Mitigation of VFTO from a Power Transformer bushing in a 275 kV Gas Insulated Substation

Very Fast Transient Overvoltage (VFTO) is a potential cause of transformer bushing failures in Gas Insulated Substations (GIS). Protecting the power transformer from this dangerous overvoltage becomes imperative for the equipment safety. This work presents the modelling and evaluation of different VFTO mitigation techniques on a 275 kV GIS using Electromagnetic Transient Program (EMTP-RV). Among the several mitigation techniques considered include Surge arrester (SA), Damping resistor (D-Resistor), Nanocrystalline rings (NC-Rings), Ferrite rings (F-rings), and Resistor-capacitor filters (RC-Filters). The analysis of results from the unprotected system revealed a peak voltage amplitude measuring approximately 2.28 pu of the busbar voltage, accompanied by a dominant frequency of 5.59 MHz, exerting considerable stress on the transformer bushing. The analysis of mitigation techniques reveals that despite efforts, the SA method still retains a dominant frequency of 1.39 MHz in its overvoltage components. D-Resistor exhibits a notable presence of 1.6 MHz components, posing a risk of imminent flashover. Conversely, F-Ring and RC-Filters effectively mitigate high amplitudes and frequencies, although with certain constraints. However, NC-Rings demonstrate superior performance by reducing peak overvoltage amplitudes by approximately 42.9 % and limiting high frequency components below 200 kHz. Consequently, the NC-Ring is a reliable technique for protecting the bushing.