FABEAD82d01 - erical Model Using Surface Charges for the...

erical Model Using Surface Charges for the Calculation of Electric
Fields and Leakage Currents on Polluted Insulator Surfaces
A.
Ahmed and H. Singer
Department of Theoretical Electrical Engineering
Technical University Hamburg-Harburg
Harburger SchloBstr. 20
2 1079 Hamburg, GERMANY
Abstract
The pollution on the surface of an insulator is
asymmetric though the insulators, in general, are axi-
symmetric. A pollution layer of wet or dry type has an
cnhancement effect on the distribution of the electric
potential, field and leakage current on the surface of the
high-voltage (HV) outdoor insulators. In the present
work, a numerical method for the computation of the
electric potential, field and surface leakage current on
the surface of the polluted HV insulators
is
developed.
The method is based on the surface charge simulation
(SCSM) and discrete charge simulation techniques
where the simulation charges are complex at a given
frequency. The method takes into account surface as
well as volume conductivities in addition to the effect of
the dry band or wet pollution. Due to the conductivity of
the insulation a complex permittivity of the insulating
material has to be regarded. The surface pollution on the
insulators,
in
general,
is
asymmetric and is simulated by
small or big patches of dry band or wet pollution.
Therefore, the field configuration has to be three-
dimensional. Special emphasis is laid on the extreme
local field variation at the boundary between dry and
wet surface patches. As an example, the method is
applied to a polluted cylindrical insulator supporting a
HV electrode above the ground plane.
Introduction
The
insulation system of a high voltage equipment has
to withstand high electric stress for decades. An
inherently critical zone of these equipments
is
the
interface between the solid insulator and the
surrounding. Hence, the study of the fundamental
phenomena in electrically stressed interfaces is of
crucial importance to enable the design of high voltage
and clean conditions the electric field follows the rules
of
the electrostatic field and can be simulated
correspondingly. Increasing surface conductivity due to
conductive contamination leads to increase in surface
equipments which satisfy the requirements. Under dry
P. K. Mukherjee
Department of Electrical Engineering
Jadavpur University
Calcutta
INDIA
leakage current which can initiate a heat conduction
process occurring on the insulator surface. The nature
and severity of the surface contamination can vary over
a
wide range
[l].
If
a
random spot dries out first, for
a
smooth cylinder, the current density around it will be
increased causing even higher heat to evaporate the
moisture. The dry spot will grow laterally, that is
perpendicular to current flow
[2],
until it covers
a
whole
circle around the surface of
a
cylindrical insulator. The
formation of a dry-band zone, which may eventually
cause a flashover of the insulator, can be viewed as the
final result of such a process [3]. Several computational
techniques have been