Very-fast transient overvoltage (VFTO) is a serious threat to the insulation of extra-high voltage and ultra-high voltage power-system equipment and should be suppressed. A simpler and more reliable VFTO suppressing method than a shunt resistor is presented in this paper, which uses magnetic rings installed on the center conductor of gas-insulated switchgear (GIS). Two kinds of magnetic materials that can work under high frequency were adopted, one of which is ferrite R2KB and the other is amorphous FJ37. Measurements in a scale model with an air-insulated busbar show that ferrite R2KB rings have a better suppressing effect than amorphous FJ37 when the ring-cluster length is larger than a certain value. The ferrite R2KB rings were applied to 252-kV GIS. The test results show that ferrite R2KB rings can significantly suppress the peak value and the time to peak of VFTO. When the ring-cluster length is 40 cm and the test voltage is 140 kV of power frequency, the overshoot factor and the front steepness of VFTO can be reduced more than 50% and 20%, respectively. The test results are exiting and indicate that using magnetic rings to suppress VFTO is feasible. Index Terms-Ferrite, gas-insulated switchgear (GIS), magnetic ring, suppression, very-fast transient overvoltage (VFTO).

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Very fast transient over-voltage (VFTO), induced by disconnector operations in gasinsulated switchgears, has become the limiting dielectric stress at ultra-high voltage levels. Much work has been done to investigate single-strike waveforms of VFTO. However, little study has been carried out investigating the repeated strike process, which would influence VFTO significantly. In this paper, we carried out 450 effective experiments in an ultra-high voltage test circuit, and conducted calculations through the Monte Carlo simulation method, to investigate the repeated strike process. Firstly, the mechanism of the repeated strike process is proposed, based on the experimental results. Afterwards, statistical breakdown characteristics of disconnectors are obtained and analyzed. Finally, simulations of the repeated strike process are conducted, which indicate that the dielectric strength recovery speed and polarity effect factor have a joint effect on VFTO. This study enhances the understanding of the nature of VFTO, and may help to optimize the disconnector designed to minimize VFTO.