Based On Magnetic Flashover Inhibition Effect Of Vacuum Surface Flashover Characteristics

Surface flashover in vacuum is a penetrability discharge phenomena occurs at the vacuum-insulator interface. With the same applied electric field, surface flashover could happen much more easily than the breakdown of insulators. So, vacuum-insulator interface at which flashover occurs is the weakest link in pulsed power system, and is a key issue in researches of high voltage insulation around the world. Generally speaking, surface flashover in vacuum can be divided into three stages:initial electrons formation electron multiplication and penetrating conducting pathway. Up to now, there are some models that have been proposed to explain the mechanisms of surface flashover in vacuum, such as the anode-initiated flashover model, the cathode-initiated flashover model, and so on. Among them, the Secondary Electron Emission Avalanche model achieves considerable approval. All these models consider secondary electron emission and avalanche as the important processes leading to surface flashover.One of the effective ways to increase the surface flashover voltage is Magnetic Flashover Inhibition, namely MFI. When a strong magnetic field has been applied with appropriate direction along the insulator surface, secondary electrons may be deflected away by the Lorentz force. As the strength of magnetic field is increased to the critical value, no secondary electrons can reach the insulator surface, and then the avalanche can no longer be sustained. The present experimental researches abroad can not provide a definite basis to give out the critical magnetic field, moreover, no intensive studies has been carried out to investigate the mechanism of surface flashover when the external magnetic field has been applied. In China, few works on MFI has been reported which make our work to be significance.In this work, we investigate the mechanisms of surface flashover by theoretical calculations and experiments. Based on the results of other works, this thesis discusses the following aspects:numerical simulations of surface flashover with an external magnetic field; development of high voltage insulation test platform, including installation and adjustment of the MARX-PFN, variable aqueous-solution high-voltage resistors and optimal designs of the planar electrode system; design and implementation of pulsed magnetic field system and the corresponding measurement system of high accuracy; a series of experiments on the relationships between magnetic field and surface flashover voltage. Preliminary experimental results have been obtained. According to the simulations, the presence of applied magnetic field can inhibit the avalanche multiplication effects of surface charge, and then inhibit the flashover. The critical value of magnetic field is related to the position at which the external magnetic field is applied. With the same electrical field, a near-cathode magnetic field will result a lower critical value for MFI than a near-anode one.The relationships between the cone angle and the surface flashover voltage, as well as the connections between the magnitude of the applied magnetic field and the surface flashover voltage, are obtained by the experiments. By analyzing these experiment results, we can see that when the magnetic field works as it forces the electrons to leave the insulator surface, then the surface flashover is inhibited. But with the reverse direction of magnetic field, the surface flashover voltage could be decreased.It is concluded, according to calculations and experimental investigations, that applying external magnetic field can effectively increase the surface flashover voltage.