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#166101 07/12/07 08:13 PM
Joined: Dec 2006
Posts: 43
W
Junior Member
Hi,
I am having a ground fault on a 15 kV cable. I was wondering if anyone has ever heard of hi-potting a cable that is still terminated to the bus?

The ground runs through the CT and the breaker is tripping on reset.

Thanks for any information or tips on hi-potting.

great website and members.

regards,
windmiller


We all live under one King
Joined: Apr 2005
Posts: 116
X
Member
Never have. The cable needs to be free and clear on each end to hi-pot it correctly.

Joined: May 2001
Posts: 717
G
Member
If you don't remove it, you will be hy-potting buss and all simultaneously. Not a big problem, but you won't know if the problem is cable or buss.

Try an ohm-meter first, it will tell you more about how far away your problem is, but if the ohms are high phase to ground, your still stuck trying to locate. You may need to find a "thumper" if it is underground.

The hy-pot will tell you if you have a bad termination, if the cable has been in service awhile and just started tripping out you will likely see track marks near the term if that is the problem.

Joined: May 2005
Posts: 984
G
Member
If you try to hi-pot the cable while it's still connected and end up hi-potting the equipment, how with the buss,relays, etc. react to that test voltage?


Ghost307
Joined: Sep 2005
Posts: 120
Z
Zog Offline
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Every cable specification out there does not recommend DC hipotting service aged (>5 years) cables. DC hipot is considered a destructive test and will most likely damage the cable. Ghost307 is right about disconnecting the cable from the bus, the cable should be disconnected with corona supression before any overpotential test.

You can locate your problem without a shutdown with an online Partial Discharge test, or if shutting down is not a major issue a VLF or Tan Delta test would be more appropriate.

When is the last time you tested your relays? What type of relays are they? What indications are you getting? What type of cable?


MV/HV Testing Specialist, "BKRMAN"
Joined: May 2001
Posts: 717
G
Member
He did not state WHAT level he was high potting to, a fairly low hy-pot will not harm an older cable.

He will want to remove any PT fuses prior to high pot. Any equipment connected to that buss is going to be rated for system voltage or is already self destructed. I would suggest hy-pot to system voltage only.

I'd still do a thorough visual of the buss and terms to check for tracking. The problem may be painfully obvious.

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It dosent matter WHAT level he is hipotting to. The ICEA, and both IEEE 400 and IEEE 576 do not recommend DC hipotting service aged cables.

In addition the IEEE 400 states “even massive insulation defects cannot be detected with DC at the recommended voltage levels”. If massive insulation defects cannot be found at recommended levels what do you expect to find at a "Fairly low" level?

Also the ICEA allows for reduced voltage DC testing for the first 5 years and no DC testing after 5 years and states that DC testing of service aged cables can lead to early failures.



MV/HV Testing Specialist, "BKRMAN"
Joined: May 2001
Posts: 717
G
Member
He is attempting to identify a problem, not commission a cable. The problem will show with a low level test, a megger, or likely, as I said before, an ohm meter.

He may want to consider an AC test as opposed to your DC.

You are of course right, better to do nothing, or hire a third party to do a tan delta test as opposed to rectify the problem.


Last edited by George Corron; 07/18/07 08:14 PM.
Joined: Sep 2005
Posts: 120
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"He is attempting to identify a problem, not commission a cable"

A DC hipot would be fine if he was commisioning a cable, A PD or Tan delta test is a condition assesment test.

"as I said before, an ohm meter"

You really think putting 5-7V across 15kV insulation will tell you anything but "O.L."???? That wont even charge the insulation. A DAR test with a 5000V megger may provide some useful info if you want a quick easy test.

"He may want to consider an AC test as opposed to your DC"

Thats what I have saying all along.


MV/HV Testing Specialist, "BKRMAN"
Joined: Oct 2005
Posts: 40
G
Member
Im like the eighth response lol. I have never heard of that method. We hi-potted a set of cables about six months ago. We pulled the load break elbows at the other end (all free from equipment and sensors) and taped Jelly jars on the end of the load breaks..to ensure they were completely isolated from any ground.


Brian Gibbons
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