Page 53 - Basic PD Theory

PD Investigations

* Phase to phase discharge
Failure mechanism Polarity Load Effect Temperature Effect Phase Location Pulse Polarity
Phase to Phase No predominance None Unpredictable 15°, 75°, 195°, Opposite in the
Discharges 255° two phases

Symptoms Detection Tests Machine types
Partial discharge, white
powder, ozone, insulation PD, visual inspection, ozone monitoring Air-cooled
discoloration
Bipolar Machine PD
1 to 3.16 pps 3.16 to 10 pps 10 to 31.6 pps 31.6 to 100 pps
7.2.2 Gap-type Discharges 100 to 316 pps 316 to 1000 pps > 1000 pps Subset 8
30000 30000
A gap-type discharge can occur when there is a constant space 20000 20000
between two components at different potential. This type of activity 10000 10000
will occur between two bars in the winding overhang, or between a
bar and the pressure finger of the stator core. The resultant PD Pulse M agnitude [m V] (k = 10.000) 0 0
pattern is usually a “cloud”, that is, a cluster that occurs at constant -10000 -10000
magnitudes without any lower magnitude pulses as shown in Figure -20000 -20000
48. The “cloud” may be clustered around the positions associated -30000 -30000
with phase-to-phase or the phase-to-ground sources according to -225 -180 -135 -90 -45 0 45 90
the location of the gap-type discharge location. Phase Angle [deg]
Figure 48: Gap-type Phase-to-phase
* Gap-type discharges
Failure mechanism Polarity Load Effect Temperature Effect Phase Location
Gap-type Discharges No predominance None Unpredictable Variable (“Cloud”)

Failure mechanism Symptoms Detection Tests Machine types
Partial discharge, white
Inadequate cable powder, insulation PD, visual inspection Switchgear, motor
spacing leads
discoloration

7.2.3 Contamination
Contamination is described in Section 1.5. Since the endwinding area
of machines is specific ally designed to be away from ground planes, PD Bipolar Machine PD
that occurs due to contamination in the endwinding may be phase-to- 100 to 316 pps 316 to 1000 pps 10 to 31.6 pps 31.6 to 100 pps
0 to 3.16 pps
3.16 to 10 pps
Subset 8
> 1000 pps
phase voltage dependent and will be 30° phase shifted from the 75 75
positions of phase-to-ground PD of 45° and 225°. The direction of the 50 50
shift is dependent on the location of the contamination and the voltages 25 25
of the affected coils and is therefore unpredictable. 0 0
Pulse Magnitude [mV]
Identification or inspection of the site of arcing may be difficult due to -25 -25
limited visibility in the area. In situations of widespread contamination, -50 -50
the PD patterns may be more complex including patterns of phase-to- -75 -75
225
180
135
ground, phase-to-phase and open-air arcing as shown in Figure 49. 0 45 90 Phase Angle [deg] 270 315 360
Though there may be some effect of temperature or load changes on Figure 49: Mixed phase-to-ground
PD activity due to contamination, it is difficult to predict the expected and phase-to-phase discharges
results. However, environmental conditions such as humidity may have

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