Study on the transient characteristics of 1100kV VFTO

The very fast transient overvoltage (VFTO) generated by switching disconnector (DS) in gas insulated substation (GIS) with the features of high amplitude, short rise-time and complex frequency components, may cause damage to the insulation of electrical equipment. Therefore, in order to improve the safety performance of GIS equipment, it is very important to study the transient characteristics of VFTO. Four typical transient parameters: rise-time t
_r
, breakdown steepness k, decay time t
_d
, and voltage overshoot coefficient H are defined for the purpose of describing the VFTO waveform characteristics accurately. The typical transient statistical law of VFTO waveform is studied. The results reveal that 1 For UHV voltage level VFTO, the rise time t
_r
is greater than 10ns but no more than 30ns. 2 The average decay time t
_d
of 1100kV VFTO is about 30μs and the average voltage overshoot coefficient H is less than 1. 3 The breakdown voltage acts out polarity effect apparently. The positive breakdown voltage is about 1042kV, which is much greater than the negative breakdown voltage with -607kV value. 4 With the increasing of breakdown voltage, the maximum value of VFTO peak waveform and the value of H become higher. At the same time, VFTO is oscillating severely, which results in the decay time t
_d
and breakdown steepness k change in the same tendency of H.