FAILURE ANALYSIS OF WINDING JOINTS IN DISTRIBUTION TRANSFORMERS

Abstract

Distribution Transformers (DTs) are the primary asset to any power distribution network. The failure of DTs has always been a concern for electrical utilities. The life of DTs is significantly affected by thermal stresses inside the transformer. The hot-spot formation is critical among various factors, especially in weak areas like winding joints. Over time, the contact resistance in winding joints tends to increase, potentially leading to a thermal runaway condition. A satisfactory winding joint should possess two characteristics; firstly, it should be able to pass the current without introducing an undesirable resistance into the circuit and secondly, it should continue to do so for a long time for better reliability. The winding joints in DTs show a high tendency to overheat during the operation and form a local hot-spot. The heat generated at joints may dissipate to nearby winding over a certain length, leading to potential breakdowns due to creep, insulation damage and short-circuit. Different types of joints like Western Union Splice (WUS), soldered WUS, crimp and butt are commonly used to connect the HV winding conductors of DTs. Aluminium (Al) and Copper (Cu) are the two commonly used conductor materials in DTs. The Al is used as conductor material in most of the DTs of lower ratings (16-250 kVA) instead of Cu. However, only a few power utilities like Tamil Nadu Power Distribution Corporation Limited (TNPDCL), India, specifies the use of Cu wound DTs exclusively, regardless of the rating. Due to the material differences, the joints using Al conductors cannot achieve the same level of effectiveness as those with Cu conductors. A field study of failed DTs was performed to identify the condition of different winding joints and the level of degradation in the joints due to service exposure. The contact resistance of Al-Al and Al-Cu winding joints of the failed Al wound DTs indicates significant degradation due to service exposure. The winding joints in this condition may tend to form hot spots, potentially reducing the lifespan of DTs. Since joints are unavoidable in the fabrication of DT winding, it is important to examine the performance of the different joints for their suitability in the construction of DTs.