This paper analyses partial discharges obtained by remote radiometric measurements from a power transformer with a known internal defect. Since fingerprints of remote radiometric measurements are not available, the formation of clusters with similar features obtained from captured partial discharge data is crucial. Hierarchical cluster analysis technique is used as a method for grouping different signals. Investigation based on Euclidean and Mahalanobis distance measures and Ward and Average linkage algorithms were performed on partial discharge data pre-processed by principal component analysis. As a result of the analysis, a clear separation of partial discharges emanating from the transformer and discharges emanating from its surrounding is achieved; this in turn should enhance the methodologies for condition monitoring of power transformers.