Voltage Distribution on Ground Fault

[111R]

I'm having some trouble wrapping my head around the voltage distribution for a ground fault. If a Delta-grounded wye transformer has an A phase line-to-ground fault a mile down the secondary line with no fault resistance, the voltage is considered to be zero at the fault point. The entire line-to-ground voltage is dropped across the conductor from the transformer to the fault point, neglecting source impedance upstream from the transformer and transformer impedance.

Since all of this current must return to the neutral of the transformer, there must be a potential difference equal to the resistivity of the return path multiplied by the fault current, correct? So the ground potential rise is equal to this value? If so, how does this voltage gradient look across the soil path back to the transformer? I've always heard that 100 ohms per meter is rough estimate of soil resistivity. So, assuming soil conditions happen to amount to this at a particular location, how can I calculate voltage rise at the fault point and step potential?

 

[Power0020]

Back to IEEE 80.

the potential will be concentrated on the current discharge point on earth, then, due to the greater soil cross section area that the fault current "sees", the drop will be very small and most of the voltage will be concentrated in few meters around the discharge point.

 

[HamburgerHelper]

Like Power0020 said the voltage gradients will drop off fast. I think they will drop off with square of the distance. It might be cubed but I know it isn't linear. The current spreads out through a very large volume of soil as it paths back to your grounded neutral if you have conductive soil. The danger is if you are in the vicinity of the fault.

I have designed ground grids inside of substations that didn't need a grounding grid and others that needed an elaborate grid and deep grounding rounds. The soil resistivity, size and shape of the substation, and assumed available ground fault current dictated how much of a grounding grid was required.

 

[jghrist]

Is there a neutral, shield wire, or cable shield? If so, some of the return current will flow through that instead of through the ground grid and earth. The GPR is the portion of current flowing through the ground grid times the ground grid resistance. Touch-voltage and step-voltage will be less than the GPR and depend on the geometry of the ground grid.

 

[JensenDrive]

You asked about step potential, a matter that diverges from your apparent question. I am used to step potential being referred to as the step potential if there is a ground fault inside a substation, engergizing the ground grid, but with ground fault trying to return to a remote source. If the current is returning to a transformer within the substation, this is actually subtracted from the current that is returning to the remote subsation and hence reducing step potentia. This is a long story requiring a decent amount of thinking, beyond what can be supplied in this forum.