Page 20 - Basic PD Theory

Basic PD Theory

a complex phenomenon depending on the size, shape, internal gas pressure, and nature of the void surface and will deviate from
cycle to cycle due to past space charge trapping [Section 2.4.2].

2.4.4 Voltage Dependence
Not only is the spark (partial discharge) an indication that a gas-filled void exists and thus a symptom of deterioration, but it also
breaks down the carbon bonds of the organic resin and accelerates aging. Because the magnitude of the voltage stress across a
void is dependent upon the applied voltage, most partial discharges only occur on high voltage (line-end) coils. Coils at the
neutral end do not have sufficient voltage to ground stress to exceed the electrical breakdown stress for air or hydrogen. Some
voids do not actually produce a pulse spark, but instead exhibit glow and pseudo-glow characteristics. Most conventional partial
discharge detectors do not detect this activity [1]. The model shown in Figure 11 is an extremely simple model, the frequency
and magnitude of discharges in a void is a complex phenomenon depending on the size, shape, internal gas pressure, and nature
of the void surface.

2.4.5 Pulse Shape
The magnitude of partial discharge depends on the void volume (W = 0.5CV ) and shape, as well * Rise-time is the time
2
as the proximities to the copper conductor and PD sensor, and the void gas pressure. The required for a pulse
to increase from
relative phase angle position depends on the void size, surface conductivity, and winding 10% to 90% of its
temperature. The rise-time depends on the cross-sectional area and the strength of the electrical pulse magnitude.
field.
The pulse from a partial discharge is unipolar and has an extremely fast rise-time (1-5ns) and short pulse-width (2-6ns) [1]. The
period of oscillation, the rise-times and magnitudes of subsequent peaks vary for each pulse. This normally depends upon the
geometry of the machine, the location of the pulse and the insulation materials. Since partial discharges are pulses, they will
produce energy across the frequency spectrum from DC to GHz. In addition to the spark pulses, there are also slower
detectable frequencies from ionic tails. Therefore, it is possible to detect partial discharge activity across several frequency bands
with advantages and disadvantages for each [1]. It also means that no one instrument can detect all of the energy dissipated due
to a partial discharge.
Most partial discharge detection devices only detect the initial pulse that has a rise-time of 1-5ns. Based * Partial discharge:
on rise-time, to a first approximation, the frequency characteristic of a pulse is 50-250MHz
frequency
f = 1/T = 1/(4*rise-time) characteristic
Thus, the rise-time range of 1-5ns for partial discharges corresponds to the frequency characteristic
range of 50-250MHz.

2.4.6 Pulse Travel
Once a pulse occurs, it has to travel somewhere to be detected by a sensor. High frequency pulses travel through a stator
winding in three different ways: transmission, capacitive coupling, and radiation.

2.4.6.1 Transmission
The most widely understood and predictable method for a pulse to travel is to transmit through the copper conductors, similar
to the 50/60Hz power signal. When high frequency pulses travel as transmission waves, they undergo severe distortion due to
the inductance of the coils. Inductance of a stator-winding coil, which is a normal and necessary characteristic for electrical
production, is a property that develops an electromagnetic field by impeding a sudden change in current. Though this has a
positive effect on the low frequency 50/60Hz power signal, it does diminish the high frequency (50-250MHz) pulses of partial
discharges. The distortion is attenuation and an increase in the rise-time (lower frequency response). In fact, the pulses are
distorted beyond recognition as high frequency partial discharges after only a few coils are traversed.
In spite of this phenomenon, two conditions make partial discharge detection possible: first, that aging, (i.e. development of a
failure mechanism) normally affects all coils to some degree; and second, that partial discharges can only occur when there is
sufficient voltage to ground stress to cause an electrical breakdown of the air in the void. This latter aspect only occurs on high

www.irispower.com

17 | P ag e