Punctured 33-kV Cables

Mechanical excavator misuse?

Rodents?

Military activity?

No. transformers installed in secured area. During operation it is punctured with explosion of arc. Only the issue is related to stress or leakage current. But I want to know what could be the technical reason with termination or cable or any other?

*The shield is connected to the ground

*The installation is only 4 months old. Each failure took place after two weeks or a month span.

*All the failure at same site. But on different transformers and different phases. No repeated failure on the same cable or termination.

*All the failed terminations are from same lot and the jointer is same for all terminations

*It could be possible of mis labled. But how to identify?

This really sounds like either the terminators are bad or the installation was improper. At this point, I would recommend contacting the manufacturer of the terminator. Give them all the information you've shared here, plus the model and lot number for the terminators. They may ask you to send them one of the failed terminators for inspection, or they may send someone to your site.

While you are waiting on a reply, you may want to take one of the failed terminators and slice it open from top to bottom, all the way to the conductor. Take the installation instructions and compare them to the layers you actually see in the terminator, particularly any wrapped material (tape or similar stuff). Are the material thicknesses as specified? Are there any air bubbles or other discontinuities? Is the layering level and even all the way around? The layers should gradually and consistently reduce in thickness from the cable insulation to the lug end. Are there any step changes? Any of those errors will cause point stress where the corona will eventually overwhelm the insulation system.

As for verifying the terminators are correct, the manufacturer's website should have information regarding material thickness and length for each piece of the terminator. Taka a new terminator from that lot and compare it to the specifications.

During operation it is punctured with explosion of arc.

It sounds like it has been subjected to a voltage that was too high for it.

Did you do that?

The voltage was normal and other 33kv feeders were operating well.

Beside mechanical&engineering errors (termination kits, method of cramping, accreditated person,etc), other shall be taken into account is ;

- moisture problem.

The humidity and dirty or dusty of where the connection point is shall be considered to determine the type of enclosure's degree of protection.

- Temperature surrounding shall be kept to similiar of 50W heaters can produce (30~40^oC), otherwise terminal box shall be modified to circulate the filtered air in order to remove the moisture.

- The transformer might be in not sufficient temperature due to low load being taken by. Generally temperature of transformer in service shall be 40~60^oC. The effect of this heat which then be metal conducted to terminal box of transformer, will act as heaters in MV board.

I have experience of pressure testing the 1 years stand by 20kV Cu cable, just by 5kV voltage injected arc start to occur, up to 10kV then arc is getting wide-spread. Then I stop the testing, cut off the old termination kits, then reterminated again.

hope this help.

cheers

The terminations are out door.

All the incidents happened in the evening between 18:30 - 23:00 Hrs. Ambient temperature is around 15 - 20 degreee C.

The installation is only 4 months old

Transformer temperature was ok. But from your experience I would like to know what could be the max. difference of Temperature between Oil & Winding Temperature

MV cable shield should be connected to earth at both ends of cable or only one end.

What I meant is casing temperature, not oil or winding (higher than casing). For transformer with Oil Filled class A ; winding temperature 90^oC (Alarm 95^oC, Trip 105^oC), whereas Oil temperature is 5^oC~10^oC below.

MV cable shield? yes, both side must be terminated to earth terminal, all of them.

nhkotte, what about Thermograph? have you done this before? After termination/jointing, NDT, DT, then switching ON, you need to monitor the "hot spot" using thermograph. Or use the infra red temperature meter although less in accuracy than thermograph but it will help you to guide to trouble solving at least.

Once again, what the cable&termination kits specification?

nhkotte,

In a word, Squirrels.

No, I'm NOT joking.

Squirrels gnawing on wires apparently account for a significant percentage of power outages.

I personally recall an incident several years ago when a squirrel gnawed into a section of cable that picked-up power through the catenary wires on my commuter train, causing the overhead wire to be damaged, and the power to fail, stranding more than 50,000 commuters.

-------------------------------------------------------

You can find LOTS of info on this problem on the internet...

See http://www.scarysquirrel.org/special/hitension/

http://www.critterguard.org/article_publicpower.htm)

http://www.cbsnews.com/stories/2007/11/20/national/main3525033.shtml?source=RSSattr=HOME_3525033

http://www.usatoday.com/news/nation/2007-03-11-suicide-squirrels_N.htm

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Just my $0.02...

The cable termination kit normally fails ,if the jointing is not properly done. HT cable jointing is a specialised job and should be entrusted to an expert and trained jointer.The type of cable and jointing kit is another important factor for failure of cable at termination. For jointing XLPE cable (now normally used for 33kV transmission) heat shrinkable jointing kits must be used.Epoxy cable jointing kits are not recommended. Many variety of jointing kits are available in the market . Use only one that can control electrical stress and provide sealed encapsulation to protect cable insulation at joint .Good quality jointing kit will provide stress control tubes.The optimum stress control is obtained by matching the impedance of the stress control layer to the impedance of the cable. The kit should be able to reduce electric stress at the screen edge. It should not allow higher potential difference to appear between the end of the stress control tubing and conductor/ lug. The only way to avoid failure of cable end termination is to use the best quality termination kits and to get it jointed by a trained cable jointer who is competent to do XLPE cable jointing job.

Yes. The terminations are heat shrinkable jointing kits on XLPE cable.

I had experienced similar repeated failures on 33KV Indoor as well as out door XLPE cables termination in my project. The matter was investigated and found that the quality of the heat shrinkable jointing kits used by the contractor was substandard. I got all defective joints replaced with "Raychem" make jointing kits . All future joints were also done with "Raycchem" cable jointing kits.The companny's jointer did all joints.( It was a new project with so many substations). There after for the past 15 years not a singe termination failed.

I feel your problem may be with the quality of kit used or with the quality of jointing done.Use of improper size of jointing kit also can be a reason for failure.The cable jointing kit should confirm to IEEE 48-1996 .The jointing also must be done systematically as per procedure given by the manufacturer and all accessories supplied with the kit should be used. The cable should be cleaned well and no sharp tools should be used to clean the semiconductor screen from insulation.The removal of length of semiconductor screen and insertion of stress tube and heat shrinkable tubes all should be done exactly as per the procedure given by manufacturer . Any deviation from this will also be cause for failure.

Hello

I have answered your Post/comments,

Generally best to use the Cable Maker of your particular cable, but try:

http://relemaccables.com/

If you are still needing help, reply with

Kind Regards....

It sounds like there is a problem with the connection points. If the connections aren't prepped as faying surfaces they may arc especially if they aren't torqued to spec.

Look for any signs of the cable having been severly bent, This could cause a dialectric breakdown and arcing to ground.

My 2 cents worth

Good luck

Possibly it looks like problem only on termination or cable joint

The cable punctures occured surprisingly only on jumper cable where two cables are connected together(jumper & load cables). two transformers (33KV side) are connected to gether and load cable is connected to one of the transformer. Do you feel any abnormality in cable connection. If any modifications required kindly suggest. For connection view look at the photo.................

Switching Surges and lightning could account for the failure if surge aresstor are not installed on the line or not earthed correctly.

Hey

Came across this

The location of the fault is important. Is it on the semicon edge? Though it is two years back, it would be interesting to analyse. Pl connect