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The electrification of society, increasing renewable energy sources and mobility charging lead to new loading patterns for power transformers. Dynamic load conditions induce enhanced mechanical stress on the transformers' windings, potentially causing degradation of the solid insulation over time and compromising the transformer's short-circuit withstand capability. Thermal expansion of the windings, caused by losses in the copper conductors, occurring as the transformer is loaded, increases the stress. Conversely, magnetic losses in the core and tie plate expansion contribute to a reduction in stress. This paper presents the effect of step changes in core losses and copper losses by on-line measurements of the clamping pressure, to better understand the mechanical stresses acting upon the solid insulation cellulose materials. Energisation is found to decrease the clamping pressure following warming up of the transformer, and loading the transformer increased the pressure as the windings increased in temperature. The converse effect was found when unloading and de-energising. The on-line monitoring system provides a new and important step towards ensuring the short-circuit performance of power transformers. [ABSTRACT FROM AUTHOR]