Pulse Shape Analysis of PD Signals – an efficient Tool to Monitor the Condition of Oils

Abstract: Partial discharges in oils are influenced by the actual dielectric properties of the liquid and especially degradation products may influence the phenomena. As a consequence of ageing processes in the liquid, oil molecules may be split into molecules of shorter lengths. The energy of a PD in the oil may heat the liquid locally and especially low molecular weight products in the oil will evaporate and form a gaseous phase within the liquid. This second phase can be generated in solids as well as in liquids. In solids the pressure in this gas phase will increase in accordance to the concentration of gas molecules. In liquids, an increase of the concentration of molecules will lead to an extension of the gas volume until the pressure within the void corresponds to the external gas pressure. Consequently, in accordance to Paschen’s law gas discharges may occur in this gas phase immediately (i.e. within μs) after a PD that generated the gas filled void. The sequence will be so quick that commonly used equipment may not be able to monitor these discharges separately. Analyses of the pulse shape of the PD signals after the band pass filter showed differences in accordance to the degree of ageing of the oils examined. Possibilities to use characteristic parameters of the pulse shape for diagnostic purposes and especially their correlation to other dielectric properties will be discussed in detail.