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Three generator-step-up (GSU) transformers successively failed in a large pumped storage plant, resulting in a considerable economic loss for the utility. A comprehensive investigation was done to find out the possible failure causes. Several aspects are considered in this study, including switching transients resulting from routine switching operations, lightning overvoltages, and harmonics and spikes due to static frequency converter (SFC) operation. The study showed that circuit breaker and disconnector restriking may affect transformer insulation due to their frequent occurrence, gas insulated switching gear (GIS) side surge arresters provide inadequate lightning protection to GSU transformers, and SFC operation has little effect on transformer insulation. This paper presents the simulation results. Methods to lower the lightning transients at the transformer terminals were provided and implemented.