Research on mechanism in formation and depression of very fast transient overvoltage

In order to research the generating mechanism of very fast transient overvoltage (VFTO) that is caused by the operation of isolator in gas insulated substation (GIS), a method of detecting exciting mode was used here. The method of exciting mode was very useful for catching the main factors which influence VFTO. Up to now, two kinds of exciting mode were discovered. One was the effectiveness of the capacitance of bushing, and the other was the change of conductor's surge impedance. Thus, a new method to depress VFTO was proposed, i.e. a section of conductor with lower surge impedance was added at the terminal of GIS. The analysis shows that the proposed method is very effective for depressing VFTO.