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Joints in Distribution Transformers

Updated April 3, 2025

Abstract — Copper-to-aluminum joints are essential in transformers, affecting their performance and reliability. The connection between these dissimilar metals can lead to galvanic corrosion, particularly under varying loads. This paper explores some methods for proper copper-to-aluminum joint design and connection: using hard aluminum, implementing bimetal interfaces, and adopting cold welding techniques. Understanding these strategies is crucial for enhancing the efficiency and longevity of electrical systems. 

Keywords — Copper-to-aluminum joints, transformers, galvanic corrosion, joint design, hard aluminum, bimetal interfaces, cold welding. 

I. GENERAL

Copper-to-aluminum joints are critical because they connect different materials and components, and can significantly impact the transformer's performance, reliability, and longevity. Proper joint design and construction are essential to avoid issues like galvanic corrosion, which can occur when dissimilar metals are joined together, especially under varying loads and various environmental conditions. 

In the transformers industry, the reliability and performance of electrical systems hinge on the quality of connections between various materials. Copper and aluminum are often used together due to their unique properties, but direct connections between them can lead to galvanic reactions that create hot spots, compromising functionality and safety. To address this challenge, manufacturers have developed several effective methods, including the use of hard aluminum for winding terminals, bimetal interfaces, and cold welding techniques. Understanding these approaches is essential for ensuring the longevity and efficiency of transformers, which play a vital role in electrical power distribution and management. As the industry evolves, innovations in joint technology will continue to enhance the performance and durability of transformer systems. 

II. BIMETAL CONNECTORS 

Direct copper-to-aluminum joints are not allowed in both oil and air environments in any case. This is because of the risk for galvanic reactions, which are triggered by temperature fluctuations associated with varying loads. Over time, these reactions can lead to the formation of critical hot spots at the joint, compromising its integrity. To prevent galvanic reactions when a copper-aluminum joint is necessary, the industry recommends three primary methods for connection: 

  1. Hard Aluminum 
  2. Bimetal Interfaces 
  3. Cold Welding 

III. HARD ALUMINUM 

 The first method is to use “hard aluminum” to connect the winding terminals to the bushing. Using the same soft aluminum conductor employed for the winding is not advisable, as it can deform under the mechanical stress of the bolted connection to the bushing.   

Connector for Cold Welding Aluminum to Copper

IV. BIMETAL INTERFACE 

 Another method is employing a bimetal interface between the materials to provide low-resistance electrical connections. This method is designed to maintain contact integrity throughout the lifespan of the joint.  

Dragon Tooth Cu to Al Connector 

V. COLD WELDING 

 The most commonly used method is the “cold welding” of the two materials. The picture above shows a commonly used connector for cold welding the two materials. H-J offers cold welders from our partner MTM to facilitate this process. These welders can be mounted directly onto winding machines or used as standalone tools.   

MTM Cold Welder Mounted on a Winding Machine

 In summary, understanding the implications of galvanic reactions allows manufacturers to implement methods to minimize the risk, which is crucial for maintaining the performance and safety of transformers. By implementing effective methods such as hard aluminum usage, bimetal interfaces, and cold welding, the industry can mitigate the risks associated with copper-to-aluminum connections, ensuring the long-term reliability and efficiency of electrical systems.

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