EHV power lines underground - Costs vs Surface 3

[BPW363]

Does anyone have a comparison of costs and challenges for EHV distribution underground. I am interested in whether or not the lifecycle cost of a burried system is really much worse than for overhead lines.

The base case system would consist of 500kV lines delivering 1000MVA. The length underground would be 10km.

I know line burial is feasible but also wonder what the challenges would be if we opted to install the 500kV to 34.5kV transformers underground as well.

Fanciful?

 

[dpc]

For 500 kV, the cost of underground might be as much as 10 to 12 times the cost for an equivalent overhead circuit.

 

[dpc]

Underground 500 kV transformer? Don't think so. Not unless you first build something about size of an aircraft hangar underground to put it in.

My cost ratio of 10 to 12 times was just for the transmission line.

 

[BPW363]

The 10 to 12 is very different to data provided by Brugg - of course they are a vendor. The fact is, some 4% of the transmission lines in the UK are underground - most for good reasons like being near airports or being new lines run in cities or urban areas.

I've read a few articles suggesting the systems cost between 2 and 5 times the surface system - this takes no account of the life-cycle costs. Do others have examples or data they can share?


Robert

 

[davidbeach]

The difference probably has to do with right-of-way acquisition costs. Overhead lines need more land but are considerably less expensive to construct while the underground line needs less land but costs considerably more. Local conditions will drive the actual cost comparison.

 

[Marmite]

You might find this link useful. This sums up the approach of the European transmission operators. For a 400kV circuit in the UK the underground circuit could cost 10-25 times the equivalent overhead line. There are also documents available on the web relating to the proposed Beauly-Denny 400kV line which support this cost comparison.

http://www2.eurelectric.org/content/Default.asp?PageID=635

Regards
Marmite

 

[dpc]

We've used an assumption of 8 times at 230 kV for many years. I had looked into this a while back and took the 10 to 12 times from here:

http://www.etso-net.org/upload/documents/ETSO POSITION ON UNDERGROUND CABLES.pdf

I could believe 2 to 5 times for a 15 kV to 34.5 kV distribution circuit, but not at 500 kV.

 

[Marmite]

Sorry, dud link.

http://www2.eurelectric.org/DocShar...c/docs/DLS/2003-200-0005-2003-200-0005-2-.pdf

I've uploaded the file just to be sure.
Regards
Marmite

 

[transmissiontowers]

We installed 10 miles of underground 138kV a couple of years ago that was about 10x and now we estimate at 20x overhead cost due to big increase in copper conductor cost.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[BPW363]

Thanks guys, this helps somewhat.

Our case is somewhat unusual though. The lines must be run through a forest with marsh-like ground surface. To complicate matters the region is classified as a 'riparian widerness'. We have an idea of how much it will cost to clear a right-of-way and of how much it will cost to drill horizontal, long-reach, conduits, the challenge is therefore to get a comparison of a) pylons and cleared ROW, vs b) underground cables, connectors, terminations etc.

Would the 2-5 ratio be correct in that case.

Also the region is very cold for most of the year so ice maybe an issue.

We know there is a risk of stakeholder objections to the lines hence the need to look at an underground option.

 

[dpc]

It will never be only twice as much. It sounds like you are trying to justify a decision you want to make.

If this is a serious project, you really need to carry out a study and do the cost comparison for this actual installation. Directional drilling of UG circuits is not cheap either. You should be able to get assistance from local contractors in doing a more accurate cost estimate.

 

[jghrist]

I agree with dpc. A detailed study is needed. For a 500 kV project, who would commit to agreeing to put the line underground based on anonymous responses in an on-line forum that says 2-5 times overhead cost is OK?

 

[itsmoked]

Is there 500kV underground cable?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dpc]

Keith,

Yes, but it is REALLY expensive. Typically it is an oil-filled system, but I hear rumors of solid dielectric (XLPE?) 500 kV cable on the market now.

115 kV and 230 kV cables are a little more practical.

 

[slavag]

Hi.
I don't know about 500kV XLPE cables, but up to 420kV
XLPE cables on the ABB market today.
Regards.
Slava

 

[QBplanner]

As far as I know in China we built a 500kV underground substation back to 1998 in northern part.

 

[BPW363]

Thanks keep the info coming.

We would of course not progress any project without a detailed engineering assessment! This thread is more for my benefit than the teams, that said there has been some useful input -especially the eureletric report.

 

[BPW363]

Slavag

there are now 500KV - 1000MVA cables to IEC 62067. They have a 2500mm2 copper core, and weight 40kg/m.

 

[bacon4life]

T&D world had an article in November 2007 comparing underground cable to overhead lines at 132, 220 and 380 Kv.

http://tdworld.com/underground_transmission_distribution/power_lines_versus_cables/

 

[Wildbairn]

The majority of cost seems to be in set-up i.e. hiring & transporting plant etc. What would be the effect of economies of scale if the length was 220km rather than 15km or even 2200km?