Abstract: A method generates a fault signal that displays whether an internal transformer fault is present. According to the method, a differential current signal that indicates the difference between the primary current and the secondary current of the transformer, taking into consideration the conversion ratio of the transformer, is ascertained. A plurality of different criteria signals are generated using the differential current signal. At least two individual fuzzy matching functions are assigned to each criteria signal. The fuzzy matching functions are analyzed, thereby generating the fault signal.