cable passing through duct banK 1

[zaza123]

During a project for replacing 34.5 KV Cables ( 400mm2) which are laid in trecoil formation of 2 sets having 3 cables each.We observed that after disconnecting we could not back pull the old cables. We wanted to keep the old cables healthy as if the New cables fail in Hi pot the old cables can be used.As we were not able to remove old cables we went for the idea to use the spare ductbanks for inserting new cabes. The spare ductbank from outer side is PVC pipes & from inner side is into the ductbank.The reason may be due to some modification years back. We observed that there was bend in the ducts when we put torch light from inside the basement.
When for testing we inserted 3 cables of 240 mm2( which is less than our size of 40mm2) we observed some hit marks on the outer body of the insulation.Is it recommended to use those ductbanks?
what if we take risk & cut our old cables clean the ducts & lay new cables.In that case what is the probability of New cables failing in Hi pot.The IR of the new cables was more than 750 Gohm & in some cases TOhm.

thanks in advance

 

[davidbeach]

Pull a mandrel through your empty duct and see if it passes through. If it does it will have knocked off an rough spots.

Why would you be replacing cable that could be reused in the same application if the new cables don't test right? Seems like a lot of needless expense.

 

[zaza123]

thanks davidbeach
What is a mandrel,
for testing we did pass three cables of 240 mm2 size.on the third cable there were some scratch marks.

Actually we are not replacing the cables,we have to give power supply from different substation. We had laid cables for that.The present supply is coming from different substation.

 

[davidbeach]

http://www.rothenbergertools.us/62.html

 

[benlanz]

Zaza123, I recommend a strong word of caution how much faith you put in a DC HIPOT/ insulation resistance test. In order for a typical DC HIPOT test to detect a cable insulation problem you basically need to cut all the way through the insulation (an insulation failure). This is one of the reasons IEEE no longer recommends a Type 1 DC test for an acceptance test. One of my utility clients just did some experiments and demonstrated that even a dime thickness of XLPE insulation left on the cable resulted in a 'good' DC test! In fact some claim 120V extension cord will pass a typical medium voltage DC HIPOT!

If there are scrapes on the outer jacket, this is not a problem. However, you need to be very concerned if you have scrapes in the cable insulation (XLPE or EPR) as a void or a slice through the outer semiconducting shield will likely significantly shorten the life of your cable. The manufacturers understand this issue very well. This is the reason that their type test requires the cable to be put through mechanical bending and thermal stresses and then be subjected to a 50/60Hz elevated voltage partial discharge (PD) test. An elevated voltage PD test is the only test which can ‘see’ inside the cable, detect, locate, and characterize this type of defect. This is fresh on my mind as I just finished reviewing the latest draft of one of the ICEA cable manufacturing standards yesterday.

The only way to prove that your cable still meets the manufacturer’s specifications is to repeat the factory 50/60Hz off-line PD test in the field. This industry best practice process would be an alternative to splicing your cable.

Please let me know if you need any additional support.

Cheers

Benjamin Lanz
Past Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants

 

[rcw retired EE]

A mandrel is a plug or cleaning device with a swivel on the front. It is pulled through a duct to clean the duct or to verify the duct diameter. Try a googel search:

http://www.cablelaying.com.au/mandrels.html