16-01-2013, 03:18 PM
Dynamic Modeling of AC Iced Insulator Flashover Characteristics
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Abstract
This paper investigates the feasibility of using a
dynamic arc modeling to predict flashover of ice-covered
insulators. The proposed mathematical model includes the
successive propagation of the arc. The input data of this selfconsistent
mathematical model are the insulator geometry, the ice
layer characteristics and/or properties, the applied voltage and
some initial values. The computed results are compared to the
minimum flashover voltage measured experimentally on a IEEE
standard insulator strings. The results indicate the feasibility of
assessment of icing severity and flashover prediction, using the
proposed model.
INTRODUCTION
HE excessive accumulation of atmospheric ice on the
power network equipments can cause towers to collapse.
One can recall the famous ice storm of January 98 which
struck the south of Quebec, part of Ontario and parts of the
Maritimes [1]. More than 1,000 power transmission towers
and 30,000 wooden utility poles collapsed. Close to 1.4
million people in Quebec and 230,000 in Ontario without
electricity. In many municipalities, power not fully restored for
at least a week. In addition to the mechanical damages, the
formation of ice or melting snow on outdoor insulators
involves a considerable reduction in their electric performance.
Indeed, atmospheric icing is a frequent cause of insulator
flashover in cold regions [2-8] and such problems occur both
in transmission lines and substations. Combined with the
outdoor pollution, the presence of ice or melting snow can be
at the origin of the appearance of corona discharges and partial
arcs, leading sometimes to flashover and consequent power
outages. In most cases, the atmospheric icing caused insulator
flashover results in a lengthy outage. Flashover phenomena on
ice- and snow-covered insulators have been reported from a
large number of cold climate countries [2-14]. Even during the
January 98 ice storm [1], a few flashover have been observed.
FLASHOVER MECHANISMS ON ICE-COVERED INSULATORS
Natural atmospheric ice deposits on insulators result generally
from a variety of conditions including hoarfrost caused by
condensation of vapor, in-cloud icing involving the freezing of
super-cooled droplets in clouds or fog, and finally
precipitation icing occurring by freezing rain, drizzle, as well
as wet and dry snow.
It should be noticed that, so far, there is almost no exploratory
research work is available on the mechanisms of flashover
occurrence on ice-covered insulators. However, it is wellestablished
that several ice parameters, including type and
density, amount and distribution, as well as conductivity of
freezing water forming the ice influence significantly the
withstand voltage of ice-covered insulators [2, 3].
TESTS FACILITIES AND PROCEDURE
Up to date, no standard method for evaluating the withstand
voltage of insulators under ice and snow conditions has been
devised. The development of testing methods for evaluating
high voltage insulator flashover voltage under icing conditions
is still at an early stage. Recently, Test methods for evaluating
flashover voltage of ceramic and non-ceramic insulators under
ice, snow and cold-fog conditions are recommended [20].
Considering that each test takes nearly one full day, it was
proposed that the maximum withstand voltage method was
more practical for evaluating the performance of iced
insulators. A detailed description of this method was presented
in our previous studies [3, 20, 21]. Maximum withstand
voltage, VWS, is considered to be the maximum voltage at
which flashover did not occur for at least three tests out of
four.
GENERAL DESCRIPTION OF THE MODELLING
Figure 4 shows a simplified description of the flowchart of the
self-consistent time-dependent mathematical model.
The input data are the insulator geometry, the ice layer
characteristics and/or properties, the applied voltage and some
initial values. The discharge time base is divided into steps dt ,
starting from t = 0. At each time step, the applied voltage
Vap(t) is calculated.
To calculate the flashover voltage, the same hypothesis of
static modeling [23] is being considered, where the flashover
takes place during a short period of time around maximum
applied voltage and so the same method [15] is applicable.
Under the estimated flashover voltage, which should be
enough to sustain an arc beyond the first arc length, the arc
development begins when the propagation criterion is met. The
internal conditions (velocity, radius, electrical parameters and
so on) as well as the residual ice layer resistance are
calculated. At each time step dt, the critical conditions for
continued propagation of the discharge are tested and, if they
are satisfied, the discharge continues to progress up to the final
jump stage. Otherwise the arc extinguishes and flashover
cannot take place. At this time a new step is considered by
increasing the applied voltage. The simulation is restarted
again with initialization of the input data. At the time when x,
arc length, will be equal to the insulator length Lf, then
flashover will take place.
THE MATHEMATICAL MODEL
One of the first quantitative analyses of arcs on contaminated
surfaces was carried out by Obenaus [24]. So far, the most
practical and useful models proposed by other authors have
been based on the commonly-known design in which a
polluted insulator is modeled by a simple electrical equivalent
circuit consisting of an arc in series with an electrical
resistance. The air gaps on ice-covered insulators (or parts of
the insulators free of ice) in series with the accumulated ice,
and which have a relatively high surface conductivity, present
situations similar to the dry bands in series with the wet part of
insulators for polluted surfaces. Thus, a comparable model can
be used. In this model, we assume the arc channel to a
cylindrical one having a radius r and length x. We also
represent it by a RLC electrical network as shown in Fig. 5.
The model is a self-consistent time-dependant one. The input
data are the applied voltage characteristics, the ice-covered
insulator geometry and characteristics.
CONCLUSION
In this paper, we investigated the feasibility of using dynamic
arc modeling of iced insulators to predict AC flashover. The
mathematical presented model is based on a simplified
simulation of the successive phases of flashover development
process on iced insulators. The flashover voltage calculated
from the model have been compared to those obtained on a
ice-covered string of 5 units IEEE standard insulators. There is
a good agreement between the minimum withstand flashover
voltage of ice-covered standard insulators obtained in the
laboratory and values calculated from the mathematical model
developed in the present work.