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Disconnector switching operations in large gas- insulated switchgear (GIS) systems may cause very fast transients oscillations (VFTO). These VFTO surges produce dielectric stress to other apparatus of the substation. This paper describes the test technique for measuring VFTO waves straight at the high voltage entry of a 550 kV/ 840 MVA generator step- up transformer. Additionally, the signal processing for elimination the test circuit transfer effect is explained. Repetitive tests identify the reliability of test circuit and test equipment. The characteristics of recorded VFTO impulses confirm the state-of- the art experience of other publications. Different case studies are applied for identifying the worst case configuration of busbar system regarding the effective VFTO stress for the generator transformer. Operating the closest disconnector while all busbar systems are interconnected causes the highest dielectric stress for the transformer entry.