Oil Filled Cables 4

[brijgoyal]

My question is regarding the maintainability and repair of oil filled cables. if a single core cable's outer sheath is damaged and oil leaks due to external excavation what minimum repairs will be needed.
1. Can the external sheath be repaired insitu.
2. Can a small piece of cable be cut and then a through joint inseted.
3. Will it be necessary to have an additional piece of cable for the installation of the through joint.
4. Is it possible to buy a small piece of oil filled cable for repair purposes.
5. Under what circumstance it becomes necessary to replace a large section of the cable.
6. If it becomes necessary to replace a large section of the cable, can it be replaced with an XLPE cable.
7 Is it okay to replace only one damaged phase with XLPE and the other two phases remain oil filled cable.
8 How do the electricity companies plan for such damage. Do they keep the necessary accessories and a small piece of cable in their stores to make them readily available.
9. The typical cable I have in mind is say 132kV 800sqmm oil filled cable.

 

[Marmite]

1. Yes. Emergency repairs to stem the flow of fluid can be carried out. Apply a tightly bound poultice of varnished terylene or silk tape to achieve a radial thickness of approximately 5-mm, extending for a distance of approximately 100-mm each side of the leak point. Permanent repair of these defects is similar to one used for repairing plumbs in that a copper repair sleeve is plumbed to the sheath. During the repair fluid leaking from the defect drains from the sleeve via a fluid union at the 6 o'clock position of the sleeve. The sleeve is then evacuated and allowed to fill with fluid aided by a small diameter hole cut in the cable sheath with a special tool through the top fluid union at 12 o’clock position on the sleeve. When completes the sleeve is insulated with a glass fibre box or equivalent and the cable backfill re-instated.
2. Only if additional length can be found in the cable. This is a very expensive way to carry out a sheath repair.
3. Yes unless you can somehow gain additional length on the cable, otherwise you need two joints. Even more expensive.
4. No. Most manufacturers would need you to purchase a minimum length. Very expensive. You could try begging from the local utility if they have some cable.
5. The damage is severe and has damaged the cable insulation causing a primary electrical fault.
6. Yes, but this requires oil stop joints, which are specially designed for the cable and expensive. An oil pipe would need to be routed across the XLPE section to maintain the hydraulic circuit.
7. Yes in principle.
8. Yes. Electricity companies do keep emergency stock of cable and joints. Sometimes as part of a consortium. An oil cable jointing company would keep sufficient parts to carry out sheath repairs as these are a routine activity.

No Primary Electrical Fault, Cable Switched Out before Minimum Pressure (PE) Reached:-

In this case, no electrical damage will have occurred to the cable and the sole concern is that moisture and air may have entered parts of the cable, which have sub-atmospheric pressure. If these parts are well away from the leak area, and the fluid pressure has been restored promptly whilst the leak area was still at a positive pressure, then no contamination will have entered the cable and further remedial work is not necessary.

However, if there is the slightest doubt about the possible presence of a second (small) leak in the part of the cable system at sub-atmospheric pressure then it must be assumed that minor moisture and air contamination has occurred. Following repair the fluid section in question should be flushed through the moisture and the RGP checked at intervals. Moisture checks should preferably be by chemical analysis e.g. Karl Fischer. If these results are satisfactory the cable can be restored to service. If contamination has occurred an increase followed by a slow decrease in moisture and/or RGP will be noted, flushing should continue until the values are satisfactory.

If more serious moisture contamination is suspected then in addition the Insulation Resistance (IR) of the cable should be checked. The best guide to an acceptable value will be cut back until dry papers are found and a repair length inserted.

If the pressure changes have taken the fluid tanks outside their working range, consideration should be given to checking the pressure-volume curve of the tanks to ensure that no gas cells have distorted.

No Primary Electrical Fault, but Cable Not Switched Out Before (PE) Reached

In this case, in addition to the considerations above, it is possible that some electrical discharge damage has occurred in the parts of the cable, which have been at sub-atmospheric pressure. Following leak repair, the fluid section in question should be flushed during which RGP measurements should be made at appropriate intervals. If discharge has occurred, the RGP will rise and then fall slowly as fluid from the sub-atmospheric region reaches the sampling point. Flushing should continue until a satisfactory level is reached.

If free gas, frothy fluid or fluid with a burnt or acrid smell is encountered, then significant discharging is suspect. In extreme cases such as this very lengthy flushing may be required before satisfactory RGP levels are reached. Consideration should also be given, in bad cases, to shutting the cable and taking samples of paper from the lowest pressure region and checking these for moisture, waxing, discoloration etc. This then becomes a major repair involving at the very least insertion of a joint and maybe two joints and a new length of cable. A high voltage test must be made after such work. In any case, if free gas has been encountered when flushing, a high voltage test will give an indication of the suitability (or otherwise) of the cable for further service.

Fluid cable repair is generally an expensive exercise, which is carried out by a handful of specialist jointing contractors.

Regards
Marmite

 

[brijgoyal]

Marmite,
Many thanks for your excellent and comprehensive response. Let me grasp this fully, I may have follow on questions later.

Regards
Brij