33kV Cable Jointing 5

[JimScot]

Our plant will be connecting to the local electrical utility via two feeders of 33kV. Each feeder will be rated at 700Amps. The distance between the utility and our plant is 5500 metres. These cables will be installed in a common power corridor. We have selected to use 2*630sqmm single cores per phase. We have estimated that we will need approximately 96 joints. We believe that these joints should be installed in a concrete chamber to allow for the possible failure, testing and quick repair.

Is there a IEEE or IEC specification or good engineering practise that would advise what to do for such a high number of joints?

 

[weh3]

Most specs require that the splicer be certified, which is usually little more than attending a demo by the rep from the manufacturer of the splicing kit. They may also require the splicer to submit his qualifications, such as the number of MV splices he has done in his career, and how many of his splices have been in place without failure for, say, five years.

Regards,
William

 

[RalphChristie]

It is good practice to first determine the positioning of joint bays. Ensure there is sufficient working space, consider passing traffic and other obstructions. If it is not possible to position the joint bays at standard cable length distances, remember that cable can be ordered in specific lengths. Consider drainage for the bays and try construct the bays prior to cable pulling to prevent any damage to the cable at a later stage.

Regards
Ralph

 

[JimScot]

We have 96 joints at 33kV. Our cables are installed within a common corridor. Is there a safety issue or restriction with respect to future inspections, Is it easier to test when we have concrete chambers rather than just laid in sand? Should we look at the cable manufacturer to perform the joints? Should we consider 63kV level joints instead if 33kV, any advantages? I have no experience of this situation and i would appreciate your advise with some practical experiences. thanks.

 

[JimScot]

Does anybody have experience of cable joints to this magnitude and only install in sand?

 

[mouli11]

I had done some 33 KV cable laying for 3500 Mtr with two runs of 3C*180 Sqmm and each each drum length of 250 Mtr. thus 28 straight through joints were provided. Most of the joints were burried in direct in ground without any Concrete chamber .However in critical loctions where excavation will be difficut at later date only were provided with concrete chamber.Since improved cable fault locaters are available ,i really do not see any requirements for concrete Chamber .If you have more funds !! no doubt go in for 99 concrete chambers. As suggeted by Ralphchristie go in for 500 mtr length drum to reduce the number of joints.

 

[mc5w]

Uh, I think that Jimscot's design uses 2 cables in parallel for each circuit, am I correct? If so, he only needs 24 manholes as each manhole would have 4 cable splices in it.

Is there any specific reason why you are doing this underground? You could just as easily do this run in overhead wiring using single core ACSR cable. Just make sure to fan out the wire strands and clean them with #220 silicon carbide abrasive paper. Also make sure that you use an overhead antilightning wire as well.

You can also get twinned ACSR cable that consists of a twisted pair of say two 4/0 or 266 KCM conductors. This allows you to use conductors that have a single layer of aluminum strands over the steel core which simplifies full current taps as only the outside of the cable needs to be cleaned of aluminum oxide.

 

[benlanz]

I would recommend an overhead line if possible. You are asking for trouble with such a long underground line. How are you going to prove that all of those joints are installed properly? A DC HIPOT or a 'soak test' are proven to be virtually meaningless as an acceptance test. The only test that can insure that joints are properly installed to IEEE 404 specification (PD free to 1.5Uo) is a PD test. You may not be able to achieve enough sensitivity to do a thorough PD test with a 5500 meter long line. So now you have a cable which has 96 points at which could have a failure due to field workmanship and you don't have a way to prove they are installed properly! I would divide the cable into 4 sections with junction boxes or switch gear. This will help in cross bonding, fault locating, and thorough PD acceptance testing. Make use you use a concentric greater than 1/3 of the conductor or flat strap metallic shield or you will run into problems fault locating and condition assessment tests in the future.

-Cheers!

 

[ScottyUK]

Benlanz omits to mention that he makes a living out of PD measurement...


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If we learn from our mistakes,
I'm getting a great education!

 

[mc5w]

Also, for overhead line wire a compnay named Sefcor makes lugs that can be WELDED to the ends of ACSR line wire and aluminum line wire using the tungsten inert gas process. This would essentially give you perfect connections provided that you make any taps by joining 2, 3, or 4 lugs together.

From the standpoint that welding is the best way to make an electrical connection aluminum is the superior material over copper. It takes something like 6 times as much heat to weld copper. For what copper costs you can usually put in twice as much aluminum cross section and have the circuit run cooler.

In fact, for a high current circuit in a factory such as DC for electroplating, I would very seriously consider using aluminum wire with welded lugs. An alternative would be tin plated aluminum busway. I have had 100% success with every instance of supplying power to an industrial machine using aluminum SER cable using box lugs.

If it is a reallycorrosive environment, such as salt mines, automotive wiring, and so forth, then using unplated copper lugs and wire does work as long as you clean out the copper oxides.

 

[benlanz]

I have to wear several different hats. I can imagine that many of you do too. If anyone would like to see my profile, it is in the public domain on this web site for everyone to see. I have no intention to do anything that not clearly above board. Please note that I only refer to technology and not specific a company or product.

I am also the vice chair of IEEE 400 which is the field guide to cable testing. This puts me a good position to view the different types of tests available, assess their benefits and limitations and recommend technology for different applications. If I happen to know the most effective way of thoroughly testing shielded power cable for workmanship defects, isn't that information worth sharing? I welcome anyone to critique the technical merit of my statements.

For clarity I have added my signature to the bottom of each post.

-cheers


Benjamin Lanz
Sr. Application Engineer
IMCORP

 

[BJC]

I recommend seperate "concrete chambers" ( there vaults in the US) for each circuit. I am not sure youre planning to do otherwise. Splices are a point of failure or a point that will have to be worked on to replace a section of cable. It's not safe to put someone in a vault working on a set of cables while the other two are energized.
If you loose a circuit you can operate at half power while the other is being repaired.

 

[jghrist]

If there is adequate working space, I don't know why working on one splice while another circuit is energized is any more dangerous than being in the vault inspecting an energized splice. I've been in vaults with as many as six 34.5 kV circuits. If you do use common vaults, use fireproofing on the cables to reduce the risk of a failure in one circuit causing a failure in the others.

 

[BJC]

jghrist

In many locatoions wiremen won't work in such a situation. You could rebuild a splice but if you have to put in sheaves etc. to set up for pulling a new cable segiment in that too much. Splices can set around with potential faults for a long time before they fail. A movement of the cabel can be the trigger. IF inspectiong an energized splice means touching and moving it around, update your insurance.

If you need a good reason to seperate systems read up on the Fire at Belliagos in Las Vegas.

http://www.reviewjournal.com/lvrj_home/2004/Apr-13-Tue-2004/news/23639094.html

There has been lots written in trade magazines on this particular fire. The root cause is supposedly "A design flaw" . They only lost a million a day, maby jimScott can afford a few days loss.

 

[beanland]

35kV class splices will serve fine. Going to an elevated voltage will not aid in reliability. In fact, they may tend to overheat unless you derate the cable for the thicker insulation in the higher voltage splice.

Think through thermal expansion and contraction. Direct burial in thermal sand is a good design to help achieve the best ampacity. 5500m of cable will expand a lot, how will you accommodate the expansion?

Concrete vaults will aid in maintenance and repair if you assume that the splices are the weak point. This may not be the case. Also, consider the ampacity effects of cable in air in a sealed or ventilated vault versus burial in thermal sand. Splices are designed for direct burial.

If you bond the neutrals, watch out for circulating current on the concentrics. Run the calculations and use cross bonding, especially at 35kV and high currents! I was involved in 100 miles of 35kV underground and we had to go back and cross bond the cables to eliminate the circulating currents to get the thermal rating we needed.

 

[JimScot]

Thanks to all, especially Benjamin Lanz.

I have not made the decision on whether to use concrete chambers which may have phase segregration and each chamber would only cover one circuit.

Testing of the joints and fault finding is clearly an issue irrespective of the choice.

If you can help me here with reference to specifications i would be most appreciated.

 

[ScottyUK]

BenLanz,

Thanks for your reply. Not all members fully explore Eng-Tips to find the additional profile information, so putting your details in your signature is a nice touch. I enjoy reading your posts - they are informative and helpful, and obviously backed up by a deep understanding. The intent wasn't to imply you were doing anything underhand.



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If we learn from our mistakes,
I'm getting a great education!

 

[mc5w]

Cleveland Electric Illuminating Company likes to use 46 KV solid dielectric cables on 34.5 KV, what is known as a 133% insulation level. This gives them a bit more lightning resistance especially since 34.5 KV lightning arrestors sometimes fail exposively.

However, using 69 KV cable on 34.5 KV does not help any since nobody makes a 34.5x69 KV primary substation transformer.

At 1 time they tried doing 34.5 KV overhead lines without an overhead antilightning wire and used lightning arrestors on the top hot wire once every 2 poles. These have had quite a few lightning arrestor just simply explode on these lines that do not have an overhead ground wire. They are now back to using overhead antilightning wires on replacement and new 34.5 KV lines.