Experimental Research on Suppressing VFTO in GIS by Magnetic Rings

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.