Very Fast Transient Over voltage(VFTO) wave common characteristics in HV& EHV GIS

 Very Fast Transient Over voltage(VFTO) wave common characteristics in HV& EHV GIS

Steep wavefront, with a rise time typically ranging from 2 to 20 nanoseconds: When restriking occurs in the contact clearance of a disconnector, the arc - reignition process is extremely rapid. As a result, the voltage waveform injected into the power grid exhibits a very high ascending or descending steepness.

Theoretically, the amplitude of Very Fast Transient Overvoltage (VFTO) can reach up to 3.0 per - unit. This extreme situation occurs when the polarities of the voltages on both sides of the open - circuit branch are opposite and both are at their maximum values. Taking into account practical factors such as residual voltage, damping, and attenuation, the VFTO obtained in actual measurements or simulation tests generally does not exceed 2.0 per - unit in most cases. Considering the worst - case scenario, the maximum overvoltage can reach approximately 2.5 to 2.8 per - unit.

VFTO contains numerous high - frequency components within the range of 30 kHz to 100 MHz. This is because Gas - Insulated Switchgear (GIS) uses sulfur hexafluoride (SF6) gas as the medium, and its insulation strength is much higher than that of air.VFTO is closely associated with the restriking and arc - quenching instants of the GIS disconnector, as well as the position of the disconnector nodes within the GIS equipment.The figure shows an example of a VFTO waveform in a 750 - kV GIS.