×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

EHV power lines underground - Costs vs Surface
3

EHV power lines underground - Costs vs Surface

EHV power lines underground - Costs vs Surface

(OP)
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?
 

RE: EHV power lines underground - Costs vs Surface

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

RE: EHV power lines underground - Costs vs Surface

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.   

RE: EHV power lines underground - Costs vs Surface

(OP)
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
 

RE: EHV power lines underground - Costs vs Surface

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.

RE: EHV power lines underground - Costs vs Surface

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

RE: EHV power lines underground - Costs vs Surface

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.

RE: EHV power lines underground - Costs vs Surface

(OP)
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.



   

RE: EHV power lines underground - Costs vs Surface

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.  


 

RE: EHV power lines underground - Costs vs Surface

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?
 

RE: EHV power lines underground - Costs vs Surface

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.   

RE: EHV power lines underground - Costs vs Surface

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

RE: EHV power lines underground - Costs vs Surface


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

 

RE: EHV power lines underground - Costs vs Surface

(OP)
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.



 

 

RE: EHV power lines underground - Costs vs Surface

(OP)
Slavag

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

 

RE: EHV power lines underground - Costs vs Surface

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?

RE: EHV power lines underground - Costs vs Surface

AC cables are much more capacitive than overhead lines, so the 220 km line would also need a couple of intermediate substations with shunt reactors to absorb vars and control voltage.

Long cables are more likely to be DC, but that requires expensive converter stations at each end.

RE: EHV power lines underground - Costs vs Surface

Cheers Bacon4life
The reason I ask is that due to the cost of offshore windfarms and the underwater cable & connectors, and the issues over planning onshore wind-farms, whether it would be feasible, if slightly more expensive to replace OH transmission lines with underground cables, and put up wind turbines in series along the previous OH transmission line.
The example would be the Denny to Beauly line which is going to be upgraded to 440kv "super-pylons"
If wind turbines need substations to connect to the Grid and underground cables need substations etc. would this not provide some joined up thinking and local generation without scenic detriment etc?

RE: EHV power lines underground - Costs vs Surface

Aside from the massive difference in cost between 400kV transmission line and underground cable, Wind turbines tend to generate at 690V and have a step up transformer in or near the tower base to increase the voltage to say 11 or 33kV for collection. It would not be feasible to have a 400kV transformer at each turbine. 400kV transformers are enormously expensive. On top of that you would need to break in to the passing 400kV cable, which would require a substation with switchgear/terminations/ protection etc, which again would be prohibitively expensive.
Regards
Marmite

RE: EHV power lines underground - Costs vs Surface

So how do windfarms work? Surely each "wind-farm" has a transformer or substation and not one on each turbine?
And underground cabling have shunt reactors to absorb & control voltage, do turbines not work in series?

Additionally there are already a network of Transfer stations,along the route of the grid.

RE: EHV power lines underground - Costs vs Surface

Each turbine has a transformer to step up the generated voltage (690V) to the collection voltage (say 11kV). There will be a number of turbines daisy chained together onto an 11kv feeder back to the main intake substation. The main 11kv switchboard at the substation has a number of 11kV circuit breakers, each controlling a daisy chain of turbines. The main intake substation will have one or more transformers transforming 11kV upto the grid connection voltage.  Normally a windfarm has a single intake substation at the grid connection point. The choice of connection voltage to the grid depends on the size of the windfarm and the economics of delivering the connection. As the voltage level increases the cost of plant and equipment increases sharply. Also at the 400kV and 275kV level the circuits take on a strategic national importance which means that the connection requirements are more onerous and expensive. If it was technically and economically feasible to tap wind turbines onto a 400kV cable every 300m or so like Christmas lights, the circuit would be taken out of service for a fault on any one of potentially 100 or so turbines. Losing 2MVA of generation could shutdown 2000MVA of transmission capacity.
Regards
Marmite
 

RE: EHV power lines underground - Costs vs Surface

Thanks Marmite, much clearer, sorry to be a pain, so is that why every windfarm I see has 2 or more turbines not working? Is this because if one stops another is booted up, so to speak, to maintain level of supply to the feeder station & grid. Surely it would be more sensible to tap into the Grid every 20-50km rather than 300m.

The reason for upgrading from 275kV to 440kv is to have remote  generation from renewables. This of course becomes redundant if more is generated locally as this is the environmental best option.
I understand that underground cabling is more expensive, however as suggested in
https://online.tu-graz.ac.at/tug_online/voe_main2.getVollText?pDocumentNr=65933&pCurrPk=33553
if we are looking at a mainly rural area (Scotland) which relies heavily on tourism and the exporting of oil,gas, electricity & water to England & Europe this may be an investment worth making.
I suggest that a daisy chain of turbines long a line is the same as a daisy chain round a hill.

Thanks & Best Wishes
Wildbairn

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources