Unknown partial discharges

[Foudrette]

Dear All,
Since one week, we try to solve a problem of partial discharges measurement on extruded cable drums.
We have observed partial discharges we can not interpretate and which do not seem described in Electra.
These partial discharges look like "candles". I mean that they have a basis larger than the top. The amplitude of the partial discharges does not increase when increasing the voltage.

I suppose it can be a problem of bad contact but in fact it is the first time we observe this phenomenom. Could it be a problem in the transformer, the inductance, the capacity ???
These discharges do not always appear even if we use exactly the same test set-up.

I can send a picture of these partial discharges if someone can give me its email address.

Thanks for all of your help,
Foudrette

 

[CarlPugh]

Have no idea what a cable drum is. However from testing transformers, the most likely cause is a sharp point.
Has anyone cut any small diameter wires in this area?
Are there sharp points on the cable drum?

Could the problem have always been there? (Is this a visual problem?) (Has you test setup been changed?)

Does this cable drum have different paint or coating,is it made from a different formula material, has the equipment used to manufacture the cable drum changed, are different people building or testing the cable drum....?

Is some object closer to the cable drum than it used to be?
This object has capacitance to ground and does not have to be connected to ground.

Are all your tests at about the same atmospheric pressure and temperature?
Has the lighting changed? (Are some of the tests made during the day and some at night?) (This would affect your ability to see the corona)
Don't know if this would have any effect, but is there a breeze (air flow) during some of the tests?

Corona detectors will sometimes detect welders, florscent lights, arcing relay contacts, brush type motors, power line disturbances...? If possible operate your corona detector for some time, say 1/2 hour and see if corona goes away or appears on a regular or irregular basis. Try turning other equipment on/off and see what effect it has.

Sounds like a fun problem. Let us know what you find.

Good Luck
Carl Pugh