Short circuit calculation though building distribution transformer?

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olc

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I'm unclear about how to figure the short circuit available at secondary terminals of a transformer.
Say the building has a 480V service and there is a 75KVA 480X208/120 transformer somewhere in the building that has say 20,000A SCCA at the primary terminals. And you know the impedance.
 
If you want the exact short circuit value on the output of the transformer than you need to know the exact available current on the primary.

If you want to know the highest possible fault current you can simply divide the FLA by the %Z.

There are many free applications, or websites, that will do the math for you.
 
olc said:
I want to figure it based on knowing the SCCA at the primary connections.
Click to expand...
Then you need to get the POCO involved. That available current will depend on upstream transformers and size and distances of lines. Many "on site" calculations just assume the transformer has an infinite capability from its supply, but in the real world that is not the case.
 
I know what the SCCA is upstream. The question is - how do you figure the SCCA on the secondary side?
 
There's probably a simpler way, but this is how I remember, converting to per unit impedances...

Your primary fault is 20,000A, which equals 16.63 MVA. Set a base MVA equal to your transformer rating, or 0.075MVA. Your per unit impedance at the primary = Base MVA/available Fault MVA = 0.075/16.63 = 0.0045099.

To get the Zpu on the secondary, you would add the primary Zpu plus the transformer Zpu (assume 3% impedance on the transformer.) So on the secondary you would have: 0.03 + 0.0045099 = 0.0345099. To convert the Zpu on the secondary back to Fault MVA, you divide the base MVA by the Zpu, or Secondary Fault MVA = 0.075/0.0345099 = 2.17MVA on the secondary. This fault would be 6,032Amps at 208V, 3ph.
 
I have always advised that you should stay with unlimited available fault current on the primary figure 100% motor contribution then figure what you fault current would be on that. That what you can decide how much trouble you are in. You may not be and save yourself the expense of an additional study. If you are in deep ----- then at least you have an idea where you need to go and what you need to change. It could be as simple as getting the actual available fault current from the POCO and reconsidering the actual motor contribution. The it may be making some minor changes in the down stream devices .
 
I prefer the MVA method of calculation.
1. Convert all sources and impedances ( transformer, cables, motors, generators) into the MVA that device could deliver to a fault if it was connected to an inifinite bus (MVAsc).
2. Add the MVA's as admittances to come up with the MVAsc at the fault point.
3. Convert the MVA to short ciruit current at the voltage of interest.

1. Your assumption that 480V 3 phase fault level is 20,000 A. MVAsc source = 20,000 x 480v x 1.732/ 10^6 = 16.63 MVA.
75 kVA transformer, assume 5% Z. MVAsc tx = 75 kVA / (.05 x 1000) = 1.5 MVA

2. Admittances ( 1/ohms) in series are added like resistances in parallel. The utility and transformer MVA's are in series feeding the fault MVA.
1/MVA fault = (1/MVAsource) +(1/MVAsc tx) = (1/16.63) + (1/1.5) = 1/1.376 MVA MVA fault = 1.376 MVA

3. Fault current at 208: Isc = MVA x 10^6 / (V x 1.732) = 1.376 x 10^6 / (208 x 1.732) = 3,820 A.

Send me a PM if you need more information on this method.
 
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