Figure 3P fault current from 3 1P xfmrs

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collier

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I have software that figures the available fault current from a 3 phase transformer (by the size and impedance of transformer and othe factors - I think)

How do you figure out the 3 phase fault current from a bank of single phase transformers? For example, if you have three 50 kva transformers, each with an impedance of say 1.7. I assume you can say this is the equivalent of a single 150kva transformer, but do you add the impedances together or what. Thanks.

Collier
 
The impedance of a transformer, in percent, is a per unit quantity calculated on it's own base. Therefore, Z = 0.017 @ 50KVA base.

Changing the base means you multiply the per unit impedance by the ratio of change in base, and this can go either up or down. So, in this case;

Z = 0.017 x (150KVA/50KVA)
Z = 0.017 x 3

or 0.51 p.u. (5.1%)

A 150KVA transformer with an Impedance of 5.1% would be equivalent to 3 x 50KVA transformers with each having an impedance of 1.7%.
 
kingpb said:
The impedance of a transformer, in percent, is a per unit quantity calculated on it's own base. Therefore, Z = 0.017 @ 50KVA base.

Changing the base means you multiply the per unit impedance by the ratio of change in base, and this can go either up or down. So, in this case;

Z = 0.017 x (150KVA/50KVA)
Z = 0.017 x 3

or 0.51 p.u. (5.1%)

A 150KVA transformer with an Impedance of 5.1% would be equivalent to 3 x 50KVA transformers with each having an impedance of 1.7%.
Click to expand...

You also change voltage base. If you look at a 120/208 volt example, the voltage base at 50 kva is 120v, at 150 kva it's 208 volts. That results in a Z base that is still 1.7% in my thinking.

if I calculate the fault current on a 50 kva 120 volt transformer with a 1.7%z, I get 24,470 amps fault current. I get the same with a 150 kva and a 1.7%z.

If I calculate the fault current on a 150 kva 5.1% bank, I get 8,157 amps.

What do you think?

Jim
 
I agree you would also change voltage base, but in this case, it does not appear that the voltage is changing. i.e. line to neutral voltage would be the same in either set-up.
 
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