Several methods have been proposed and investigated so far on mitigation of Very Fast Transient Overvoltages (VFTO) in Gas-Insulated Switchgear (GIS). The state-of-the-art methods are primarily based on dissipation of the energy associated with electromagnetic waves that the VFTO originate from and are composed of. Present paper reports on an alternative concept of VFTO mitigation based on the principle of controlling voltage conditions preceding voltage breakdown in SF6 gas that leads to VFTO generation. The paper introduces different control algorithms and shows how the algorithms can limit VFTO maximum value and total number of voltage breakdowns during operation of the GIS disconnector. The concept is applied for mitigation of VFTO in ultra-high voltage (UHV) GIS. As the study case, an 1’100 kV test set-up is used as recently reported for Wuhan (China) GIS station, with the disconnector characteristics obtained from 1’100 kV development tests.
VERY Fast Transient Overvoltages (VFTO) originate from voltage breakdowns in SF6 gas that inherently accompany any operation of Gas-Insulated Switchgear (GIS) disconnector . The VFTO process is characterized mainly by the VFTO peak value, frequency of its main components, and the number of occurrences during the disconnector opening or closing operations. The frequency components of VFTO are related to the time duration of the voltage breakdown in SF6 gas and to the travelling wave conditions along the GIS. The VFTO peak values result from the voltage conditions at the time instance just preceding of the voltage breakdown (spark ignition) and to the travelling wave conditions along the GIS as well . Fig. 1 presents an example of VFTO waveform obtained from development tests of 1’100 kV disconnector.
Analysis and mitigation techniques related to VFTO attract high attention among industry and academia, specifically in recent years when the power grid faces the advent of extra-high voltage (EHV) and ultra-high voltage (UHV) class GIS. For these high voltage levels the VFTO peak values can exceed the GIS insulation withstand voltage and thus can become a design factor of the GIS components . It implies that the VFTO generated during the disconnector operations needs to be accurately investigated to ensure proper design of the GIS components and to support a decision making process on the potential application of VFTO mitigation techniques. In some GIS solutions the VFTO needs to be mitigated in order to maintain the VFTO peak values within the limits that are acceptable for a specific design of components.
A. Methods of VFTO attenuation
Several methods have been proposed so far for mitigation of VFTO originating from the GIS disconnector operations. Also, review works, such as  and , have been published, giving an overview of the state-of-the-art methods. The state-of-the-art methods of VFTO mitigation include disconnector equipped with a resistor inserted in the disconnector contact system , and application of magnetic rings of different types (ferrite , , , amorphous , or nanocrystalline , , , ) in the GIS busducts. With respect to the methods based on the energy dissipation in the magnetic materials, recently a new magnetic material was proposed and tested for VFTO attenuation as reported in . The recently published new methods include also resonators with a sparking element, with the resonance frequencies that are fitted to the main VFTO components , the GIS busbars equipped with surge arresters , and the disconnector with a new arrangement of the contact system . All of these methods are based on a common principle of attenuation of the energy associated with the electromagnetic waves that constitute or have constituted the VFTO.
A state-of-the-art method on VFTO mitigation that to some extent involves controlling of the voltage conditions that precede the voltage breakdowns in the disconnector contact system, is based on reducing the voltage associated with the trapped charge (the so-called trapped charge voltage, TCV) that remains at the load-side of the disconnector after the opening operation is completed , , . The most severe voltage conditions for the disconnector type testing are defined in , for the first voltage breakdown during the closing operation for the TCV = -1.1 p.u. (where 1 p.u. = Vr∙√2/√3; Vr – rated voltage). According to , , , the TCV can be controlled by proper disconnector design. The most common design changes that are reported for controlling of the TCV are related to the disconnector moving contact speed.
B. Controlling of voltage ignition in switching devices
It is known from previous works in the subject that the voltage breakdown can be controlled in a switching device. Applications of selected techniques of voltage breakdown control have already been implemented in MV switching devices in vacuum . One of the solutions is to introduce a trigger electrode into the contact system of the vacuum interrupter chamber to initiate additional voltage flashovers between the contacts at the assumed time instances. Also, triggered spark gaps are in use for dielectric tests of HV insulation to conduct tests with voltage impulse chopped at any time instance on the front or on the tail . This method is also used in HV trigatrons, for which specific operating characteristics are discussed in  as dependent upon the detailed construction of the device.
Different phenomena are recognized as potentially useful to ignite a voltage breakdown in the contact system of the switching device. Experiments with laser-triggered electrical breakdown with a proposal of different triggering electrode configurations are reported for gases  and for liquids . The voltage breakdowns induced by microwaves are reported in .
C. Paper overview
This paper presents a concept of VFTO mitigation by means of controlling voltage conditions that precede the voltage breakdowns in the GIS disconnector contact system. The analysis is presented of the VFTO mitigation for different control algorithms applied to 1’100 kV test set-up as recently reported for the Wuhan (China) GIS station . The Wuhan test set-up has been thoroughly described in , , , , , and the disconnector design-specific characteristics has been recently described in . In the present paper we outline the test set-up briefly and focus on the VFTO mitigation analysis with the use of the proposed control algorithms.