Soil resistivity vs moisture in earth grid design
Soil resistivity vs moisture in earth grid design
(OP)
Hi,
I need to design a substation's (88/11kV 3x45MVA) earth grid. My problem is that I live in a relatively dry country (South Africa) and that we have had quite a lot of rain during the last couple of weeks. Our dry months are during our winter (May - August) and early spring (September and early October). I do not have the time to wait until winter for the soil to dry in order to measure the ground's resistivity. I understand that moist soil's resistivity can be as much as 100 times less than dry soil depending on the type of soil. Both IEEE 80 and 81 give imperical graphs on how moisture affect resistivity.
Is there any method I can use to compensate for the effect of the moisture and still have safe step and touch potentials during the dry months?
Waldo
(sorry for the long message)
I need to design a substation's (88/11kV 3x45MVA) earth grid. My problem is that I live in a relatively dry country (South Africa) and that we have had quite a lot of rain during the last couple of weeks. Our dry months are during our winter (May - August) and early spring (September and early October). I do not have the time to wait until winter for the soil to dry in order to measure the ground's resistivity. I understand that moist soil's resistivity can be as much as 100 times less than dry soil depending on the type of soil. Both IEEE 80 and 81 give imperical graphs on how moisture affect resistivity.
Is there any method I can use to compensate for the effect of the moisture and still have safe step and touch potentials during the dry months?
Waldo
(sorry for the long message)






RE: Soil resistivity vs moisture in earth grid design
RE: Soil resistivity vs moisture in earth grid design
With a little luck (good for you, the ground grid designer, bad for whoever needs the substation), construction will not be far enough along in May to prevent modifications to the design, if necessary, after later resistivity measurements.
Step and touch potentials are not directly proportional to the resistivity because with higher resistivity, the grid resistance goes up and more of the fault current returns through shield wires and neutrals than through the grid.
RE: Soil resistivity vs moisture in earth grid design
RE: Soil resistivity vs moisture in earth grid design
Thanks RalphChristie. From your bio I see are in the Northern Cape (or were when you registered). I am in Joburg and unfortunately Eskom can not help as the sub is in City Power's supply area.
RE: Soil resistivity vs moisture in earth grid design
I suppose you've either spoke to the wrong person or he/she doesn't want to help you. Although City Power runs the substations in that area, I would be very surprised if Eskom does not have soil data too for that region. Try both the distribution guys and the transmission guys. I am not that familiar with the Gauteng-region, but can't you contact/visit someone at Simmerpan?
Regards
Ralph
RE: Soil resistivity vs moisture in earth grid design
Beware that this curve is not intended for universal application. Therefore, you should calibrate it for seasonal variation comparing results with local soil measurements performed previously in different weather conditions.
Finally, keep in mind that the design resistivity with single or multiple soil layer model is only an approximation of the actual soil conditions.
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RE: Soil resistivity vs moisture in earth grid design
RE: Soil resistivity vs moisture in earth grid design
The enclose graph provide additional details with different soils. Appears that the RUS graph was developed based on a clay soil since is comparable with the information provided in the IEEE Std 142.
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