Power Transformer Design and Construction

Discover the essentials of power transformer design and construction. Learn about key components and factors that ensure efficient and reliable operation.

July 29, 2024

Power transformers play a crucial role in the transmission and distribution of electrical energy. Their design and construction involve several key principles and materials to ensure efficiency, reliability, and longevity. This article explores the fundamental aspects of power transformer design, including core types, windings, insulation, and cooling systems.

Core and Coil Assembly Configurations

An important aspect to consider in power transformer design is how the core is formed around the windings. The core and coils can be configured with either a shell-type or core-type design, each with distinct characteristics and applications.

Core and Coil Assembly Configurations

With a shell-type configuration, the core surrounds the windings. This design creates a closed pathway for magnetic flux to flow around the windings, resulting in several advantages:

Magnetic Flux Path: The closed magnetic flux pathway reduces energy losses, making the shell-type design more efficient than the core-type design.
Application: Shell-type cores are commonly used in distribution class padmounts and substations, often featuring a wrapped 5-legged core.
Mechanical Strength: The shell-type core provides excellent mechanical strength, reducing the risk of damage during transportation and installation. This robustness also enhances the transformer’s ability to withstand short-circuit forces.
Thermal Performance: The configuration allows for better heat dissipation, facilitating efficient cooling, which is crucial for maintaining optimal performance and extending the transformer’s lifespan.

Core-Type Core

In a core-type design, the windings surround the core steel. This configuration has distinct characteristics:

Magnetic Flux Path: Unlike the shell-type, there is no return path or closed loop for the magnetic flux around the coils, which typically results in higher energy losses.
Material Requirements: The core-type design requires more copper or aluminum winding material compared to the shell-type configuration.
Application: Core-type designs are often used in applications where the compact size and cost efficiency of the winding material are less critical.

Windings

The windings in a power transformer are responsible for transferring electrical energy between circuits through electromagnetic induction. The primary winding is connected to the input voltage source, while the secondary winding delivers the transformed voltage to the load. Types of windings include:

Copper Windings: Copper is widely used due to its excellent electrical conductivity and thermal properties. Copper windings ensure minimal energy losses and efficient heat dissipation.
Aluminum Windings: Although less conductive than copper, aluminum windings are lighter and less expensive. They are used in some transformers to reduce costs, especially in larger units where weight is a concern.

Insulation

Insulation in transformers is critical to prevent electrical short circuits and ensure safe operation. The insulation materials must withstand high temperatures and electrical stresses. Common insulation materials include:

Paper and Pressboard: These cellulose-based materials are impregnated with insulating oil to enhance their dielectric properties and thermal stability. They are widely used in power transformers.
Oil Insulation: Transformer oil serves a dual purpose: it provides electrical insulation and helps in cooling the transformer. Commonly used oils include mineral oil and synthetic oils. Key characteristics include:
1. Electrical Insulation: Transformer oil has excellent dielectric properties, which help in insulating the windings and preventing electrical discharges.
2. Cooling: The oil circulates through the transformer, absorbing heat generated by the windings and core and dissipating it through radiators or cooling fins.
3. Moisture Absorption: Transformer oil also helps in absorbing moisture from the cellulose-based insulation, maintaining its dielectric strength and preventing electrical breakdown.
Synthetic Insulations: Materials such as Nomex, Mylar, and other advanced polymers are used in specialized applications where superior thermal and dielectric performance is required. Key characteristics include:
1. High-Temperature Resistance: These materials can withstand higher temperatures than traditional cellulose-based materials, making them suitable for high-performance transformers.
2. Dielectric Strength: Synthetic insulations offer excellent dielectric properties, ensuring reliable insulation under high electrical stresses.
3. Durability: These materials are highly durable and resistant to aging, providing long-term reliability and reducing maintenance requirements.

Cooling Systems

Efficient cooling is essential for maintaining the performance and lifespan of power transformers. Various cooling methods are employed depending on the transformer’s size and application:

Oil Natural Air Natural (ONAN): In this passive cooling system, natural convection circulates the oil within the transformer, transferring heat to the external air. It is suitable for smaller transformers with lower heat dissipation requirements.
Oil Natural Air Forced (ONAF): This method uses fans to force air over the transformer’s radiators, enhancing heat dissipation. It is used in larger transformers where natural convection is insufficient.
Oil Forced Air Forced (OFAF): Pumps circulate the oil within the transformer, and fans blow air over the radiators. This active cooling system is employed in very large transformers to manage significant heat loads.
Oil Forced Water Forced (OFWF): Water-cooled heat exchangers replace air cooling in this method, providing even more efficient cooling. It is used in extremely large transformers or installations with high ambient temperatures.
Non-Mineral Oil Natural Air Natural (KNAN): Similar to ONAN, The core and coils are cooled by surrounding in synthetic oil (FR3). Heat transfer of oil by natural air convection.
Non-Mineral Oil Natural Air Forced (KNAF): This method combines synthetic as the cooling medium with forced air to enhance heat dissipation. Fans are used to blow air over the radiators, improving cooling efficiency.

Additional Considerations

In addition to the core components and systems, several other design principles are essential in power transformer construction:

Voltage Regulation: Maintaining stable output voltage despite variations in load is crucial. Tap changers allow for voltage adjustments to ensure consistent performance.
Short-Circuit Withstand: Transformers must be designed to withstand mechanical and thermal stresses during short-circuit conditions without damage.
Sound Level Control: Transformers generate noise due to magnetostriction in the core. Designing for low noise involves using materials and construction techniques that minimize sound production.

Conclusion

The design and construction of power transformers involve meticulous attention to detail and the use of high-quality materials. By understanding the principles of core types, windings, insulation, and cooling systems, manufacturers can produce transformers that are efficient, reliable, and durable. These components work together to ensure the safe and effective transmission and distribution of electrical energy, meeting the demands of modern power systems.