Page 22 - Basic PD Theory

Basic PD Theory

2.4.7.1 Voids within the Insulation 50/60 Hz phase-to-
Machines that have not been improperly impregnated ground voltage
* Voids within the or that have been operated for several years at high
insulation – no polarity temperatures tend to develop voids within the
predominance (45°/225°)
groundwall insulation. As described before, a partial
* Internal delamination discharge will occur across such a void when both an 0 o 45 o 180 o 225 o 360 o
* Manufacturing defect over voltage condition exists and a free electron is
present. Once the partial discharge occurs, the voltage
across the gap will stabilize at the level required to sustain the discharge activity. During
this process, the charges within the void will redistribute according to the applied charge. Partial
As the AC cycle reverses in polarity, these charges will cause another over voltage Discharges
condition in opposite polarity to be established and another partial discharge to occur. If Figure 13: Voids in the Bulk of the
both sides of the void have similar insulation materials then the charge distribution will Insulation
be equal during the positive and negative cycles [12]. In theory, as shown in Figure 13,
there will be two observable PD pulses in each AC cycle of equal magnitude and opposite polarity per void within the bulk of
the insulation. These pulses clump at the classic positions for phase-to-ground dependent pulses, that is, negative pulses near 45°
and the positive pulses near 225° with reference to the 50/60Hz phase-to-ground voltage. In some cases, the polarity of the
pulses are reversed from these “classic” positions, see Appendix F. Prime Numbers.

2.4.7.2 Voids Near the Conductors
A machine that is frequently load cycled or severely
50/60 Hz phase-to- * Voids near the copper
ground voltage overheated develops voids near the copper conductors – negative
conductors. A void bounded by the copper polarity predominance (45°)
conductor and insulation, exhibits a different
phenomenon than those within the bulk of the * Thermal cycling
0 o 45 o 180 o 360 o insulation. Though the basic breakdown * Internal delamination
225 o mechanisms are the same, because the electrodes
are of dissimilar materials polarity predominance will occur. The mobility of the
positive ions on the insulation surface is much lower than the negative ions on the
Partial conductor surface [12]. The result is a predominance of negative ions migrating
Discharges through the gap to the positive insulation surface. In this case, there will usually be
an observable predominance of negative PD pulses clumped near 45° during the
Figure 14: Voids Near the positive AC cycle, as shown in Figure 14.
Conductors
50/60 Hz phase-to-
2.4.7.3 Voids Near the Core Iron ground voltage
Loose coils, poor semi-conductive coatings, and
* Voids near the core iron
– positive polarity problems with the grading/semicon interface can all
predominance (225°) lead to surface discharge between the stator bar and the 0 o 45 o o 360 o
grounded core iron, called slot discharges. As with 180 225 o
* Semicon arcing those near the copper conductors, these discharges also
* Looseness occur between electrodes of different materials. Here,
* Interface problems the immobile positive charges on the insulation and
mobile negative charges on the grounded metallic Partial
electrode lead to pulses occurring on the negative AC cycle. Because the metallic Discharges
electrode is grounded, the observable PD pulses will be predominantly positive Figure 15: Voids Near the Core Iron
clumped near 225°, as shown in Figure 15. [12]

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