High Magnetic Induction, High Overload Capacity Oil-Filled Power Transformer Cores

The Function of the Core in an Oil-Filled Power Transformer

The core of an oil-filled power transformer provides the low-reluctance magnetic path that couples the primary and secondary windings through electromagnetic induction. It is built from thin laminated sheets of electrical steel to limit eddy-current losses, and the laminations are insulated from each other and stacked so that the magnetic flux path is continuous. The core accounts for the no-load loss of the transformer (hysteresis and eddy-current losses) and contributes to the sound level through magnetostriction. A high magnetic induction design reduces the core cross-section for a given flux, which lowers the weight and material cost, while a high overload capacity design keeps the core away from saturation during fault and overload conditions.

Core Structures

Three-phase three-leg core

The three-phase three-leg core consists of three vertical legs and two yokes. Windings are placed on each leg, the structure is simple to manufacture, and it is the most widely used configuration for three-phase oil-filled power transformers of various ratings.

Three-phase five-leg core

The five-leg core adds two outer side yokes to the three-leg design. It provides a lower-reluctance path for zero-sequence flux, which reduces the zero-sequence impedance and improves performance under unbalanced loads and ground faults. It is specified where the system places special requirements on zero-sequence current behavior.

Single-phase two-leg core

The single-phase two-leg core has two legs and two yokes and is used in single-phase oil-filled transformers, including special single-phase equipment and small-capacity systems.

Core Materials

The dominant material is grain-oriented silicon steel, whose grain structure is oriented in the rolling direction to give high permeability and low loss along the flux path. Grades are classified by thickness and loss, and the material tests follow GB/T 2521 or ASTM A876. Amorphous alloy cores use a rapidly solidified ribbon with very low no-load loss, at the cost of a lower stacking factor and lower saturation induction, and they are used where energy-saving requirements dominate. Nanocrystalline cores combine high saturation induction with low loss and good temperature stability, and are used in special high-end designs.

Manufacturing Processes

Fully inclined joint stacked core: the sheets are stacked with fully inclined (45°) joints, which smooths the magnetic circuit, reduces reluctance and lowers both no-load loss and noise.

Stereoscopic wound core: the three legs are arranged as an equilateral triangle in three dimensions with a continuous, gap-free magnetic circuit; the flux path is short and consistent, losses and noise are reduced, and the third-harmonic component is effectively suppressed.

Laser-cut core: laser cutting of the sheets gives high dimensional accuracy and clean edges, improving the material utilization and the consistency of the finished core for high-precision designs.

Performance Characteristics

Low-loss cores reduce the hysteresis and eddy-current losses of the transformer, which lowers the operating cost and the temperature rise. Low-noise cores use optimized clamping, cushioning between sheets and careful annealing to reduce the vibration caused by magnetostriction, which matters in residential and commercial areas. High-overload-capacity cores are designed so that the magnetic and mechanical margins are sufficient to avoid saturation and deformation under short-time overloads, improving the reliability of the transformer in systems with fluctuating loads.

Silicon Steel Grade Reference Table

The grade table follows the conventional industry classification of grain-oriented silicon steel; the loss values are referenced to P17/50 and P17/60 test conditions (17 kG peak induction at 50 Hz and 60 Hz), and the material testing method follows GB/T 2521 or ASTM A876. Customized sizes and grades are available from manufacturers on request.

FAQ

Why is the core laminated instead of solid?

Laminating with thin insulated sheets breaks the eddy-current paths inside the steel, which drastically reduces the eddy-current component of the core loss. Thinner laminations give lower loss at the cost of a lower stacking factor.

What does high magnetic induction mean for the design?

It means the steel can operate at a higher flux density (for example 1.88 T minimum induction in the high-induction grades) without excessive loss, allowing a smaller core cross-section for the same flux.

How does the core structure affect the overload capacity?

The magnetic margin (operating flux below saturation) and the mechanical clamping strength determine whether the core stays stable under overload; a high-overload design keeps the working induction low enough and the clamping rigid enough to avoid saturation and deformation.

What is the difference between P17/50 and P17/60 losses?

The loss is measured at 17 kG peak induction, at 50 Hz or 60 Hz respectively. At 60 Hz the eddy-current loss is higher, so P17/60 exceeds P17/50 for the same grade, as shown in the table.

Which core material has the lowest no-load loss?

Amorphous alloy has the lowest no-load loss of the three material families, followed by domain-refined grain-oriented silicon steel; the choice balances loss, saturation induction, stacking factor, cost and mechanical robustness for the application.

In summary, the performance of an oil-filled power transformer core depends on the structure, the steel grade, and the manufacturing process. High magnetic induction grades reduce the core size, domain-refined and high-induction grades lower the loss, and a sound clamping design preserves the overload capacity; together they define the no-load loss, noise and reliability of the finished transformer.