This contribution describes an evaluation method for phase resolved patterns of partial discharges over time. Therefore, their specific shape characterizes patterns in question. The method searches for these characteristics within PRPD patterns by comparing characterization and PRPD using 2-dimensional cross-correlation. Cross- correlation represents a measure for the similarity between different PRPD patterns. A use case determines the usability of the method in practice. The result is the time dependent occurrence of specific patterns, respectively specific PD sources. Long-term monitoring data of an aged generator step-up transformer with several PD sources of varying activity is used as PD dataset. PDs are recorded by the use of the electromagnetic PD measurement method, also known as the UHF-method.