Cable Damage

How did the phase-to-phase insulation resistance test go before the cable was re-energised following its first repair, or was it a case of "switch-it-on-again-and-hope-for-the-best"?

I saw that in a bank of fuses, it didn't take long to infere that the first failure caused an arc, that deposited a carbon coating (conductive) over the black fuse holders, thus was not visible; a quick megger test confirmed my suspicions.

Afterwards I've saved a lot of downtime in other faulty devices like starters and power contactors.

Regards

If the same cable termination keeps failing on one cable, it's not the termination causing it - it's the cable. Install a new cable with new terminations, properly hipot test it after terminating, and you will probably eliminate your problem. But to keep trying to "band-aid" fix a faulty cable to save a few dollars (or whatever your currency) instead of doing a proper repair/replacement is foolish in the long run.

After Each repair before and after the fixing of terminal kit the cable was meggered using a 5 KV megger.The result of each phase to ground and phase to phase were normal.That is why doubts remain.We have meggered the stator winding with out the cable being connected and physically inspected the winding.every thing is normal

May I correct some misleading information. IF, If, that cable is XLPE, 3 core with individual screening, PLEASE, DO NOT TRY TO MEGGER THE PHASE-PHASE CONNECTIONS. You are honestly wasting your time and getting false reaings, it cannot be done on this type of cable. One cannot assume on a cable, hence I requested further information. I advise, information being profered is misleading, without cable details and termnination details. (Perhaps I missed the details along the line), but, one needs to know exactly what cable is being used, and it really sounds to me as if the cable is XLPE, the semi con was not stripped of properly and it blew in the crutch area, which is in fact a phase to earth fault, and it is that quick to blow. As stated, if you advise the cable detail I can help you, alternately you have solved the problem. Cheers.

NB; Dont mix XLPE with PILC cable testing, they are NOT the same test regimes at all.

To me it looks like a high resistance joint.

Review the end kit installation.

Without a physical examination of the damaged terminal, the type of Cable used, the type of terminal kit used, the finished terminal and the way it is fixed on the terminal from the generator, it is not possible to give a proper advice.

1st, the damaged terminal and cable end must be looked at to see if the way it looks can suggest the reason and the way the damage occurred.

2nd, the cable end will need to be cleared far enough to remove the stressed parts and reach healthy regions before a new termination is attempted.

3rd, having analysed the probable cause of the initial damage/arcing, avoid it:

could be a bad surface contact or bad positioning of the lug end on the gen terminal, etc... Clean all the area from any dust or carbon/conductive dust.

Even if no load is applied, there will be some inductive/capacitive leaks with the earthing band on the cable. (or the cable is not 11kV proof anymore, due to some overheating spot ...? Check the cable at higher voltage testing at least at 11kV, not just 5KV.

and of course, to add the obvious:

Inspect and verify that the tools used to replace the terminals are clean, in good condition, not causing damge, etc. etc. etc.

Also verify that the "proper kit used" is indeed the proper kit.

I too agree with what YAHLASIT had said maybe not fuses but the vicinity of that joint may have a weak area which for some reasons dont appear during insulation tests.maybe an injection test will reveal something.

This is my field of expertise. Firstly, what cable are you using? Secondly, what termination kit are you using? Is the cable single cores or 3 cores? Armoured or unarmed? Can you post a photo of the cable and end box to the gen set. If you can tell me this I shall solve your issue. And tell me if you have both ends earthed down via a gland or other means. This is what I do for a living, right up to 500kV cables. If you are keen I am too, ready to help you fix the problem. Looking forward to your response. Cheers.

I don't know what tests you did on the cable but it sounds like more than one core got damaged the first time around. There are so many variables not known to the reader which could be giving rise to the fault and replacing the entire cable may not sort out the problem. The characteristics of the drive, generator and the load could be effecting the fault. I think you should call on a specialist before spending any more time and money as there is a lot of expensive equipment in the generating train.

Hi,

IQ has asked for some relevant additional information and if possible this should be provided.

None the less, sounds to me like the termination is defective. While 'proper kit' might be used, is the jointer installing it properly. I have had cable failures before due to incorrect installation of 'proper termination kits'. Mainly due to problems with semi conducting layers and stress cones.

Also check again that the 'proper termination kit' matches your cable type.

these pictures are after the first damage

The cable is individually screened, steel wire armoured, XLPE cable. Raychem terminations. But, this kit has been bastardised to work. OR, someone forgot to supply you with constant force springs for the earthing kit that should have come with the kit. Binding wire, copper, has been used. (This was fine on PILC cables, but binding wire needs to be neat, tidy, tight and properly installed on XLPE, epsecially at the crutch area where this binding is shown). The core has blown at the bottom of the stress cone, causing a phase to earth fault, as the earth is around each core. (Screens). These screens are then connected to the armour to circulate the stress, (circulating currents) down to earth at a gland or earthing point for the armour.

Looking at both damaged cores, it is evident that the prepearation works was not great and that the stress cones should have been moved further up the cores. Both red and blue phases have similar damage and the tracking started at the lug. Have a look under the heatshrink, just behind the lug and make sure the semicnductor was stripped off the core to the to the stress cones. (Stress cones. They are long black tubes approx 120mm long and marked with Rachem identifying numbers which denate semi con material. The stress cone should be in contact with the semi con and copper tapes on each core, and earthing braids connected to the copper tapes, by means of a constant force springs. The stress cone is installed on the semi con which should be at least 25mm inside the stress cone. The semi cion edge should be tapered. From above the stress cones there should be NO SEMI CON materials on the core, right up to the lug. It should be white XLPE insulation only. With the edge of the insulation tapered as it meets the lug barrel. The lug barrel needs not less than 6mm clearance between the barrel and the insulation. This void is them filled with the red void filler tape which needs to be well stretched. Then install the short sleeve over the barrel and shrink it down. AND; Fill the hole in the lug that allows you to see the core end inside the lug barrel. It stops moisture getting into the core.

The XLPE below the instulation tubes, (long red tubes), should be white, (Maybe not now as it will be carboned, treed and tracked). It should be smooth, polished and clean and not rough and unsanded. (Not Scored, rippled and scratched).

When crossing cores at the crutch area, make sure the cores are seprarated by not less than 25mm between the cores, epsecially if you make the stress cones away down low. (Not a hassle to move them up and makes for a better termination and clearances). If you have two cores touching near the stress cones, at 11kV, the voltage measured on the core will be around 23-26kV. N.B. not cable voltage, core voltage. No good on an 11kV system.

I shall confirm that the kit you have is correct for the cable and voltage being used, the installaion is poor, and it is clear that the cable was badly prepared for terminating. My sound advice is this, the cables are now shot and full of carbon. It is probably down about 2m passed the crutch, so you can cut back a min of 5m, check ther cores for carbon and if it is clean and free of carbon from the flashing that has taken place, rejoint a section in and terminate the end again doing it in the correct manner. If this cannot be allowed, then pull both cables out and ditch them. Install 2 new cables. Move the stress cones up each core about 200mm-300mm, above the crutch. You need not less than 100mm from the top of the stress cone to the bottom of the lug barrel (At 11kV, 60mm is the minimum length of clean, white XLPE insulation needed to prevent tracking occurring to the semi con). I have found it cheaper to replace the cables.

DO NOT pressure test, (HiPot test), the phases in the manner phase to phase to earth. Each core is down to earth already with the screens, so connect 3 phases together, test all to earth at 12kV for 15 mins only on 11kV rated cables using AC and 18.6KV using DC. One test only and it takes 15 mins only.

Make sure the armour is glanded and it goes to earth, make sure the earthing kits for each core, goes to earth. These core screen earths can be connected to the armour and them the armour taken to earth with a suitably sized braid strap, with a lug crimped on if possible. If this cable is a short run, you can leave the earths off on one end only and circulate the earth fault currents to the opposite end, normally where the breaker is situated.

This termination did not take long to reach a high temperature and tracking started immediatlty the voltage was applied. (Check that yellow phase core, as it has little damage, score the red insualtion on the core, apply heat and it will split and open up, exposing the cable core below, check to make sure the semi con was stripped off the core). The heat and damage near the lug barrel suggests the semi con was not removed from the core or it was not removed back far enough. To much carbonising,splitting and rupturing of the red insulation tubes suggests this.

I say you are lucky to still have a gen set. Make sure the panel is well cleaned of carbon. If anything here is NOT CLEAR, let me know and I shall explain further. The kit is right, the wokmaship is suspect, the cable is now useless, remove and install new.

after second damage

Cable after first damage.The damaged part cut off

I forgot to metion this too, If you use a megger, again connect all three phases together and megger to the screens. Remember, each core is down to earth by means of the screens around each core. The semi con is in contact with the screen, and each screen is touching the next screen/core. So each core is effectivley down to earth. Any other tests will be fruitless.

ok thank you.will get in touch again

I just posted for anyone, some real cable damage caused by fire. PILC and XLPE ali sheated cables, 3 core, CU and Al cores. The temperature was so hot in a cable tunnel that the ali and lead sheaths melted an formed puddles on the tunnel floor. The ali cores and copper melted and and the puddles were a real mix of metals.

the cable was replaced by a new one.With new termination kit.

The person who installed the kit said that there is black coating on the individual core of the cable.It has to be removed by scrapping as if not removed a path is formed between the terminal and ground.

Cable working now but do not know if that person is right

The black layer is a semi conductive plastic, It need to be removed and it should be strippable. I have given instruction here how to do this termination, and scrapping will not be good enough, if you do scrape it down use glass. Or a semi con stripping tool and sand the white XLPE down smooth. It will give Pd if this is not done and eventually blow again to the crutch. Closest earth connection and the cable is finished for ever. I can see from the cable make up it is: core, semi con, XLPE, semi con, Brass/copper screen, bedding. outer sheath. The semi con needs to be removed on each core. Best