Page 50 - Basic PD Theory

PD Investigations

Be aware, however, that the increase in load current will also increase the winding temperature, which may lead to a reduction in
PD due to inverse temperature effects. Therefore, when checking for loose windings, run the PD test with the machine at different
loads but at the same voltage, winding temperature, and hydrogen pressure. Note, for a winding with only a “looseness”
problem, mainly the positive PD will change with load; the negative component will only change slightly, if at all. Changes in the
negative PD with changes in load may be an indication of conductor vibration due to significant voids near the conductors.

* Bar/Coil movement
Failure mechanism Polarity Load Effect Temperature Effect Phase Location

Bar/Coil Movement Positive Direct Inverse 225°

Symptoms Detection Tests Machine types
Partial discharge, ozone, PD, visual inspection, wedge tap, ozone Hard groundwall systems – epoxy and
loose wedges monitoring polyester

7.1.2.2 Voltage Stress Coatings
If you have a positive predominance and no load dependence, you might want to test at two temperatures of at least
20°C difference to see the effect on PD. With stress control coating problems it is possible PD will go up with
increasing temperature, a direct effect. As the temperature of the stator winding increases, the resistance of the carbon-based
coatings decreases, the result is an increase in surface (positive) PD activity. This problem is generally a very slow failure
mechanism, but often leads to high ozone production in air-cooled machines.
Interface Deterioration is described in Section 1.3.2.2. PD that occurs at the semi-conductive and grading coatings is a surface-
type activity that is phase-to-ground voltage dependent. The activity is predominantly positive and centered near 225°. Unlike
loose windings, this activity is not usually affected by load changes. An increase in temperature is usually found to lead to an
increase in the PD activity resulting from this area, or a direct temperature effect. If a PD pattern is predominantly positive, centred
near 225°, and increases with temperature, it is most likely from the semi-conductive and grading coating interface deterioration.
Deterioration from the voltage stress coatings is usually a slowly developing ageing mechanism, but can
produce high volumes of ozone in air-cooled machines.
A severely deteriorated interface or arcing directly at the slot exit may lead to pulses clumped at 0° and 180° sometimes
exhibiting a hook pattern (sometimes referred to as “rabbit ears” pattern) as it tracks the AC cycle (Figure 45) and other times
simply vertical spikes (Figure 46).
Bipolar Machine PD Pulse Density Linear Plot
Bipolar Machine PD
1 to 3.16 pps 3.16 to 10 pps 10 to 31.6 pps 31.6 to 100 pps
1 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
750 750 100 to 316 pps 316 to 1000 pps > 1000 pps Subset 8
150 150
-500 lse M agnitude [m V] -250 0 250 -100 Pulse M agnitude [m V] -50 0 50
500
500
100
100
250
50
0
0
-250
-50
Pu
-750
-750 -500 -150 -100
-150
0 45 90 135 180 225 270 315 360 0 45 90 135 180 225 270 315 360
Phase Angle [deg] Phase Angle [deg]

Figure 45: PD at Voltage Stress Coatings Figure 46: PD at Voltage Stress Coatings
( “Rabbit ears”) (Spikes)

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