For inhomogeneous fields like in many practical
insulation configurations, the switching-impulse
breakdown voltage of a given air-gap usually decreases
with the decrease of atmospheric humidity. The
phenomenon behind is complicated and is influenced by
the gap structures, i.e. the electric field distribution and
the type of discharges. A large number of contributions
on these subjects exist in literature and have been well
covered in the Cigré reviews [1][2]. For a uniform gap,
the effect of humidity on the breakdown voltage is
negligible. For non-uniform gaps such as rod-plane like
gaps the influence of humidity is found to be of
significant. From quasi-uniform gaps to non-uniform
gap the level of humidity influence is not very clear. A
correction procedure recommended by IEC is commonly
used for converting the measured voltage or the test
voltage from non-standard to standard reference
atmosphere, or vice versa [3]. This correction procedure
was obtained by a semi-physical model and by
laboratory test results on, mostly, rod-plane type of gaps.
The applicability of the humidity correction on relative
uniform gap is no clear. For the indoor high-voltage
installation, the shape of the electrode is often modified
to obtain a more compact design than the case that a
rod-plane gap will be used. It is therefore important to
verify the accuracy of the recommended correction for
these cases