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Electrical Characteristics of Bentonite 1
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Bentonite is a clay substance used in areas with high soil resistivity. However conduction in bentonite clay only takes place via the movement of ions. Ionic conduction can only occur in a solution, which means the bentonite clay must be moist to provide the required resistance levels. When bentonite clay loses moisture, its resistivity increases and volume decreases. This shrinkage results in a discontinuity in the contact between the bentonite clay and surrounding soil, which further increases system resistance.
When wet it has a resistivity of about 250 ohm-cm and increases the effective diameter of the rod.
There are many improvement by adding chemicals as sodium and potassium salt.
See:
When wet it has a resistivity of about 250 ohm-cm and increases the effective diameter of the rod.
There are many improvement by adding chemicals as sodium and potassium salt.
See:
I cannot answer the question specific to betonite but I will offer some following general principles:
1) Lowering resistivity is done in several ways:
- 1st you can lower resistivity by installing more rods. This item will only work to a small extent as each rod exerts what I believe is called a "sphere of influence" around it. The closer the rods are to each other in the same area, the less value you get from them.
- 2nd, you can chemically treat the soil. Several companies make a products that will do that by in essence of transferring the chemical (bentonite) to the soil. One company (Lightning Eliminators and Consultants)makes a rod that can be refilled with chemical.
2)Note that it is VERY important to understand what level of ground resistance you wish to achieve. What we do at a substation (in the way of a ground mat) is VERY different from the objective we have when we design a grounding systems for telecommunications. There is a good deal of consensus on substation grounding (I believe the standard is IEEE Standard 80 but there is much less written as guidance or standards for communications grounds. In essence, the reason for this is somewhat simple: communication systems are at high frequency (and conductors and the like exhibit more of their capacitive behavior) where power systems operate at only 60Hz (where conductors exhibit more inductive behavior).
As to the reference, the best reference is the old IEEE Color Book called the IEEE "Green" Book. I believe that IEEE may have withdrawn the Color Book series but if you can find it, it serves as a good references drawn from years of applications engineering. Check Amazon.com or half.com for spare copies.
1) Lowering resistivity is done in several ways:
- 1st you can lower resistivity by installing more rods. This item will only work to a small extent as each rod exerts what I believe is called a "sphere of influence" around it. The closer the rods are to each other in the same area, the less value you get from them.
- 2nd, you can chemically treat the soil. Several companies make a products that will do that by in essence of transferring the chemical (bentonite) to the soil. One company (Lightning Eliminators and Consultants)makes a rod that can be refilled with chemical.
2)Note that it is VERY important to understand what level of ground resistance you wish to achieve. What we do at a substation (in the way of a ground mat) is VERY different from the objective we have when we design a grounding systems for telecommunications. There is a good deal of consensus on substation grounding (I believe the standard is IEEE Standard 80 but there is much less written as guidance or standards for communications grounds. In essence, the reason for this is somewhat simple: communication systems are at high frequency (and conductors and the like exhibit more of their capacitive behavior) where power systems operate at only 60Hz (where conductors exhibit more inductive behavior).
As to the reference, the best reference is the old IEEE Color Book called the IEEE "Green" Book. I believe that IEEE may have withdrawn the Color Book series but if you can find it, it serves as a good references drawn from years of applications engineering. Check Amazon.com or half.com for spare copies.
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7anoter4,
I appreciate the valuable information you have provided. My next question is about modeling this in a grounding calculation program, in particular, CDEGS. Since you mentioned that resistivity of Bentonite increases when dry, my concern would be modeling a ground well with 2.5 ohm-m resistivity for Bentonite.
Jghrist,
Could you advise on how you would model a ground well in CDEGS?
sjmgd977,
I do have green book and IEEE-80. The station I am doing a study for is a pre-existing Utility substation and has plenty of 10 ft ground rods. It currently can withstand 25kA but we are trying to inject 50kA instead. My next approach is to try ground wells since the resistvity of the station drops to about 10 ohm-m after 7 ft of depth and the surface resistivity is around 150 ohm-m. Also, I am not sure about how economical chemically treating the soil would be since the station is about 600 ft x 500 ft. However, I am interested if you can convince me otherwise.
I appreciate the valuable information you have provided. My next question is about modeling this in a grounding calculation program, in particular, CDEGS. Since you mentioned that resistivity of Bentonite increases when dry, my concern would be modeling a ground well with 2.5 ohm-m resistivity for Bentonite.
Jghrist,
Could you advise on how you would model a ground well in CDEGS?
sjmgd977,
I do have green book and IEEE-80. The station I am doing a study for is a pre-existing Utility substation and has plenty of 10 ft ground rods. It currently can withstand 25kA but we are trying to inject 50kA instead. My next approach is to try ground wells since the resistvity of the station drops to about 10 ohm-m after 7 ft of depth and the surface resistivity is around 150 ohm-m. Also, I am not sure about how economical chemically treating the soil would be since the station is about 600 ft x 500 ft. However, I am interested if you can convince me otherwise.
The chemical rod sends (I believe the correct term is "leaches")on a continuous basis. I have not done substation grounding so I can't comment on what you have to do there. I have only used it in SCADA applications and not in a ground mat but I see no reason why it cannot work.
I do believe that this is a correct approach because you know what the end result is (it lowers resisitivity). While the chem rod looks like a grounding rod, you don't have to connect it to anything as you are lowering soil resistivity.
The real issue with this approach is how many rods you need and how far apart they need to be.
My suggestion is to talk with the people at Lightning Eliminators and Consultants or Lyncole XIT and see what they tell you.
I also some information in the Electrical Power Systems Calculations Handbook by H.Wayne Beatty so I have to believe that there is some validity to this approach for a ground mat.
I do believe that this is a correct approach because you know what the end result is (it lowers resisitivity). While the chem rod looks like a grounding rod, you don't have to connect it to anything as you are lowering soil resistivity.
The real issue with this approach is how many rods you need and how far apart they need to be.
My suggestion is to talk with the people at Lightning Eliminators and Consultants or Lyncole XIT and see what they tell you.
I also some information in the Electrical Power Systems Calculations Handbook by H.Wayne Beatty so I have to believe that there is some validity to this approach for a ground mat.
CDEGS has the ability to model cylindrical volumes of soil. You could model the 2.5 ohm-m cylinder of soil with a ground rod in it.
If your surrounding soil is significantly higher resistivity than 2.5 ohm-m, and the diameter of the well is not much more than the rod, I think it would be adequate to model the ground well as a ground rod with an increased diameter.
If your surrounding soil is significantly higher resistivity than 2.5 ohm-m, and the diameter of the well is not much more than the rod, I think it would be adequate to model the ground well as a ground rod with an increased diameter.
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