Page 56 - Basic PD Theory

PD Investigations

7.3.3.1 Excitation pulses
PD couplers sometimes detect the high frequency thyristor-firings of static excitation systems. These pulses are usually of equal
pulse repetitions, low magnitudes and occur repetitively across the AC cycle (Figure 51). Usually, they are only discernible when
there is low PD activity on the unit and the test range of the PD Bipolar Machine PD
test instrument is set to its most sensitive (lowest range) setting. 0 to 3.16 pps 3.16 to 10 pps 10 to 31.6 pps 31.6 to 100 pps
This is not usually indicative of a problem with a stator winding, 100 to 316 pps 316 to 1000 pps > 1000 pps Subset 8
but if pulse levels are high this may indicate that there is a 75 75
problem in the exciter. Although this type of pattern is called 50 V] 50
“excitation pulses”, similar signals may also be detected from 25 25
machines close to variable frequency drives in the electrical
system. Pulse M agnitude [m 0 0
-25 -25
Disturbance pulses are not indicative of PD -50 -50
sources within the winding insulation. -75 -75
0 45 90 135 180 225 270 315 360
Phase Angle [deg]
7.3.3.2 Cross-coupling Figure 51: Excitation Pulses
PD pulses are high frequency pulses that may capacitively couple
from one phase to another across the endwinding or circuit rings. In addition, a coupler connected to one phase may detect
pulses radiated from a source on another phase. This behaviour is called cross-coupling and can be detected by observing PD
pulses shifted ± 120° from the positions of the source PD. Phase-to-ground PD cross-coupled from another phase would
cluster at 345°, 105°, 165° or 285°. Cross-coupled inter-phase activity would be ± 30° phase-shifted from these positions as
shown for C-phase (middle plot at 315°) in Figure 36.
Cross-coupled PD is dependent on the geometry of the stator winding and does not necessarily indicate a problem with the
insulation. Additionally, based on the mode of pulse travel, the pulses may invert in polarity.

Cross-coupled pulses are not indicative of PD sources,
and therefore, are not usually associated with ageing mechanisms.

7.3.3.3 Broadband Clusters
Due to the physical attributes of some installations, there may
be sources of low-magnitude broadband clusters detected by
the couplers. When this situation exists, the pulses are low in
magnitude and normally manifest themselves across the AC
cycle (Figure 52). In some instances, the magnitude of the
detected pattern could be reduced when a more reliable (or
shorter) ground path for the PD couplers was chosen.
Typically the magnitude of this activity is quite low and as the
winding ages, PD originating within the winding will reach
higher magnitudes and be visible beyond this noise-like
disturbance. Figure 52: Broadband Clusters

Broadband clusters are not indicative of PD sources
within the winding or problems with the PD detection system.

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