DC Hi-pot voltage buildup

[alwaink]

Hello,

I encountered a voltage increase during the DC hi-pot test for our ESP (electrical submersible pump) power cable. Normally when doing a DC test at 10kV for 1 min, it'd stay there and pass the test...or drop back down to fail. However, in this instance, when testing this 4kV rated cable, after setting the test at 10kV, it'd keep gradually increasing to more than 12.5kV by itself. What could be the reason for this voltage buildup?

Help appreciated!

 

[alwaink]

Correction: it was an AC hi-pot test.

Thank you

 

[benlanz]

Is the cable in question a shielded power cable? Is the cable disconnected during the AC HIPOT test?

 

[alwaink]

Yes, it's a rubber&plastic insulated, galvanized steel shielded cable. I'm sure the cable wasn't disconnected during the AC hipot test.

 

[electricpete]

I have seen a funky ac hi-pot set that was not very stable. The one I saw used some kind of resonant circuit to generate the high voltage. I think perhaps it was very sensitive to small variation in circuit parameters, possibly even changing temperature. The contractors who brought the test set were constantly fiddling with it during our 1-minute constant voltage test. Kind of scary. Can you imagine the whoops factor if they fried our equipment because their test equipment got out of control.

That particular hi-pot set I think was home-made. It had several different pieces.

I have also seen factory made ac hi-pot sets nicely packaged in a tall cabinet - worked like a charm and voltage was perfectly stable.

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[benlanz]

I know I am not answering your question but, your applicatio raise more questions. Can your pump handle an over voltage of . A 4kV system (5kV class)is approximately 2.4kV to line to ground. You are applying > 4 times the operating voltage during your test! Perhaps your pump rated for a DC HIPOT but I would be surprised if it had a AC HIPOT rating of more than 1.5Uo.

The voltage build up could be a number things. I think eletricpete's idea is a valid one. Is the AC voltage source a resonant transformer? It is possible that at higher voltages the charging current can effect the gap in the magnetic circuit of the transformer. This then changes the resonance circuit. It is plausible that the circuit actually became more in tune during the test and therefore caused an increase in voltage.

 

[rbulsara]

Could it be charging of the cable capacitance to peak value of 10kV rms? (nearly peak..)

I am not an expert in AC hi-pot test equipment.

 

[alwaink]

Thanks, everyone. I guess I'll have to find out what our AC source is. This situation only happens to this particular cable. When we test the others, everything's fine. Weird.