Determining the soil resistivity to design a good substation grounding system

Design a safe substation grounding system

It is essential to determine the soil resistivity and maximum grid currents to design a substation grounding system. The touch and step voltages are directly proportional to these values. Overly conservative values of soil resistivity and grid current will increase the cost dramatically.

Underestimating them may cause the design to be unsafe.

Soil Resistivity

Soil resistivity investigations are necessary to determine the soil structure. There are a number of tables in the literature showing the ranges of resistivity based on soil types (clay, loam, sand, shale, etc.) .

These tables give only very rough estimates.

The soil resistivity can change dramatically with changes in moisture, temperature, and chemical content. To determine the soil resistivity of a particular site, soil resistivity measurements need to be taken.

The most widely used test for determining soil resistivity data was developed by Wenner and is called either the Wenner or four-pin method. Using four pins or electrodes driven into the earth along a straight line at equal distances of a, to a depth of b, current is passed through the outer pins while a voltage reading is taken with the two inside pins.

Based on the resistance R, as determined by the voltage and current, the apparent resistivity can be calculated using the following equation, assuming bis small compared with a:

ρ_a = 2πaR

where it is assumed the apparent resistivity ρ, at depth a is given by the equation.

Interpretation of the apparent soil resistivity based on field measurements is difficult. Uniform and two-layer soil models are the most commonly used soil resistivity models. The objective of the soil model is to provide a good approximation of the actual soil conditions.

Sunde developed a graphical method to interpret the test results.

The equations in IEEE Std. 80 require a uniform soil resistivity. Engineering judgment is required to interpret the soil resistivity measurements to determine the value of the soil resistivity ρ, to use in the equations. IEEE Std. 80 presents equations to calculate the apparent soil resistivity based on field measurements as well as examples of Sunde’s graphical method.

Although the graphical method and equations are estimates, they provide the engineer with guidelines of the uniform soil resistivity to use in the ground grid design.

Conducting a Wenner 4-point soil resistivity test

When conducting a Wenner 4-point soil resistivity test, we need to consider the effects that the “Sphere-of-Influence” will have on our test, in two (2) ways:

1. The distance our test is being conducted from any buried metallic objects, railroad tracks, fence lines, etc. This distance should be equal or greater than the maximum (“a”) spacing of our test. In other words, if you are conducting a Wenner 4-point test with a maximum probe spacing of 60-meters (a 180-meter traverse), there should be no interfering objects (fence, buried metal pipes, etc.) within 60-meters of any part of our test.

2. The probes we use to conduct the test, will have their own sphere-of-influence that they will generate based on the depth they are driven in to the earth. For hand calculations, the probe depth may not exceed 1/20 of the spacing of the Wenner test. Advanced computer algorithms can adjust for these differences, but the 1/20th rule is a good one.

Information taken from: esgroundingsolutions.com

Reference: Electric Power Engineering Handbook by Leonard L. Grigsby (Get it from Amazon)