Damage to Single Core Cables

If you bond together the flex straps at both ends, a current will circulate as if they are bonded and grounded at both ends. Imagine the armor as a winding and if the three windings are connected together at the ends three_ phase currents will flow and the sum of these currents will be zero only at the connection point.

You have to isolate each strap from the ground and from each other at the not_grounded end.

If you have both ends of the cable earthed, and they are single cores, as stated, remove/disconnect the far end earths and this will reduce the circulating earth fault currents you have created in the XLPE cable.

Separate the earths from the terminations to thier own earths, Do not connect them together then take them to earth. Make sure the termiation kit is an 11/6.6kV(7.5 kit) and that the constant force springs, (tension springs) and securely wound around the core and that the heat shrink, I think you have used, has been shrunk properly, smoothly, equally. Even wall thickness.

Your end result will be that the cable burns at the stress cone edge, it will go straight to earth via the semi con and pop to earth. Also, check you earth fault currents and check the cross section of what you call the concentric neutral, against the cross section of the earth straps. Normally the earth straps vary fromn 35mm2 to 70mm2. You may have to install bigger earth starps or two to suit the cable earthing/screening tapes and wire. Also check to make sure the outer sheath is not graphite coated, if it is it is a semi conductor and the earthing is poor along the cable. And make sure you use Brundy paste on the ali/copper connections. If your cable is rated at 6.6/11kV, you Hi Pot test is too high. 18.6Kv is your test voltage, if DC and 11kV if AC. You may have created a problem, on over voltage testing at the earth point of the termination. Your sheath fault current was varying due to load and it was highest at the termination earth point, where it should be, due to poor earthing, right at the termiantion. Cheers

Flex Straps have been seperated from each other, and ground.Burnt flex strap has been repaired for now.We are waiting for a shutdown to do more tests and investigation.

Questions-

1 - If load current was 400amps on each conductor. What was valve of sheath currents flowing in each phase and at connection point of flex straps that were connected and not grounded.

2 - Do you think we may have cable damage anywhere ?

3 - What type of tests should we be doing during shutdown ?

*Note - Cable Glands on Line ( Pole ) side were in free air and armour/ concentric neutral were grouned on this end only.

Thankyou All for Comments so far. Just wandering what we might expect to find during shut down.

The value is around 200Amp on the shield wire, (concentric neutral), The wires can take a higher KAmp, but the time is reduced tremendously i.e 10kA for 0.1 second. Make sure the earth strap used is of the same cross section as the copper tapes in the cable. If possible, use the next cross section earth braid up, or double the braid over and securely bind in or use a constant force spring to secure the doubled-over braid, to the shield wires. Over wrap the constant force spring with 3M,No 23 tape. (self fusing tape) just to smooth the spring edges and secure the braid connection to the copper tapes. On the earth braid one needs a damned good connection to the cable tapes/wires as there is a great deal of stress and force at the connection point. Make sure the XLPE is clean, no scuffs and scratches, make sure their is no carbon building up on the insulation and pay particular attention for treeing/ferning on the insulation. (You will need to strip down the termination to see this on the XLPE). Don't be applying pressure test, (Hi Pot), at over voltage, just stick to the service voltage used. You could try doing a sheath test at 10KV for 1 minute, but I doubt this will provide you with any evidence of a fault. I would expect on a pressure test, (Hi Pot), the leakage current should not be higher than 5ma on each cable. If you want to check further for Pd, take a radio, put it slightly off station and while the cable is under test, you will hear the Pd on the radio as interference, and you will hear the termination discharging both on the radio and on the termination. (Not a super test but, It works if you don't have Partial Discharge tester). NB, MAKE SURE YOU EARTH THE CABLE BEFORE WORKING WITH IT AND KEEP IT EARTHED. XLPE builds up a capacitance charge and it bites 24 hrs later. To get your sheath fault amps one needs more details than 400amp! need Tfx amps, etc etc. Cable damage may only be within the crutch area of the termination, and it will be carbon you will look for, If there is carbon, you can try cleaning it with a non conductive cleaner and lint free cleaning pads. DONT WIPE the semi-con then wipe the xLPE with the same pad, you will transfer semi-con to the XLPE. I have found it pointless and quicker, cheaper and more sensible to replace the cable. (Long term). The pole side was earthed and the switchgear side was also earthed via a CT. You said? Disconnect the pole side earth from earth. Leave the armour and tapes earthed together only and keep the earthing at the sub station side so circulating to one side only, not both sides, and not taking in your line currents as well as your cable currents. Hope this all makes sense to you, and I hope it helps you. Sometimes it is just a bad/weak earth connection and the braid being to small to handle the load. You could try the radio as above now, if the cables are energised, you should hear any discharge taking place, even a walkie-talkie works to hear discharge. !!!!!This is only indicative and not a proper test or test method!!!!! Have fun.

This site crams my paragraphs together and changes the spacing. Sorry if it is difficult to read, I can safely blame the SYSTEM and not me. Cheers

You can throw a <br> to separate your own paragraphs manually.

I have to do that on my iPad for some forums.

I sometimes throw a wobbly and give up with computers. But, I shall try the <br>. I hope the info is helping the lads with these cables. Hope it is also helping others, so if I am loosing folks, they must tell me. Far easier to speak than type instruction and advice. Mayhap's I need to do a common terminology list for people and then there is less confusion when typing i.e. OPGW, is referred to as 'fibre optic ground wire (FOGW)'. The real acronym is OPGW, Optical Power Ground Wire. But, THANK YOU for your kindly advice and help, its appreciated. Cheers.

Thankyou IQ for your detailed comments.

Just to let you know cable is non shielded, has no semi con, has a concentric wire shield of 2/0, wire shield was connected to ground on pole side only,not at switchgear. They were connected together but isolated from ground at this end.

Cable Structure - PVC Jacket - Aluminum Interlocking Armour - Inner PVC Jacket - Concentric Copper Ground Wires - XLP Insulation - Conductor Shield - ( Not Semi Conductive) - and 750 MCM Copper Conductor.

What would be approx. sum of currents flowing in this case

We do have some slack in cables and could do new terminations. Also this being a non shielded cable we can remove concentric ground wires back closer to top of gland entry point, bend concentric wires back, cut & tape for isolation. Cable can be terminated with out stress cones.

Please Comment. Again Thanks for all comments

The armor and grounding total circulating currents-all three phases connected together both ends-calculated as per IEEE-525 could be about 40A -if the main conductor current would be 400 A. But in short-circuit case for 15 KA per phase the circulating current could be 1400 A. If the fault clearing time will be 0.5 sec a 4 awg copper conductor will melt down.