Short Circuit Current

SCC calculation is important to determine rating of breaker, cable feeder, busbar, protection relay  setting, etc. How to calculate SCC (Short Circuit Current) ?There are three methods to solve it i.e. Ohmic Method, PU Method and MVA Method. In this session I want to share about one power full method, i.e. MVA Method. Why MVA method? There are several reasons :

•  This method doesn't require MVA base to calculate
•  No need to convert impedance based on the voltage
•  Easy and not complex
•  Suitable for the whole system one line diagram (OLD)
•  The equation is simple and easy to remember.
•  MVA is the most accurate method.

Let's look at the one line diagram below.

Following are steps to convert the OLD  with MVA Method:

• Generator : MVA/Xd" = 1500/1 = 1500 MVAsc
• Line : kV^2/x = (60 kV)^2/(3,87) = 1230 MVAsc
• Transformer : MVA/x = 15/0,078 = 198 MVAsc
• Motor : MVA/Xd" = 15/0,2 = 75 MVAsc

Generator and motor are a source of SCC.  Now, how to calculate SCC 3 phase at the system above if the fault at bus 12 kV?

The generator, transformer and line are connected in series so that we have to sum up it like a parallel network  and if those connected in parallel we have to sum up it like a series network.

• 1/(MVAsc series) = 1/(MVAsc Generator)+1/(MVAsc Line)+1/(MVAsc Transformer) = 1/(1500)+1/(1230)+1/(198) = 153 MVAsc (MVAsc series)
• MVAsc series is connected in parallel to the motor at bus 12 kV. So, here is the result :
• MVAsc total = MVAsc series + MVAsc motor = 153 + 75 = 228 MVAsc
• To get SCC (Isc 3ph) at bus 12 kV, please use this formula :
• Isc = MVAsc total/(1,732*kV) = 228/(1,732*12) = 11 kA

You can try to solve LL, LG, LLG, and LLLG. To check whether the result is correct, you can use ETAP or similiar software.

If any question or suggestion please deliver to me. I am glad to hear your comment. Before closing my first posting at least there are two questions that we can to discuss futher :

• Is lighting panel could be a source of SCC when the fault happened ?
• How about the line and transformer? are they a source of SCC too?

Thank you.
Please comment after reading my posting.

Regards,
HI