Page 45 - Basic PD Theory

PD Investigations

7
Chapter

7 PD Patterns

Basic tips on how to evaluate PD Patterns

Many failure mechanisms have their own PD pattern; therefore, detection of this pattern can help in determining which failure
mechanism may be predominant.
PD plots may be complicated if two or more failure mechanisms or processes are occurring in the winding at the same time. For
example, contamination and a loose stator winding could be
occurring simultaneously. In these situations and in the Bipolar Slot Total
presence of high PD magnitudes, subsequent PD testing, 0 to 3.16 pps 3.16 to 10 pps 10 to 31.6 pps 31.6 to 100 pps
off-line PD tests, other electrical tests and visual inspection 100 to 316 pps 316 to 1000 pps > 1000 pps Subset 8
may be needed to conclusively separate the different failure 30 30
processes. 20 20
Below are descriptions of the most common stator winding 10 10
insulation problems viewed in isolation. Two or more such 0 0
deterioration processes may superimpose PD pulses in a -10 -10
complex pattern. Pulse Magnitude [mV] -20 -20

7.1 Discharges in the Slot -30 -30
180
135
225
Inspection of the PD data reveals the phase angle 0 45 90 Phase Angle [deg] 270 315 360
relationship between the PD pulse and the phase-to-ground
reference voltage. PD that originates within the slot portion Figure 39: PD in Slots
of the coil or bar is centred at 45° for the negative pulses and 225° for the positive pulses (Figure 39). These are the phase-to-
ground PD positions for pulses dependent upon the phase-to-ground voltage of the specific phase being measured, and are referred
to as “Classic PD”. Phase-to-ground voltage stresses occur all along
the coils as they pass along the length of the slots in the stator core.
Hence, this type of pattern is indicative of PD occurring in the slots
and is referred to as “PD in slots” or phase-to-ground PD.
Modelling the actual characteristics of a pulse is difficult since the
void dimensions, void gas and pressure, inductance, capacitance,
geometry and many other issues can affect the magnitude and
frequency of a pulse [1]. However, some basic pulse characteristics
can be predicted based on the void location as described below. (See
Section 2.4 for more information on Models)
The polarity of the classic pulses and their predominance tells a lot
Figure 40. PD Polarity based on Void Position about the relevant failure mechanism as shown in Figure 40. Due to

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