The impact of drying and oil impregnation conditions and of temperature cycles on the clamping force of power transformer windings

This study reports on the effect of processing conditions on the static clamping force of transformer windings and on the variation of the clamping force with time. Small scale disc winding models were submitted to various drying and oil impregnation procedures, as well as different clamping and temperature conditions. The results confirm the importance of properly controlled processing parameters. Furthermore, the impact of time, and of temperature variation as they typically occur at load cycles, was investigated. Significant change of the clamping force was observed, which means that the static force deviated considerably from the value as set initially, prior to oil impregnation. Even within a short period of 10 days, a slight overall decrease of the clamping pressure was measured.