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Iron Core Content and Design Features in Oil-Immersed Transformers

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Iron Core Content and Design Features in Oil-Immersed Transformers
  • By ZTELEC GROUP
  • 2025-06-30

Oil-immersed transformers are one of the most widely used types of power transformers in electrical transmission and distribution systems. One of the critical components of these transformers is the iron core, which plays a vital role in the electromagnetic energy conversion process. The design, structure, and material composition of the iron core directly affect the transformer's efficiency, noise level, magnetic losses, and overall performance.

Oil-Immersed Transformer

1. Importance of the Iron Core in Transformer Design

The iron core of a transformer is responsible for providing a low-reluctance path for magnetic flux. During operation, alternating current in the primary winding generates a magnetic field that induces a voltage in the secondary winding via the core. A well-designed iron core ensures minimal energy loss, reduces heat generation, and supports long-term stable operation.

2. Common Materials Used for Transformer Iron Cores

The choice of material for the iron core is crucial for achieving high magnetic permeability and low core losses. The most commonly used materials include:

Silicon Steel Sheets: High-quality cold-rolled grain-oriented silicon steel (CRGO) is widely used due to its high permeability and low hysteresis loss. The silicon content typically ranges from 2.5% to 3.5%, which improves resistivity and reduces eddy current loss.

Amorphous Metal Alloys: For applications that demand ultra-low no-load losses, amorphous alloy cores are used. These materials feature a disordered atomic structure that minimizes energy loss during magnetization and demagnetization cycles.

Non-Grain-Oriented Steel: Used in low-voltage and low-power transformers, this material has isotropic magnetic properties but higher losses compared to grain-oriented steel.

3. Iron Core Content Ratio in Oil-Immersed Transformers

The iron core content of an oil-immersed transformer usually accounts for about 20% to 30% of the total transformer weight, depending on the voltage level, rated power, and design efficiency requirements. For example:

– A 500 kVA oil-immersed transformer typically contains around 400–500 kg of silicon steel core material.

– In high-efficiency designs, the core mass may be slightly increased to reduce magnetic losses and improve performance.

The balance between core weight and copper winding is essential. A heavier core reduces no-load loss but may increase the manufacturing cost and transportation burden. Therefore, designers must optimize the iron core content to achieve both energy efficiency and economic feasibility.

4. Structural Design Features of Transformer Iron Cores

Oil-immersed transformer cores are typically designed in either the core-type or shell-type configuration. Each has specific design benefits:

Core-Type Transformers: The windings surround the core limbs. This structure is widely used in distribution and power transformers due to easier cooling and mechanical robustness.

Shell-Type Transformers: The core surrounds the windings. This design offers better short-circuit strength and is commonly used in special applications such as furnace transformers and traction transformers.

Additional core design features include:

Step-Lap Joints: Reduce magnetic flux leakage and noise by providing smoother transitions between laminated core sheets.

Mitred Core Construction: Angled core joints that reduce magnetic reluctance and improve flux flow continuity.

Transformer Design

5. Laminated Core Design and Loss Reduction

The iron core is built from thin laminated steel sheets (usually 0.23 mm to 0.30 mm thick) to reduce eddy current losses. Each lamination is insulated from adjacent sheets to block eddy current loops. The lamination direction is carefully aligned to optimize magnetic flux flow and minimize stray losses.

Modern core designs often adopt the following strategies:

– Use of laser-scribed grain-oriented silicon steel to guide magnetic domains.

– Precision shearing and stacking of laminations to reduce air gaps.

– Vacuum annealing of the core to relieve stress and enhance magnetic properties.

6. Influence of Core Design on Transformer Performance

The iron core has a direct impact on the following performance parameters:

No-load Loss: Determined by hysteresis and eddy current losses in the core. A well-designed core reduces no-load loss by 10–30% compared to traditional designs.

Temperature Rise: Lower core loss leads to lower heat generation and helps maintain thermal balance in the oil-immersed system.

Noise Level: Magnetic flux changes in the core cause mechanical vibrations, which can generate audible noise. Step-lap joints and vibration damping reduce transformer hum.

Energy Efficiency: High-efficiency iron cores contribute to achieving Tier 1, Tier 2, or DOE2016 efficiency standards in power systems.

7. Technological Advancements in Core Manufacturing

Recent advancements have significantly improved the design and manufacturing of transformer cores:

– CNC-controlled lamination cutting for precise dimensions and tight tolerances.

– Automated stacking and core clamping systems to ensure mechanical alignment and magnetic uniformity.

– Integration with digital twin simulation for optimal core shape and loss prediction.

In oil-immersed transformers, the iron core is a central component that directly affects energy conversion, thermal performance, and long-term operational stability. By optimizing the core material, content ratio, and structural design, manufacturers can deliver high-performance transformers that meet modern energy efficiency requirements. As power systems evolve toward smarter and greener grids, the role of transformer core design will become even more critical in ensuring reliable and sustainable power delivery.

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