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Review

A Review of Research Progress in Very Fast Transient Overvoltage (VFTO) Suppression Technology

by
Huan Wang
1,2,
Yinglong Diao
2,
Xixiu Wu
3,* and
Bolun Du
2
1
School of Electrical and Electronic Engineering, Hua Zhong University of Science and Technology (HUST), Wuhan 430074, China
2
China Electric Power Research Institute Co., Ltd., Wuhan 430074, China
3
School of Automation, Wuhan University of Technology, Wuhan 430070, China
*
Author to whom correspondence should be addressed.
Energies 2025, 18(9), 2147; https://doi.org/10.3390/en18092147
Submission received: 9 March 2025 / Revised: 11 April 2025 / Accepted: 16 April 2025 / Published: 22 April 2025
(This article belongs to the Section F: Electrical Engineering)
Browse Figures
Versions Notes

Abstract

:
The very fast transient overvoltage (VFTO) is characterized by steep wavefronts, high amplitude, and wide spectrum. These characteristics can lead to partial discharges or insulation breakdowns in potted insulators, which can lead to localized overheating or turn-to-turn insulation damage in transformers. The strong electromagnetic radiation generated by VFTO may also interfere with relay protection and communication systems, triggering risks to grid operation. This paper introduces the existing mainstream VFTO suppression methods, such as damping resistance method, disconnecting switch operation method, inductive method, etc., from the aspects of circuit structure, parameter design and suppression effect, and focuses on the influence of the material selection, parameter adjustment, and mounting structure adjustment of ferrite ring on VFTO suppression effect. In addition, the lightning arrester method and the method of utilizing overhead lines for VFTO suppression are also briefly discussed. The article concludes with a comparison of the advantages and disadvantages of each method and an outlook on the future research direction of VFTO suppression technology.

1. Introduction

A gas insulated substation (GIS) greatly reduces the floor space because all the equipment is enclosed in the grounded metal shell, eliminating the wire connection between the equipment and the corresponding insulation distance in the traditional open substation. This feature makes GIS especially suitable for use in urban centers, industrial zones, and other areas with tight land resources, which can effectively save land resources and reduce the cost of substation construction, and is therefore widely used [1,2]. In recent years, with the increasing demand for electric power and the increasing voltage level of electric power equipment, the application of high-voltage level GIS has become more and more widespread [3,4]. For a GIS, its insulation fitness level is determined by very fast transient overvoltage (VFTO). It has been shown in the literature that the amplitude of VFTO in a GIS can be up to 2.5~3 times of the rated operating voltage, and the frequency is high and the bandwidth is wide, giving rise to a great threat to the safety of the GIS body and the surrounding equipment. VFTO arises from the operation of disconnecting switches in a GIS. The disconnecting switch is not equipped with an arc extinguishing device, and its operation action process generates multiple ignition arcs. The generated VFTO will propagate on the busbar, forming a fold-reflection waveform as shown in Figure 1. Figure 2 shows a 1000 kV substation during circuit breaker operation, where VFTO propagation through the cable caused a discharge in its enclosure, resulting in insulation damage to the disconnect switch, leading to a breakdown of the electromagnetic lock and tripping of the grounded knife gate.
The hazards of VFTO are mainly reflected in the following aspects:
  • High amplitude and steep wavefront cause damage to equipment insulation. The higher the overvoltage amplitude, the steeper the wavefront, the more powerful the local electric field distortion of the equipment, prone to local breakdown [5,6,7]. As the voltage level increases, the insulation margin of electrical equipment is relatively reduced, and the VFTO amplitude increases with the increase in voltage level, so the higher the voltage level, the higher the probability of insulation failure caused by VFTO.
  • Folding and reflection propagation effect. VFTO in the propagation process of folding and reflection will produce a new electromagnetic transient phenomenon, causing the GIS external insulation damage. VFTO in the folding and reflection process will produce transient enclosure voltage (TEV). Some statistics show that 50% of GIS accidents are caused by TEV [8].
  • High-frequency and wide-band radiation electromagnetic interference effects. VFTO has a high frequency, wide bandwidth characteristics; these instantaneous ups and downs of high-frequency signals will be propagated in various forms, such as through the GIS basin insulator gap to the outside radiation, or through the casing in the form of the casing antenna to the GIS space radiation, enhance the GIS external space transient electromagnetic field, and interfere with the normal operation of the substation’s control, protection, and automation of the secondary equipment, such as the normal operation of the substation [9,10,11,12,13].
  • Invasion effect on transformer. When VFTO invades the transformer winding, it will oscillate and propagate in the winding, forming a very unevenly distributed voltage along the winding. The steepness of the VFTO wave is larger than the lightning surge voltage, so the uneven distribution of the VFTO steep wave voltage along the transformer winding is more complex and harmful than the lightning surge voltage [14,15]. In addition, VFTO contains several megahertz oscillation frequency, and part of the transformer winding natural frequency overlap, causing the winding local electromagnetic resonance.
To summarize, VFTO can directly damage the insulation of the equipment through its own high amplitude and steepness, and can also interfere with the normal operation of the substation through the derivative effect propagated by VFTO. At the same time, VFTO can also form a higher amplitude and steeper overvoltage in the transformer winding, which jeopardizes the insulation of the transformer winding. Therefore, it is of great significance to carry out VFTO suppression research to improve the insulation coordination level of GIS to ensure the safe and stable operation of power systems.
This paper first analyzes the hazards of VFTO on power equipment and its suppression necessity. Subsequently, the principles, effects, and limitations of VFTO suppression techniques such as the damping resistance method, the disconnecting switch operation method, the inductive method, the surge arrester method, and the overhead line method are systematically introduced. Through the analysis of this paper, it can be seen that the damping resistor method is the most effective but the technology is dependent on imports, the magnetic ring method is autonomous but there is a saturation problem, and the lightning arrester method is low-cost but the mechanism is not yet clear. The purpose of this paper is to provide technical support for finding better VFTO suppression methods, developing efficient and stable localized VFTO suppression schemes, and guaranteeing the safe operation of power grids.

2. Current Status of Research VFTO Inhibition in China and Abroad

2.1. Damping Resistance Method

The use of resistance to dissipate the energy generated by switching operations is the most common method of suppressing over voltage. Switching operations change the power system from one state to another, and overvoltage is a manifestation of the energy transformation between these two states. Overvoltage can be suppressed by installing a resistor to dissipate the energy from the state transition.
The resistance used to dissipate energy is called damping resistance. The mechanism of VFTO generation shows that VFTO is caused by the repeated breakdown discharge of SF6 inside the gas chamber during the operation of the disconnecting switch in the GIS. Utilizing this principle, Toshiba Corporation of Japan proposes to add on–off GIS disconnecting switches to the ports, damping resistors for VFTO suppression. The specific installation location is shown in Figure 3, which is next to the static contacts of the disconnecting switch and connected in parallel with the static contacts [16].
When the disconnecting switch is closed, the movable contact and the damping resistor are first contacted and discharged, and the energy is consumed by the damping resistor. When breaking, the arc discharge between the movable contact and the static contact is stretched with the breaking distance. When the arc contacts the damping resistor, the arc discharge energy is transferred between the movable contact and the damping busbar, and the damping resistor absorbs the SF6 discharge energy and accelerates the SF6 arc extinguishing. It can be seen from the isolation switch closing and breaking process, after installing damping resistor, the isolation switch closing and breaking operation absorbs and consumes SF6 arc discharge energy through the damping resistor. On the one hand, the voltage difference between contacts reduces the voltage wave steepness; on the other hand, the existence of damping resistor leads to SF6 rekindle difficulty, reduces the number of SF6 rekindle times, makes the number of VFTO folding and reflecting reduced, and finally reduces the VFTO amplitude and frequency.
Tokyo Electric Power Company and Toshiba Corporation in Japan added damping resistors to the isolation switches of the 1000 kV system, and later calculated the key parameters of VFTO. The results show that the resistor resistance value around 1000 Ω can suppress VFTO below 1.6 p.u. [17]. In the paper [18], a damping resistor was added to the isolation switch of a 500 kV system, and the adjusting resistance was increased from 10 Ω to 1000 Ω. It was found through simulation that the amplitude and wavefront steepness of VFTO decreased with the increase of the resistor value, and when the resistor value was 500 Ω, the amplitude of VFTO was reduced by 45.4% and the wavefront steepness was reduced by 79%. In 2007, the State Grid Corporation of China calculated the 1000 kV Jindong Southeast GIS isolation switch retrofitted with damping resistor, and the results show that the amplitude of VFTO can be reduced to 1.6 p.u. by using a 500 Ω damping resistor. The actual measurement of VFTO after the operation of this substation found that the amplitude was reduced to 1.29 p.u., and the frequency was less than 1 MHz [19]. Both the simulation and the measured results show that VFTO can be suppressed by adding a damping resistor with a resistance value of (100–500) Ω to the isolation switch.
The effect of adding damping resistors to GIS isolation switch to inhibit VFTO is remarkable, but there are still shortcomings:
  • The addition of damping resistors leads to a more complex structure of isolation switch, which enhances the failure rate of the equipment.
  • In order to add damping resistors, isolation switches can only be vertically mounted, which increases the mounting area, and is not conducive to the compactness and miniaturization of the equipment.
  • The material formula of damping resistors as the core technology is in the hands of the Japanese.
The material formula of damping resistor as the core technology is in the hands of the Japanese, which is impossible to break through. All the damping resistors in China’s GIS projects are produced by Japan, and the installation is also completed by Japanese technicians. Under the background of localization and independent development of intellectual property rights, adding damping resistors is not the first choice of VFTO suppression method in China.

2.2. Disconnecting Switch Operation Method

The magnitude of VFTO is mainly determined by the breakdown voltage between the movable and static contacts of the disconnecting switch, and the operation process of the disconnecting switch is an important factor affecting the breakdown voltage. Some scholars have proposed to inhibit VFTO by utilizing the operation of the disconnecting switch, including changing the operating speed of the disconnecting switch, controlling the phase angle of the disconnecting switch, and changing the mode of operation of the disconnecting switch.

2.2.1. Changing the Operating Speed of the Disconnecting Switch

The speed of the disconnecting switch can change the SF6 gas ignition time and the residual voltage at the port of the disconnecting switch, thus reducing the number of SF6 gas breakdowns and the VFTO amplitude.
When the switch action speed division was isolated and closing time reduced, SF6 gas re-ignition arc number reduced, the number of breakdowns decreased. Isolated switch action speed slow time division and closing time increases, the leakage increases, the voltage difference between the isolated switch ports decreases, so that the VFTO amplitude decreases. The Swiss ABB measured the residual voltage generated by the slow-action isolation switch through experiments, and found that 99% of the residual voltage is around 0.4 p.u. The above two methods in terms of re-ignition respectively of residual charge analyze opening and closing speed on the suppression effect of VFTO number and amplitude [20,21]. The China Electric Power Research Institute investigated the effect of the opening and closing speed of the disconnecting switch on VFTO by simulation, and the maximum VFTO amplitude versus speed curve when the disconnecting switch is opened is plotted as shown in Figure 4, while the maximum VFTO versus speed curve when the disconnecting switch is closed is plotted as shown in Figure 5.
According to the curves in Figure 4 and Figure 5, it can be seen that the operating speed of the disconnecting switch has a large influence on VFTO. When the speed of the disconnecting switch is reduced to below 0.8 m/s, the slower the operation speed, the lower the maximum VFTO generated during operation. As the speed reaches or exceeds about 0.8 m/s, the maximum VFTO generated during disconnect switch operation approaches the maximum possible value of 2.4 p.u. for this circuit [22]. According to the study of the paper [23], it is recommended that the tripping speed of the disconnectors manufactured by Pinggao Company should be optimized to (0.7–0.9) m/s, and the operating speed of the disconnectors manufactured by Xikai Company should be optimized to (0.3–0.4) m/s. Optimized disconnecting switch speeds can reduce the VFTO amplitude by 10%.
However, there are shortcomings in changing the operating speed of the disconnecting switch:
  • The VFTO suppression effect is limited. The optimum suppression effect can only reduce the VFTO amplitude by 10%.
  • There is an irreconcilable contradiction between changing the operating speed of the disconnecting switch between the amplitude and the number of breakdowns, and the residual voltage.
  • The requirements for the speed control system and the control strategy are extremely high, and there is a lack of research on this technology
  • The optimum operating speed of disconnecting switches from different manufacturers and batches varies significantly, and a large amount of research is needed to determine the optimum operating speed.

2.2.2. Controlling the Phase Angle of the Disconnecting Switches

There are fewer studies on controlling the phase angle of disconnecting switch opening and closing to suppress VFTO. The paper [24] utilized a three-dimensional simulation model to calculate the capacitance parameters of a single-phase system with different opening distances of the disconnecting switch, as shown in Figure 6.
The relationship between the breakdown capacitance and opening distance of the isolating switch of the 252 kV GIS test platform is obtained from the capacitance parameter curve, and the fitting equation is:
U = 285 + 22.8 d + 0.429 d 2   ( d ≤ − 9 ) U = 175 + 12.8 d + 0.728 d 2   ( d > − 9 )
According to the relationship between breakdown voltage and the equations at different opening distances, the effects of different initial phase angles on the first breakdown voltage of closing and the last breakdown voltage of opening are analyzed, as shown in Figure 7.
According to the curve change in Figure 7, it can be seen that changing the initial phase angle has a certain inhibition effect on VFTO, whether it is the opening or closing of the gate, which verifies the effectiveness of the method. However, the inhibition mechanism of VFTO by changing the phase angle of the disconnecting switch opening and closing is not clear and requires an accurate control system, so the relevant research is still in the initial stage. Moreover, since the initial phase angle of each phase of the three-phase system is different, the possibility of modifying the operation mode of the control system needs to be achieved, so there are still technical difficulties in applying the method to the three-phase system.

2.2.3. Controlling the Mode of Operation of the Disconnecting Switch

VFTO overvoltage is closely related to the GIS disconnecting switch fit time and the installation position of the disconnecting switch in the GIS, as well as the operation wiring mode of the GIS. Therefore, some scholars have carried out research on the effect of different operating modes of isolation switches on VFTO in GIS. Xi’an Jiaotong University calculated the VFTO generated by the disconnecting switches in the GIS of Wujiang 550 kV hydropower station under three different operation modes. The calculation results show that operation mode 3 produces a maximum voltage gradient in the transformer windings that exceeds the permissible range, so operation mode 3 should be avoided in operation [25]. The authors had calculated the magnitude of VFTO amplitude under different operation modes in Huainan EHV GIS station, and selected the operation modes of double main transformer with double outlets (Type I), single main transformer with double outlets (Type II), and single main transformer with single outlets (Type III), and the results of the calculations are shown in Figure 8.
According to the curve in Figure 6, it can be seen that the maximum VFTO magnitude of 1.878 p.u. occurs in operation mode 1. The case in which the maximum VFTO amplitude is 1.787 p.u. occurs in operation mode 2. The maximum VFTO amplitude of 2.023 p.u. occurs in operation mode 3. It can be seen that the maximum overvoltage is generated in operation mode 3.
Changing the disconnecting switch operation algorithm shows that the choice of different operation methods can avoid the occurrence of excessive VFTO amplitudes. This approach is low-cost and highly operational, but different operation schemes are required for different substations. Moreover, even with the optimal operation, the maximum VFTO amplitude exceeds the insulation requirements, so that the effect of changing the disconnecting switch operation is not obvious.

2.3. Inductance Method

2.3.1. Ferrite Ring Method

The ferrite magnetic ring method is the most representative method to suppress VFTO by utilizing the inductive properties of power equipment or components. This method was proposed by Tsinghua University in 2002 [26,27], which utilizes the high-frequency inductive resistance effect and eddy current loss effect of ferrite magnetic ring to realize the suppression of VFTO. Under high frequency, the ferrite nonlinear permeability characteristic will produce eddy current and hysteresis loss, which will absorb and weaken the energy of VFTO and reduce the amplitude of VFTO. At the same time, the ferrite permeability at high frequencies will produce a strong inductive resistance effect. When the VFTO traveling wave passes through the ferrite ring, the wave head of VFTO will be flattened due to the inductive properties exhibited by the ferrite ring, which reduces the steepness of the wavefront of VFTO, and thus plays a role in suppressing VFTO. In order to make the VFTO wave pass through the ferrite ring well, Tsinghua University proposes to install the ferrite ring on the busbars at both ends of the GIS isolation switch, as shown in Figure 9.
The ferrite ring method is the most representative method to suppress VFTO by utilizing the inductive characteristics of power equipment or components, which is proposed by Tsinghua University. The method utilizes the high frequency inductive resistance effect and eddy current loss effect of ferrite magnetic ring to realize the suppression of VFTO. Tsinghua University has carried out a series of theoretical analyses, simulation tests, and simulation studies for the magnetic loop method [28,29,30]. In 2004, Tsinghua University investigated the magnetic saturation characteristics and VFTO suppression effect of magnetic rings of three materials, namely, ferrite, metal magnetic powder core, and amorphous core, through simulation tests. The results show that metal magnetic material is the best, amorphous core is the second best, and ferrite is the worst. In addition, Tsinghua University tested the VFTO suppression effect of R2KB ferrite magnetic ring and FJ37 amorphous magnetic ring with different lengths, and the results are shown in Table 1.
According to the data in Table 1, it can be seen that there is no significant change in the suppression effect of the amplitude with the increase in the length of the magnetic ring, while the rise time increases with the increase in the length of the magnetic ring. When the magnetic ring is larger than 1.0 m, the suppression effect of ferrite magnetic ring is better than amorphous magnetic ring. However, the ferrite material saturates more easily as the voltage level increases. The experiment verifies the conclusion that the VFTO suppression effect increases with the increase of the length of the magnetic ring.
In addition to Tsinghua, many scholars have also carried out related technical research, such as optimizing the design of magnetic ring size, analyzing the influence of the position of the magnetic ring on the suppression effect of VFTO, and adopting the method of using small magnetic ring to add nanocrystalline large magnetic ring which is not easy to be magnetically saturated [31,32,33,34].
Analyzed from a circuit point of view, the role of the ferrite ring can be equated to an inductor and a resistor in parallel. At industrial frequency (50 Hz), the inductive and resistive properties exhibited by the magnetic ring have a negligible effect on the system. At high frequency (MHz), the magnetic ring shows large inductive resistance characteristics, so that the bus high-frequency circuit parameters change, increasing the traveling wave energy loss at the same time, preventing the number of folding and reflecting VFTO, to achieve the suppression of VFTO. In order to further study the mechanism of VFTO suppression by ferrite magnetic ring, domestic research institutes have carried out the study of equivalent circuit of ferrite magnetic ring, proposed the high-order L-R magnetic ring model shown in Figure 10, the study of the selection of material parameters of ferrite magnetic ring, and the validation test of ferrite magnetic ring for VFTO suppression in the extra-high-voltage GIS test circuit. The test shows that when the length of the magnetic ring string is >0.5 m, its suppression effect on the voltage traveling wave amplitude will no longer change significantly with the increase of the length of the magnetic ring string, but the traveling wave rise time will increase with the increase of the length of the magnetic ring string. When the string length is ≥1 m, the combined suppression effect of the ferrite ring on voltage traveling wave is better than that of the amorphous ring. This shows that the method of using magnetic rings to suppress VFTO in UHV GIS is feasible in principle.
International research on the magnetic ring method to inhibit VFTO research is mainly ABB. The ABB’s research on the magnetic ring method is more systematic, not only to carry out research on the principle of the magnetic ring method, but also to carry out research on the production process and material selection of the magnetic ring. It has used nanotechnology to make nanocrystalline magnetic rings, which are made of iron-based nanocrystals with silicon, boron, and other additives, transformed the amorphous structure into the desired nanocrystalline state by heat treatment, and conducted experiments on the suppression of VFTO for such rings with different number of rings, materials, and sizes. Some of the ABB experimental results are plotted in Figure 11. The ABB experiments show that:
  • The damping effect increases linearly with the increase in the number of rings, and the amplitude of the VFTO decreases by about 20% when the number of rings is 8.
  • Rings with a higher relative permeability have a better damping effect.
  • Since the magnetic flux of the ring decreases with the increase in the radius of the ring, the suppression of the ring with a larger diameter is not as effective.
The magnetic ring method of ABB and the ferrite ring method of Tsinghua University both utilize the nonlinear inductance and resistance generated by the magnetic ring’s magnetic conductivity at high frequencies to suppress the VFTO, but there are differences in the way the magnetic rings are installed. The magnetic ring method of Tsinghua University is to install the magnetic ring outside the bus conductor, and the ABB is to install the magnetic ring inside the GIS main conductor, as shown in Figure 12.
The difference between the two methods of installation leads to a large difference in the selection of the ring material and the design of the ring structure, which is ultimately reflected in the equivalent circuit having a significant difference. Tsinghua’s magnetic ring method of equivalent circuit for the higher-order equivalent circuit, while for ABB magnetic ring method equivalent circuit core idea is to establish a distributed parametric circuit; each ring is equivalent to the resistance and inductance of the parallel first-order circuit model. A number of magnetic rings in series can be equated to a number of first-order equivalent circuit of the series, that is, constituting a distributed parametric equivalent circuit, as shown in Figure 13.
The ABB magnetic ring method installs the magnetic ring inside the busbar, which is more complicated to install and more difficult to realize in engineering compared to Tsinghua’s magnetic ring method, and the maintenance is even more difficult in the later stage, which is difficult to be promoted to the engineering application.
Although a great deal of research has been done on the magnetic ring method, the easy saturation characteristics of the magnetic ring have seriously restricted the development of the method. For this reason, domestic scholars have taken many measures to improve the saturation characteristics of magnetic rings. The paper [35] proposed a VFTO suppression method for the combination of small and large magnetic rings. By conducting a series of experiments on the baseline group without magnetic ring, a 40 cm small magnetic ring group, an 80 cm small magnetic ring group, and a 80 cm + 10 cm combination of multiple magnetic rings, it is verified that the combination of magnetic ring structure can provide better VFTO suppression effect. However, the saturation characteristics of the rings are still the primary factor limiting the improvement of the suppression effect.

2.3.2. RF Detuner

The HF resonator is added to the outside of the busbar and consists of a long cavity (L) and a narrow gap (C) at the left end, and its resonant frequency can be expressed as:
f L C = 1 2 π L C
The damping capability of the HF resonator for the VFTO relies on the matching of its resonant frequency FLC with the main frequency component of the VFTO, and the addition of the collector resistor in the gap of the resonator to realize it [36]. The paper [37] verified the damping effect of this method by comparing simulation and measurement, as shown in Figure 14. The addition of the HF resonator is verified by experiments so that the energy of the VFT wave is reduced by 27%, and the VFT main frequency is reduced by 60%, but at the same time, the first peak of the VFTO is slightly increased, instead of increasing its harmonic.
Due to the extremely high matching requirements between the resonant frequency and the VFTO frequency, the structural design of the HF resonator is extremely demanding, and the structural dimensions of the GIS bus conductor need to be specially treated. Currently, the design of HF resonator only stays on the study of the extraction method of resonance frequency, including the use of time and frequency domain methods based on the full Maxwell’s equations, and the overall structural design of HF resonator has not yet been studied.

2.4. Lightning Arrester Method

A metal oxide arrester (MOA) has excellent nonlinear volt-ampere characteristics and steep-wave response characteristics, so the addition of a MOA is a good overvoltage protection device. The lightning arrester method utilizes the nonlinear resistive property of a MOA under impact to suppress VFTO, so the research of the lightning arrester method focuses on the establishment of the equivalent model of a MOA. The International Conference on Large Grids Organization (CIGRE), Toshiba Corporation of Japan, and the Institute of Electrical and Electronics Engineers (IEEE) have proposed different MOA models through experimental validation [38,39,40,41]. A MOA model is shown in Figure 15.
Xi’an Jiaotong University discussed the MOA model under steep wave impact [41], and concluded that the dynamic voltammetry and the time-delay characteristics of the grain boundary layer must be taken into account when establishing the equivalent model of MOA. Meanwhile, Xi’an Jiaotong University compared the simulated and measured values of the IEEE recommended model and the PINCETI model [42], and concluded that both models have high computational accuracy. The paper [43] summarized four MOA equivalent models: capacitance to ground, nonlinear resistance, nonlinear resistance considering the body capacitance and stray capacitance of the valve sheet, and MOA high-frequency transient model, as shown in Figure 16.
Shenyang University of Technology analyzed the VFTO at 1100 kV voltage level based on this model and showed that the amplitude of VFTO oscillation was continuously reduced to 0.8 p.u. after adding MOA [44]. Xihua University determined the optimal location of MOA by analyzing the relationship between the main frequency and amplitude of VFTO and the location of MOA [45]. The China Electric Power Research Institute (CEPRI) obtained a 20% increase in the residual voltage of MOA valves under VFTO compared to the lightning residual voltage through experimental studies and showed that its parameters have a great influence on the suppression effect by calculating the VFTO characteristics of MOA installed in a 252 kV GIS.
A metal oxide arrester has simple structure with small size and large current throughput capacity, and the retrofitting of surge arrester does not need to make changes to the GIS structure, which is a low-cost and easy to realize method. However, the nonlinear impedance characteristics of MOA are related to the excitation, electromagnetic field around the surge arrester and the residual voltage under the inrush current, and the suppression mechanism is not yet very clear. The suppression effect of surge arrester on VFTO is mainly focused on the amplitude, and the suppression effect on the wavefront steepness has not been discussed yet.

2.5. Overhead Line Method

The transformer is the equipment most affected by VFTO. Researchers and scholars have proposed that an overhead line be installed between the GIS outlet casing and the main transformer. This method is equivalent to stringing a large inductor in the front end of the main transformer, using the inductance and the additional capacitance generated by the impact corona of the overhead line to flatten the wavefront steepness of the VFTO [46,47,48]. Zhejiang University explored the influence of overhead lines on VFTO wavefront steepness in a 500 kV substation through low-voltage tests, and the simulated test circuit is shown in Figure 17 [49]. The relationship between the length of the overhead line and the VFTO wavefront steepness is shown in Figure 18.
According to the curve in Figure 18, it can be seen that the wavefront steepness of VFTO decreases with the growth of the overhead line, but when the length of the overhead line grows to a certain length, the decreasing trend tends to level off. Changsha University of Science and Technology establishes a 550 kV GIS simulation model under the condition of corona and inductance of the overhead line, and the simulation results show that a 20 m overhead line can suppress the wavefront steepness to within the standard requirements [50]. On this basis, the Power Science Research Institute of State Grid Zhejiang Electric Power Co., Ltd (Hangzhou, China) simulates and analyzes with the real parameters of a pumped storage power plant, and finds that cables and overhead lines have a certain suppression effect on the amplitude and steepness of VFTO [51], but when the cable length is greater than 250 m and the overhead line is more than 4m, the transient attenuation is weakened, and the decline tends to be leveled off.
In summary, in terms of suppression mechanism, both the resistance method and the disconnecting switch operation method start from the VFTO generation mechanism and realize the purpose of VFTO suppression by influencing the SF6 gas discharge process [52,53,54,55]. Inductance method, MOA, installation of overhead lines, and other methods are used to change the circuit parameters on the VFTO propagation path, change the VFTO folding and reflecting impedance, and inhibit the propagation of VFTO.

3. Suppression Effect Comparison

The resistor method suppresses the generation and propagation of VFTO by absorbing the SF6 gas discharge energy. A resistance value of 500 Ω can reduce the VFTO amplitude by 45.4% and the wavefront steepness by 79%. Tests show that in a 1000 kV power system, the resistor method can control the amplitude of VFTO within 1.29 p.u. and the main frequency within 1 MHz. The advantage of the resistor method is that the suppression effect is significant in three aspects: amplitude, steepness, and frequency. The disadvantages of the resistor method are that it leads to a more complicated structure of the disconnecting switch, which increases the failure rate of the disconnecting switch, and the resistor needs to be installed vertically, which occupies a large amount of space. In addition, the core technology relies on Japan.
The disconnecting switch operating method is used to achieve a reduction in the number of SF6 gas breakdowns by adjusting the operating speed, phase angle, or mode of operation. The amplitude of VFTO can be reduced by 10% when the isolating switch breaking speed is 0.3–0.9 m/s. Adjustment of the initial phase angle can also reduce the VFTO amplitude, but with limited effect. By changing the isolating switch operation method, the amplitude of the VFTO can be suppressed to within 2 p.u. under optimum conditions. The advantage of the isolated switch operation method is that it is low cost and requires no additional equipment. Its disadvantages are limited suppression (amplitude reduction of only 10%), difficulty in controlling the three-phase system, and poor generalizability.
The inductive method utilizes the inductive and eddy current losses of the magnetic ring at high frequencies to achieve VFTO suppression. A ferrite ring with a length of 1 m can reduce the VFTO amplitude from 1.83 p.u. to 1.0 p.u., and suppress the wavefront steepness from 10.4 kV/ns to 5.04 kV/ns. The magnitude of amorphous material is better than that of ferrite material, but it has the disadvantage of easy saturation. The advantage of the inductive method is that it is an autonomous Chinese technology with comprehensive suppression effects (amplitude and steepness). The disadvantages of the inductive method are that the magnetic ring is easily saturated, complicated installation, and difficult maintenance.
The lightning arrester method utilizes the nonlinear resistive properties of the MOA to achieve suppression of the VFTO amplitude. The surge arrester method reduces the VFTO magnitude to 0.8 p.u. in 1100 kV systems. The lightning arrester method is effective only for nearby equipment and does not provide significant steepness suppression.
The overhead line method flattens the steepness of the VFTO wavefront through inductive and corona effects. The 20 m lengths of overhead line in combination with long cables (>250 m) suppress the steepness of the VFTO to within the standard requirements. The overhead line method has the advantage of no additional equipment and is suitable for transformer protection. Its disadvantages are poor amplitude suppression, low practicality, and saturation of the effect as the length increases.
The comparison of several common VFTO suppression methods mentioned above is shown in Table 2.
According to the information in Table 2, it can be seen that among the existing VFTO suppression methods, the damping resistive method and the inductive method have better suppression effects. However, the resistive method is limited by foreign technology and needs to solve the saturation problem. From an economic point of view, the arrester method and the disconnecting switch operation method have the lowest cost for VFTO suppression, but their suppression effect should not be. From the engineering applicability point of view, the inductor method and the lightning arrester method are more suitable for promotion, but the actual operation needs to be combined with specific scenarios to carry out parameter optimization.

4. Discussion

This paper analyzes the VFTO suppression techniques, such as damping resistance method, isolation switch operation method, inductive method, and other related contents. Among them, the damping resistance method reduces VFTO amplitude and frequency by absorbing SF6 arc discharge energy, but there are problems such as increasing equipment complexity and core technology constraints; the disconnecting switch operation method inhibits VFTO by changing the operation speed, controlling the phase angle of opening and closing the gate, or optimizing the operation mode, but the inhibition effect is limited and the technical requirements are high; the ferrite toroidal method in the inductive method makes use of high-frequency susceptibility and eddy current loss to weaken VFTO energy, but the easily saturated characteristics of magnetic materials limit its engineering application. In the inductive method, ferrite magnetic ring method utilizes high-frequency inductive impedance and eddy current loss to weaken the VFTO energy, but the easily saturated characteristics of magnetic materials limit its engineering application. This paper summarizes the inhibition principle, model structure characteristics, and inhibition effect of various methods, which provides a theoretical basis for VFTO inhibition methods. Facing the threat of VFTO to the operation safety of power equipment, composite suppression methods can be used, such as adding damping resistors to the isolation switch ports, and optimizing the isolation switch operating speed and phase angle, improving the suppression effect through a variety of synergistic methods and reducing the dependence on a single technology, to provide a more effective guarantee for the safe operation of power equipment.

Funding

This research was funded by the National Natural Science Foundation of China grant number U1866201.

Conflicts of Interest

Authors Huan Wang, Yinglong Diao and Bolun Du were employed by the China Electric Power Research Institute Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Schematic diagram of refractive–reflective voltage traveling wave interaction.
Figure 1. Schematic diagram of refractive–reflective voltage traveling wave interaction.
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Figure 2. VFTO invasion of disconnect switches in GIS.
Figure 2. VFTO invasion of disconnect switches in GIS.
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Figure 3. Schematic diagram of isolation switch retrofitted with breaking resistance.
Figure 3. Schematic diagram of isolation switch retrofitted with breaking resistance.
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Figure 4. Maximum VFTO amplitude vs. speed at time of breaking the gate.
Figure 4. Maximum VFTO amplitude vs. speed at time of breaking the gate.
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Figure 5. Maximum VFTO vs. speed at closing of the gate.
Figure 5. Maximum VFTO vs. speed at closing of the gate.
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Figure 6. Capacitance parameters of isolation switch at different opening distances.
Figure 6. Capacitance parameters of isolation switch at different opening distances.
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Figure 7. Breakdown voltage at different initial phase angles for unloaded line without residual charge. (a) Trend of first breakdown voltage at time of closing, (b) Trend of last breakdown voltage at time of tripping.
Figure 7. Breakdown voltage at different initial phase angles for unloaded line without residual charge. (a) Trend of first breakdown voltage at time of closing, (b) Trend of last breakdown voltage at time of tripping.
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Figure 8. Effect of different operating modes on VFTO. (a) VFTO Amplitude for Type I Operation, (b) VFTO Amplitude for Type II Operation, (c) VFTO Amplitude for Type III Operation.
Figure 8. Effect of different operating modes on VFTO. (a) VFTO Amplitude for Type I Operation, (b) VFTO Amplitude for Type II Operation, (c) VFTO Amplitude for Type III Operation.
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Figure 9. Schematic diagram of magnet ring mounting position.
Figure 9. Schematic diagram of magnet ring mounting position.
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Figure 10. Model of different order circuits of magnetic ring. (a) First-order model, (b) Second-order model, (c) High order model.
Figure 10. Model of different order circuits of magnetic ring. (a) First-order model, (b) Second-order model, (c) High order model.
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Figure 11. VFTO suppression effect of magnetic ring. (a) Magnetic rings with different number of rings, (b) Different magnetic permeabilities.
Figure 11. VFTO suppression effect of magnetic ring. (a) Magnetic rings with different number of rings, (b) Different magnetic permeabilities.
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Figure 12. Magnetic ring mounting position at ABB.
Figure 12. Magnetic ring mounting position at ABB.
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Figure 13. Equivalent circuit model of Tsinghua’s magnetic ring ABB.
Figure 13. Equivalent circuit model of Tsinghua’s magnetic ring ABB.
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Figure 14. Damping capacity of resonator HF. (a) Magnitude suppression effect, (b) Frequency suppression effect.
Figure 14. Damping capacity of resonator HF. (a) Magnitude suppression effect, (b) Frequency suppression effect.
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Figure 15. MOA model proposed by IEEE.
Figure 15. MOA model proposed by IEEE.
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Figure 16. MOA model. (a) Capacitance to ground, (b) Non-linear resistance, (c) Nonlinear resistance considering valve sheet capacitance and stray capacitance, (d) MOA transient model.
Figure 16. MOA model. (a) Capacitance to ground, (b) Non-linear resistance, (c) Nonlinear resistance considering valve sheet capacitance and stray capacitance, (d) MOA transient model.
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Figure 17. Simulation test circuit.
Figure 17. Simulation test circuit.
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Figure 18. Overhead line length vs. VFTO wavefront steepness.
Figure 18. Overhead line length vs. VFTO wavefront steepness.
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Table 1. Suppression effect of magnetic rings of different lengths.
Table 1. Suppression effect of magnetic rings of different lengths.
Toroidal MaterialsLength of Magnetic Ring/mTest Voltage Level
50 kV75 kV100 kV
End Voltage MultiplierRise Time/nsEnd Voltage MultiplierRise Time/nsEnd Voltage MultiplierRise Time/ns
Ferrite Ring0.21.312.51.412.41.518.0
0.51.180.01.150.01.130.7
1.01.1120.01.190.01.175.0
2.01.1170.01.1140.01.1125.0
Amorphous Magnetic Ring0.21.420.01.422.01.524.0
0.51.316.01.326.01.328.0
1.01.350.01.352.01.252.0
2.01.472.01.470.01.372.0
Table 2. Comparison of common VFTO suppression methods.
Table 2. Comparison of common VFTO suppression methods.
MethodAmplitude Suppression EffectSteepness Suppression EffectFrequency Suppression EffectAdvantageDisadvantage
Damping resistance method45.4% (500 Ω)79% (500 Ω)remarkablecomprehensive effectcomplex structure, technology dependent on imports
Disconnecting switch operation method≤10% (speed optimization)No datanoinexpensivelimited effectiveness, complexity of control
Inductance method1.83→1.0 p.u. (ferrite)10.4→5.04 kV/nsremarkableamplitude and steepness can be taken into accounteasily saturated, difficult to install and maintain
lightning arrester methoddrop to 0.8 p.u. (1100 kV)Inconspicuousnoeasy to implementpoor steepness suppression, mechanism unclear
Overhead line methodineffective (requires long cable assistance)20 m overhead line with 250 m cablenono additional equipmentpoor amplitude suppression and limitations on overhead line lengths
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Wang, H.; Diao, Y.; Wu, X.; Du, B. A Review of Research Progress in Very Fast Transient Overvoltage (VFTO) Suppression Technology. Energies 2025, 18, 2147. https://doi.org/10.3390/en18092147

AMA Style

Wang H, Diao Y, Wu X, Du B. A Review of Research Progress in Very Fast Transient Overvoltage (VFTO) Suppression Technology. Energies. 2025; 18(9):2147. https://doi.org/10.3390/en18092147

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Wang, Huan, Yinglong Diao, Xixiu Wu, and Bolun Du. 2025. "A Review of Research Progress in Very Fast Transient Overvoltage (VFTO) Suppression Technology" Energies 18, no. 9: 2147. https://doi.org/10.3390/en18092147

APA Style

Wang, H., Diao, Y., Wu, X., & Du, B. (2025). A Review of Research Progress in Very Fast Transient Overvoltage (VFTO) Suppression Technology. Energies, 18(9), 2147. https://doi.org/10.3390/en18092147

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