Step-up transformer = Star-grounded LV / Star-grounded HV. How to calculate EPR?

[tofulover]

Hi,

I am looking at a pre-existing electrical system which is an off-grid microgrid.

This consists of 3 off low voltage 415V 500kVA generators feeding into 1 off 2000kVA 415V/11000V step up transformer to then connect to 11kV infrastructure for a small village.

The 3 off generators at LV 415V are connected to an LV electrical switchboard which has the TN-C-S earthing topology. In Australia we called this the MEN system.
We have been informed that the SLG at the LV side of the TX is 20kA (being LV) and at the HV side of the TX is 0.5kA (being 11kV).
The step-up transformer is Wye-neutral-grounded at the LV side and also Wye-neutral-grounded at the HV side. Due to the proximity of the existing earthing, the client has connected the two earthing together.

The task that I am looking at is to establish what is the EPR of the system.
Do I consider only the 0.5kA SLG fault on the HV side as the EPR?
I am not too sure what is the impact to EPR by the cross-bonding between the TX HV earth and LV earth.

Any thoughts?

 

[Kiribanda]

You have mentioned that that two earthing systems are bonded together. No worries. That means any earth fault current on the 415V side will rise the EPR at the 11kV side too. Therefore, you have to consider 20kA earth fault current which is maximum out of the two values and calculate the EPR. If it is higher than the allowable level then you have to reduce the earth grid resistance.

 

[tofulover]

You have mentioned that that two earthing systems are bonded together. No worries. That means any earth fault current on the 415V side will rise the EPR at the 11kV side too. Therefore, you have to consider 20kA earth fault current which is maximum out of the two values and calculate the EPR. If it is higher than the allowable level then you have to reduce the earth grid resistance.

may be a silly question from me - wouldnt the 20kA SLG at LV side - most of the fault current will return via the earthing cable and then via the MEN back to the neutral of the generators being the source?

 

[Kiribanda]

What you say is correct. If the 415V system is totally with cables, yes the earth fault currents will go through the cable amour etc and the fault current discharged to the earth is very small meaning you are taking the split factor as almost zero. Unless I see the detailed dwg of the earthing system or model it on CDEGS, I cannot confirm the correct split factor.