Split factor Calulation

[asadiqbal]

Hi Folks,
This is my first post in this community.

I would like some help regarding split factor calculation for a GIS substation my company is building. As i havent done this earlier I require some help in this region. This substation has overheadlines coming in and all underground cables leaving the substation. I've looked into IEEE papers and only information for GIS substations with Underground cabling system is found in IEEE-80-2000. The details are as follows:
1. 132kV System level
2.40kA fault current for 1sec.
3.two circuits of OHL feeding the substation
4. all outgoing 132kV are underground cables.
5.system frequency 50Hz
6. lower level underground cables outgoing at 11kV

Pls assist me as soon as possible.

Asad

 

[Besoeker]

GIS substation?

 

[rcwilson]

Asad- Welcome to the forum.

Split factor is the ratio of ground fault current that returns to the remote source through the earth compared to total ground fault current. Example: a 60% split factor means 60% of the fault current flows in the earth from the local substation ground grid to the remote power source and 40% returns to the source through the overhead lines' ground wires, the shields of the underground cables or through the grounded neutral of local transformers.

Split factor is needed to calculate the Ground Potential Rise (GPR) which is equal to the fault current in the earth times the ground grid resistance to remote earth. E=IxR = GPR= (Fault Current x Split Factor) x (Grid Resistance).

The IEEE tables were developed from simulations of multiple transmission line and substation configurations. If you can't find a table that approximates your design, you will have to use a software program like SES Splits to simulate the transmission lines, the cables and your ground grid to estimate the split factor.

Another way to do it is to over design your grid to limit the GPR assuming 100% of current flows in the earth.