Very Fast Transient Overvoltage(VFTO) in Gas-Insulated Substations
Guru prakash R.
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Gas Insulated Substations (GIS) have found a broad range of applications in power systems because of their high reliability, easy maintenance, small ground space requirements etc.
In spite of the merits of GIS it has its unique problems, among which is susceptibility of the insulation system under very fast transients (VFT) caused by operation of switching devices.
Very fast transient overvoltages (VFTO) arise within a GIS at any time and there is an instantaneous change in voltage. Most often this change occurs as a result of the opening or closing of a disconnector switch (DS) or Isolator. Other events, such as the operation of a circuit breaker (CB), the occurrence of a line-to-ground fault or the closing of an earthing switch can also cause VFTO.
However, during a DS operation a high number of re-strikes and pre-strikes occur due to the low operating speed (opening time in seconds) of DS compared to a circuit- breaker. Therefore, DS switching is the main source for generating VFTO.
A 400 kV GIS has been modelled and simulated. Voltage at various location has been measured and the voltage at entrance of the substation has been shown below for disconnector operation.
Voltage reached almost 1.4 P.U
The generation and propagation of VFTO from their original location throughout a GIS can produce internal and external transient overvoltages . The main concerns are internal overvoltages between the conductor and the enclosure that’s what we have seen above.
Transient Enclosure Voltage -TEV (External Transient Voltage)
Transient Enclosure Voltage (TEV) is special case of VFTO, characterized by short rise time, short duration high voltage transients which appear on external of earthed enclosure of the GIS.
TEV does not emerge onto the external enclosure surface at the point of initial disturbance . This will happen during transiently couple onto the exterior surface of enclosure at physical discontinuities ( SF6 to air , Cable to SF6 Bushing )
SF6 to air , Cable to SF6 Bushing , is the main sources of TEV
Same case voltage is measured at the enclosure of the GIS at the termination of SF6 to air at Line end and SF6 to oil at transformer end
TEV
VFTO may damage the insulation of GIS and its adjacent primary facilities, and TEV may raise the issue of personnel shock hazard and electromagnetic interference to secondary equipment.
Observation of sparking between grounded enclosures and support structures, failures of protective devices, inadvertent operation of relays etc. are common manifestations of TEV.
TEV was controlled in three steps firstly proper design and arrangement of the substation mat, secondly by minimizing the surge impedance and electrical length of grounding strip and lastly by surge arrester which reduced the TEV peaks to a desired value and also damped out the undesirable oscillation. Usually GIS equipped with Enclosure Surge arrester to limit this TEV
VFTO can be controlled by some common methods are given Below
- · Opening And Closing Resistor
- · Installation Of The Ferrite Ring
Reference
CIGRE-Very Fast Transient Overvoltages (VFTO) in Gas-Insulated UHV Substations Working Group D1.03
E-Hazard Partner and Electrical Safety Consultant
2 年Can you explain how exactly the transient is developed specifically because of the GIS environment? Why, say, is this different than a switching transient in a VCB?
Lead Electrical Engineer at Wood
4 年very good information
??????????????????????????????????.?????? platform & groups founder/Switchgear Senior Sales Engineer/ Switchgear Technical & Marketing Consult
4 年Good article For more information about VFTO please visit GIS category in switchgear content website: https://switchgearcontent.com/category/gis/
Results-Driven PMP? Certified Lead Electrical/Project Engineer with Expertise in Power Systems, Renewable Energy, and Microgrids for Oil & Gas, Power Plants, and Infrastructure Projects
4 年an excellent way of conveying technical things. Appreciated. could please add a snapshot of SLD modeling?