Mohammed El-Amine SLAMA Étude expérimentale et modélisation ...

Experimental study and modelling of the influence of chemical
constitution and repartition of pollution on flashover of high voltage
insulators.
Abstract
This work aimed on optical and electrical characterization and mathematical
modeling of flashover of polluted insulators, taking into account the applied voltage
waveform, the distribution and chemical composition of pollutant deposit. Under
lightning impulse voltage (1.2 / 50µs), the morphology of the discharge and the
associated currents depend on voltage (shape, amplitude, polarity), the distribution
of pollution and its conductivity. The measured average velocity of the discharge
depends on the polarity applied to the tip and the pollution conductivity. The critical
time corresponding to the critical conditions is about 9/10 of total flashover duration
regardless of the polarity of the voltage and the pollution conductivity. The chemical
composition of the pollution layer has little effect on the critical voltage unlike the
critical currents. The values of the characteristic constants n and N depend on the
chemical nature of the deposits and the voltage polarity. The developed model
shows that the discharge constant characteristics n and N are dynamic parameters
and depend on the elements of the equivalent electrical circuit system and thermal
parameters of the discharge. The developed relationships of critical conditions of
flashover link the electrical parameters and thermal equivalent circuit and the
propagation condition of the discharge. This approach allows us to understand the
effect of the chemical constitution of pollution on the values of n and N. The
application of this model for various voltage waveforms and for several types of
pollution, gives satisfactory results. The assumption that the column of the discharge
contains only water vapor and air is a good approximation of the critical conditions.
Under lightning impulse voltage, the current circulate into a effective section of the
pollution surface. The introduction of the concept of critical effective thickness of
pollution and its application to the calculation of critical parameters gives good
results compared with the experimental values. We have also shown that the
effective thickness of the deposit is proportional to the resistivity of the pollution and
depends on voltage polarity and pollution configuration. In the case of discontinuous
deposit of pollution and / or non-uniform propagation conditions of discharges are
local and their characteristic parameters vary depending on the configuration and
the conductivity of the deposit and the polarity of the applied voltage. The
developed model gives good results.
Key-words: High voltage - Insulator– Pollution – Electrical discharge – Flashover –
Chemical constitution of pollution – Discharge constants – Discharge temperature –
Critical conditions – Effective thickness – Pollution repartition.
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