Study on Mechanical Strength of Winding Joints in Distribution Transformers

This study investigates the winding joints of Aluminium (Al) and Copper (Cu) wound Distribution Transformers (DTs) for mechanical behaviour. Winding joints in DTs are subjected to large mechanical stresses generated by inrush current, short-circuit current and external transients. The conventional methods of winding joint formation in DTs increase the chance that joints result in poor performance and less reliability. Loose joints results in higher thermal due to an increase in contact resistance, which poses safety concerns, ultimately escalating to the risk of arcing and potential fire hazards. The Western Union Splice (WUS), soldered WUS, crimp and butt joints are identified as commonly used winding joints in lower rating DTs (16–200 kVA) and compared on the basis of ease of joining, space requirement and applicability in DTs. The Al-Al, Al-Cu and Cu-Cu winding joints were prepared using Al and Cu conductors of 25 kVA DT. Tensile test of winding joints was performed using the INSTRON-5982 UTM machine as per IS-1608 part-1, The results indicated that the optimum number of turns for WUS joints are 18, 14 and 12 for Al-Al, Cu-Cu and Al-Cu winding, respectively. For soldered WUS joints, the 8 turns joint shows maximum strength for the Al-Al joint, while for Al-Cu and Cu-Cu, the 4 turns joint shows maximum strength. For crimp joints, 2-point crimping shows optimum strength and is stronger than other joints of respective metal combinations. Although the highest strength for Al-Al and Al-Cu winding joints is 74.8 and 68.96 MPa, respectively, these are approximately one-third of the maximum strength of Cu-Cu winding joints. The Cu winding joints offer better resistance to different stresses and thus may be preferred in the manufacturing of DTs to achieve better strength of winding joints and reduced failure of DTs.