Winding Joints - A key towards reliability in Distribution Transformers

15 Nov 2024

Distribution Transformers (DTs) - An overview

Number of DTs in India ~ 15 millions with aggregate capacity of 340 GVA (In 2021-22) [1]

• 16 lakhs DTs failed with 11% failure rate

• Repair/replacement cost is ? 32 Billions

• Revenue loss to utilities are ? 6.72 Billions

Joints, connections and terminations

• A winding joint should be able to pass the current without introducing an undesirable resistance into the winding and it should continue to do so for a very long time.

• Estimated that about 50% of the failures are due to defects in joints, connectors and terminals.

• Problem is more severe in case of aluminum wound transformers due to poor creep behavior of aluminum.

- Creep temperature limit for copper ~ 1350C
- Creep temperature limit for Aluminium ~ 70C

• There is no practice to detect dangerous hot spots at connectors/terminals during service.

• Only way to prevent them by taking care in the design, manufacturing and installation of the transformers.

Technical challenges associated with DT winding joints

• Aluminium oxidizes with air to form insulating layer of Al2O3

- Difficult to develop a satisfactory joint.

Contact resistance of joints may increases on the long run.

• Overheat due to loading & local carbonization.

• Increased creep rate of winding due to higher temperature.

• Mechanical joints (Crimping and Western Union Splice) generates sparking, if contact interface becomes loose.

Winding joints in DTs

Tensile strength of DT winding joints

• The crimp joint shows the highest strength in Al-Al joints

• The Cu-Cu winding joints have significantly higher strength compared to corresponding Al-Al and AlCu winding joints.

[2] Shashank Gupta, D.K. Dwivedi, Manoj Tripathy, Study on Mechanical Strength of Winding Joints in Distribution Transformers, in 10th IEEE Uttar Pradesh Section International Conference on Electrical, Electronics and Computer Engineering (UPCON), 2023.

Creep test results of crimp joints of 25 kVA DTat temperatures (100°C and 140°C) and stress (40 MPa and 55 MPa)

• At higher stress and temperature conditions, the Al wound DT shows a much lower creep life due to creep in winding joints.

• The Cu wound DTs is negligibly affected by the creep in winding crimp joint.

[3] Shashank Gupta, D.K. Dwivedi, M. Tripathy, “Creep Failure Analysis of Crimp Joints in Aluminium and Copper Windings of Distribution Transformers”, Engineering Failure Analysis, Vol. 156, 2024.

[4] Shashank Gupta, D.K. Dwivedi, M. Tripathy, “Creep Failure Analysis of Western Union Splice Joints in Distribution Transformer Winding”, Journal of Materials Engineering and Performance, Springer, 2023.

Field Study: Aluminium winding joints failure 

• Creep in HV winding conductor joints greatly affect the performance of the distribution transformers.

• The life of the Copper wound distribution transformers is more than Aluminum wound distribution transformers.

• Copper is the better choice of conductor for DT winding to ensure high reliability