Lightning overvoltage suppression of power transformer in UHV GIS by ferromagnetic ring

Lightning overvoltage protection is of great importance to the safety of UHV GIS. The widely used arresters can only suppress the magnitude of lightning overvoltage but lack efficiency on alleviating the steepness. In practice, insulation failure of UHV transformer is often reported. A new protection method is proposed in this paper using ferromagnetic ring as supplement to arresters to suppress lightning overvoltage in UHV GIS. Moreover, equivalent model based on the characteristic of ferromagnetic material and factual dimension of the ring is established for calculation. Simulation of an actual 1000kV GIS is carried out to thoroughly evaluate the lightning overvoltage suppressing effect of ferromagnetic rings with different materials and structure dimensions.