Repeated Cable Failures in 34.5 kV Cables

If the cable is 4000 m long then has to be 6-7 junction boxes on each cable.

Junction box is located -usually -in a manhole. If the manhole was flooded the water could penetrate the cable in the junction box vicinity.

If the duct is not "embedded duct" but is laid directly underground and the pipe is PVC made [and the cable is not armored] then could be termite attack.

If the failure is occuring in a particular place within the same junction box change it immediately...its seems like moisture has entered there. Changing entire cable length is not possible but in this case being 30 yrs old i would suggest you to change it part by part......

From the information given (and unless something has happened recently to damage or degrade the cable insulation) the cables appear to be at or over their end of life (in your particular application) and are starting to fail.

Have you performed a detailed analysis of the failed sections of the cables to find out what the failure could be due to?

I would talk directly with the cable manufacturer (or someone similar if they have gone out of business) and get their advice on the expected cable life.

Some years ago I read of long cables needing surge protection, on the cable side of the breaker, as the cables are a big capacitor, if the breaker opens and recloses there could be 1.76x line volts or there abouts applied due to phase change.

I didn't see a diagram showing if you have a generator or motor on the end of this cable ? The gen/motor should have surge protection at its terminals.

What type of repairing u have done so far...?

it seem s the issue is still there & u are ignoring it

The cable is old. But, the failure modes you describe don't suggest failure from insulation breakdown. It sounds like you are getting "hot spots" in your cable assembly. You should check for high levels of harmonics in the feed source and in the load. You might also have a faulty ground bond at a transformer at either end of the run. Check your transformers, if they are running at temperatures over their rated operating levels for the loads they carry you probably have a harmonic problem. If your environment includes allot of VFD's or high frequency power supplies then you can almost be certain that is your problem. That combined with the age of the cable adds up to "hot spy" failures in your cable!

"You might also have a faulty ground bond at a transformer at either end of the run".

I think DogOfWar could be right. Usually Medium Voltage System it is an ungrounded system or High Resistance Grounded System. In this case one phase accidentally grounded raises the potential between Ground and two other phases to 34.5 kV [instead of 34.5/sqrt(3)] and if the cable shield is grounded the insulation is strong stressed.

Hey brother, it is time to replace the cable. It is said that XLPE cables have suffered from "treeing". This is encouraged by testing such as Hipot. Also as said by another contributer, if you have VFD's then dont even think further...proceed to replace. I always do an RCA to know "what" happened and this is recommended as a learing exercise that we share with the less learned members of the Maintenance or Operations team.