Propagation behaviour of acoustic partial discharge signals in oil-filled transformers

In-service monitoring of power apparatus has become increasingly important for electric power utilities. In power transformers, many techniques are used for this purpose including gas-in-oil analysis and acoustic and electrical partial discharge detection. One of the most popular techniques is the detection of acoustic partial-discharge (PD) signals that propagate through the transformer oil. This technique is also widely used for the location of PD sources inside transformers. Unfortunately, the propagation of acoustic signals in large transformer tanks is not sufficiently known because it had previously only been studied in small tanks and the results extrapolated to large tank. This paper presents some of the results of a study done on a medium-size power transformer (25 MVA). Acoustic PD signals were generated by immersing either an electrical PD source or a piezoelectric crystal excited by a square-wave generator in the transformer oil. The ensuing tests included acoustic propagation in obstacle-free oil and oil with obstacles, over distances of up to 3 m. A very close fit to a simple analytical function was verified in obstacle-free oil. The reflection behaviour as a function of the angle of incidence of the wavefront on the tank wall was studied. A relationship was established between the apparent charge of electrical PD signals and the level of acoustic signals produced