Grounding grid current flow back to source

[EPEngineer]

Hello everyone,
I have a situation here regarding grounding. There is a new 2x132/11/11kV substation fed by 2x132kV cable circuits. Cable sheaths (95mm2 each) are connected at both ends, length is only 4200m. We also have 2x150mm2 ECCs connected at both ends. Cable sheaths and ECCs are within the duct and connected to both grounding grids at the same points. I want to know that in case of a fault at downstream end, say 21.5kA, what proportion(%age) of the fault current would flow back to source through cable sheaths and ECCs so as to put the actual ground current into the model.

Many Thanks,

Aqeel

 

[davidbeach]

What is an ECC?

 

[EPEngineer]

Sorry David, ECC stands for earth continuity conductor. I should also mention that I am in UK.

 

[jghrist]

This is not an easy question. It depends on the mutual impedances among the conductors, the sheaths, the earth, and, if the duct is metallic, the duct, as well as the grid resistance at each end and the soil resistivity.

SES has software that will find the solution. Their website is at

http://www.sestech.com/

You may also be able to solve the problem with ATP/EMTP which is a public domain transients program. Go to

http://www.sestech.com/

for information.

Otherwise, about all you can say is that most of the current will flow through the sheath and ECC because the return current tends to flow in the path closest to the conductor.

 

[EPEngineer]

Thanks jghrist,
I am using CEDEGS software by SESTECH. There are several modules with different capabilities. In a nutshell, it is not possible to model the two grids with return path in that software as there are lot of coated conductors as well.
SES helpline told me that I could ignore the 2x150mm2 ECCs as most of the return current will flow through the sheaths due to induction and proximity of sheath with conductor. I was unable to understand this as cable sheaths and ECCs both are connected to two ground grids at same points, almost have same equivalent resistance and both are insulated. Ignoring ECCs would reduce the equivalent resistance to half. In case of earth fault, the current flow back to source should be through conduction not induction. Please correct me if I am wrong.
I have sort of a basic knowledge of earthing based on IEEE-80 but I was unable to dig out any, easy to understand and use, formula for this return current calculation. Please correct me if I am wrong.

Anyone from UK knowing about Engineering Recommendations S34?

Looking forward for your help.

Regards

 

[jghrist]

I would model the system as a central station with a terminal using SPLITS and TRALIN or FCDIST. FCDIST cannot model underground cables, but you can get a close approximation by modelling the cable just above ground. I have done this with satisfactory results. You have to use SPLITS and TRALIN if you want to model the cable underground.

The current flow back to the source is by conduction, but inductive coupling impedes the flow. Inductive coupling is the basis for the reactive component of impedance. There is less reactance when two conductors are close together, so more current flows in paths where the return current is close to the conductor.

I don't think there are any easy to understand and use formulas for the return current calculation. IEEE-80 Section 15.9 discusses the computation of current division, but the discussion and examples are for overhead lines. There are several references to to papers on current division. The general methods may be applicable to underground cables as well as overhead lines, but all of the simplifications and empirical data is based on overhead lines.