Page 21 - Basic PD Theory

Basic PD Theory

voltage coils (line-end coils). Thus, detection of partial discharges in the high voltage coils will assuredly detect typical aging and
find the coils most at risk, i.e., the high voltage coils. What cannot be detected because of this distortion characteristic is a
singular flaw in a lower voltage coil.
2.4.6.2 Capacitive Coupling
Unlike transmission whereby the pulse has to travel through an inductor, in capacitive coupling the pulse travels through
capacitors. For a high frequency signal, a capacitor appears as a short circuit and would allow the current to pass through virtually
unimpeded. Although this would be ideal for detection of partial discharges since they would travel without any distortion
throughout the winding, the prediction of capacitive coupling is impractical.
Capacitance is the electrical property that stores an electrical change by impeding a change in voltage. A capacitor consists of
two metal plates separated a specific distance by a dielectric such as insulation or air. The problem with calibrating the behavior
of a pulse in a winding is that the quantity and magnitudes of the capacitance elements are unknown. Capacitance exists:
* in every coil between the conductor and the core with insulation as the dielectric
* in the end arm area between the conductors of two adjacent coils with air and insulation as the dielectric
* in between the circuit rings with insulation and air as the dielectric
The magnitude of the capacitive coupling depends on: the thickness of the insulation, the space of separation, and the integrity of
the organic resin; all of which are constantly changing with temperature, humidity, and load. Thus, although we know that
capacitive coupling occurs and we can detect it, it is not possible to predict how a pulse will capacitively couple through a
winding. However, once the behavior is established through empirical testing, it can then be monitored.

2.4.6.3 Radiation
The last method of pulse travel that will be discussed is radiation: when the occurrence of a high-frequency pulse creates a wave
that travels through a medium. This is the same phenomenon observed when you hear static on your radio when passing under
a power line. Because of this characteristic, a radio-frequency antenna placed in close proximity of a partial discharge that has
corona, will detect the disturbance. The difficulty in using this method for PD monitoring is that like transmission, the pulse
distorts quickly, losing its identity as a high-frequency partial discharge. Thus, in order to be sure that it was a true spark from a
void, you would still have to monitor at high frequencies and be very close to the source of the PD that is occurring in the high
voltage coils.

2.4.7 Pulse Polarity

Modeling the actual characteristics of a pulse is
difficult since the void dimensions, void gas and
pressure, inductance, capacitance, geometry, among
other issues can affect the magnitude and frequency
of a pulse [1]. However, there are some basic pulse
characteristics, which can be predicted based on the
void location as shown in Figure 12.

Figure 12: Pulse Polarity based on Void Location

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