DC Thumper Application Question

[farmape]

Consider a 15kV aged underground (radial) shielded cable system. 13.2kV AC service. Many splices, taps, and terminations. Ground fault exists, location and phase unknown. Insulation resistance tests confirm this. What is the opinion on thumping this circuit WITHOUT clearing the cables (i.e. included in the test would be switchgear terminations and line-line PTs). DC thump voltage would be 7.5 or 15kV, both less than service voltage. The concern is the PTs, possible thump damage. Those with experience thumping, do you require the cables cleared? I have thumped many times with cables terminated, but never with PTs connected, and it concerns me. Thank you.

 

[steelerfan28655]

Why wouldn't you sectionalize and thump each section without anything connected?
By doing this you will also do a high-pot test on each section of line.
I'm assuming that all three phases are currently de-energized.

Happiness is a way of travel, not a destination.

 

[davidbeach]

How would you not know the phase with the ground fault?

 

[farmape]

Thank you for the replies.
.
Why wouldn't you sectionalize and thump each section without anything connected? ... I will answer this question with a question ... Why should I take the effort? This is the question.
.
How would you not know the phase with the ground fault? ... 3 phase CTs residual connected to a GF protection relay initiates trip. DC IR test ("megger test")(phase to ground) indicates GF. The three phases are tied together via L-L PTs.
.
Yes if the cables were disconected and sectionalized the fault could be found. I am trying to speed things up. Why not thump and hope the fault shows itself. If the insulation system is sound, would thumping a 15kV (AC) system at 15kV (DC) (or 7.5kV DC) damage the integrity of the good insulation (to include switchgear, PTs, cables)

 

[PHovnanian]

I'm thinking that leaving a bunch of distribution transformers connected could attenuate the DC so that a fault at some distance would not break down.

Even the cable capacitance of a large underground system could cause problems. Crank up the sending end voltage enough to overcome this and equipment close in could be damaged.

 

[gattie]

Whats your objection to racking out (disconnecting) the PTs?

 

[Zogzog]

You should always disconnect the cable and isolate any PT's before an insulation test, let alone thumping.

 

[Jebb]

I've done this a few times, (by accident) leaving things connected. You usually end up with weird results, depending on what was left connected and then spending more time trying to figure out what’s wrong then actually finding the fault. Fault finding is much easier when you work methodically rather than randomly. Also the customers complain when their lights blink for hours every time you “thump” if you don’t disconnect them first.

Typically on a distribution ckt we’ll try to disconnect the equipment using switches, removable links, elbows, etc. and not go through the trouble of unbolting terminations unless absolutely necessary.

Happy fault finding!!!!!

 

[dpc]

I've had clients run into problems when they've accidentally thumped PTs and small control power transformers. The results are generally not useful, and it doesn't help the transformers much either.



David Castor

http://www.cvoes.com

 

[dpmac]

Maybe its a bad PT and you will be thumping the cable for no reason.

We have always done our best to perform cable fault locating on the cable itself (whenever possible). It always seems to take more time when we try to save time when fault locating.

That being said I have thumped the crap out PT's, CT's transformers, vacuum breakers, air magnetic breakers, lightning arrestors, etc for many years. To the best of my knowledge no equipment failed while I was on site.

 

[farmape]

Thanks again for the input. My thoughts were inline with most, it probably wouldn't damage the components, but probably wouldn't provide conclusive results, expecially with limited thumping power. Circuits may have upwards of 40+PTs (non-drawout). Initially I was concerned with damage to a PT, but the circuit resistance may be too large, thus diminishing the thump current ... and the whole point of the thump.