69kv Cable Underground

[richanton]

I have to run about a mile of 69kv 1000MCM cable underground on a college campus. At first thought, I would use duct bank, but this will require several splice points since that size cable is limited by reel size and lots of manholes. Can this cable be direct buried. What standard would provide information on this as this voltage is outside the scope of the NEC. Has anyone used power trench for an application like this.

 

[davidbeach]

Since you mention the NEC, you are likely in the United States or elsewhere where ANSI standards would also apply; you probably need to become familiar with the NESC (National Electrical Safety Code), an IEEE document used by utilities .

In my opinion direct bury would be asking for trouble over the long run. But, do you really need 1000kcmil cable? What would a college campus need with an 80MW circuit? It would seem that small wire, which would provide longer lengths, would provide a better result.

 

[alehman]

If the cable is rated by the manufacturer for direct burial, it can be direct burried. Direct burial is permitted by the NEC if the cable is appropriately rated and installed properly (69kV is not outside of the scope of the NEC). THe National Electrical Safety Code also provides requirements for installation. Whether this is a good idea or not is another question.

You may wish to consider:
1. Risk of dig-in during future excavations.
2. Difficulty of replacing if there is a fault.
3. Corrosion.
4. Damage from burrowing animals.
5. Thermal conductivity of surrounding media and its effect on cable ampacity.
6. Difficulty of future modifications.
7. Cost.

 

[richanton]

The college currently has peaked at 37Mw, although the portion of the site the new feeder is going to is only about 17MW. The reason for the 1000MCM is we are expanding the existing ring bus and that is the conductor size that was utilized plus that maximizes the future load growth capabilities. The campus has a lot of research facilities and is rapidly expanding.

 

[davidbeach]

richanton,

80MW distribution circuits sounds like an awful lot of eggs in one basket. Multiple, smaller, circuits would seem to provide better reliability, but if that size of circuit is already a given, I would try to avoid direct burial, even if the cable is rated for direct burial, and install it in duct banks. I would suggest looking at an installation of one conductor per duct for ease of installation and minimize the likelihood of an accident involving more than one phase.

 

[dcdtn]

richanton,

Would you happen to know about what the cost is for that one mile run? Coincidentally, I am looking at options for relocating an existing 1 mile 69-kV overhead circuit, and have no idea about underground costs at that voltage level.

In my case, the 69-kV circuit is insulated at 161-kV and I am wanting to use it for a new 161-kV circuit. Which means I have to either bury the 69-kV circuit, reroute it, or tear down and rebuild from a double circuit to triple circuit tower (there is also an existing 161-kV ckt on same tower).

 

[richanton]

Haven't done a real good estimate yet, but it's in the million dollar range. The other option I'd like to look at is prefab cable trench, but it's probably not a good option for a campus. For an industrial location, it would save a lot of money.

 

[cuky2000]

I have to run about a mile of 69kv 1000MCM cable underground on a college campus. At first thought, I would use duct bank, but this will require several splice points since that size cable is limited by reel size and lots of manholes.
For this application appear that cable length is not the limiting factor. See the enclose info for details.

http://cuky2000.250free.com/Cable.jpg

Can this cable be direct buried? Yes. However, direct burial cables are recommended for low density areas and low construction rates. For colleges campus with significant new constructions planed, the cable should be designed in a safe and high level of continuity. Therefore, concrete encase may be a suitable option to reconsider in the design since may provide the required mechanical protection and safety requirement of the campus population.

What standard would provide information on this as this voltage is outside the scope of the NEC. For US cables are often specified according to IEEE, ICEA (Insulated Cable Engineers Association, Inc.): or AEIC (Association of Edison Illuminating Companies): CS7-93 Specifications for cross-linked polyethylene insulated shielded power cables rated 69 through 138 kV.
[sub]NOTE: Even though this voltage is not directly included in the table provided in the NEC, you probably will be in the jurisdiction of local electrical inspector. Take advantage of the provision in the NEC provision to size the cable under “engineering supervision” statement and other similar provision. [/sub]

Has anyone used power trench for an application like this. Not. This is not a cost effective for this type of application.

 

[cgrodzinski]

The system 69kV is usually custom designed as it is "transmission level" voltage. It means that cables and their surrounding have to be evaluated, calculated and tested for this purpose. It is possible to install your cable in trench (direct buried). It is one of technique of installation and the cost associated with it is usually less than of a duct bank. However, you have to realize that there is more than a simple trench or duct for such circuit. There are also cable construction, backfill, bonding that have to be taken into consideration and if they are not properly addressed at the time of engineering they can damage the cable in a very short time. You can contact several standards published IEEE that cover these topics. The standards are prepared by Insulated Conductor Commitee (ICC). If you have any detailed question please contact me at EHV Power (ehvpower.com). You will find my contact on this web page.

 

[mc5w]

What is your next lower voltage level? This seems to be an awfully high voltage for feeding individual buildings. If you buildings are fed with 2,400Y4,160 volt to 19,920Y34,500 circuits, you might want to think about eliminating the 69 KV level. Ohio Edison has replaced some 69 KV primary 7,200Y12,470 volt secondary distribution substations with 138 KV primary substations.

Is your 69 KV ring bus confined to just this campus or does it serve other customers?

With the advances in SF6 switchgear, insulators, and solid dielectric cables, you can often run a 138 KV or 161 KV circuit for less money than 69 KV. About the only real difference is spacing between wires and open ( air insulated ) busbars.

You also need to consider that with a 1,000 KCM conductor you will have a hard time getting more than about 3/4 of the wire strands to conduct unless the connection is welded. You can get aluminum lugs that are intended to be tungsten inert gas welded to the end of ACSR transmission conductor. In fact, I would not consider any connection method for this wire size other than welding. Since aluminum is easier to weld I would shy away from copper. If you want to play around you could splice aluminum conductor using electroslag welding but that would really only work on transmission conductor.

Copper is much harder to weld than aluminum which is one reason why I do not like knee jerk prejudice against aluminum wire. About the only welding method that works on copper is exothermic welding which would not work for an underground cable. This is because it takes about 6 times as much heat to weld copper than aluminum. In fact, it takes less heat to weld steel than to weld copper.

A lot of "aluminum" wiring failures were later determined to be caused by something else and it turned out that there were other factors that caused a lot of copper wiring failures. Among them were cheap steel terminals in 69 cent outlets, not cleaning off the aluminum oxide, and so forth. Also, Federal Pacific, Wadsworth, and Zinsco circuit breaker would develop arthritis and allow a circuit to carry 150% to 300% of its rating. Reynolds also made a rather soft aluminum alloy that aggravated things and Alcan has been able to prove that their alloy is by far superior.

Also, Dr. Jesse Aronstein (

http://www.inspect-ny.com

) ran some test that showed that a wire brush is 100% INEFFECTIVE at removing aluminum oxide. The most effective agent is #220 silicon carbide abrasive paper. I also have directions on how to get ALL the wire strands of copper or aluminum wire to conduct over at

http://home

dot earthlink dot net/~mc5w .

 

[benlanz]

Just a little advice...

No matter which design you choose, do yourself a favor and perform a thorough PD test to insure everything is installed to IEEE standards. Only then can you be reasonably assured that the cable will have a long installation life. A 'soak test' or a DC HIPOT are virtually meaningless as an acceptance test.

-Cheers