Characteristics of positive leader discharges in long air gaps play a critical role in the insulation coordination design for extra high voltage/ultra high voltage (EHV/UHV) transmission systems. These characteristics are affected by ambient humidity. Currently

humidity effects on these characteristics are still unclear and the humidity correction methods lack data support. Under 8 humidity conditions

the U50 of a 4-meter rod-plate long air gap under positive standard switching impulse voltage are measured. The result reveals that the humidity correction methods specified in current standards such as IEC are not fully applicable to the humidity correction of positive leader discharge in long air gaps. To analyze the mechanism of humidity effects on positive leader discharge behavior in long air gaps

2 humidity points for each of the high and low humidity conditions are selected

and synchronous observations for the discharge process are conducted. The results indicate that under low humidity conditions

the positive leader discharge progresses in a continuous mode. As humidity increases

the average development speed of the positive leader decreases

while the gap breakdown voltage increases. Under high humidity conditions

the positive leader discharge progresses in an alternating continuous and stepwise development mode. As humidity increases

the stepwise development process occurs more frequent

the average development speed of the positive leader increases

and the gap breakdown voltage decreases.