Assessment of Very Fast Transient Over voltage in GIS Substation through Physical Measurements and System Studies

Gas-Insulated Substations (GIS) pose significant challenges to insulation coordination, particularly at Extra High Voltage (EHV) levels. POWERGRID Corporation of India Limited operates a large fleet of 400 kV and 765 kV GIS substations across the Indian transmission network. In recent years, repeated operational issue related to insulation were experienced at selected 765 kV GIS substations, primarily during energization of bus reactors, interconnecting transformers (ICTs), and transmission lines.

This paper presents a comprehensive study combining on-site VFTO measurements with electromagnetic transient (EMT) simulations to identify the root causes of these issues. Based on the study outcomes and operational experience, mitigation measure including enhanced earthing, optimized switching sequences and marginal optimization of SF₆ gas pressure was implemented at GIS. The substation has been operating reliably, with no further insulationrelated issues reported. The paper discusses the adopted VFTO reduction strategies and their effectiveness in enhancing the reliability and operational performance of EHV GIS installations.