Negative partial discharge (PD) pulses clumped near 45° during the positive portion of the AC cycle — indicated as a polarity less than 0.67 — suggest voids between the winding’s copper conductors and insulation. A machine with a frequently cycled load, or severe overheating, develops voids near the copper conductors.
PD near the turn insulation in multi-turn coils may lead to winding failure quicker than similar PD in Roebel bars. As with any internal delamination, there tends to be an inverse temperature effect. If the voids are large, it is possible for the negative PD to increase with load due to the higher conductor vibration associated with the higher mechanical forces.
Suggested actions
- Review PD trends to determine the extent to of the internal voids and whether PD activity is increasing or decreasing.
- Because of the location of the voids, that is, near the copper conductors, there are no reliable repair mechanisms for this problem; at some point, the machine will require rewinding. However, it may be possible to slow deterioration by altering operating procedures by reducing load cycling or by adding more sophisticated cooling techniques that restrict the range of the machine operating temperatures.