Transients at GSU transformer terminals. I. Historical case analysis

In a large pumped storage plant, three generator step-up (GSU) transformers successively failed in recent years and considerable economic loss is resulted for the utility. Therefore, a comprehensive investigation is done to find out the possible failure causes. Several aspects are considered in this study, including overall review of historical GSU transformer failures, modeling and simulation of switching transients resulting from various possible routine switching operations and lightning overvoltages that may affect the GSU transformer insulation, and modeling and simulation of harmonics and transients due to static frequency converter (SFC) operation that may be seen by the transformers. This paper extensively reviewed the historical GSU transformer failures, and discussed possible effects of switching transients, lightning, very fast transients (VFT), harmonics, on the transformers, some suggestions are given for the troubleshooting of the GSU transformer failures in the pumped storage plant. The detailed modeling and simulation are presented later in another paper.