Influence of operating voltage on breakdown characteristics of HVDC GIL under impulse voltage

In HVDC GIL, impulse overvoltage is not generated alone but generally superimposed on the DC operating voltage. Therefore, it is of great necessity to study the influence of DC operating voltage on breakdown characteristics of HVDC GIL under impulse voltage. In this paper, breakdown characteristics of GIL under superimposed voltage of DC and impulse are studied. The results indicate that pre-stressed DC voltage of positive polarity has little effect on the breakdown voltage of gas gap. When DC voltage is in negative polarity, the influence of DC voltage is related to the polarity of impulse voltage. Under superimposed voltage of negative DC and negative impulse, breakdown voltage is higher than that under impulse voltage alone. Under superimposed voltage of negative DC and positive impulse, breakdown characteristics of GIL depend on the size of the fixed particle. With the increase of particle length and decrease of particle radius, the influence of DC voltage becomes larger. Moreover, the light signal proves that the space charge produced in corona discharge under DC voltage has significant influence on the discharge process of gas gap under superimposed voltage. The results of the experiments show that superimposition of DC operating voltage and impulse voltage can be a critical condition for HVDC GIL, therefore superimposed test of DC and impulse is proposed to be part of on-site test for HVDC GIL to ensure the insulation ability under the service condition. Since up to now there is no valid standard for HVDC GIL, this research can serve as reference for the future formation of test standards.