Short Circuit when we do not have R X of T/F and Source

[IndianTechEngg1001]

I have a Transformer (11/.4kV) 1000kVA 3Phase, Dyn11, Dry Type, Secondary which can supply 25kA Short Circuit Current. Now Secondary is connected to a Bus Bar whose R = 0.48mOhm and X=0.265 Ohm which is connecting a panel. From panel a cable is connected R= 5.3mOhm X=2.6 mOhm.

My concern is how to arrive at SC Current at the end of Busbar and at the end of Cable. What will be the formula?

Normally when I calculate Sc currents I take V and Divide it by over all Z (Source+ Transformer+ Busbar+ cable+ etc +etc) this gives me the Isc. But this time I am not having Source and Transformer R and X. Please help.

 

[rbulsara]

You can find the source impedance from the available SCC value, which you have.

Refer to IEEE Red Book or some authentic book on the subject for formulas.

Rafiq Bulsara

http://www.srengineersct.com

 

[IndianTechEngg1001]

Thanks I will refer Red Book. Also I referred book INDUSTRIAL POWER SYSTEMS HANDBOOK - Donald Beeman from there I got that we can take source x/r as 12. When I am calculation SC Currents I am getting comparable results using formulas Z=V(L-N)/Isc, X/R=12, X={Z(X/R)/Sqrt((X/R)(X/R)+1)}, V(L-N)=230V, Isc=25000A I am getting R and X values.

Am I doing correct things? And Is there any reason why X/R value stated as 12?

 

[rbulsara]

It was based on historical data, perhaps. Utility X/R ratio can vary anywhere between 10-20, depending upon where in the system you are. 12-15 is a reasonable range to assume at medium voltage levels in the USA, as a starting point. You need to consult the utility company for more accurate data.



Rafiq Bulsara

http://www.srengineersct.com

 

[IndianTechEngg1001]

I am extending the question. I am using ETAP to analysis the system and while specifying data for Swing Bus ETAP is asking me to insert values of 3 Phase SCMVA, Single Phase MVA and 3 phase and single phase X/R. My 3 Phase MVA is 350MVA, 3 Phase X/R is 12.

What will be Single Phase MVA and X/R?

ETAP help file gives following formula.

MVA3P = Ö3 * kV * I3P
MVA1P = Ö3 * kV * I1P

I am confused what values to substitute for 1 Phase.

 

[Kiribanda]

IndianTechEngg1001,
Since the single phase SC MVA or the Thevenin's driving point impedance is not given you can reasonably assume the 3-ph SC MVA to be equal to the 1-phase SC MVA at the source (swing)bus.Because your 3-ph and 1-ph sc currents at secondary of the transformer will not make a "huge" difference to your objective.

Sorry for this question please.Are you not familiar with the theory part behind the SC calculation method or you "JUST" playing with the software?

 

[IndianTechEngg1001]

Yes I am studying Sc from 909 and also applying this on ETAP.

Thanks for your reply.