Comparative investigations on UHF and acoustic PD detection sensitivity in transformers

This publication deals with UHF and acoustic partial discharge (PD) detection sensitivity assessment of paper/oil insulation configurations which are typical of power transformers. Even though the basis of this topic has been developed some time ago, the knowledge of the accuracy and limits of the sensitivity is of increasing importance for large power transformer test engineers who are confronted not only of reliable measurement of PD but also locating the source of discharges. Discharges representative of that occurring in a transformer were recorded by simulating them in a 220 kV test tank. The models simulated the basic insulation configuration of a transformer with multiple pressboard barriers and oil gaps. The influence of the effect of barriers on the sensitivity of UHF and acoustic PD detection were studied. Classical phase-resolved PD patterns were recorded alongside for analyzing the results of experiments. Experimental outcomes on these models were further verified on a 220 kV winding in a 400 kV test tank. Studies reveal that acoustic. sensitivity is seriously impaired by the presence of barriers, i.e. sensitivity of discharges shielded by insulating barriers is low as compared to those discharges in open oil volumes. For comparable distances, this sensitivity limitation is absent for UHF PD detection. Furthermore, merits and limits of individual methods are discussed that may lead to future research.