mv cable terminations

[jimmy2times]

i would like to get opionion on attached photo showing some damage to cable terminations. application vsd, secondary of 12 pulse transformer, 1.7kv, cable is 3.3/6kV grade. installation is 1yr in service

this was noticed as part of routine inspection.

terminations on one winding seem to be ok but other winding seems to be heat damage around earth screens

what are thoughts on leaving this vsd in service with this, there is insulation monitor on DC link with early alarm for low insulation and also trip for too low earth faults.

 

[ScottyUK]

Screen earthed at both ends and large image current circulating in the screen?

 

[jimmy2times]

That was my initial thought cable screens are earthed both ends, cable run only 10m. Someone else pointed out that the termination method my not be correct though, an area of screen has not been exposed to atmosphere and that termination kit is not appropriate for that construction of cable

 

[jimmy2times]

So after stripping cable back cable contractor reckons constant force spring applied incorrect. Spring first wrapped around screen, then earth braid (to earth bar) applied over first layer of spring then another layer of spring wrapped around again. Apparently this is common error made in application of this 3m kit. Anyone else have experience of this? I'd thought the spring should be good conductor and thus not matter. There would still need to be circulating current to cause this heating damage right?

 

[GroovyGuy]

Hey Jimmy,
My first thought was same as Scotty's.
Question: These are single-conductor cables, correct? How many amps is each cable carrying? (Actually that makes 2 questions).
I understand that the cables are only 10m in length, but if I am surprised to see both ends of the shields grounded. I would think that the harmonics might be contributing to the induction of current to the shields. (ie don't forget that each conductor is carrying 6-pulse type of current waveform - very high in harmonics).

I would be hesitant to leave this for too long. Is this part of the circuit within the protection zone of the ground-detection circuit? (sorry, that is Q3).
Your saving grace is the fact that the cable is rated for 6kV, but operated at 1.7kV.
Regards,
GG

"Wish I didn't know now what I didn't know then." -- Bob Seger

 

[jimmy2times]

Approx 1600a free air rating for 3 parallel circuit. Load max 1280a. So approx 0.8 derate as installed

I was in process of working out derate for bunch, temp and circ current losses. Run is actually more like 20m. Run is not ideal either last 2m either end comes out of three times trefoil arrangements to three x 3 parallel conductors of each phase, so 20% of run is non trefoil.

Was using the guide for cable circ current losses as found in oxonite engineering handbook (as referenced in another thread i found).

So for 6 pulse content should i factor in an additional margin in the circuit load for non-linear waveform, e.g +10% ??

 

[jimmy2times]

Re: q3. Only ground fault monitoring on dc link, free firing diode front end. This would look back into transformer windings during operation right? No fault was detect though, as in OP picked up as part of routine inspection

Re q1. Yes single core