HV Polymeric Cable installations

[ItAintMe]

Does anyone have experience with installation and operation of HV (11kV) polymeric cable systems - EPR, XLPE etc? I am interested in experiences with overheating due to backfill drying out when operating cables at around 90+oC, or mutual heating, especially of paper insulated cables run in the same trench as newer polymeric cables? Do you use corrective backfill, or have you had experience (good or bad) with installations in plain sand or other backfill materials?

thanks
IAM

 

[MarkAthay]

You have several problems to contend with.

1. PILC and extruded cables have different temperature ratings. PILC if I remember right is only rated for 80 degrees C. TRXLPE is rated for 90, and EPR has a nameplate of 90 but most manufacturers feel comfortable pushing it to 105. The PILC will not be happy in a common duct bank or trench with the other cables.

2. High operating temperatures DO drive the moisture out of the soil. Combine that with dry weather or drought and you have a disaster cooking. Remember Aukland? I have seen numerous 15 kV cables where the jackets melted off the cables and congealed the 3 phases into one fat blob, both direct buried and in a duct bank. Not a nice situation.

3. Mutual heating must be definately accounted for. I saw where a customer had multiple parallel runs of 500 copper at 4 kV where the TRENCH temperature was 140 degrees F the day AFTER it was opend to replace the failed cables. Even if the cables are not all part of a common circuit, and peak at different times, some derating must be done to avoid catastrophic cable damage.

How do you solve problems 1 and 3 to avoid problem 2? There are several computer programs out there to assist in calculating the "real" ampacity of the cables. You should avoid prolonged operation at the full 90 degrees C to try to keep the moisture around the cables. Running the cables under a landscaped area is ideal for this. Running the cables under asphalt is not great, but we all have to do it.

If native soils will not ive you the ampacity you need after the computer calculations, then a thermal backfill can help a lot.

This is not a simple problem, and will require some work to come up with the "best" solution. IEEE has a book of cable ampacities that is very thorough. Your cable manufacturers undoubtedly have tables for the cable ampacities, or their engineering department may be willing to help you. Most cable manufacturers are very willing to help you out with your particular application. If you don't have to go very far, the simplist solution might be the "brute force" method. Use bigger cables than you think you'll need.

Mark in Utah

 

[ItAintMe]

Thanks very much for the valuable feedback Mark. We have a Plant Rating department in our organisation which has a reputattion for the work they have done in cyclic ratings of all types of electrical plant. They are the ones suggesting we need to be very careful. What we are trying to do is see if we can actually avoid the blunt approach - we want to replace 500 mm2 AL PILC with 400 mm2 AL XLPE. At 70oC (PILC max. temp), we lose about 6% compared with the 500 PILC, but at 90oC obviously, we can get quite a bit more power out of the cables, and at less cost!

Our native soil is mostly clay and sand. Fully dried out G values range from around 0.8 to 6.0 oC W/m. We rate based on a value of 1.2 - hence the concern. We get good rainfall in Sydney, and we've never had any problem running PILC cables at 70oC with plain sand backfill (G up to 4 fully dried). We are just entering unknown territory if we do push to 90oC (alone or with other cables). We are setting up a trial using distributed temperature sensing, running optical fibre cable with the power cables but plan a year to get meaningful results.

I am just snooping around for anyone who's already been there and done it. You're comment on melted 15kV sheaths is very relevant. Can you give me any more info on soil and operating conditions? 140oF trench temperatures are definitely scary!!

thanks again
Keith

 

[jbartos]

Suggestion: There appears to be several aspects to be evaluated and analyzed:
1. Chemical compatibility of the cable surface with respect to the environment
2. Temperature effect of the cable surroundings.
3. Ampacity rating/derating based on the cable rating, installation, and design margin to have reasonable cable aging or life cycle.
4. The overall cost.

 

[ItAintMe]

Yep you're right. We've chosen to go over to polymeric cables from PILC, and we now have a couple of months to do a lot of work! We're just trying to avoid doing too much of what someone else has already done. We're talking to some ppl in canada who have a business doing soil analysis and recommending suitable thermal backfill materials - they are checking with US clients to see how many of them use thermal backfills, and how many if any just use sand or excavated material, and in that case, what their experiences have been running at 90oC. The cables use High density polyethylene outer sheath which seems pretty good so far. Our plant rating group have done quite some work on soil dry out characteristics some 20 years ago, but that was focussed around 70oC operation - which we've had considerable success with. We just need to see how dry it gets at 90. We already know local clay, once dried out, cannot be remoistened, so we definitely can't just chuck that back in there. Sand works really well (better than expected) up to 70, but we don't know how dry its going to get. If it does dry out- G rises dramatically, and we can lose up to 40% of our ratings. Hence the need to find the right balance between operating temperature and cost of remedial work on backfill. We are using a high purity XLPE to give us as long a life as we can manage, so we don't want to go burning the stuff out early on in it's life.

 

[gordonl]

I'm in a large industrial, and we've been using direct buried 15kv in sand for 30+ years. We haven't had any problems, but our water table is high (we're on a river edge) and we seldom load our cables to 90C rating for extended periods.

 

[ItAintMe]

thanks all for the comments and feedback. It's been very helpful

 

[MarkAthay]

Here are a few of my experiences....

1. Concrete-encased duct banks tend to work well in cycled loads with different circuits in them. We've been conservative in our cable loading limits, while exceeding them only for emergency conditions.

2. Conservative cable ratings has "saved us" in the past. 750 kcmil Al 15 kV cable has a rating of about 455 amps in conduit as a circuit by itself, and in a duct bank we'd limit it to about 350 amps. This is the 3-circuit rating of the cable. The 6-circuit rating is 280 amps. Direct buried this same cable in a trench by itself s good for 540 amps. This usually creates a buffer between what we think we know and what is actually there.

3. Sand tends to dry out quickly, but will also allow the inward migration of moisture afterwards. All of the really bad cable "cooks" I've seen have been in sand or sand-encased conduit. Here in Utah things can get pretty dry and hot in the summer, and without some source of moisture besides what little rain we get can be hard on sandy soil.

4. Some areas have used bentonite clay to help in retaining moisture around the cable. Clay is great for holding the moisture, but lousy in soaking it back up afterwards. I've heard of studies that other utilities have one that show several years for the soil to "heal" after severely overheating the soil for prolonged periods of time.

5. EPR cables can handle overloads a LOT better than XLPE cables. XLPE or TRXLPE goes through a phase change around 140 degrees C from a solid to a gellatinous material. When it does this it greatly expands. Concentric neutral cables will see the insulation be pushed out against the neutral wires. Tape shielded wires will see the tape split in half by the internal pressures being generated. EPR is a rubber, and when it overheats the plasticizers (the stuff that keeps the rubber flexible) are gradually drived out of the rubber. The cable will not fail at this point unless it is moved, and then the insulation is cracked. This happens at a temperature well above the melting point of the PVC jacket. Melting and damage to the cable jacket does not necessarily signify that the cable itself is damaged, at least in the case of EPR.

The cable that was in the very hot trench was 500 Cu TRXLPE 4kV cable with a tape shield. The conductor had sagged through the insulation part way, the insulation had swelled and split the tape shield, and the PVC jacket had hardened and cracked off.

Unless you have a very long run of cable to do, with very expensive cable, the "brute force" method may well be the most cost-effective method to use. Often there are just too many unknowns in the soil conditions along the route to be able to control what is happening.

How long is the cable run? How much current are you trying to carry between the two points? Is it all underground, or is some of it in overhead cable tray or such? I can give you my $0.02 worth of some suggestions if you want.

Mark in Utah