Page 49 - Basic PD Theory

PD Investigations

Voids near oids near oids near Polarity Internal nternal nternal
I I
V V
copper pper pper
voidsdsds
co co predominance of voi
voi
Classic PD? None
None
None
Ne
Ne
Negativegativegative

D Positive
Discussed earlieriscussed earlier

Due to pulse behavior, positive predominance normally indicates PD originating on the surface of the insulation system. Often
the problems associated with surface PD are repairable by common maintenance practices such as rewedging, cleaning, dips and
bakes, and repairs of the voltage stress coatings.
7.1.2.1 Bar and Coil Movement
Bar and Coil Movement is described in Section 1.4. The most obvious sign of bar or coil movement is a noticeable increase in
PD activity, especially positive activity, with an increase in load from a no load to full load, or a direct load effect (Figure 44).
If you have a strong positive predominance, you should run the test at different loads to ascertain the effects of changes in the
mechanical forces on PD. If tests are taken at loads that differ by at least 40%, it may be possible to observe the effects of
magnetic forces on coil vibration. These tests are often referred to as full load and reduced load tests.

Load dependent PD may be an indication of winding looseness, which can damage a winding fairly quickly

This may be due to the increase in magnetic forces (Fm ∝ I ) causing an increase in coil movement and thus surface (positive)
2
PD activity. The effect is most significant with loose thermoset (hard) windings such as epoxy-mica insulated windings.
Thermoplastic (soft) windings expand when heated and tend to secure themselves within the core. If PD load dependency is
present, it is always a concern that loose coils can lead to premature winding failure. These coil or bar movements, if present, can
rapidly damage the resistive surface coating on the coil sides, and if left unchecked, they can wear away the insulation itself. In
extreme cases, this failure mechanism can destroy an otherwise good winding in only a few years. Be aware that the initial stage
of looseness is called “contact sparking,” and can often happen on coils throughout the winding, not just the high voltage ones.
The load effect may also occur with the vibration sparking (or spark erosion) failure mechanism [2]. See Section 7.3.2 for
information about Vibration Speaking.
Bipolar C2 Bipolar C2
0 to 3.16 pps 3.16 to 10 pps 10 to 31.6 pps 31.6 to 100 pps 0 to 3.16 pps 3.16 to 10 pps 10 to 31.6 pps 31.6 to 100 pps
100 to 316 pps 316 to 1000 pps > 1000 pps Subset 8 100 to 316 pps 316 to 1000 pps > 1000 pps Subset 8
750 750 750 750
500 500 500 500
250 0 250 250 0 250
0
0
Pulse Magnitude [mV]
Pulse Magnitude [mV]
-250 -250 -250 -250
-500 -500 -500 -500
-750 -750 -750 -750
0 45 90 135 180 225 270 315 360 0 45 90 135 180 225 270 315 360
Phase Angle [deg] Phase Angle [deg]
Hydro generator: 55 MW, 112 RPM, 75 °C Hydro generator: 5 MW, 112 RPM, 75 °C

Figure 44: Direct Load Effect

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