Download Answer: Very fast-front overvoltages (VFFO) are caused by switching

GROUP REF. : A3
PREF. SUBJECT : PS1
QUESTION N° : Q1-4
NAME : Fan, Jianbin / GE, Dong
COUNTRY : China
REGISTRATION NUMBER :
Q.1-4 The authors of A3-106 do not refer to very fast phenomena, such as VFTO (very fast
transient overvoltages), despite the fact that at the SC A3 Session 2010 attention has been asked
for VFTO in air insulated substations (AIS). Maybe the large capacitance, 2,000 pF, eliminates
the onerous effects of VFTO, caused by disconnector switching? What is the position of the
authors and other experts with respect to VFTO requirements for air insulated equipment
applied for 800 kV and above?
Answer: Very fast-front overvoltages (VFFO) are caused by switching disconnector or faults
within GIS or MTS. According to experiences in China, VFFO rarely emerges in AIS.
For GIS or MTS, disconnector switching is common in substations operation. During the
closing and opening of a disconnector, a number of pre-strikes and re-strikes occur due to the
low speed of disconnector compared to the circuit breaker. Each strike generates VFFO.
Therefore, the switching of the disconnector is the main cause of VFFO.
VFFO is characterized by a fast increase of the voltage to its peak value, resulting in a front
time below 0.1μs. This front is usually followed by oscillation with frequencies above 1MHz.
Dominant frequency components may reach up to 100MHz. VFFO may damage the
insulation of GIS, power transformers and voltage transformers. The maximum amplitude is
generally less than 2.5 p.u. and may reach up to 2.5 p.u. in special cases.
In UHV AC systems, the ratio of the BIL to the system voltage is lower than that in EHV
systems. Thus VFFO needs special attention in UHV systems.
Maximum system voltage
VFFO
BIL
BIL in pu
550kV
2.5 pu
1550
3.45
800kV
2.5 pu
2100
3.22
1100kV
2.5 pu
2400
2.67