Technical Article

Transformer Core Design and Cooling Classes

July 19, 2021 by Alex Roderick

Two key transformer design parameters are the type of core and the type of cooling. Common types of cooling include dry and oil-immersed transformers, with or without forced-air ventilation.

The type and shape of the core influence the efficiency of the transformer. Since all transformers have power losses, transformer cooling is part of the power rating. 

Cores

The design of transformer cores has an effect on the efficiency of the transformer. Cores are constructed of legs and yokes. The vertical legs support the coils, and upper and lower yokes connect the legs. The ends of the laminations used to construct the core are often cut at a 45° angle instead of square (see Figure 1). This allows the lamination layers of the legs and yokes to overlap slightly at the corner. This helps improve the magnetic conduction path through the core.

Figure 1. The laminations used to construct cores are often cut at 45° angles to allow overlap to improve the magnetic conduction path.
Figure 1. The laminations used to construct cores are often cut at 45° angles to allow overlap to improve the magnetic conduction path.

 

For a step-down transformer, the secondary coil is wound first and placed closest to the core iron with a layer of insulating material between them. The primary coil is then wound and placed over the low-voltage coil with a layer of insulating material between them. This construction places the conductors energized at the high voltage at a greater physical distance away from the iron, which is normally grounded. For a step-up transformer, the primary is wound first, and the secondary is wound on top.

The core is electrically interconnected with core clamps, steel structure, and enclosing case, all of which are connected with a lead to the plant or system ground. Three common types of transformers are core-type, shell-type, and toroidal transformers.

 

Core-Type Transformers

A core-type transformer has windings placed around each leg of the core material. A thick layer of insulating material is wrapped around the legs to prevent electrical contact between the wire of the coil and the iron of the leg. Three-phase transformers typically use a 3-leg design. Single-phase transformers typically use a two-leg design. Core-type transformers are generally less expensive than other types as less iron is used and the enclosure is smaller.

 

Shell-Type Transformers

A shell-type transformer has extra legs and a metal body surrounding the core. The extra conductive material helps keep the core leakage flux to a minimum. Common shell-type transformers include 3-, 5-, and 7-leg cores. Three-phase transformers typically use a 5-leg or 7-leg design. The two auxiliary legs of the 7-leg design provide for symmetry among the three magnetic circuits of the core. However, the extra legs of a shell-type transformer result in increased capacitance between the primary and the secondary. This design allows a more uniform distribution of flux between the various legs of the core. The more uniform distribution of flux results in reduced harmonics. Single-phase transformers typically use a 3-leg design.

 

Figure 2. Core and Shell Type Single and Three-Phase Transformers. Image Courtesy of SpinningSpark (CC 3.0)
Figure 2. Core and Shell Type Single and Three-Phase Transformers. Image Courtesy of SpinningSpark (CC 3.0)

 

Toroidal Core Transformers

A toroidal core transformer has a doughnut-shaped core with copper wire wound all around the core (see Figure 3). The toroidal core is constructed by taking a long strip of magnetic material and rolling it tightly into shape like a spring. Since the core is constructed of one piece of magnetic material, this design offers the best possible coupling between the primary and the secondary. 

The primary and secondary wires are wrapped through the center hole, around the core, and back through the center hole. The wire can be wrapped to provide complete coverage of the core and completely keep the magnetic field confined to the core to improve efficiency. Toroidal transformers are also quieter than other transformers. Toroidal transformers are most commonly used in relatively high-frequency applications, such as audio and R.F. components.

 

Figure 3. A toroidal transformer has a doughnut-shaped core with the windings wrapped around the core. Image Courtesy of GlobalSpec
Figure 3. A toroidal transformer has a doughnut-shaped core with the windings wrapped around the core. Image Courtesy of GlobalSpec
 

Transformer Cooling

Part of the rating of a transformer is determined by the dissipation of the heat losses generated in the transformer and by the temperature rise of the transformer when delivering a certain load. Because of the close concentration of the conductor in the windings, heat is a factor in transformer installation and design. If excessive heat is permitted to remain in or around the transformer, the insulation on the conductors that comprise the windings is subjected to damage and perhaps failure. Any means that can economically dissipate the heat generated in the coils and magnetic circuit more readily permits an increase in the rating of a transformer having a definite physical size or permits a decrease in the dimensions of a unit having a definite kVA rating. Transformers may be designated as dry-type, where air is used to cool the coils, or liquid-immersed, where the coils are immersed in oil.

 

Types of Coolants

While dry-type transformers are cooled by air, sometimes additional cooling is needed to reach the desired load rating. This is accomplished by immersing the coils in a liquid coolant. The oils typically used as coolants also act as electrical insulators, so they have a very high dielectric strength.

Mineral oil is a common coolant used in transformers. Mineral oil has a higher dielectric strength than air, so it acts as an insulator. It also has a higher heat capacity than air, so it acts as a coolant. However, mineral oil is somewhat flammable. 

Askarels are a class of oils that were developed to replace mineral oils in locations where flammability was a concern. Askarels are considered to be non-flammable. However, they can decompose when exposed to heat to form hydrochloric acid and toxic chemicals like dioxins. In addition, some types of askarels contain polychlorinated biphenyls (PCBs). The use of askarels in new transformers in the U.S. was banned in 1977. Many older transformers in the field still contain poisonous askarels. Extreme care must be taken when working on older oil-filled transformers.

High-temperature hydrocarbons and synthetic esters are seeing increased use as transformer coolants. Both types of materials are very stable at relatively high temperatures, but the high cost somewhat limits their use to only the most critical applications. Silicones and halogenated fluids have been used in the past but are seldom used now because both exhibit biological persistence in the environment if spilled.

 

Cooling Classes

Historically, the cooling class of a transformer was given by a 2-to 4-letter designation indicating air (A), water (W), mineral insulating oil (O), or a synthetic nonflammable insulating liquid (L) and whether natural convection (N) or forced convection (F) was used. The cooling class of dry-type transformers is defined in IEEE C57.94-1982 (R-1987) (see Figure 4).

 

Figure 4. The cooling class of dry-type transformers tells whether the transformer is ventilated and whether the transformer is self-cooled or forced-air cooled
Figure 4. The cooling class of dry-type transformers tells whether the transformer is ventilated and whether the transformer is self-cooled or forced-air cooled

 

The cooling class of liquid-immersed transformers historically had a similar designation. The cooling class for liquid-immersed transformers has recently been replaced. The cooling class of liquid-immersed transformers is now defined in IEEE C57.12.00-2000. This standard provides a 4-letter designation that indicates specific criteria relative to the type of oil, how the oil is circulated, what is used to cool the oil, and how the oil is cooled externally (see Figure 5).

 

Figure 5. The cooling class of liquid-immersed transformers describes the type of oil, how the oil is circulated, what is used to cool the oil, and how the oil is cooled externally.
Figure 5. The cooling class of liquid-immersed transformers describes the type of oil, how the oil is circulated, what is used to cool the oil, and how the oil is cooled externally.

 

Temperature Limits

The temperature rise in a transformer is the difference between the hotspot maximum core temperature at full load and the temperature when not operating. The temperature rise must be limited so that the local temperature does not exceed the insulation rating. The amount of transformer temperature rise depends on the transformer design, ambient conditions, and the load. Common design factors that influence the amount of temperature rise include the diameter of the winding wire and the size and design of the core.

 

Figure 6. The temperature rating of a transformer is determined by the allowable temperature rise and the hot spot allowance temperature rise.
Figure 6. The temperature rating of a transformer is determined by the allowable temperature rise and the hot spot allowance temperature rise.

 

There are several common transformer ratings based on the allowed temperature rise (see Figure 6). For example, Class 105 (A) insulating materials are limited to use in transformers that are designed to have a continuous full-load temperature rise not exceeding 55°C over a 40°C ambient temperature. The standards allow for the hottest spot temperature of 10°C over the normal temperature rise. Therefore, a Class 105 (A) transformer under full load has an average conductor temperature of 95°C when operating in a 40°C ambient temperature and a maximum conductor hottest spot temperature of 105°C. Other classes have different values for the allowable temperature rise and the hotspot temperature rise.