Page 19 - Basic PD Theory

Basic PD Theory

2.4.2 Field Theory
The capacitive model described in Section 2.4.1, does not predict the
variability of PD patterns, especially
 Why we have different pulse phase analysis distributions?
- -  Why PD occurs before voltage zero crossing?
 Why PD decreases as voltage applied longer?
E E ai ai E E Electric field theory is a better way to predict and describe PD behavior.
E E 0 0 + + wi wi The electrical stress, E , in a void, is the sum of two stresses. Firstly,
v
it includes the electrical stress from the field in the surrounding
material, E , which is determined by the Laplace, or geometrical field
ai
as described in Section 2.4.1. Secondly, it includes the electrical
stress from the charges trapped on the void surface from previous
E = E + E
E = E + E wi wi , or the Poisson component.
ai ai
v v
PD, E wi
Figure 10: PD Field Theory
The Poisson component, based on the void surface charge distribution, is
affected by the void surface conductivity. Surface conductivity depends
on the material, temperature, past history of the discharges and time between the discharges. As the surface conductivity
increases, then the partial discharges decreases since the reduction in space charge will decrease the net field in the void.
Eventually, the voids surface may become fully conductive and the detectable partial discharge will stop all together, or transform
into a glow or pseudo-glow pulse. Most conventional partial discharge detectors do not detect this activity [1].
2.4.3 Physics of PD

When the applied 50/60Hz increases sinusoidally, the apparent electric stress across the void increases until it exceeds 3kV/mm
or the equivalent breakdown voltage in the void. Over voltage is the state at which the voltage across a void exceeds the
breakdown voltage required for the void size and gas. The larger the over voltage achieved, the more intense are the space
charge effects in the void. Although a void may be in an over voltage state, breakdown will not occur until a free electron due to
cosmic or natural radiation appears within the gap and starts an avalanche of the electrons. The avalanche is a flow of electrons
across the gap which gives rise to a very fast rise-
time (a few nanoseconds) current pulse [1], called 50/60 Hz phase-to-
a partial discharge (PD). ground voltage

The dependence on the free electron for a partial
discharge makes the occurrence of PD a Breakdown
statistical event and therefore not predictable [1]. voltage of void
Once the breakdown occurs, the voltage across sustain
the gap collapses to a voltage level sufficient to
sustain the discharge. sustain

Most instruments only detect the initial Voltage across
breakdown pulse. No further detectable
discharges will occur until the gap voltage has the void
reversed in polarity and another over voltage
condition established. Shown in the simple
model Figure 11, for each void there will be a
detectable PD twice an AC cycle. However, the Figure 11: Partial Discharge Occurrence
occurrence, magnitude, and pattern in a void are

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