LV Short circuit study-

[swiminelectricity]

Hi, Folks,
I am running ETAP short circuit study (11kv-22KV-6.6KV-0.4KV), I ignored the most equipment impedance values, resulting in much bigger values.It is hard for me to get the data for the existing system. How can model my system in order to get the right data.By the way, S-C value I have got for 400V system is ~80KA(IEC909). Does it make sense?

 

[Mazoon]

To be more accurate and depend on your work, you have not to ignore the impedances.

 

[alehman]

You must include impedances for your calculations to be meaningful. 80kA may be reasonable. What is the kVA and impedance of the transformer supplying 400V?

 

[swiminelectricity]

trans is 2MVA 5.5%impdance.

 

[jghrist]

Then 80 kA is not reasonable. With an infinite 6.6 kV bus, the fault current on the 400 volt bus would be 52.5 kA. Any system impedance would lower this.

 

[cuky2000]

Assuming motor feedback contribution of four times full load current of transformer, the total SC @ 400V bus is around 64 kA.

80 kA appear to be too high for this application.

 

[alehman]

I agree. Assuming 100% motor load at 6x contribution, I get about 70kA. It's unlikely there would be that much motor contribution.

 

[rbulsara]

Swiminelectricity:

Responder above assumed only one tranfromer and it may be correct. But please confirm that.

You indicate multiple primary side volages in our original question. What does that mean? Do you have multiple units , paralleld on the secondary side? Do you have any generator connected to the secondary side? What else is connected (in the software onelie that you created) on the secodary side of the 2MW transfromer you mention?

Also when you say you ignored equipment impedance, did you mean the transformer impedance?

I suspect the system in your software is not setup right, the output of a software is as accurate as the input. You must make sure that relevant switches are in correct position (in service or out) on the oneline.

 

[Mazoon]

In ETAP, you can select such equivalent parameters form the software library or typical data for such load or transformer sizes etc.
Furthermore be aware of Study case editor mode, for example you have to consider loading category for each sub system.

 

[cflatters]

Hi

The only way to get a meaningful output is to have realistic model data,

a) Accurate dat deom your DNO for the supply connection Z1, Z2, Z0 etc (check the reference base for impedances quoted - often quoted on 100MVA base)
b) actual impedance data from manufacturers test sheets for transformers
c) actual data for generators or larger motors.
d) actal data for cables
e) realistic data for motor loads especially X"d = approx 1/LRC as a PU value (except if you are using a lot of inverters where this could be a lot lower - say 1PU LRC of fault current contribution)

Draw the network out by hand, do some hand calcs to satisfy yourself that results are in the right order ( a common sense check). Check the output data files to ensure that the motor contributions are sensible

Ik" approx LRC for induction motor, check also that the IEC909 Table 1 voltage factors have been applied correctly, Typically Cmax = 1.1 above 1000v, 1.05 below 1000V

Hope this helps