Page 47 - Basic PD Theory

PD Investigations

* Internal delamination
Failure mechanism Polarity Load Effect Temperature Effect Phase Location
No or Negative
Internal Delamination None Inverse 45° and 225°
predominance

Symptoms Detection Tests Insulation types
Partial discharge, insulation PD, tan δ, power factor, capacitance, visual All stator winding types
discoloration inspection (Asphaltic mica)

7.1.1.2 Manufacturing Defect
Manufacturing defects are described in Section 1.3.1. Like internal delamination, these voids may be near the conductors or
are often scattered throughout the insulation without dominance of voids either close to the copper or at the surface. There may
be a slight negative or perhaps no noticeable polarity predominance of the PD with activity centred near 45° and 225°. As stated
before when describing internal delamination, internal voids usually lead to a decrease in PD with an increase in temperature, that
is, an inverse temperature effect, but no changes with load (Figure 41). See Section 2.4.7.1 and polarity predominance.

* Manufacturing defect
Failure mechanism Polarity Load Effect Temperature Effect Phase Location
No or Negative
Manufacturing Defect None Inverse 45° and 225°
predominance

Symptoms Detection Tests Machine types
Partial discharge PD, power factor, tip-up, tan δ, capacitance Global VPI, resin rich coils

Temperature monitoring is usually possible as most modern units 3.3 kV and above have temperature sensors (thermocouples or
RTDs) on bearings and embedded in stator slots. It is now cheap to connect sensors to data acquisition systems. You can also
use thermovision cameras to detect some over temperature problems. Increasing temperature over time under same load and
ambient conditions can detect problems.

7.1.1.3 Voids at Copper Conductor
Thermal Cycling and Excessive Surges/Starts are described in Section 1.5. A void bounded by the copper conductor and
insulation exhibits a different phenomenon than those within the bulk of the insulation. Though the basic breakdown
mechanisms are the same, the electrodes are of dissimilar Bipolar Machine PD
materials and so polarity predominance occurs. In this case, there 0 to 3.16 pps 3.16 to 10 pps 10 to 31.6 pps 31.6 to 100 pps
will usually be an observable predominance of negative PD pulses 100 to 316 pps 316 to 1000 pps > 1000 pps Subset 8
clumped at 45° during the positive AC cycle, as shown in Figure 750 750
42 [1]. See Section 2.4.7.2 and negative predominance. 500 500
Due to pulse behavior, negative predominance normally indicates 250 250
PD originating near the conductor surface inside the insulation 0 0
Pulse Magnitude [mV]
system. Because of the location of the voids, that is, near the -250 -250
copper conductors, there are no reliable repair mechanisms for -500 -500
this problem. It may be possible to retard the deterioration by -750 -750
altering operating procedures, such as fewer load swings or the
180
135
225
addition of more sophisticated cooling techniques that restrict the 0 45 90 Phase Angle [deg] 270 315 360
range of the machine operating temperatures. PD near the turn
insulation in multi-turn coils may lead to winding failure quicker Figure 42: PD near Copper Conductor
than similar PD in Roebel bars. Although there may be voids and other defects in the insulation between the turns, as long as they are not on the

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