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Fault Current Split between Grid and Shieldwires

Fault Current Split between Grid and Shieldwires

Fault Current Split between Grid and Shieldwires

(OP)
IEEE 80 Section 15.9 suggests using Endrenyi's method for calculating equivalent impedance of the overhead static wire (Pg 80).

It also shows some values implying equivalent impedance equation to be of the following type

Zeq-l = 0.5 x Zs-l + SQRT ( Rtg x Zs-l)

where

Zs–l is the self impedance of the 115 kV overhead static wire in Ω/span

Rtg is the impedance to remote earth of each transmission ground electrode in Ω

Can anyone explain how this equation was developed or even better if they can share a copy of Endrenyi, J., "Fault current analysis for substation grounding design," Ontario Hydro Research Quarterly,2nd Quarter, 1967.

I am trying to figure out how transmission or distribution tower footing resistance and a remote terminal station grid resistance could affect current devision between shield wire and the ground grid at the station under study.
 

RE: Fault Current Split between Grid and Shieldwires

Good question.  If the overhead ground wire is connected to the ground at each pole we end up with a ladder or lattice network.  The pole grounds are connected between poles by the earth. The resistance of that connection is represented by the pole or tower footing resistance.  That path is in parallel with the overhead ground wire and both paths have impedance due to coupling with the phase wires on the tower.  The rungs in the ladder network are the towers.  The "legs" are the overhead wire and the earth.  The final rung in the ladder is the resistance to ground of the remote grid, assuming the ground wire is connected to the remote grid.

I visualize the circuit by looking at an overhead ground wire connected only to the ground grid at each end. The impedance of that return path is usually low compared to the earth due to better coupling with the phase wires carrying the fault current into the substation.

When the overhead wire is grounded at each tower, a portion of the current in the earth can run up the first tower and flow to the remote end on the overhead wire. Additional current transfers to the overhead wire at the second tower, and each subsequent tower.  The net effect is to increase the portion of the fault current that is entering the ground at the faulted substation.

I haven't analyzed all possible scenarios.  There may be combinations of footing resistance, line impedance and grid impedance that decreases the amount of current into the ground.  

I didn't answer your question, but I hope I provided some insight.
 

RE: Fault Current Split between Grid and Shieldwires

IEEE 80 has a tutorial that comes with it, bought separately, there are some worked examples. ETAP has some explanations as well.

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