This paper was produced for the 2019 NAFEMS World Congress in Quebec Canada
Noise pollution is a subject of increased importance, particularly in urban areas or even in areas with substantial wildlife. The characteristic sound of power transformers can not only cause discomfort to the people in its vicinity, but also disturb the balance of the ecosystem in general. Therefore, reducing the noise level of power transformers is of great importance since these devices are indispensable in a power network and also, with the increasing power supply demands, need to be placed nearer and nearer populated areas. An inappropriate mechanical design, especially for the tank, can result in structural resonance and/or increased noise radiation efficiency.
A power transformer is mainly composed by a submerged active part, in synthetic or mineral oil, confined in a steel tank. This oil volume then forms an acoustic cavity in which the vibration induced pressure waves will propagate, getting partly reflected on the systems boundaries, interfering between each other, considered as a practical example of a diffuse field, and thus forming acoustic modes. These boundaries, mainly the transformer tank panels, are relatively thin but are also reinforced with stiffeners, which create a boundary with variable stiffness. Modelling such a complex system requires some simplification in order to quicken the numerical solving process and obtain solutions in a reasonable time without compromising results.
In this paper, a modal and harmonic analysis are presented, using Finite Element Method, aiming to evaluate the effect of the oil volume as an interior fluid-structure acoustic interaction between active part, oil and tank. For simplification, typical harmonic simulations of power transformer behaviour disregard the effect of the oil. Such simplification introduces significant differences not only in the modal results, but also in the frequency response. Their modal density is very high, usually having a great number of harmonics in the audible frequency spectrum. Therefore, the effect of the oil might have considerable importance on the vibro-acoustic numerical analysis of a power transformer, though its inclusion requires a greater computational cost.