Static and quasi-static behavior of dry and oil-impregnated pressboard

Pressboard has been used as insulation material in transformers due to its outstanding mechanical behavior. Spacers, collars, press rings and common spacer rings are subjected to a number of mechanical loads that include out-of-plane (through-thickness) compressive load. Once the assembly process is finalized and the oil filling is completed, the pressboard components in the windings are subjected to both a constant compressive load applied during manufacturing and to a superimposed pulsating compressive force during service. Pressboard, as most of the fiber-based materials, can be described as an open pore structure characterized by irregularly shaped voids. The internal structure of the material affects the overall mechanical behavior. The oil filling in the pores is said to contribute to the overall stiffness of pressboard when compressive (through-thickness) load is applied. However the size of the contribution to the overall through-thickness stiffness of pressboard is not clear. The present report investigates the difference between the mechanical behavior of dry and oil-impregnated pressboard. The experiments elucidate the contribution that oil impregnation gives to the overall stiffness of the pressboard system. Compressive tests on model systems have been carried out and precise measurements of the deformation of pressboard were performed.