Abstract
During manufacturing of a power transformer, a fixed clamping pressure is exerted on the winding structure to prevent the winding from buckling due to a short circuit fault. Over the long service life, the transformer is subjected to different loads. The load current influences the clamping pressure in many ways: essentially by repetitive mechanical stresses of the cellulose materials (e.g., pressboard, paper etc.) due to thermal expansion of copper winding and by continuous moisture exchange between the cellulose materials and the insulation oil due to their temperature variations. Although the presence of moisture in cellulose may temporarily increase the clamping pressure, it exacerbates the ageing process. As such, the mechanical properties of pressboard may degrade over time, resulting in a loose clamping structure where the mechanical integrity of the transformer may jeopardize. This paper reports the experimental setup of a test rig to investigate the effect of load variation on the development of clamping pressure. To do so, a stack of dry pressboard samples and heatable brass plates are clamped together and is placed in transformer oil. The heat-flow in the brass plates is controlled, thus emulating different winding losses. Effect of three different loads, i.e., the losses in the winding is controlled to investigate its effect on clamping pressure. The thermal expansion of the brass plates is observed to strongly influence the clamping pressure in a short time scale. Although long term evolution of the clamping pressure is not investigated yet, nonetheless, no significant change in clamping pressure is observed once the temperature of the rig comes back to its initial room temperature.
