Maximum earth grid current 2

[ohms]

Hi everyone,
We are doing the earthing calculation for 33/6.3kV SS.
There are 2 nos. 33/6.3kV , 30MVA transformer Dyn11.

For calculating the maximum grid current, should we consider
- Maximum switchgear fault current
0r
- Transformer 3 phase fault current at Y side

thanks for helpful replies.

 

[cuky2000]

A conservative approach could be taking as the maximum short circuit rated equipment, often driven by the SC rated of the switchgear.

If less conservative approach is desired for design purpose this could be a fraction of the maximum available short circuit plus a margin for grow.

This fraction of current is typically called split factor as shown in the link below.

http://cuky2000.250free.com/Gnd_Example_Current_Div_Factor.jpg

 

[jghrist]

Usually, you need to look at the maximum ground fault current on the high side of the transformer, with appropriate split factor as indicated by Cuky. The current on the low side is usually not a factor because the return current does not flow through the earth. There may be cases where the worst case is a low side fault away from the station where return current flows through the earth. If the outgoing 6.6 kV lines have neutrals, generally most of the return current will be through the neutral except for currents distant from the station, but in these cases, the fault current is reduced.

 

[ohms]

Thanks,
Your replies really helped me.