HOW DO YOU CALCULATE SHORT CIRCUIT LEVEL ON A LOW TENSION BUS ?

[deeveeyes]

how does one calculate the short circuit level on a Low Tension bus when both utility and Captive Power (Diesel Genset) is connected to the same bus.&nbsp;&nbsp;There are all the three possibilities of operation: Utility only, Captive Power only and both utility + captive Power. <br><br>I need this information to size the filter banks on a LT Bus feeding various VFDs.

 

[peterb]

The way to do this is to calculate the contribution from each source and then add them together as needed.<br>Start with the utility - assuming that you have the transformer high side fault level in MVA (say 500) and that you know the transformer impedance from the nameplate (say 1000 kVA, 13800-480V, 5% impedance) -<br>Utility source impedance on 1000 kVA (1 MVA)base = 100x1/500 = 0.2%<br>Add the transformer impedance in series with the utility <br>Z = 5.0 + 0.2 = 5.2% = 0.052 per unit<br>Calculate the fault MVA on the transformer secondary<br>MVAf = 1/0.052 = 19.23 MVA<br>Fault current = 19230/sqrt3x480 = 23.13 kA<br><br>Next calculate the contribution from the generator:<br>Find the subtransient reactance (X&quot;d) of the generator from the nameplate or data sheet - say 500 kVA generator with X&quot;d = 20% = 0.2 per unit<br>Generator fault contribution = 500/0.2 = 2500 kVA<br>Generator fault current = 2500/sqrt3x480 = 3.01 kA

 

[jbartos]

Suggestion:<br>The above calculations are correct, if you do not have any rotating machinery (e.g. induction motors, synchronous motors, etc.) on the low tension side (e.g. 480VAC). If you do, then you have to add the fault contribution from the rotating machinery to the above total current. The induction motors may have standard contributions 5 x equivalent motor full load current (approximately). See Donald Beeman &quot;Industrial Power System Handbook&quot; McGraw-Hill Book Co., 1955, page 86.