High volts pressure test failure

[Gyo]

I have tested two identical 6.6kV motor, water-filled and windings are XLPE-insulated. One passed, and one failed the test. This is a rare occurence. The failed test has a flashover at 5kV very close to the star point.

Historically, this motor had failed the test once before at the same location, and water may had entered through the damaged location as the copper conductor was exposed. After leaving the windings to dry overnight, the damaged section was cut away, insulation resistance displayed high value and new star joint was moulded. Second test was then performed, and breakdown happened again close to the star point.

Is water inside the wire to be blamed for the failed test? Why breakdown happen close to the star location, is it because it is being placed close to or touching the stator case? Any thoughts are appreciated.

 

[Gyo]

All test were done in ac.

 

[KllrWolf]

How exactly are you insulating the joint? Is it an epoxy covered splice, taped, or molded rubber Are you doing all tests with the joint submerged? Was the original the factory splice and you are trying to copy it? Did you just repair the section of the insulation, or replace it all? Did you make sure you did not mess up the wire insulation when removing the old insulation? Was everything perfectly clean, including the workers hands? Even the oils from the hands can mess up a splice if the proper precautions are not taken.

Dealing with wet wound motors can be difficult to impossible if not experienced working with them.

 

[RRaghunath]

The 6.6kV motor is expected to have same level of insulation at the star end as at the phase end (uniform insulation). Hence, the failure during HV test suggests that this was not followed in this case.

 

[Gyo]

The motors are located in a rewind shop in China, and I am not sure of the exact condition. From what was being described, the damaged section of the star joint was cut off, then reconnected with wraps of PE tape, heated together to form a joint. The person who did them was highly experienced, so I can only assumed that the conditions were clean.

Test was done with the stator in a tank filled with water, windings including joints were submerged.

 

[KllrWolf]

While I can not tell without more information, it sounds as if they are not using the correct materials or thickness for the repairs. They could also be trapping air in the repaired section.

We use special tapes for underwater splices that do not require heating, so I do not know much about your method. You could also look and see if the heating is causing the insulation to flow in a way that causes a thin point in it.