testing distribution cables

[fredrick2121]

Does anyone have an experience like this. You are in the field and you are dc hi potential testing some new ( or existing ) distribution shielded high voltage cables. You crank up the voltage on your test set, to whatever, say, 25,000 volts dc, and take your readings of the leakage current as instructed. Then, at the end of the test, you back off the control, remove your test set, and short the center conductor and the shield together to remove any residual charge on the wires ( since they act as capacitors and store the charge in this case ). Thinking the cables are fully discharged, you remove your shorting wire, and go about your business doing other things. Then, when it comes time to form the cables into place, you get a nasty shock, and realize that the cables have mysteriously built back up some charge in them somehow. This leads to formulating some policy on leaving your shorting wire in place, etc. My question is:
Does anyone have experiences like this, and if so, can you post it here ?
Thank you,

Frederick

 

[dpc]

I don't have any direct experience, but Megger instruction books recommended discharging for at least four times as long as the voltage was applied in the previous test.

They also recommended keeping equipment with a lot of capacitance (cap banks, transformer windings) short-circuited until they are re-energized.

 

[DanDel]

Yes. The jumpers applied from ground to the conductor must be left in place for a significant amount of time(most written testing procedures will state that it must be for at least the same amount of time as the duration of the hipot test, 5, 10, or 15 minutes, depending on the test). This is especially true when the cable is very long. I would guess that this was your situation; a long cable can store a significant amount of energy(joules) which your body will absorb when it became the ground connection.
I always leave the ground jumpers on until I have to move them for re-connection; then I place the jumpers on the lug to which I am connecting the cables. Only after everything is connected do I remove the jumpers.
The cables can also pick up a charge from being run near other cables carrying current.

 

[scottf]

As is said above, capacitor/capacitances can take a long time to fully dis-charge. This is based on the overall capacitance, which, with the cables resistance, controls the "time-constant" of the circuit.

 

[kraigb]

All of the above posts are entirely correct that sufficient grounding practices are the best method to handle this behavior in the insulation.

There is actually a diagnostic technique that depends on the return voltage that you have described. The return voltage technique is based on polarization (while you are DC testing) and depolarization (when you remove the DC test voltage) of dipoles within the insulating material.
After the dipoles in the insulation have been forced to orient in the direction of the electric field fo rthe duration of the test, they have some tendency to try to re-orient themselves in that direction to an extent. The degree to which they repolarize is a measure of the retained dielectric strength.

This characteristic has also accounted for a few "space charge" failures that occur when the cable is re-energized following a DC test where the cable was not effectively grounded long enough. The difference in potential between polarized dipoles and the re-applied AC voltage has been known to exceed the dielectric strength of the insulation in that location, causing a cable failure.

 

[fredrick2121]

Thank you all for your most excellent posts, I am aware of the phenomenon and the hazards and risks that it entails. I do not do this testing anymore myself, but am now gathering information for a research project, which information will help support/not_support a hypothesis involving energy storage in concentric capacitors, per se. I am asking if you have experience with this, and if so, to post the experience, describing what was done, how it was done, and what was seen/felt, etc.
The energy storage capabilities of high voltage cables are fairly easily calcuable, as are the discharge times of the cables. My own experience has mainly been with XLP and EPR dielectric cables, in the 5KV/15KV range.
If you wish, I can keep you appraised of the progress of this research as well.
Thank you for your contribution.

Frederick

 

[busbar]

If you're looking for procedural boilerplate, IEEE Std 400-1991 may be reviewed for applicability.

NETA testing standards seem to have minimal cautionary advice.

 

[jbartos]

Suggestion: Visit

http://www.okonite.com/engineering/high-voltage-testing.html

etc. for more info

 

[najib77]

The above testing of the cable is called the Hipot of the cables to check the insulation level of the cable and its bearing voltage, the procdure for it is that

The dc volatge injected or applied is calculated as

VOLTAGE TO BE INDUCED = System Volatage x 1.73
For example the 11 kv cable is to be tested at 20 kv voltage.
The duaration for the applied volatge is 3 to 5 minutes, if the cable is new and not yet used in the network, then 5 minutes is recommended if old and already used the 3 minutes time is required as the insualation level of the strength of insulation is reduced due to passage of time.
After application of the the lekage current is to be noted the reference value is 2mA.
That the lekage current must be less then this value.if the lekage current is above this, the cable under test cannot be used in the network, the breakdown may be happened anytime. Sardar.najib

 

[fredrick2121]

I am keenly interested in any past and ongoing research in that topic, the one mentioned of the polarization of the dielectric dipoles. Kraig, do you recall exactly where you read about this ?
If anyone else has any knowledge or experience of this phenomenon of the recharging of the cable after bleeding off the charge from the hipot test, I would like to hear about it.
Again, thanks for any information you may have.
Fredrick