Finite element analyses of electromagnetic forces and thermal stress effects on transformer winding due to switching impulse voltage

Recurrence switching impulses in power system induce overvoltage attenuation along a transformer winding, leading to abnormal stresses. This study presents a comprehensive approach using Finite Element Method (FEM) to analyse the behaviour of a 33/11 kV disc-type winding under recurrence switching impulses. Firstly, the effects of standard switching impulse (SSI) and non-standard switching impulse (NSSI) voltages on electromagnetic forces were examined based on analytical and numerical methods. The electromagnetic forces were calculated analytically based on the winding’s geometrical design and numerical simulations were conducted in Ansys Maxwell. Structural analysis was conducted in Ansys Workbench to evaluate the impact of these forces on disc windings. Then, the interrelated electrical, thermal and structural response of disc winding under SSI voltage were analysed. Ohmic losses were computed through electric transient analysis and transferred to thermthermal stress. The thermal stress was used in structural analysis to assess disc winding deformation and displacement. Buckling analysis was conducted to evaluate winding stability, while stress-strain and load-displacement curves to examine the mechanical behaviour of the winding materials. Lastly, a safety factor analysis was conducted to assess the disc winding's structural reliability to electromagnetic forces and thermal effects. The electromagnetic force analysis shows that the highest force experienced by the disc winding is the axial force that reached 207 mN under NSSI voltage, approximately five times greater than under SSI voltage. SSI voltage results in localised hoop tension, axial displacement and conductor tilt, while NSSI causes similar deformation and displacement but on a global scale. The ohmic losses are unevenly distributed across the winding, with high concentrations at the topmost disc layers and outer circumference with hot-spot temperature of 428.6 °C. This leads to significant temperature and substantial structural deformations in that area. The thermal stress causes copper conductors to expand and form wavy bends. The fixed supports exert pressure as it restrain the expansion of the conductors and cause a bend. Kraft insulation paper deformations include crinkles, distortion and tears. The buckling analysis indicates localised instability at the top disc and winding circumferences under electromagnetic force while thermal dependent buckling leads to global instability. The safety factor analysis highlights critical stress levels under thermal effects, with values as low as 0–1, that indicate a high risk of mechanical failure.al analysis to determine temperature distribution and

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