Page 46 - Basic PD Theory

PD Investigations

space charge effects, a pulse will occur in a specific direction based on the proximity of the void to a metallic substance. For this
reason, it is possible to determine where the pulses originate, and therefore, the location of the voids in the insulation system.
Several failure mechanisms produce voids predominantly at a certain location in the insulation.
By observing the pulse polarity it is often possible to determine which failure mechanism is dominant. Note that polarity
predominance should be based only on the classic PD patterns, and in the absence of classic PD, polarity predominance is not
applicable.

7.1.1 Internal Discharges

7.1.1.1 Internal Delamination
Internal Delamination is described in Section 1.5. Initially, these voids may be near the conductors and the PD exhibits a
negative predominance, but over time these voids are distributed throughout the insulation and may discharge regardless of the
“direction” of electrical stress across them. Because of this, such voids may not exhibit any PD polarity predominance and the

V V
Voids near oids near oids near Polarity Internal
co co
copper pper pper predominance of voids
Classic PD? None
Ne
Negative
Negativegative
Positivesitivesitive
Po
Discuss laterscuss later Po
Di

PD clusters near 45° and 225°. Therefore, a fairly equal distribution of positive and negative discharges at these phase positions
is indicative of general internal delamination or ageing (Figure 41). See Section 2.4.7.1 and polarity predominance.

Deterioration from internal voids is usually a slowly developing long-term ageing mechanism.

Due to the difference between the thermal coefficients of Bipolar Machine PD
expansion of copper and insulation, the voids originating in the 0 to 3.16 pps 3.16 to 10 pps 10 to 31.6 pps 31.6 to 100 pps
slot portion usually reduce in volume as the winding temperature 100 to 316 pps 316 to 1000 pps > 1000 pps Subset 8
increases, producing an inverse temperature effect. The extent of this 30 30
change in PD with temperature is dependent on the degree of 20 20
delamination that may be present. There is usually minimal change 10 10
in PD with load. Operating temperature can greatly influence the 0 0
Pulse Magnitude [mV]
results from a partial discharge test. Various materials respond -10 -10
differently to changes in temperature. These changes can be subtle -20 -20
for small variances of +/-5 °C, or drastic, doubling and tripling, for -30 -30
greater discrepancies of ± 20 °C. The sizes of voids within the 0 45 90 135 180 225 270 315 360
Phase Angle [deg]
stator winding insulation are usually inversely proportional to the Figure 41: Internal Delamination or Manufacturing
operating temperature. As the temperature increases, the copper Defect
and groundwall bonding material expand closing voids and thus
decreasing the PD magnitude of internal sources [1]. This inverse effect is more significant on asphalt and polyester windings, but
can also be observed on epoxy windings. A large inverse temperature effect suggests more severe Internal Delamination. Note
that stator slot RTD temperatures do not quickly reflect changes in the temperature of the copper conductor or the internal
insulation.

www.irispower.com

43 | P ag e