MEDIUM VOLTAGE CABLE TESTING

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mohsan514

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Could some one reply the following..

1- Differenc between MV cables HI-POT test (VLF type) & Partial Discharge test (PD).
2- Advantages and disadvantages of each
.
3- which one is better and recommended for old MV cables.

Thanks

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IEEE 400 is the standard for MV cable testing. Due to research proving that DC hi potting basically doesn’t detect anything in cables and the fact that it actually causes damage in service aged cables the standard actually recommends against doing DC hi pot tests. AC (VLF or regular) doesn’t cause damage at least. PD testing is two types...online and offline. Offline uses a VLF as a source while online just uses naturally occurring transients,
 
Thank you for valuable information...
At one of my project we have done offline PD test .. cable is about 25 yrs old .. during test we did not get satisfactory results. As per our testing engineer there is problem in termination.. because during installing Rychem termination kit the the armoured is connected with shield layer of cable... which could not be saparated untill removing the termination kit...
I have doubt on his claim .. is there any affect on PD test result if shield and armour is coupled...
Could someone advice plz..

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mohsan514 said:
Thank you for valuable information...
At one of my project we have done offline PD test .. cable is about 25 yrs old .. during test we did not get satisfactory results. As per our testing engineer there is problem in termination.. because during installing Rychem termination kit the the armoured is connected with shield layer of cable... which could not be saparated untill removing the termination kit...
I have doubt on his claim .. is there any affect on PD test result if shield and armour is coupled...
Could someone advice plz..

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Click to expand...

PD is caused where there are voids in the insulation or surface tracking usually due to contamination that are arcing but the surrounding insulation is intact...hence partial as opposed to full discharge.

Not sure why you wouldn’t strip the armor back. It’s not right but since it’s just part of the cover and outside of the shield it sounds pretty goofy to me too. The thing is that the shield creates a coaxial cable system. The voltage on the conductor creates an image on the inner surface of the shield. The outer surface voltage is zero. The termination just helps with the transition from shielded to unshielded. The drain is there because shielded cables are capacitors...it drains the charge off. The armor is already grounded even inadvertently and should be zero volts from the shield so it is just intentionally grounded this way. I can see a problem if the drain is not connected to the shield but otherwise chances are they’re both grounded somewhere anyway. So yes I’m not buying it either. It’s not necessarily right but probably not your issue.

A more likely suspect if indeed it is in the way it was done is that the mastic seal around the armor is incomplete due to the grooves. That allowed humidity to destroy the capacitive grading putty (green stuff) which is hygroscopic (sucks up moisture then breaks down). The other big problem area that I would say is responsible for the most termination failures is when the semicon cutback is done ragged or incorrectly or nicked, or if the insulation isn’t cleaned properly. This is a critical stress area, even higher stress than the area around the shield since voltage flux (stress) is greatest near the conductor and relatively small near the shield.

Either way time to reterminate. Take a sawzall and cut the whole thing off first. Then carefully disassemble it and it should be blatantly obvious what failed and with a little knowledge why. Raychems product support has always been excellent to me. If you don’t have a lot of experience and ask they might help support diagnosis but honestly since you caught it early it might be easy to do,
 
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