中文摘要：

为获取负极性长间隙放电梯级先导特征参数，进行了多种间隙结构、多种间隙尺寸的负极性长阃隙放电试验，得到了第1梯级先导长度、暗区持续时间的统计结果及梯级先导的发展速度。统计结果表明：短间隙（〈2．5m）下的负极性放电仅1个梯级，梯级先导长度一般小于间隙长度的20％；长间隙（〉6m）下的负极性放电有多个梯级，梯级先导发展速度在10^4m／s量级～10^5m／s量级之间；同一次放电，梯级先导发展速度随先导长度增加而加快。

英文摘要：

In order to obtain the characteristics of the stepped leader of negative discharge in long air gaps, we carried out a series of long air-gap experiments under negative switching impulse with varieties of gap structures and gap sizes, then counted the length of the first stepped leader and the duration of dark period from the experimental data, and got result of the progress velocity of stepped leader. The statistical results indicate： there is only one step in negative discharges in short gap which is shorter than 2.5 m, and the length of the stepped leader is shorter than the 20% of gaps. ; there are several steps in negative discharges in long gap which is longer than 6 m, and the progress velocity of the stepped leader is between 10^4 m/s and 10^5m/s; the velocity of stepped leader will be faster and faster when it extends in a discharge.