Substation Grounding Design

[Mila2015]

I used ETAP software to design 44KV substation ground grid. For the shape of my particular substation, I had to use Finite Element Method, and not the IEEE 80. My question is regarding Meth sizes, and conductor spacing. The IEEE 80 design requires that minimum spacing between conductors is 2.5m (I believe NEC requires 6ft). THE FEM used in ETAP doesn't have this requirement. THis is because IEEE 80 requires fixed shapes, while FEM is very flexible and calculates values based on actual design.
Due to the very high soil resistivity values, in order to meet "touch" and "step" potential values, and not have to end up with a very large grid, my spacing between conductors was closer to 5ft. This design met all safety requirement, and client's max resistance value. Anyone sees issue with being under 6ft spacing in this situation? Thanks for the help!

 

[jghrist]

I know of no requirement in either IEEE 80 or the NEC that the spacing between grid conductors be of a particular value. You may be thinking of the NEC requirement for multiple electrodes to be not less than 6 feet apart. This applies to individual electrodes like ground rods. This does not apply to the distance between elements of one electrode, like wires in a ground grid.

 

[Mila2015]

jghrist, I believe IEEE-80 does have a minimum of 2.5m between parallel ground conductors. I agree, NEC's requirement is regarding rods (not conductors). I do have rods installed in conductor intersections, and are 5ft apart. This was the only way I can be under "touch" and "step" potential limits determined by IEEE-80 standard. I believe NEC-250 applies to buildings, and it only cares about meeting resistance limits. There is no mention of "touch" or "step" potential limits! I think NEC assumes all ground fault current will use EGC as a return path, not soil.

 

[cuky2000]

There is no restriction to limit the grid spacing for the substation ground grid design. The main criteria for grid spacing per the IEEE Std 80 are to meet the maximum allowable touch and step potential for a maximum design clearing time of the protective device.

Here is one example of many grid design with grid spacing larger than 2.5 m that could meet the safe criteria: Link

http://ses.ca/HowToHelp/AGPHowTo/HTML/Input_Data_Specification.htm

 

[cuky2000]

Be aware that an equally spaced grid may be overdesigned toward the center and underdesigned toward the perimeter.
An optimal grid spacing design can be achieved with non-uniform grid spacing making the surface potential distribution more uniform

See example in the following Link