Monte Carlo Simulation of Surface Flashover
Surface flashover is an electrical breakdown that occurs on or just above
the surface of an insulator when a sufficiently high voltage stress is
applied. This phenomenon is distinct from bulk breakdown which occurs
within the insulating material. It is believed that the following
mechanisms are involved in surface flashover. First, electrons are
emitted at the triple junction of cathode, insulator, and vacuum due to
field enhancement at microscopic irregularities and dielectric mismatch.
The electrons are accelerated in the field between the cathode and
anode. Some of these electrons impact the surface of the insulator
where---depending on the particular material, the angle of incidence, and the
impact energy---0,1,2, or possibly more secondary electrons are emitted
with a distribution of angles and kinetic energies. Clearly, if more than
one secondary electron is emitted, then that part of the insulator surface
will acquire a positive charge.
It is believed that such a positively charged surface does develop on some
(if not most) of the insulator. Once the surface acquires a positive
charge, the trajectories of the emitted secondary electrons are likely to
reimpact the surface in such a way that more than one additional secondary
is emitted for each impacting electron. Thus, an avalanche of electrons
hopping along the surface of the insulator toward the anode occurs. This
avalanche knocks off gas molecules that were adsorbed on the insulator
surface. When the pressure in this layer of gas reaches a critical value,
ordinary gas phase breakdown due to electron impact ionization of the gas
occurs.
In order to study the charging phase that occurs prior to flashover, we
developed a Monte Carlo simulation of the process for a simplified geometry. Since a simulation would be
followed for millions of avalanches, large quantities of suitably random
numbers were needed. Appropriate probability distributions for the
important physical quantities were developed. However, not all of these
distributions were invertible, thus we made extensive use of the Von Neumann rejection technique. A set
of results showing the surface charging at different times in the simulation
(top/early to bottom/late) and for different electric field strengths
(left/weaker to
right/stronger) from the M.S. thesis of Dr. Gregory B. Osterman is shown in
the figure.
The Paper Chase
If you are interested in obtaining more information about this topic, try
the following:
- G.B. Osterman, "Monte Carlo Simulation of Surface Charging Prior to
Surface Flashover," M.S. Thesis, Texas Tech University (1990).
- G.B. Osterman and T.L. Gibson, "Numerical Simulation of Surface Charging
Prior to Flashover in Lexan and Lucite," Proceedings of the 14th
International Symposium on Discharges and Electrical Insulation in
Vacuum, Santa Fe, N.M. 368-71 (1990).
This page designed and maintained by T.L. Gibson
thomas.gibson@ttu.edu
Page Last Modified July 20, 2006
Return to Gibson Home Page