am doing each step, if you know what I mean.

Thanks.

 

[Knobhead]

Post what you've got so far.

 

[Spark2be]

1. Find voltage to be able to calculate the full load current

400 x 5
100 = 20v

2. Normal full load current
400
20 = 20

3. Short circuit power
750000 x 20 = 15 mva

4. Short circuit current
15000000
400 = 37.5kva

now I think that's correct but I am having trouble with the particulars of steps 1 & 2.

thanks.

 

[Knobhead]

You need to calculate the transformers full load current and then divide by the reactance to give the maximum fault current.

This only holds good for a theoretical zero impedance source.

 

[Spark2be]

That should have read 37.5ka (not kva)

thanks for the advice Tony.

So is the answer to step 2 in amps?? Volts divided by volts = amps?? Apologies for being a bit dumb lol.

 

[Knobhead]

Transformer full load current = (P/3)/(U/√3)
Transformer maximum fault current = FLC/ reactance

I get ((850000/3)/(400/√3))/0.08 = 15.33KA

 

[Spark2be]

Sorry I'm confusing two similar questions, the question is now correct.

 

[Knobhead]

Obvious question on transformer calc, I don't think so! If you want help please get the questions right in the first place.

A thank you would be nice.