Very Fast Front Overvoltage (VFFO)
1. Very fast Front Overvoltage
Very fast front overvoltage (VFFO) is a transient overvoltage, and it is also unidirectional. It takes 3 ns to 100 ns time (Tf) to reach the peak value with the superimposed oscillations at the frequency (f1) 30 kHz to 100 MHz and frequency (f2) is 30 kHz to 300 kHz. The VFFO shape is shown in Fig. 1.
The VFFO can occur due to the breakdown of disconnectors contacts, faults within the GIS, switching and flashover inside the GIS. The amplitude of VFFO depends on the construction of disconnectors (for disconnector operation), GIS configuration and connections.
Fig. 1. VFFO Shape.
References
International Electrotechnical Commission. "Insulation co-ordination—Part 2: Application guidelines." International Standard, IEC (2018): 60071-2.
IEC, TR. "60071-4: 2004, Insulation co-ordination–Part 4.