Analysis of VFTO and Its Mitigation in a GIS through Additional Earthing Grid

Abstract

Very Fast Transient Overvoltages (VFTOs) are one of the major insulation stress phenomena encountered in Gas Insulated Substations (GIS). VFTOs are generated primarily during disconnector switching operations and are characterized by extremely steep wavefronts, high frequency oscillations, and short durations. These transients can cause insulation degradation, electromagnetic interference, transformer winding stresses, and flashover incidents in GIS installations. According to IEC classifications, VFTO frequencies typically range from tens of kilohertz to several tens of megahertz. This paper presents a practical case study from the GIS Installation, where an additional copper flat based earthing grid was installed to enhance grounding performance. Following the implementation of the supplementary earthing system, no flashover incidents were observed. An engineering analysis is presented to explain how the improved grounding system reduced the magnitude and impact of VFTOs. The advantages, limitations, and field observations are discussed, providing guidance for future GIS design and retrofitting projects.

Introduction

The text discusses Very Fast Transient Overvoltages (VFTOs) in Gas Insulated Substations (GIS) and describes a flashover incident that occurred at a 765 kV GIS installation.

Background

Gas Insulated Substations are widely used because they are compact, reliable, require relatively low maintenance, and use less land. However, switching operations involving disconnectors and earthing switches can produce VFTOs. These occur because of repeated prestrikes and restrikes between disconnector contacts, creating high-frequency transient waves that travel through the GIS.

VFTOs can reach approximately 1.5–2.0 per unit (p.u.) and may cause:

• Internal insulation stress and accelerated aging.
• Transformer winding insulation damage.
• Bushing failure.
• Flashover at weak insulation points.
• Electromagnetic interference with protection and control systems.

765 kV GIS Flashover Incident

The reported incident occurred at 00:19 on December 30, 2023, during the first-time charging of ICT#3 at a 765 kV GIS installation. During the closing of isolators, a flashover occurred in the 71189A R-phase disconnector/earthing-switch gas compartment.

The event resulted in the tripping of 765 kV Line #2 through distance protection, with a recorded fault current of approximately 13.9 kA. SO? was detected in several associated gas compartments, and burnt marks and powder residue were found inside the affected DS/ES chamber.

Corrective Action

The installation experienced concerns related to flashover and transient stresses. As a corrective measure, additional grounding conductors made of copper earth flats were connected to the existing earth mat. After this modification, subsequent operation showed improvement, with no further flashover occurring.

VFTO Generation Mechanism

VFTOs are generated during disconnector switching through a repeated sequence:

Changing contact gap → Arc/restrike → Steep travelling wave → Wave reflection/refraction → Superposition → Local overvoltage

As the disconnector contacts move, the gap changes and repeated arc restrikes occur. Each restrike generates a very steep-front travelling wave. These waves reflect and refract at discontinuities within the GIS, and their superposition can significantly increase local insulation stress.

Typical VFTO Characteristics

These high-frequency transients can also produce Transient Enclosure Voltages (TEV) and Transient Ground Potential Rise (TGPR).

Conclusion

A field assessment conducted at the 765KV GIS Installation indicates that the installation of an additional copper flat earthing grid significantly improved the grounding characteristics of the substation. The enhanced grounding reduced ground impedance, transient enclosure voltages, transient ground potential rise, and electromagnetic disturbances caused by Very Fast Transient Overvoltages. The most important practical observation was the absence of flashover incidents after commissioning of the supplementary grounding system. From both theoretical analysis and field evidence, it can be concluded that the improved earthing network provided an effective low impedance path for high frequency transient currents, thereby reducing insulation stress throughout the GIS. This case study demonstzates that enhanced grounding is a practical, economical, and effective mitigation technique for VFTO suppression in EHV GIS installations and should be considered as part of comprehensive insulation coordination and reliability improvement programs.

References

[1] IEC 60071, Insulation Coordination for High Voltage Systems. [2] J. P. Shah, \"Modeling and Analysis of Very Fast Transient Overvoltages in 400 kV GIS,\" ERDA Technical Paper. [3] M. A. Abd Allah et al., \"New Techniques for VFT Mitigation in GIS.\" [4] CIGRE B3_10542_2026, \"Assessment of Very Fast Transient Over Voltage in GIS Substation through Physical Measurements and System Studies.\" [5] R. Fu et al., \"Very Fast Transient Overvoltage Calculation and Evaluation for 500 kV GIS,\" IET Generation, Transmission & Distribution. [6] IEEE 1313.2, Guide for the Application of Insulation Coordination. [7] CIGRE Working Group B3, “Assessment of Very Fast Transient Over Voltage in GIS Substation Through Physical Measurements and System Studies,” CIGRE Session Paper B3 10542, 2026. [8] R. Fu, J. Wang, J. Wang, M. Zhou, C. Li, Y. Zhao, and Y. Fan, “Very Fast Transient Overvoltage Calculation and Evaluation for 500 kV Gas Insulated Substation with Double Circuit and Long GIS Busbar,” IET Generation, Transmission & Distribution, vol. 17, pp. 1 14, 2023

Copyright

Copyright © 2026 M Prabhakar, V Anil, I Chandramouli, Ch. Sreedhar, G. Srikanth. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.