Page 44 - Basic PD Theory

PD Investigations

Increasing PD
A A Increasing PD B B High PDigh PD
H

Higher than PD increase >
typical? 150%?
Monitor
No No every 3
Yes Yes months

Further
investigation
required

6.3 Phase Comparison
Purpose: To determine if the PD levels are comparable among the phases
Requirements: Evaluate the Q m values from each phase for relative magnitude
When the PD levels and patterns are similar among the phases, then uniform deterioration is suspected; otherwise, an isolated
problem is more likely. Typically, coil looseness, internal delamination, contamination, and problems with the voltage stress
coatings are widespread, or uniform, whereas, improper manufacturing and phase to phase discharges are often isolated problems.

6.4 Pulse Distribution
Purpose: To determine if the PD sources are likely within the insulation system
Requirements: Evaluate the shape of the PD clusters
The pulse distribution with respect to the AC phase position in the 3D plots can assist
in determining the source of any problems in the stator winding. Normal pulse
distributions are Gaussian, with a standard deviation, (σ), of 15°, and are indicative of
spherical shaped voids within the slot section of the core. Narrow band pulses usually σ = 15
originate from PD occurring at sharp metallic corners, Gaussian curve
such as connections. Wide band pulses may indicate
multiple failure mechanisms or perhaps elongated voids (see Section 7 on PD Patterns). PD
patterns scattered across the ac cycle suggests either Contamination, noise, or perhaps a
combination of several failure mechanisms occurring simultaneously. Void shapes are rarely
standard and often result in unusual patterns that cannot be easily described. One example of
Non-Gaussian curve unusual patterns is when the PD is skewed, non-Gaussian, because of space charge effects
within the voids.

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