Can high voltage (i.e.: high tension) lines run underground? I thought this was not possible since they are not insulated, as I understand it. Yet, if this is true, there IS a place in the NW suburbs of Baltimore where every last inch if land was developed…although a power line right-of-way existed first.
Well, the right-of-way still exists, but the unsightly tower was removed and the powerlines go underground for maybe a 1/4 mile before emerging back on towers. How is this possible?
They are not insulated when they are overhead, but they can be insulated and run underground. Just takes more money. 15kV lines are routinely run underground in built up areas. I have never had personal experience with higher voltages being put underground, but there is no physical reason they couldn’t be, just economic ones.
Underground 15kv cables are commmonplace and,usually, run through a series of manholes and air ducts.
There are systems where voltages up to 138kv are contained in oil filled ducts-also running between manholes.
Any voltages between those two figures are common but when you get to the pipelines you’re talking thousands of dollars per foot.
EZ
For the same size cable, aerial service has a higher ampacity as it can freely radiate heat into the atmosphere. The insulation and protective cover on buried cable reduces that ability, hence a larger cable is required to do the same job. IIRC, up to 33 kV was often buried by the power company for which my Dad was an engineer.
This site implies that a 345kV line is being run underground for 24 miles in Connecticut.
And this one describes some underground lines of up to 500kV in New York, Denmark and New Zealand.
underground power lines are actually very common in cities. They are, as previously mentioned, insulated and typically have a nitrogen pressurized oil filled sheath around them. I assume this is to assist in heat dissipation. The underground vaults where these things run are… interesting places. A little scary. I’ve been in a lot of them in DC.
actually, I just remembered. The pressurization is also to keep water out of the lines as well.
Yup. I’ve been involved with environmental work at the Norwalk end for the last two years.
Pressurised oil helps spread any warm spots, I serioulsy doubt that the pressure is intended to keep out moisture as such, it might be a desirable byproduct but these cables are watertight, I don’t imagine even molecule migration of water into the cables is a significant issue.
The other main and most important fucntion of pressurised oil is to act as electrical insulation, if the oil were not pressurised, the cable would have to be several times the width and much less practical, to all intents and purposes they would become rigid pipework, but even taht would not really be practical as it would have to be constructed whilst in position rather than manufactured elsewhere and rolled from a reel.
Electricity companies don’t like running power lines underground because of the resultant large capacitance between the conductor and earth, and only do so when they’ve got no option.
IIRC, the electrical grid link between the UK and France is DC for just this very reason.
The capacitive effect in an underground cable[IE sheath-insulation -conductor] and the same relationship in an overhead line[IE earth-air -conductor] are natural conditions and are used to a utilities advantage.
An underground cable energized at ,say,11kv, has a capaciive voltage rise that is proportionate to it’s length and diameter.
All of these capacitive effects contribute to a better power factor and are,in effect, static generators.
EZ
Telecom operators used to install twisted-pair conductors pressurized with nitrogen - called Aircore. I don’t think anyone does this any longer. Maintenance is a bitch. Here’s some guys selling a product used to repair pressurized cable leaks: http://www.polywater.com/pressure.html
Instead of pressurized cable, everyone is using a filled cable for buried twisted-pair conductors now. The’re filled with a petroleum jelly. In certain areas, the cables are also armored with a metal mesh to prevent dmaage from gnawing by sub-surface rodents.
Another reason for DC links is that they’re asynchronous, meaning no care needs to be made in synchronizing the two different country’s power grid frequencies. In theory, a DC link could be made between the US and UK, bypassing the whole “one’s at 60 Hz and the other’s at 50Hz” problem. Of course, the distance is much to far to be practical.