Abstract
During the manufacturing of core-type power trans formers, the winding structure is clamped by horizontal clamping beams to withstand operational and through-fault current forces. The effects of evolving grid conditions, e.g., frequent switching cycles and increasingly dynamic load profiles, on the evolution of winding clamping pressure of transformers in operation are largely unknown. This paper investigates how different thermal cycles affect clamping pressure in a pressboard spacer–brass plate stack clamped within a test rig. To minimize the surface effects caused by the sieve structure of the pressboard spacers, the top and bottom cellulose plies have been machined off. In this paper, both oil-impregnated and non-impregnated dry pressboard spacers are studied. All spacers are first vacuum dried under a constant sizing pressure of 7 MPa, followed by oil-impregnation when applicable. Afterwards, the spacer-brass plate stack is assembled in the test rig with an initial clamping pressure of 4 MPa. To investigate the effect of different thermal cycling on the clamping pressure, three different heating rates are applied: low, medium, and high power. At least 15 heating cycles with a cooling phase in between are applied to each test case. It is observed that the loss of clamping pressure is most pronounced during the first heating cycle and gradually diminishes with subsequent cycles. When the initial effect is excluded, no clear relation between the heating rates and the loss of clamping pressure is observed. No significant influence of oil-impregnation on the evolution of clamping pressure is observed under the present test condition. The results highlight the importance of material treatment and initial clamping procedure during the transformer manufacturing process, as these factors significantly influence the clamping pressure and its long-term stability.