69kv Cable Underground
69kv Cable Underground
(OP)
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.






RE: 69kv Cable Underground
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.
RE: 69kv Cable Underground
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.
RE: 69kv Cable Underground
RE: 69kv Cable Underground
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.
RE: 69kv Cable Underground
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).
RE: 69kv Cable Underground
RE: 69kv Cable Underground
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.
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.
Has anyone used power trench for an application like this. Not. This is not a cost effective for this type of application.
RE: 69kv Cable Underground
RE: 69kv Cable Underground
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 ( 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 .
RE: 69kv Cable Underground
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