Investigation On The Characteristics Of SF₆ Gas Discharge Caused By Disconnector Operation In GIS

Electromagnetic transient phenomenon caused by SF6 gas discharge due to GIS operation is one of the main factors which can result in insulation fault and system instability problems. The phenomenon is caused by the breakdown of SF6 medium during the process of GIS disconnector operation. When it comes to research status on electromagnetic transient phenomena,the characteristics of SF6 gas discharge was ignored, and failed to consider the changeable characteristic in practical isolating switch operation process of dielectric breakdown,which makes it not being researched deep enough. Therefore, this paper studies the breakdown characteristics of SF6 insulation medium when GIS disconnector is in operation condition.Firstly, critical breakdown mathematic model of SF6 insulation medium is built by using streamer breakdown theory, which takes into account the effects to SF6 critical electric field, brought by factors such as density, pressure, temperature and so on. The effectiveness of the model is verified by comparison analyses with measured datas and engineering experience in calculation.Secondly, this paper attaches much work on the calculation analyses of SF6 flow field and electric field in the process of GIS disconnector operation, in order to get the curve of critical breakdown voltage. The frequency effect to loss value, breaking velocity, and open/close operation effect to flow field are all taken into account when the SF6 flow field is analyzed, obtaining the critical electric field under different electrode separation. The conclusion is that breakdown happens more easily in open process than in close process, and the higher the frequency, the more the loss. When calculating the electric field of operation process, we at first present the importance to improve the distribution of inner electric field which results from optimizing the minimum range of dynamic and static contact shielding. Then, we get the critical breakdown voltage by parametric analyses in which voltage is the variable based on obtained critical electric field under different electrode separation. At last, the effect of motion state of dynamic contact shielding to critical breakdown voltage in the open process is further discussed.Lastly, this paper analyzes engineering application state of SF6 critical breakdown model, in which critical breakdown model application in analysis of electromagnetic transient, and select of EVT insulation alternative medium is discussed based on the above accomplishment. The simulation and calculation shows that medium breakdown state before operation can be reflected well when SF6 discharge rekindle model is brought into VFTO calculation. The optimal position analysis of GIS busbar under high frequency shows that equipment overall insulation performance can be improved by properly lowering the dominant conductor position of GIS busbar in the inside shell. The feasibility of evaluating insulation medium performance of alternative gas is verified by theoretical analysis and experiment comparison to EVT insulation alternative medium, under critical breakdown model of gas medium. meanwhile, the SF6 gas critical breakdown model of this dissertation is widely applicable.