The core of a transformer constitutes the magnetic circuit. To minimize hysteresis and eddy current losses caused by alternating magnetic flux, the core is made by stacking high-quality silicon steel sheets with a thickness of 0.35 mm or thinner. Currently, cold-rolled grain-oriented silicon steel sheets with high magnetic permeability are widely used to reduce the volume and weight of the transformer, which also allows for savings in conductor material and reduces heat losses due to conductor resistance.

The transformer core consists of two main parts: the core limbs and the yokes. The core limbs hold the windings, while the yokes connect the core limbs, forming a closed magnetic circuit. Depending on the arrangement of the windings within the core, transformers can be classified into two types: core-type and shell-type transformers.

Single-Phase Two-Limb Core Transformers

These transformers have two core limbs, connected by upper and lower yokes to form a closed magnetic circuit. Both core limbs are wound with high-voltage and low-voltage windings. Typically, the low-voltage winding is placed closer to the core, while the high-voltage winding is placed outside, meeting insulation requirements more easily.

Core-Type Three-Phase Transformers

Core-type three-phase transformers can have either a three-limb or five-limb core structure. The five-limb core structure (also known as the three-phase five-limb core or three-phase five-leg core) is essentially a three-limb core with two additional outer limbs (side yokes) that do not carry windings. The cross-sectional area and height of the upper and lower yokes in the five-limb core are smaller than those in a typical three-limb core, which helps reduce the overall height of the transformer.

In a three-phase three-limb core, the windings for each phase are placed on separate limbs, with the three limbs connected by upper and lower yokes to form a closed magnetic circuit, similar to the winding arrangement in single-phase transformers. In a three-phase five-limb core, the additional outer limbs act as side yokes, and the windings for each voltage level are placed on the three central limbs, while the side yokes remain unwound, creating the three-phase five-limb transformer structure.

Magnetic Circuit Independence and Zero-Sequence Impedance

In a three-phase five-limb core, the magnetic flux of each phase can close its loop through the side yokes, making the magnetic circuits of each phase largely independent, unlike the interdependent magnetic circuits in a typical three-phase three-limb transformer. Consequently, when the transformer is subjected to unbalanced loads, the zero-sequence magnetic flux generated by zero-sequence currents in each phase can also close its loop through the side yokes. As a result, the zero-sequence excitation impedance in a three-phase five-limb core is equal to the positive-sequence impedance under balanced operation.

Core Structure Preferences

Small and medium-capacity three-phase transformers generally use the three-phase three-limb core structure. For large-capacity three-phase transformers, where transportation height is often a constraint, the three-phase five-limb core structure is more commonly used.

Shell-Type Transformers

A shell-type single-phase transformer features a central core limb and two side limbs (also known as side yokes). The central limb is as wide as the sum of the widths of the two side limbs. All windings are placed on the central core limb, with the two side limbs encircling the windings like an "outer shell," hence the term "shell-type transformer." This design is sometimes referred to as a single-phase three-limb transformer.

A shell-type three-phase transformer can be viewed as three independent single-phase shell-type transformers placed side by side.

Structural Differences

Core-type transformers have a simpler structure, with a greater distance between the high-voltage windings and the core, making insulation easier to manage. On the other hand, shell-type transformers have a more robust structure but are more complex to manufacture, with the high-voltage windings closer to the core limbs, making insulation more challenging. The shell-type structure allows for stronger mechanical support of the windings, enabling them to withstand greater electromagnetic forces, making it particularly suitable for transformers carrying large currents. The shell-type structure is also used in large-capacity power transformers.

Cooling in Large-Capacity Transformers

In large-capacity transformers, cooling oil ducts are typically incorporated into the core to ensure that the heat generated by core losses can be effectively dissipated by the circulating insulating oil, thereby achieving efficient cooling. The cooling oil ducts can be aligned either parallel or perpendicular to the plane of the silicon steel sheets.