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.