230 kV XLPE UG Cable in Steel Encase Pipe

[cuky2000]

Does any one have a reference of a short (1 mile) underground cable system, 3 single conductors cables XPLE, 230 kV route in single steel encase piping?

Most of the typical installation that I am aware of is designed using a trench, duct bank with multiple single cable per conduit, tunnel or cable direct buried.

It is any drawback to preclude the installation encasing 3 single solid dielectric cables in a single steel pipe?

 

[RajT]

There are no technical objections as long as the pipe OD is suitably sized and the cable are run in trefoil formation.

 

[gmazumder]

you may not br sble to run for such a long length in a trefoil condition as suggested by RajT. It is not suitable to run single core cables in metallic ducts and specifically if you cannot maintain the trefoil condition.

hope this helps

 

[cuky2000]

CLARIFICATION: The total length is about 1 mile. Our civil colleagues are suggesting the possibility to use direct drilling under 200 ft of wetland to drive a single encase steel pipe to used as a raceway for 3-single XLPE conductors.

We are considering the possibility to have a hybrid installation with regular duct bank with single cable in conduit and joined on a transition manhole to 3 conductors single routed in steel piping to end in a structure with cable termination (pothead) and surge arresters.

We are confronting a resistance from our colleagues on the utility not familiar with this none conventional installation. We are requesting opinions and searching for practical application and ideas to push this project through.

 

[RajT]

I would say that from technical aspects it looks fine.

 

[MikeDB]

I am not an underground cable expert, but I can sure understand the utility's reluctance.
- The cost to put 230kv underground must be horrible
- My catalog only lists cable up to 138kV, how many times has this been tried at 230kV?
- Using XLPE instead of EPR
- What size conduit is required? >12"?
- Can they use directional boring on conduits that large?
- Putting single phase cable in steel or reinforced concrete will cause heating problems of the steel due to induction -- a definite no-no.
- Putting all three phases in one conduit may limit power flow due to the temperature rise in the cables and poor disipation of heat.
- What is the maximum length of cable for that size, you may need several splices to go a mile.
- Pulling tensions for the installation of the cables is also a concern on cables this big, even on straight runs.

Good luck, I would be interested in how this project turns out.

 

[cuky2000]

MikeDB,

Thanks for your input and share with us your concern. This is motivating us to address some of the potential problem. So far the project is proposing three alternatives: Pipe type cable (HPFF), Self Contained Fluid Filled (SCFF) and Solid Dielectric (XLPE). In this face of the project we do not have all the answers however, there are some of your question have been discussed and I would like to share with all of you in this forum:

- Q1] The cost to put 230kv underground must be horrible. A-1] The objective is to relocate the existing overhead line underground. The developer performed a cost/benefit analysis and offering the utility to pay all cost associated with TL relocation.

- Q-2] My catalog only lists cable up to 138kV, how many times has this been tried at 230kV? A-2] This is one of the main reason of our post do to the lock of large amount of cases in the US
[sub]A2-a. The power industry is moving toward the acceptance of solid dielectric cable above 138 kV. Utilities such as FP&L, PG&E, PJM, NYPA and may other are using XLPE. The Neher & McGrath method for calculation is commonly accepted and IEC is also used as an optional reference since national standards such as AEI std. CS7-93 2. goes up to 138 kV.

A2-b Even thought in the US there are limited installations (~2000 ft) of solid dielectric cable operating at 230 kV, outside the US there were 170+ circuit miles in service in 1993. There are excellent reliability performance reports from France were more than 50 per cent of the in-service cables in the field 63 kV-400 kV are polymeric cables (LDPE-HDPE and XLPE).

A2-c- In the US there is 4000+ circuit miles of pipe type cable (HPOF) with 3 conductors encased on 8” and larger steel pipe with many section of hwy and RR crossing use encase pipe with solid dielectric cable.

A2-d 230 kV cable may be available from: Pirelli, Nexan (Alcatel), ABB(Kabel), Siemens, Mishubishi, Toshiba….(there limited case in service up to 500 kV and test up to 1000 kV).[/sub]

- Q-3] Using XLPE instead of EPR. A-3] We choose XLPE instead of EPR because its comparative dielectric losses and possible better compatibility with the engineering practice in the US.

- Q-4] What size conduit is required? >12"?. A-4] The preliminary pipe size is expected to be around 8 inches.

- Q-5] Can they use directional boring on conduits that large? A-5] With Horizontal Directional Drilling (HDD) it is possible. Initially small pilot hole is drilled follow by drilling a bigger size hole.

- Q-6] Putting single phase cable in steel or reinforced concrete will cause heating problems of the steel due to induction -- a definite no-no. A-6] Pipe type cable is normally routed in steel encased pipe. There are fair number of HV underground solid dielectric cable in service using encased in steel pipe for RR or HWY crossing application. We run a preliminary calc. that may us believe that this could be resolved.

- Q-7] Putting all three phases in one conduit may limit power flow due to the temperature rise in the cables and poor dissipation of heat. A-7] It is expected that the cable will be routed as symmetrical as practical to cancel most of the EMF field. that produce the heat. The presence of water and the high thermal conductivity of the steel pipe will help to remove some heat. A completed detail study will be performed to address those potential cases. Worst case scenario will be using pipe type cable encase in steel pipe on the entire underground portion.

- Q-8] What is the maximum length of cable for that size, you may need several splices to go a mile. A-8] The project will be highly satisfied if could get up to 3 splices.

- Q-9] Pulling tensions for the installation of the cables is also a concern on cables this big, even on straight runs. A-9] There are experience using 3000+ ft of cable for pulling purposes. However this case should be analyzed in the case-by case basis depending on the cable route, friction and other characteristics.

 

[MikeDB]

I'm still not sold on the idea and unless the utility you are referring too has a lot of HV and EHV underground on their system, I would expect them to be very opposed to introducing it. Most utilities are very conservative (we are not talking about enron or dynergy) and do not want to be the first on the block to try something way out on the leading edge, especially if the line is a critical part of their system.

Do you mean an 8" conduit per conductor? If so, it would need to be none metallic. There are two different heating effects to be considered. One is the heating of adjacent steel conduit or rebar due to the electric fields. The other is the heating of the cable itself due to I^2R or losses. You can minimize the first by putting all three phases together in the same conduit, but that just makes the second more of a problem. That may be why most EHV installations are gas or oil filled pipes.

Assuming the overhead line is at least 795 kcmil, to reach the same ampacity, the underground cable would probably need to be 1,250 kcmil or more. I am guessing that a cable like that would be 4 to 5" OD and weigh more than 10 lbs per foot. The minimum conduit size for all three phases would be 12" (4" x 2.79).

What was the estimate to put the line underground? I would think that it would run over $1,000,000 USD to put a mile underground and retire the overhead line.

I think that XLPE is more common as it is less expensive. I would think that treeing would be a major concern at 230kV. Isn't XLPE the worst for that?

 

[ItAintMe]

There is another issue here which I have been dealing with recently. Putting a relatively short section of cable (especially such a critical part of a network as a 230kV cable) into an overhead line really gives the utility the worst of both worlds. They have the long repair times of an underground cable, and exposure to the (comparatively) high fault/interruption rate of an overhead network. There are also issues with step and touch voltage rise hazards at the underground to overhead transition point, effectiveness of earthing, need for cross bonding of the cores to reduce sheath losses. In the event of a fault on the cable, if it is in a long pipe with no ready access, then they are also up for the replacement cost of one or more phases over the whole length - the company proposing the works will not be around to pay for that in the future. New Zealand don't permit short sections of cable at these voltages to be put into overhead networks, and as far as I can find so far, it's not done in Australia either.

 

[gordana]

3 PVC conduits in trefoil (single cable per conduit) insert in the steel pipes and fill with bentonite between conduits

 

[jbartos]

Suggestion: Visit

http://public.ansi.org/ansionline/D...Comment/Standards Action/2003 PDFs/SAV342.pdf

for IEEE 82-2002 addressing high pressure high-pressure
pipe cable, low-pressure gas-filled cable, self-contained liquid-filled cable, solid-paper cable, and other taped cable designs.

 

[pablo02]

As I recall, Consolidated Edison of New York has done similar installations -- but, I think they used a cooling medium to pull the heat out of their installations... I no longer have any contacts there, but you might give them a call regarding contacts for such a design...

 

[sdlloyd]

With regard to XLPE I would suggest you look at the following:

1. The effect on cable AC resistance and ampacity.
2. The high level insulation stress of 230kV cable required that will fit in the pipe you describe.
3. The stress on the insulation shield and its effect on the accessories.
4. Thermomechanical forces and movements in the pipe and forces on the joints
5. The cable build over the insulation (for charging current and mechanical strength)
6. The method of restaining joints
7. The route profile and "cable walking"
8. Cable design of water barrier
9. Bonding arrangements, circulating and fault currents, particularily at joints.
10. Choice of cable pulling lubricant

If you do it, let us know what happens. I for one would be very interested.

http://www.cableconsulting.net

 

[RJF]

We have two 6000 foot runs of 138 KV cable in the US that we put in about 10-12 years ago. The cable ran from a Utility 138KV station into our facility. It was expensive but gave us a way to get though an area that management did not want an overhead line running through for a variety of reasons. We used Brugg (Swiss) XLPE cable and use PTI (

http://www.shawgrp.com/PTI/index.cfm)

as a consultant. At that time there were not a lot of people doing this with XLPE in the US. However, there was quite a bit installed in Europe at that time. We looked at EPR(too lossy) and pipe type cables. Most of the run consists of concrete duct banks with 6 inch PVC ducts. For a few hundred feet where we went under a railroad track, we bored a hole which was lined with steel, pulled the conduits through with spacers and then pumped the whole thing full of concrete. I would think that any cable like this is a custom design considering BIL, ampacity requirements, ductbank construction, grounding,fault levels, etc. Our cables each have a differential protection system with fiber optic "pilot wire" as a first line of protection. You have to be very careful in doing your splices and terminations. Each 6000 foot run has one splice in the middle. We hired the cable manufacture to do the splices and terminations. We also installed a 4th duct in each run just in case we had a cable failure that damaged the duct, bought a spare piece of cable and built the termination structures at both ends to allow the 4th duct to be used for any phase.

 

[Henryk]

This thread may be interested in an EU-funded project "Artemis" on XLPE cable that completed March 2003.

http://www.le.ac.uk/eg/research/groups/power/highvolt/artemis/artemis.html.

We (Polymer Research Centre - Surrey University :

http://www.surrey.ac.uk/prc)

looked at treeing, migration of materials under high field conditions etc. Have to say that Alcatel and Pirelli materials, even after 325kV lifetime tests, looked fine.