Test Study On The Effect Of Space Charge On Discharge Of Air Gap

Shielding failure is identified as the main cause of lightning outrages for both Ultra High Voltage (UHV) and Extra High Voltage (EHV) transmission lines. During the experiment study of lightning striking points, many scholars find that space charge is generated from corona discharge under the influence of thundercloud electrostatic field and from the development of streamers or leaders. And the space charge may have an effect on the striking points, which is probably one of the main reasons for abnormal shielding failure on the UHV&EHV transmission lines.In order to study the effect of space charge formed by air gap discharge on the discharge striking points under switching impulse, a rod-plane air gap test platform is set up, using rod electrodes with different diameters to roughly represent the changes of the amount of space charge to study the effect of space charge on discharge characteristics under switching impulse. First, the discharge characteristics of rod-plane air gap are studied under the standard switching impulse with different polarity and the discharge propagation is recorded by high speed video cameras to compare the different effects of space charge on discharge striking points under positive and negative voltages. The results show that the discharge propagation path is from the positive upper plane electrode to the lower rod electrode. The striking points on the rod electrode are different when the head shapes of the rod electrode are different, which demonstrates that space charge has a selected effect on the striking point location on the rod electrode, and the effect of space charge is different when the head shapes of the rod electrode are different. Under negative switching impulse the discharge propagation path is from the positive lower rod electrode to the upper plane electrode, which is opposite to the results under positive switching impulse. As a consequence, there is no selected effect of space charge on the striking points on the rod electrode. Second, based on the different effects of space charge under different switch impulses with different polarity, the pre-breakdown experiments of rod-plane air gap with different rod shapes are conducted under positive switching impulse. There is a serial non-inductance sampling resistance connected to the air gap used to measure the pre-discharge current of the air gap. Then the space charge current can be extracted and the amount of pre-discharge space charge is obtained by integral current method. The results show that the amount of pre-discharge space charge decreases as the diameter of the rod increases under the same voltage. That means the amount of space charge is different when the head shapes of the rod electrode are different under the same condition, which leads to the different distortion effects of space charge on the background electric field. It is the reason that results in the different striking points on the rod electrode. At last, according to the experiment results, the lightning shielding performance of transmission lines is analyzed considering the effect of space charge.