Creep failure analysis of crimp joints in aluminium and copper windings of distribution transformers

The present work investigates creep performance and contact resistance of Distribution Transformers (DTs) winding crimp joints under the influence of temperature and stress, aiming to reduce the failure rate of DTs. The integrity of crimp joints is crucial, as the field study revealed that a majority of crimp joints become unstable and show elevated contact resistance due to service exposure. These changes affect the steady-state creep rate of the crimp joint and the surrounding winding conductors, ultimately diminishing the operational lifespan of DTs. For experimentation, crimp joints with varying crimp points were fabricated for a 25 kVA DT using aluminium and copper conductors with diameters of 0.83 mm and 0.6 mm, respectively. For all types of joints i.e., Al-Al, Al-Cu and Cu-Cu, the 2-point crimp joint was identified as optimum in terms of tensile strength and contact resistance. The creep tests for the crimp joints were conducted under varying temperatures of 100 degrees C and 140 degrees C with stress levels of 40 MPa and 55 MPa as per ASTM E139-11. The steady-state creep rate and contact resistance of Al-Al crimp joints increased with an increase in temperature and stress. The creep life of DTs due to Al-Al and Al-Cu crimp joints is significantly less than the designed life of a typical DT. On the other hand, creep behaviour of copper winding crimp joints supports a significantly higher DT creep life even at a higher temperature of 140 degrees C and elevated stress of 55 MPa.