Remote Grounding Electrodes

[kmh1]

I have a question regarding remote ground electrode arrays as used for reducing the station ground resistance of high voltage substations installed in difficult grounding conditions such as rock or sand where soil treatment or deep well electrodes are not an option. Some utility installations that I am aware of have used a grounding plate installed in a swamp, river, or water body and connected to the ground grid with copper cable (insulated or uninsulated?) to access a low resistivity grounding medium. I have also heard that there may be safety hazards associated with this type of installation. Does any know the appropriate criteria for evaluating this as an option for reducing station ground resistance to acceptable levels?

 

[cuky2000]

Buried plate or counterpoise in the water body is an excellent electrode if this is located near the station.
(See table 3 in the enclose link)

http://www.sestech.com/software_packages/pdf/User2000_F.pdf

The terrain surrounding the water body in many instances is wet native soil with low resistivity may require careful consideration from the safety point of view since could create a potential hazardous area in the event of a short circuit or lightning discharge.

This is caused because the allowable step and touch potential is reduced proportionally to the low soil resistivity. Even if the surface is covered with rock with high resistivity, there is a risk of soil or sediment contamination in time.

One way to mitigate this situation is isolating the potential dangerous area with an appropriate security fence to restrict the access to unauthorized personnel. This option could be expensive, hard to control and difficult to monitor the surface resistivity even with periodic testing.

SUGGESTED ALTERNATIVE:
Other options may be using multiple ground rods as described in the enclose information selected from the REA bulletin 1751.

http://cuky2000.250free.com/Multiple_Electrodes.pdf

This could be supplemented with counterpoise buried in the ground diagonally to the station perimeter fence and also considering the foundations as part of the ground grid. Consider design Ufer ground if possible.

http://www.ieee-pcic.org/archive/Fagan_Lee_Paper.PDF

For rocky soil, rods could be installed in angle if the conditions are adequate. For sandy soil, many utilities are using long ground rods. Electrical contractors usually used successfully sledge or jackhammer to drive rod in hard soils.

This project appears to be a good candidate to use software with capability to model foundation and calculate the potentials in the entire station and selected surrounding areas.

http://www.sestech.com/software_packages/pdf/User2000_H.pdf

 

[kmh1]

Thank you for your response, cuky2000. The local electrical inpsection authority is reluctant to allow the installation of remote electrode arrays in water bodies that are accessible to the public due to the potentially lethal potential gradients that you allude to in your response. Since it is not feasible to fence off an entire lake the next option that I see for this particular application is to go with the high station ground resistance (85 ohms)and high GPR (24kV) and perform addional computer modeling or current injection testing to determine the extent of any step/touch potential issues and then mitigate those. Hopefully it is just a matter of installing multiple ground rod arrays on the corners of the switchyard grid and/or additional perimeter grounding conductors/counterpoise as you have suggested. There are no building foundation grounds available in this installation.

 

[cuky2000]

Hi kmh1,

I am not sure if 85 Ohms with a GPR=24 kV is an acceptable design. Typically an small stations resistance is less than 5 Ohms and for large HV facility the resistance typically the acceptance values is less than 1 Ohm.

Prior to break ground, consult with the local inspector/utility and research more about this issue.

 

[advidana]

how about using a ufer ground