Page 75 - Basic PD Theory

References

insulation failure a complete electrical discharge that occurs between two metallic electrodes
Internal delamination the debonding of tape layers due to thermal stresses
result of coil vibration where the portion of the coil adjacent to the core abrades and that
ladder-effect
near the air vents does not - the damaged coil resembles a ladder
the coil connected to the circuit ring or phase bus - will have maximum phase-to-ground
line-end coil
voltage stress
linear pulse density plot (LPD) Linear 2-D plot that displays the pulse magnitude versus the reference phase-to-ground
AC sine wave. Color is used to identify pulse repetition rate
when there is a substantial change in PD with a change in load -- if posisitve
load effect predominant as well, this is an indication of loose coils
coils that are inadequately secured within the slot and thus vibrate due to the mechanical
loose coils
and magnetic stresses in a machine
the principal insulating material of stator windings - it is a durable but brittle mineral
Mica
product
mica flake insulation systems or tapes where large pieces of mica adhere to a tape backing
mica paper tape to which small bits of mica (mica splittings) are attached to form a roll of insulating
tape used in low temperature (Class B) applications
an early insulation system consisting of large mica sheets held together with varnish or
mica folium shellac binders

multi-turn coil Coils made of several turns per coil
negative predominance when the amount of negative classic PD, near 45°, is higher than the amount of positive
classic PD, near 225° -- indicative of voids near the conductors
When an increase in the operating or ambient condition causes a decrease in PD – can be
negative effect
attributed to load, temperature, hydrogen pressure or ambient humidity
no load test test at no load or reduced load for determining load effect by comparing results to a full
load test
when the amount of negative classic PD, near 45°, is approximately equal to the amount
no polarity predominance
of positive classic PD, near 225° -- indicative of voids within the insulation bulk
pulses originating from outside of the insulation system, or pulses that do not exhibit PD
noise characteristics
the technique used to ensure that only machine PD is displayed and not PD-like noise
noise separation
from sources outside the machine
non-classic PD, which consists of small magnitude pulses occurring across the ac cycle.
noise-like PD This is usually not indicative of a problem, and as the winding ages “true” PD will
eventually be observed beyond the noise-like PD.
non-classic PD pulses pulses that are not clumped near 45° and 225° - see endwinding, cross-coupling and
excitation PD

NQN normalized quantity number - represents the area beneath the curve of the 2-D plots
weighted by the magnitude of the pulses
organic substance used to bond insulating tape layers - asphalt, polyester, and epoxy are
organic bonding material the most common
over voltage When the voltage stress across a void exceeds the breakdown voltage
ozone O 3, a by-product of electrical discharges in air

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