Ground Grid Study

[Humble2000]

Hydro provides customer with maximum and minimum short circuit currents as well as max Z and Z0 values.
If the ground grid study needs to be done. I was told to use Max( L-L-G, L-G) for ground fault.
Why do we need max and min values? Where Min is used in calculation?

 

[DanDel]

Typically, minimum Short Circuit values are used to verify pickup of protective devices at the lowest amount of fault current available, and this is usually for GF devices.

Minimum values are also used when performing Load Flow or Motor Starting studies.

 

[apowerengr]

Ground grid studies typically require a determination of how much fault current will use the ground grid under study vs. other parallel paths which may exist to the source (neutrals or static/shield conductors). Typically the maximum L-G or L-L-G fault current is much larger than that portion which actually contributes to grid potential rise because the remainder of the current uses other paths available. If the fault occurs at the grid under study (max If) then the current flows in the grid not through it to earth............thus minimal grid potential rise to remote earth.