20,000' tunnel grounded conductor question

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bracemc

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hello-We are working on a tunnel project with a 20,000' long tunnel being mined by a tunnel boring machine. We are having grounding issues that are affecting various things in the tunnel, including the possibility of ground fault events. We have power stations every 1000'. We need to extend a grounded conductor the entire length from the switchgear (15kv). The ground actually has 29 amps on it. The voltage being run is 12.47k. I used a standard voltage drop calculation to figure the voltage drop and the wire size, but that really doesn't work because that is phase vd. Does anybody have any advice on how to run the ground using a correct voltage drop, etc. This is a new situation for us, and my engineer taksed me with figuring it out (good learning lession I guess). Grounding is not my strongest of areas. Any advice would be greatly appreciated.

Thanks-

Brace McGillivary
Electrical Designer
 
I would look to designing this like a utility would design an underground distribution system. Maybe consult the NESC (national electrical safety code), the code that the utilities use. This installation is really covered by the NEC, but it does not do a good job with rules for medium voltage systems. I understand that there is a task group looking at a new medium voltage code article for the 2011 code.
 
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Brian,
You have a good point, this installation may be considered a mine and the NEC would not apply...the MSHA rules would govern the installation.
 
brian john said:
The poster is asking for legitimate information, many times on jobs like this it may be new issues to all involved as tunnels this size I would assume are rare.

I would do as Don said also Check Mining documentation.

Such as

http://www.msha.gov/S&HINFO/TECHRPT/GROUND/MNMELECT.HTM

What type of issues are you having.
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Thanks for the suggestion. I have opened the link and it looks very promising. The tunnel in question is part of a new combined sewer overflow (CSO) system. The 2 main tunnels are very long and 14' and 22' in diameter with substations every 1000'. We are only concerned with the 22' diameter one. The other has been finished for a over 2 years.
 
Don is correct, the NEC does not have all the rules for this type of application. The NEC and NESC differ greatly in how grounding is done.
 
bracemc said:
I used a standard voltage drop calculation to figure the voltage drop and the wire size, but that really doesn't work because that is phase vd. Does anybody have any advice on how to run the ground using a correct voltage drop, etc. This is a new situation for us, and my engineer taksed me with figuring it out (good learning lession I guess).
Click to expand...

Voltage drop is voltage drop, and it doesn't matter if the conductor is your 'phase conductor' or your 'grounded conductor'.

What is confusing you is the use of the term 'drop'. Drop implies that the voltage gets lower. But in the case of a grounded conductor, the voltage _change_ caused by the resistance of the wire is seen as an _increase_ in voltage.

At the end of the run, the ungrounded conductor will have a voltage somewhat lower than at the supply end, and the grounded conductor will have a voltage somewhat elevated relative to the supply end.

-Jon
 
bracemc said:
hello-We are working on a tunnel project with a 20,000' long tunnel being mined by a tunnel boring machine. We are having grounding issues that are affecting various things in the tunnel, including the possibility of ground fault events. We have power stations every 1000'. We need to extend a grounded conductor the entire length from the switchgear (15kv). The ground actually has 29 amps on it. The voltage being run is 12.47k. I used a standard voltage drop calculation to figure the voltage drop and the wire size, but that really doesn't work because that is phase vd. Does anybody have any advice on how to run the ground using a correct voltage drop, etc. This is a new situation for us, and my engineer taksed me with figuring it out (good learning lession I guess). Grounding is not my strongest of areas. Any advice would be greatly appreciated.

Thanks-

Brace McGillivary
Electrical Designer
Click to expand...

Brace
1. It this a 3 phase installation? 3 cables + neutral?
2. The voltage 12.46 kv, 7.2 kv phase the neutral?
3. When you say Voltage drop are you referring to primary Voltage Drop?
4. "power station' is a transformer every 1000 ft. 3 phase of 1 phase? Is the faults from the transformer secondary or primary?
5. When you say " I used a standard voltage drop calculation to figure the voltage drop and the wire size, but that really doesn't work because that is phase vd." Primary or Secondary?
 
Bracemc,
Be careful the enviroment you are working in is an area that the code notes that it dose not cover.
90.2 Scope
(B) Not Covered This Code does not cover the following:
(2)Installations underground in mines and self-propelled mobile surface mining machinery and its attendant electrical trailing cable
Check out IEEE Transaction paper on "Mine power-system ground research" by Cooley, W.L. Hill, H.W., Jr. Dept. of Electr. & Comput. Eng., West Virginia Univ., Morgantown, WV;
This paper appears in: Industry Applications, IEEE Transactions on
Publication Date: Sep/Oct 1988
Volume: 24, Issue: 5
Abstract
The unique problems associated with mine power system grounding are described, and the efforts that have gone on in this area over the years are traced. Special emphasis is given to efforts funded by the US Bureau of Mines. These include efforts aimed at improving ground-check monitor performance, designing and measuring the resistance of safety ground beds, analyzing ground-system safety and effectiveness, studying the effects of stray currents, identifying situations over which the effectiveness of standard designs may be lost, and communicating theoretical and field-verified findings to the mining industry. A well-referenced list of ground-system design considerations which should be familiar to those responsible for the design of mine power systems is included. An extensive annotated set of references which lists most of the grounding research published as the result of the Bureau of Mines research efforts is given
 
this subject actually should have some interesting view points in it. I have a question as you dive deeper into the earth the grounding capabilities should become better would this then cause the voltage to travel from the poorer grounding point on the surface.

I hope that makes sense. as you use the ground to stabilized the transformer.

so the actual resistance to ground should be less the deeper you go as the amount of moisture in the soil increases this should actually make the earth more conductive. I would then think that complete system isolation would be desired and the ground wire should actually be paralleled to increase efficiency.

this should actually breed some serious discussion.
 
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