In previous work, published data for electrical tree initiation
as a function of tip radius has been analyzed [9]. The conclusion
was that, to cause failure, the space charge limited field must
extend to about 1.5 µm in the direction of the electric field, which
is the same magnitude described in the previous paragraph for
the size of the initial discharges. Thus, we believe that this criterion
is a reasonable basis for the evaluation of defect severity.
Prediction of the Space Charge Limited
Field Extent
Based on the previously explained principles, we must be able
to predict the extent of the space charge limited field (SCLF) to
assess the severity of a defect. For axi-symmetric defects, we
can do this using 2-D transient nonlinear finite element analysis
with field-dependent conductivity. The key parameters in such a
computation are the defect geometry, the background field, and
the space charge limited field of the dielectric. However, transient
nonlinear finite element analysis requires very specialized
software and runs relatively slowly, although we have now improved
the computational speed of our software by a factor of
100 to 1000 for highly nonlinear problems [10]. Thus, an anaFigure
3. Photon emission from the space change limited
region surrounding a semicon tip in polybutadiene [5]. The
“needle” is ~100 µm thick semicon cut to a ~1 µm tip radius.
Thus the needle represents more of a line source than a
spherical tip. The distance from the needle tip to ground was
about 2 mm. As seen in the inset, when the square root of the
charge is plotted vs coltage, a straight line results. The
intersection of this line with voltage axis indicates a “critical
voltage” of about 900 V.
Figure 4. Basis for computing the extent of the space charge
limited field. The green area must equal the sum of the red
area and the blue area so that the integral of the field is equal
to the voltage for both the Poisson and Laplace fields. The
extent of the space charge limited field (SCLF) is computed on
the basis of making the green area equal to the red area plus
the blue area, assuming that the field, by which the Poisson
field (top of red area) exceeds the Laplace field (bottom of the
red area), decays exponentially with a distance constant of 6
µm. However, this does not include the blue triangle, the area
of which depends on the extent of the SCLF. Thus, an iterative
approach must be used to find the length of the yellow line (the
initial difference in field that decays exponentially with
distance), which results in the red area plus blue area equaling
the green area. This approach provides excellent agreement
for the extent of the SCLF with transient nonlinear finite
element computations.