TEXT 10. Power transformer. Construction

The construction of a power transformer will vary throughout the industry to a certain degree. The basic arrangement is essentially the same and has seen little significant change in recent years, so some of the variations may be discussed here.

The Core

The core, which provides the magnetic path to channel the flux, consists of thin strips of high-grade steel, called laminations, which are electrically separated by a thin coating of insulating material. The strips can be stacked or wound, with the windings either built integrally around the core or built separately and assembled around the core sections. Core steel may be hot or cold rolled, grain oriented or non-grain oriented, and even laser-scribed for additional performance. Thickness ranges from 9 mils (1 mil = 1 thousandth of an inch) upwards of 14 mils. The core cross-section may be circular or rectangular, with circular cores commonly referred to as cruciform construction. Rectangular cores are used for smaller ratings and as auxiliary transformers used within a power transformer. The type of steel and arrangement will depend on the transformer rating as related to cost factors such as labor and performance.

Just like other components in the transformer, the heat generated by the core must be adequately dissipated. While the steel and coating may be capable of withstanding higher temperatures, it will come in contact with insulating materials with limited temperature capabilities. In larger units, cooling ducts are used inside the core for additional convective surface area and sections of laminations may be split to reduce localized losses.

The core will be held together by, but insulated from, mechanical structures and will be grounded to a single point, usually some readily accessible point inside the tank, but may also be brought through a bushing on the tank wall or top for external access. This grounding point should be removable for testing purposes, such as checking for unintentional core grounds.

The maximum flux density of the core steel is normally designed as close to the knee of the saturation curve as practical, accounting for required over-excitations and tolerances that exist due to materials and manufacturing processes. For power transformers, the flux density is typically between 13 and 18 kG

with the saturation point for magnetic steel being around 20.3 to 20.5 kG.

The two basic types of core construction used in power transformers are called core-form and shell-form.

In core-form construction, there is a single path for the magnetic circuit.

In shell-form construction, the core provides multiple paths for the magnetic circuit. Due to advantages in short circuit and transient voltage performance, shell forms tend to be used more frequently in larger transformers where conditions can be more severe.

The Windings

The windings consist of the current carrying conductors wound around the sections of the core and must be properly insulated, supported, and cooled to withstand operational and test conditions. The terms winding and coil are used interchangeably in this discussion.

Copper and aluminum are the primary materials used as conductors in power transformer windings. While aluminum is lighter and generally less expensive than copper, a larger cross-section of aluminum conductor must be used to carry a current with similar performance as copper. Copper has higher mechanical strength and is used almost exclusively in all but the smaller size ranges, where aluminum conductors may be perfectly acceptable. In cases where extreme forces are encountered, materials such as silver-bearing copper may be used for even greater strength. The conductors used in power transformers will typically be stranded with a rectangular cross-section, although some transformers at the lowest ratings may use sheet or foil conductors. A variation involving many rectangular conductor strands combined into a cable is called continuously transposed cable (CTC).

There are a variety of different types of windings that have been used in power transformers through the years. Coils can be wound in an upright, vertical orientation, as is necessary with larger, heavier coils, or can be wound horizontally and uprighted upon completion. As mentioned before, the type of winding will depend on the transformer rating as well as the core construction.