HV Conduit on top of the ground

[jtester]

I have a governmental client who wants an underground 25 KV line run up a mountain to an observatory. The route is across National Forest land, so rocky that they want me to design the line on top of the ground, and cover with concrete.
The area is 10,000' above sea level, lots of snow, and freezing and thawing.

I am aware that the NEC and NESC do not require a specific burial depth if concrete is used, but I am still very uneasy about it. I have designed hundreds of miles of overhead and UG high voltage, but not under these specific circumstances.

Has anyone heard or done something like this? Any info will help.

Thanks

jtester

 

[tkb]

Re: HV Conduit on top of the ground

That should look good in a national forest.
What about expansion and deflection???

 

[mc5w]

Re: HV Conduit on top of the ground

Are you sure that the power problem is not amenable to solar cells and windmills? The 2 power sources together provide a very reliable means to recharge batteries. That high up there probable is enough wind that batteries would only be needed to buffer load changes.

If this hill is so steep that people cannot trip over the line, why not carry galvanized rigid conduit on fenceposts? Since there is a road to the top you do not have to worry that much about people using the conduits to climb up the mountain.

You could put in some crossarms with some barbed wire on them or just plain old mesh fencing to keep people from using it as a climbing device. X-bucks would also work if foundations are a problem.

If this hill is steep enough then you can claim that the line is only accessible to unauthoraized persons or at least nonaccessible enough than you only need to elevate the GRC about a foot or 2 above the ground and protect it with a short fence. You might need to leave a gap underneath the fence to let small animals run. I would put up some markers to keep mountain climbers and skiers from running into it.

Also, PVC conduit does NOT work in blast freezers. The conduit contracts so much that the solvent cemented joints come apart at both ends of say an 8 or 10 foot section.

Also, the oil and gas people get away with laying 2 inch pipe on the ground when running through wooded areas.

Cable supports while going up the hills would also be a problem.

An alternative is that Dusquesne Light Company in Pitssburgh, PA has some 14,400Y24,940 volts home runs that run as shielded cable strapped to an overhead messenger wire. In your environment you might need to make 1 out of 4 or even all of the poles A-frames to withstand wind and ice. Or, run a heck of a lot of guylines not that you need guy lines going uphill anyways. A shielded cable only needs to be 10 feet off of the ground if pedestrians will be going underneath.

No matter how you do this be prepared to do a lot of hand digging.

A third method is that the oil and gas people have trench diggers that will dig through rocky soil when they do want to bury a pipeline. You could use a winch at the top of the hill to help the trench digger go downhill without falling. You still would have a cable support problem necessitating a handhole or manhole every 100 or 200 feet.

I also do not see why you cannot use conventional pole and overhead construction along the access road for this observatory.

A fifth method would be to pipe fuel oil up the mountain and put in a bank of diesel generators that provide both electricity and heat.

 

[allenwayne]

Re: HV Conduit on top of the ground

Just run it encase in conctere and collect the $$$,Those green tree people might have a beef but they can take that up with the contractor

 

[highkvoltage]

Re: HV Conduit on top of the ground

You could use PVC . They make a stand off bracket that allow the conduit to move. With expansion fittings on both ends this should cure the conduit coming apart If it is very long run you could put a juction box in the middle with exspansion fittings on both sides with a loop of wire in the box. You would only have to go down about 30" to install these supports. They also make rock achoring system in which the brackets could be easily attached to.

 

[thinfool]

Re: HV Conduit on top of the ground

How far do you have to go?
We were recently involved in building a satellite dish on top of a mountain here in Kentucky.
The Power Co. contracted with a directional boring company to bore about 400' thru solid limestone, shale and clay to bring the primaries from a service pole at the bottom of the hill to the padmount xfmr.
I didn't get to watch...wish I had. And I would love to know what it cost.

 

[jtester]

Re: HV Conduit on top of the ground

Thanks all for the good ideas. The run, straight from riser pole to top of the hill, is about 1000'. It is too steep to walk directly down, however you can traverse the hill at an angle without falling.
I am concerned about PVC because the winter temperatures can be colder than -15F.
mc5w's suggestion of poles is an intriguing idea. The customer wants shielded power cable because of the research being done, and I suspect that a couple of towers and somehow hang the cable from a messenger betwen towers is an option.

Thanks again

jtester