Propagation and mitigation of Very Fast Transient Overvoltage in Gas Insulated Substation

Abstract: Very Fast Transients are surge overvoltages created for example when opening or closing a disconnector switch (DS). Because of the very high frequency stress they exert on the equipment and their magnitude (up to 3 pu), they constitute an important issue in the design of ultra-high voltage Gas Insulated Substations. This thesis offers a general understanding of Very Fast Transient Overvoltages (VFTO or VFT) and explores different mitigation techniques, mainly based on dissipation of the energy associated with it. Two solutions are mainly discussed for attenuating the VFTO: applying a semi resistive and magnetic coating or using magnetic rings. Both rely on the non-linearity of skin effect and magnetic losses and are treated in similar manners. An electromagnetic model of both systems is proposed as well as a way to model the influence of the device on the transient behaviour of the system. The performances of both systems still have to be determined by experimentation but they both should significantly reduce the impact of the VFTO.