Dry-type transformers fundamentals

What is a transformer?

A transformer is an electrical device that is used in AC power distribution systems to safely “step up” or “step down” voltages to meet the incoming power requirements of connected equipment. Transformers achieve this goal without modifying the frequency or amount of electrical power to help ensure connected devices can operate reliably.

Transformers are static devices, meaning they contain no continuously moving mechanical parts. This contributes to their long lifecycle and minimal maintenance requirements. They can range in size from enormous devices that weighs several tons, commonly used in power substations, to very small devices that weigh only a few ounces and are commonly found in compact electrical equipment.

Thanks to their design flexibility and critical role in the electrical system, transformers are used in nearly every power distribution system application including commercial buildings, schools, healthcare & institutions, water/wastewater, industrial, oil & gas and data centers.

How do transformers work?

The typical transformer has several secondary windings or coils of insulated wire conductor wrapped around a laminated steel core. When voltage is introduced to one coil, called the primary, it magnetizes the iron core. A voltage is then induced in the other coil, called the secondary or output coil. The change of voltage (or voltage ratio) between the primary and secondary depends on the ratio of the turns in the two coils.

There are two primary types of transformers that are defined by the insulation used within the enclosure: dry-type and liquid-filled. On this page, we will explore the basics of dry-type transformers and their most common configurations.  

These transformers use natural or fan-assisted circulation through ventilation openings to maintain temperature requirements.

Also known as compound-filled or potted transformers, encapsulated designs enclose windings within a mixture of solid-insulating materials such as resin, gravel, sand or epoxy to help dissipate heat.

The completely enclosed design of non-ventilated transformers is made for applications where the atmosphere may contain conductive, corrosive or combustible materials that could damage the transformer, and works by dissipating heat through the surface area of the enclosure. 

1. K-Factor rated

K-factor transformers are specifically designed to withstand the harmful overheating effects caused by harmonics generated by nonlinear (non-sinusoidal) loads. These loads include computers, laser printers, copiers, and other office equipment, as well as video monitors and other electronic equipment. K-factor transformers are not simply oversized transformers. The core and coils of K-factor transformers are designed for reduced induction levels, which results in less stray losses. K-factor transformers do not reduce or mitigate harmonic levels but are specially designed to withstand the harmful heating effects of the harmonic currents produced by nonlinear loads. Oversized (200% rated) neutrals and electrostatic shielding are typical features of K-factor transformers. 

2. Harmonic mitigation

Harmonic currents produced by nonlinear loads can cause additional heating which may overload transformers, generators and conductors. Harmonic mitigation transformers, when properly applied within an electrical system, will cancel out harmonic currents to keep the loads operating the way the manufacturer designed them and keep the facility’s electrical system free from voltage distortion. These devices are suited for installations rich in harmonic loads, such as educational facilities (K–12 and universities), government buildings, commercial office buildings, medical facilities and call-center applications. 

3. Breaker Integrated

Breaker Integrated Transformers (BITs) provide a simple, cost-effective approach to reducing arc flash risk and increasing flexibility for mounting secondary panels. By integrating the secondary molded-case circuit breaker into the low-voltage dry-type distribution transformer package, arc flash incident energy values for secondary panels can be greatly reduced (usually to less than 8 cal/cm2). Additionally, with the transformer secondary protection integrated into the transformer, placement of the secondary panel is not required to be within 10 feet to meet NEC requirements. This allows total flexibility in the placement of the secondary panels to be closer to loads, reducing individual branch conductor lengths/voltage drop or placement in any available location when existing space is limited. BITs can be supplied with integrated primary breakers, secondary breakers or both, providing ultimate flexibility to meet specific application disconnect and overcurrent protection needs.

4. Motor drive isolation

Motor drive isolation transformers are designed for three-phase variable  speed drive load profiles. Sized by horsepower and common motor voltages, motor drive isolation transformers are braced to withstand the mechanical stresses associated with AC or DC  variable speed drives. The two-winding drive isolation transformers provide:

• Electrical isolation between the incoming line and drive SCR circuitry
• Voltage conversion on input line to standard drive input voltages
• Help to minimize line disturbances caused by variable speed  drive rectifiers
• A reduction in short-circuit currents and voltage line transients 

Motor drive isolation transformers are manufactured as ventilated or encapsulated. Ventilated transformers include a normally open, dry contact temperature sensor installed in the coils that provides advance warning of potential transformer overheating. 

5. Class 1, Division 2 hazardous area

Many petrochemical plants, refineries and other industrial facilities have atmospheres unsuitable for the safe application of either liquid-filled or general-purpose dry-type transformers. Transformers for Class I, Division 2, Groups A, B, C and D locations meet the stringent standards set forth by NEC® Article 501, with NEC-recommended installation procedures for dry-type transformers rated for under 600V nominal operation.

These solutions are used in locations where volatile flammable liquids or gases are handled, processed or used, and might become harmful to the transformer following failure of ventilation equipment.

6. Marine-duty 

Marine-duty transformers are “Type Approved” by the American Bureau of Shipping (ABS) for on-board use in steel vessels (not for propulsion systems or combat vessels). These transformers are typically installed below deck in electrical or mechanical rooms where the ambient temperature is greater than normal. Marine-duty transformers are especially designed for operation in 50 °C ambient locations and copper windings are standard.

7. Buck-boost transformers 

Buck-boost transformers are suited to applications where the available voltage needs to be slightly boosted or bucked (reduced). When buck-boost transformers are wired as autotransformers, they can be used to accomplish this bucking or boosting of voltage automatically. Buck-boost transformers are single-phase encapsulated transformers, available in three-voltage combinations:

• 120 V x 240–12/24 V
• 120 V x 240–16/32 V
• 240 x 480 V–24/48 V

Specific combinations of these buck-boost transformers are applied to output the desired voltage level and load kVA requirement. Since buck-boost transformers are only slightly boosting or bucking voltage levels and are typically wired as autotransformers, the kVA size of the actual buck-boost transformers is typically very small compared to the kVA requirement of the load. Buck-boost transformers only range in size from 0.05 kVA through 7.5 kVA, but can be applied to boost or buck the voltage to loads as large as hundreds of kVA.

These transformers can also be used at their nameplate voltages for applications such as low voltage interior or landscape lighting.

When you need more than “off-the-shelf,” Eaton's Transformer Flex Center is available to help meet unique general-purpose and distribution transformer needs that cannot be met with standard offerings.

With the ability to engineer or modify transformers to meet nearly any application requirement, our expert team solves critical application challenges.