×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Grounding grid current flow back to source

Grounding grid current flow back to source

Grounding grid current flow back to source

(OP)
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

RE: Grounding grid current flow back to source

What is an ECC?

RE: Grounding grid current flow back to source

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

RE: Grounding grid current flow back to source

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.

RE: Grounding grid current flow back to source

(OP)
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

RE: Grounding grid current flow back to source

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources