PD component testing

[DocShock]

I am performing PD testing on short sections of sheath 22kV cable which has been factory tested to have <10pC at 22kV but I get very high PD readings 300-1000pC at 8.2kVwhich then decreases after several minutes to 2-3pC - does anyone know what effect is causing the decrease of the PD activity?

PD sometimes decreases prior to failure but this is new cable and the effect is repeatable by taking the cable to 9.2kV beyond inception then reducing and holding at 8.2kV

 

[HVDragon]

can you send pictures of the types of te PDs?

Somethin terrible is stalking between the high voltage potentials

 

[DocShock]

Unfortunately I have no photographs as the kit was in the UK and I'm back in the US.

I was using a HIPOTRONICS DDX9101 unit hooked to a XLPE cable 12,20,(22)kV.

 

[HVDragon]

where was the cable? what was the lenght? what were the conditions during the measurement?

Somethin terrible is stalking between the high voltage potentials

 

[DocShock]

Cable length ~1m, test performed inside a Faraday cage with the sample suspended ~1m from the ground plane.

 

[ausphil]

Are you testing a 1m length?

Are you testing between the conductor and the screen, or is the screen floating?

Have you terminated the ends properly to be PD free? (I ask this because being a 1m length, the terminations would probably cover most of the length of cable, and you would be left with very little length of insulation where there is a screen over it - it would almost end up like an unscreened cable. If the ends have not been terminated properly, you would end up with problems.

If you have terminated properly, don't forget that you are now also testing the terminations and the cable, which is different to the factory, who may well have used a water termination. Chances are that the cable is fine.

Finally, why are you PD testing a 1m sample before it goes in the ground? This is a little over the top, unless you are suspicious about something happening during transport.

Ausphil

 

[DocShock]

We have been testing the PD of connectors so we wanted to confirm that the PD readings did not arise from the cable itself. The main issue was that we saw the PD activity decrease with time with no apparent cause - so I was wondering if anyone had seen this before. I was testing between the core and the shield but did not use stress relief around the shield or the core as the voltages we relatively low - we had 2 cable samples pass with very little PD but the third sample exhibited this decreasing activity with time.

 

[HVDragon]

Can You recognize the type of the PDs? I believe that You have bad contacts and after some time the contacts are welding, so this is the reason for decreasing of PDs. Can You point where was the position of PDs over one period of voltage?

Something terrible is stalking between the high voltage potentials

 

[DocShock]

The PD activity slowly decreased, then without changing anything, increasing the applied voltage above the inception point and decreasing to the test voltage, the activity increased and then showed the same slow decline - so the whole thing is repeatable. I think I may have to do more investigation into this!