I/R test failure of 66 kV oil filled cable

[pradtt]

Hi
we recently repaired a 66 kV oil filled cable which was damaged by third party
Upon doing the final I/R test, the blue phase to earth reading was 92 M ohms. R and W phases were both in the range of 2.5 G ohms. the phase to phase readings were as follows R-W 3 G ohms , R-B and W-B 440 M ohms
The problem is that the oil pressure is remaining constant. hence there is no damage to the outer sheath. But the hidh resistance fault prevents a proper breakdown when doing the fault location, and the cable is buried appro 5-6 feet hence a very weak signal is recieved by the acoustic set. we even used the set to try and burn out the fault but the resultant was the I/R reading on the B phase went to 169 M Ohms. But it failed at 52 kV during the DC Hi-pot test. again the problem with detecting the fault.
any suggestions ???
your help is highly appreciated
prad

 

[ausphil]

If you are breaking down the cable during a dc hi-pot test, you should be able to do an arcing fault location on the cable when it flashes over. This is essentially the same as the traditional TDR method but uses a coupling unit (read CT) round the outside of the cable to determine that the flashover has passed the start of the cable. You do one with just the faulted phase in the circuit, and the other with a good phase in the circuit as well. The split tells you the distance to the fault from the far end. This will get you in the right vicinity, then by connecting all the cores in circuit and charging them all up, when the fault breaks down, then you have 3 lots of energy thumping into the fault. You should probably hear it.

If you cannot get repeatable breakdown of the cable, try repeating the HV tests after lowering the oil pressure. Your breakdown voltage will be lowered, but you must be careful to ensure that you don't cause other problems with this though.

We have had breakdown of oil filled cables that have no oil alarms or leaks. It ended up being a valve being closed at the fill point, which allowed the cable to leak oil, but the pressure sensor thought that there was no problems because it was being fed by the storage tanks, and only feeding to the closed valve. You may want to check to see that all the valves are in the correct position - not maintenance position. The oil leaked out of the cable and dried out a joint at a high topographical point on the run, and weeks later, new joint, new road excavation and jointer with lots of overtime.

 

[pradtt]

ausphil
We found the fault, it was located very close to the ceiling end after the tri-fucating joint.
In an attempt to repair the cable, we removed the outer pvc and lead jackets from the cable, followed by the screen and damaged insulation. the core was examined and no defect seen. we then did an I/R on the said phase which resulted in a reading of 500M ohms. we replaced the insulation and screen using 3M's insulating and semi-conducting crepe paper, 404 and 401 resp. of course we made a lead sleeve and pressured the cable back up. The resultant I/R following the works was 630 M ohms, the next morning we tested the cable again and the resultant was 350 Mohms. The then proceeded to flush the cable again. following which the I/R reading dropped to 190 Mohms.
I believe that the crepe paper we used was not compatible with the oil filled cable.
Any suggestions
(thanks for your earlier comments)