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This article presents an overview of UHF PD measurements of a generator step-up transformer over a three-year period. Several PD sources are active in the transformer, and fast changes in the PD pattern and levels are observed. An approach for the evaluation of measurement data is introduced, and the long-term development of PD is discussed in the article. The aim is to discriminate different PD sources from the PRPD pattern and classify them in terms of their level and frequency of occurrence on a long-term basis. Thus, a classification of different sources and their severity is attempted.