Abstract:
This research presents the positive impulse breakdown characteristics of air-gaps under atmospheric
conditions. According to IEC 60-1(1989) [1], the maximum humidity accepted under the positive
impulse breakdown voltages for humidity correction factor (k) is 15 g/rn. However, the humidity of
Thailand is mostly higher than 15 g/m". Therefore, this research a study of breakdown
characteristics of air-gaps under positive impulse voltages and various atmospheric conditions was
carried out. In the experiment, sphere-sphere gaps were used with the sphere diameter of 12.5,25
and 50 cm where as the gap distances was varied in the range of 1-6, 1-12, and 3-12 em for each set
of spheres respectively. The absolute humidity condition in this test was varied between 10-21 g/mJ.
Then, The results obtained were used to find the new humidity correction factor (IS) equation for
positive impulse voltage. This equation was applied to correct the breakdown voltage of rod-plane
and rod-rod gaps with gap distance varied between 3-15 em. The length of rod was 20 ern with tip
angle of 45? and the size of plane was 4x4x2 cm, The range of the absolute humidity of this test
was also varied between the 10-21 g/m'. Finally, this equation was analyzed and compared with IEC
60-1(1989) standard. It was found that the new humidity correction factor (ISm) obtained
analytically is better than the correction factor specified in IEC 60-1(1989) standard for the test
condition used in this thesis.