VFTO mitigation – switchgear

There are three main methods for VFTO damping, which are the high frequency resonator method, ferrite magnetic ring method and the inductive arrangement of surge arrestors (Li

et al. 2019). All methods are relatively new and limited test data is publicly available. There is also an additional method under recent research, called the spiral tube damping busbar.

However, some discrepancies between different sources exist on how good or applicable the methods are for different solutions are.

6.4.1.1 High frequency resonator

A slightly older concept is the application of specially formed shielding parts inside the GIS to serve as a high frequency, also referred to as radio-frequency (RF), resonator. Reduction effect is only achieved if the resonance frequency and the value of the resistor across the gap fit the VFTO parameters very well. (Burow et al. 2014) The figure 6.9 illustrates the effects of this mitigation method.

Figure 6.9 Measured VFTOs with or without high frequency resonator. (Riechert et al. 2012)

As can be seen from the figure, some frequencies were dampened to a significant degree.

This can especially be seen with the dominant 7.5 MHz harmonic component marked as R1 in the figure. The parameters of the resonator could also be changed to increase its damping efficiency. An important aspect is also that the damping effect of the high frequency resona-tor is not only limited to disconnecresona-tor switching operation generated VFTOs, but also other switching or fault events that are not as common as causes of VFTO. (Riechert et al. 2012) 6.4.1.2 Ferrite rings

Implementation of ferrite rings is a relatively easy to realize mitigation measure, where rings of ferrite material are arranged on the GIS inner conductor. Simulations and some small-scale measurements have been conducted, with promising results for VFTO mitigation po-tential. However, in HV solutions where the VFTO travelling waves reach larger values, the high magnetic field saturates the ferrite material completely, leading to considerably worse dampening effect. Measurement results for a 550 kV GIS are shown in the figure 6.10.

Figure 6.10 Comparison VFTOs without ferrite rings (1) and with 6 ferrite rings (2). (Riechert et al. 2012)

As can be seen from the figure 6.10, the potential of ferrite ring applications for VFTO mit-igation for HV GIS applications is limited. For lower voltage applications where the ferrite rings do not saturate, ferrite rings can have good damping qualities. (Riechert et al. 2012) New method, a material referred to as nanocrystalline alloy rings, has been researched to provide better magnetic qualities than ferrite rings, especially for HV applications. Rings are similarly arranged on the inner conductor of the GIS where good damping qualities have been found. It has also been found that damping qualities can be increased with adding more rings around the GIS conductor. (Burow et al. 2014)

6.4.1.3 Inductive arrangement of surge arrestors

This damping method is based on the helical slotted part of a GIS conductor that serves a small inductance. Surge arresters are installed inside the conductor parallel to the helical slotted part. This arrangement is illustrated in figure 6.11.

Figure 6.11 Inductive arrangement, where the surge arrestors are installed parallel inside the helical slot-ted conductor. (Burow et al. 2014)

The VFTOs are damped due to the surge arresters absorbing energy if the voltage drop across the arrangement exceeds the residual voltage. The arrangement must be designed

considering the optimum number of arrester discs and the parameters of the slotted conduc-tor. (Burow et al. 2014)

6.4.1.4 Spiral tube damping busbar

This new application of spiral tube damping busbar has showed promising results in recent studies. The method is based on hollowing the conventional busbar to a spiral tube, and then the busbar conductor is turned into a series circuit with multi-turn hollow inductance coil and multiturn gap. This parallels the damping resistance with the spiral tube inductance cir-cuit, absorbing the transient energy. Main structure of the busbar consists of three parts, the spiral tube damping busbar, non-inductive damping resistors and an epoxy glass support to ensure the mechanical strength of the busbar. These parts are presented in the figure 6.12.

Figure 6.12 Spiral tube damping busbar (a) and an epoxy support with damping resistor (b). (Li et al.

2019)

Figure 6.13 The different parts of a spiral tube damping busbar assembled. (Li et al. 2019)

The damping busbar, structure illustrated in figure 6.13, does not affect current or voltage at rated operational frequency and will operate only at higher frequencies. Advantages of the damping busbar are that it is a relatively easy installation as it is formed based on the con-ventional busbar, and that it has no magnetic saturation problems. In tests, the suppressing effects of VFTO have been significant and the peak magnitude as well as the high frequency

components are reduced remarkably. Therefore, the results are very promising for the im-plementation of this novel method. (Li et al. 2019)

Transient enclosure voltages (TEV) effect can be minimized by keeping short and straight ground leads to minimize inductance, by increasing the number of connections to the ground, by installing voltage limiting arrestors with spacers or by applicating shielding to prevent internally generated transient voltages to reach the outside of the enclosure. Cable sheath earthing should be kept as short and as straight as possible to minimize the interference to control cables. (CIGRE 519 2012)