Experimental Characterisation of the Dynamic Mechanical Properties of Pressboard

The structural strength and dynamic behaviour of transformer windings are mainly influenced by solid insulation material conditions. Since during transformer's operation the windings are subjected to dynamic forces and the pressboard used as insulation material tends to degrade, characterisation of its dynamic mechanical properties is necessary to improve control and early fault identification. In this study, the out-of-plane dynamic mechanical properties of pressboard are analysed as function of excitation frequency and ageing. A set of pressboard samples were subjected to an accelerated ageing test to obtain six different ageing conditions defined by their degree of polymerization according to IEC 60450. Then cyclic uniaxial compression tests were performed to determine the mechanical properties of pressboard by analysing the hysteresis curve as function of frequency and ageing. According to the experimental results, the pressboard can be modelled as a nonlinear elastic spring and a viscous damper connected in parallel. It is shown that ageing affects proportionally the stiffness and the damping. With the increase in frequency, the pressboard tends to increase its stiffness and mimics the behaviour of an ideal elastic material.