Humidity is a function of the amount of water vapor in the
air. This is dependent on a myriad of factors such as:
location, ambient temperature, atmospheric pressure and
winds.
The variation of electrical breakdown strength with the
humidity of air is dependent on the water molecule content
of the gas which impacts the ionization (α) and attachment
coefficients (η). Several mechanisms have been proposed in
the literature to explain the variation of breakdown voltage
with humidity at power frequency.
Water molecules increase the formation of negative oxygen
ions which can be meta-stable. These meta-stable ions cannot
maintain the excited energy level for extended periods of time,
and shed the electron within 100 ps [1]. Though short lived,
these ions increase the electron absorption rate and the
attachment coefficient (η). As a result, breakdown voltage
increases with humidity at power frequency.
A second mechanism proposed is based on the direct
attachment of electrons to water molecules. Single
molecules of water cannot absorb an electron, but clusters of
H2O molecules can form negative ions [6-8].
Another mechanism that explains the higher breakdown
strength with humidity is an increased number of conversion
reactions in which collisions of oxygen ions with water
molecules create stable ions [9]. For higher humidities, the
breakdown voltage is about 8.5% higher than that obtained
for dry conditions at power frequencies (60/50 Hz) and DC.
This is true for gaps up to 1 m long [1].
The level of understanding for the VLF/LF bands is not as
extensive as that for power frequency and is the motivation
of the present project