The effect of humidity correction on the AC sparkover voltage characteristics of rod-rod gaps in air up to 30 cm

Values of the humidity correction factor to be used for the evaluation of sparkover voltages are different pending on the standard applied. The main focus of this paper is the discussion and illustration of differences between the two calculation methods of humidity correction factors provided by the 1978 and 1995 versions of the IEEE Standard Techniques for High Voltage Testing (IEEE Std. 4). After the sparkover voltage at a specific gap spacing of a rod-rod gap or a similar gap arrangement is measured at specific atmospheric conditions, it has to be compared to the voltage to be applied during the test, at the same conditions. Conclusions from the comparison will be different when the procedures of the 1995 version of the standard are applied, as opposed to the procedures of the 1978 version. Consequences of this are important for the design and selection of high voltage equipment. Numerical examples, and tabulated values of the “new” humidity correction factor and the reciprocal value of the “old” humidity factor are given at the end of the paper