What Are the Different Types of Transformers?

Transformers all perform the same basic job: they change electrical voltage from one level to another. But the physical design, installation, insulation system, cooling method, connections, voltage class, and application can vary significantly.

For sourcing purposes, knowing the type of transformer is one of the best places to start. It tells you which specifications matter next and helps narrow the search much faster.

GridGear works primarily with six transformer families:

• Pad-Mounted Transformers
• Substation Transformers
• Unit Substation Transformers
• Dry-Type Transformers
• Pole-Mounted Transformers
• Specialty Transformers

Pad-Mounted Transformers

Pad-mounted transformers are enclosed transformers designed to sit outdoors on a concrete pad, usually as part of an underground distribution system. Primary and secondary cable connections are housed inside a tamper-resistant cabinet, which makes pad-mounted transformers common in commercial developments, campuses, data centers, industrial sites, utility systems, and other locations where equipment may be accessible from ground level.

Pad-mounted transformers are commonly liquid-filled and may be configured for radial or loop feed, with dead-front or live-front primary connections.

Common identifying information

• kVA rating
• Primary voltage
• Secondary voltage
• Phase
• Radial or loop feed
• Dead-front or live-front
• Primary bushing interface
• Fluid type
• Winding conductor material, if specified
• Existing footprint or fit constraints when replacing a unit

IEEE C57.12.34 addresses three-phase, pad-mounted, compartmental-type distribution transformers and is one of the principal standards references for this equipment family.

Substation Transformers

Substation transformers are generally larger transformers used in utility, industrial, infrastructure, and large commercial power systems. They are often installed in secured electrical yards or substations and may have exposed bushings, radiators, fans, gauges, controls, and other external equipment.

Substation transformers can range from distribution-size units into large MVA-class power transformers. Electrical matching can become especially important when replacing existing equipment.

Common identifying information

• kVA or MVA rating
• High-side voltage
• Low-side voltage
• Phase and frequency
• Cooling class
• Tap changer type, such as DETC or LTC
• Impedance
• BIL
• Winding connection or vector relationship
• Winding conductor material, if specified
• Fluid type
• Physical interfaces or footprint constraints

Where do mobile transformers fit?

For GridGear's public-facing equipment guidance, a mobile or trailer-mounted transformer is best understood as a substation or power transformer mounted on a transportable platform rather than as a completely separate electrical category.

Mobility matters operationally, but the transformer is still defined by its electrical ratings, voltages, impedance, BIL, cooling, taps, connections, and other power-transformer characteristics.

Unit Substation Transformers

A unit substation transformer is designed to operate as part of a close-coupled power-distribution assembly rather than as a completely independent transformer.

A typical unit substation combines:

1. Primary distribution equipment
2. A transformer
3. Secondary switchgear or a switchboard

Because the equipment is physically integrated, the interface to the adjacent gear can matter just as much as the electrical ratings when replacing a unit.

Common identifying information

• kVA rating
• Primary voltage
• Secondary voltage
• Phase
• Construction or insulation type
• Which side is close-coupled
• Primary and secondary termination type
• Throat, bus, flange, or transition requirements
• Winding conductor material, if specified
• Existing footprint and adjacent-equipment constraints

A unit substation transformer may be liquid-filled or dry-type. "Unit substation" describes how the transformer is integrated into the distribution system, not necessarily the insulation or cooling technology used inside the transformer.

Dry-Type Transformers

Dry-type transformers do not use liquid as the primary insulating and cooling medium around the windings. Instead, they use air and solid insulation systems.

Common dry-type construction methods include:

• VPI
• VPE
• Cast coil

Dry-type transformers are widely used in commercial and industrial facilities and are frequently incorporated into unit substations.

Common identifying information

• kVA rating
• Primary voltage
• Secondary voltage
• Phase
• Construction type
• Enclosure
• Indoor or outdoor installation
• Cooling arrangement
• Winding conductor material, typically copper or aluminum
• Temperature-rise requirement, if specified
• Impedance
• BIL where applicable
• Physical fit or sound constraints when relevant

Does copper vs. aluminum winding material matter?

Transformer windings may use copper or aluminum conductors. Some project specifications or replacement requirements explicitly call for one material, so winding conductor material can be important when matching an existing specification or sourcing a replacement.

Copper should not automatically be treated as "good" and aluminum as "bad." Properly designed transformers can use either material successfully. The complete transformer design, ratings, temperature performance, losses, mechanical design, and application matter more than conductor material alone.

If the project specification requires copper or aluminum, include that requirement in the sourcing request.

IEEE C57.12.01 provides general requirements for dry-type distribution and power transformers.

Pole-Mounted Transformers

Pole-mounted transformers are distribution transformers designed for installation on utility poles. They are commonly single-phase units used to step distribution voltage down for residential and smaller commercial loads.

Three single-phase pole-mounted transformers may also be installed as a bank to provide three-phase service.

Common identifying information

• kVA rating
• Primary voltage
• Secondary voltage
• Single-phase unit or three-unit bank
• Conventional or CSP design
• Fluid type
• Primary fusing and protection
• Winding conductor material, if a specification requires it

IEEE C57.12.20 covers overhead-type distribution transformers within the ratings and voltage ranges defined by that standard.

Specialty Transformers

"Specialty transformer" is a broad category for transformers designed around a particular electrical function or application rather than one of the standard installation families above.

Examples may include:

• Grounding transformers
• Generator step-up transformers
• Drive-duty transformers
• Rectifier or converter-duty transformers
• Autotransformers
• Harmonic-mitigating transformers
• Other application-specific designs

For specialty equipment, the application is often the first identifying specification. From there, voltage, capacity, impedance, BIL, winding configuration, cooling, construction, and other application-specific requirements determine what will work.

Because specialty transformers vary substantially, GridGear evaluates these opportunities case by case.

Transformer Types Can Overlap

Transformer terminology is not always mutually exclusive.

For example:

• A dry-type transformer can also be part of a unit substation.
• A substation transformer can be stationary or mobile/trailer-mounted.
• A transformer may be described by installation style, insulation system, application, or electrical function.

So the most useful question is not always simply, "What type is it?"

It is:

That is why the next step after identifying the transformer family is usually to examine the nameplate and any available drawings or specifications.

What Information Matters Most When Sourcing a Transformer?

Once the general transformer type is known, the most useful electrical identifiers usually include:

Primary identification

• kVA or MVA rating
• Primary or high-side voltage
• Secondary or low-side voltage
• Phase
• Frequency

Important matching information, especially for replacements

• Impedance
• BIL
• Winding connection or vector relationship
• Tap arrangement and range
• Cooling class where applicable
• Winding conductor material when specified

Type-specific information

The remaining details depend heavily on transformer family. A pad-mounted transformer may require feed and bushing information. A unit substation may require throat or close-coupling details. A dry-type unit may require construction and enclosure information. A substation transformer may require cooling, tap-changer, bushing, and interface details.

You do not need every field before GridGear can begin evaluating the requirement.

Need Help Identifying or Sourcing a Transformer?

If you know the transformer type and ratings, send them. If you do not, send the information you have.

A clear nameplate photo, one-line drawing, specification sheet, old quote, or equipment photos can often provide enough information to begin narrowing the requirement.

GridGear works with buyers to source hard-to-find transformers across new surplus, unused, remanufactured, and used equipment markets.

Request a Transformer Quote

Technical References

This resource is GridGear-authored educational content informed by industry standards and technical references. Relevant standards include:

• IEEE C57.12.34 — Pad-Mounted, Compartmental-Type, Self-Cooled, Three-Phase Distribution Transformers
• IEEE C57.12.01 — General Requirements for Dry-Type Distribution and Power Transformers
• IEEE C57.12.20 — Overhead-Type Distribution Transformers

Standards define equipment requirements more precisely than this introductory guide and should be consulted when formal engineering or procurement compliance is required.