DC Thumper Application Question
DC Thumper Application Question
(OP)
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.






RE: DC Thumper Application Question
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.
RE: DC Thumper Application Question
RE: DC Thumper Application Question
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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.
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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.
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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)
RE: DC Thumper Application Question
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.
RE: DC Thumper Application Question
RE: DC Thumper Application Question
RE: DC Thumper Application Question
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!!!!!
RE: DC Thumper Application Question
David Castor
www.cvoes.com
RE: DC Thumper Application Question
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.
RE: DC Thumper Application Question