Dry Type Transformer: Types, Insulation & Applications | Yawei

A dry type transformer transfers electrical power without using conventional transformer oil for insulation and cooling. Instead, it uses solid insulation materials, resin, air, or, in some special designs, gas.

This makes dry type transformers a practical option for many indoor and environmentally sensitive installations. You will often find them in commercial buildings, factories, data centers, hospitals, renewable energy projects, rail systems, and other facilities where fire safety and installation conditions matter.

There is no single design that fits every project, though. Resin-type transformers, VPI transformers, polyarylamine insulated transformers, SF₆ gas insulated transformers, and cable-type dry transformers all take slightly different approaches to insulation and cooling.

So, when someone says "dry type transformer," it is worth asking what type they actually mean.

What Is a Dry Type Transformer?

A dry type transformer uses non-liquid insulation to isolate its windings and core. Since the transformer still produces heat from core and winding losses, it also needs a suitable cooling system to keep the temperature under control.

A typical dry type transformer contains a magnetic core, high-voltage winding, low-voltage winding, insulation system, terminals, mechanical supports, and cooling arrangement. Depending on the design, temperature sensors, fans, an enclosure, and protection equipment may also be included.

One obvious advantage is that there is no conventional transformer oil tank. This eliminates the risk of oil leakage and can make dry transformers a good fit for indoor electrical rooms and areas where liquid insulation is undesirable.

That said, "dry" does not mean maintenance-free. The transformer still needs proper ventilation, inspection, cleaning, temperature monitoring, and protection. The installation environment can make a big difference.

Resin-Type Dry-Type Transformers

Resin-type dry-type transformers are widely used in industrial and commercial power systems. Resin forms a major part of the winding insulation and helps protect the conductors against moisture, dust, contamination, and mechanical stress.

There are several resin-based technologies, and they are not all exactly the same.

Resin-Filled Casting

With resin-filled construction, the winding is surrounded or filled with insulating resin. Once the resin cures, it forms a solid insulation structure around the winding.

This can be useful in environments where moisture or contamination is a concern. For example, industrial facilities may have dust, humidity, or airborne pollutants that could cause trouble for less-protected winding systems.

But resin quality matters. Quite a lot, actually. Voids, cracks, trapped air, or poor curing can weaken the insulation system and may contribute to partial discharge in high-voltage applications.

Resin Casting

Resin casting is a manufacturing process in which the winding is placed inside a mold and insulating resin is introduced around it. The resin is then cured under controlled conditions.

The process may sound straightforward, but there is plenty going on behind the scenes. Resin temperature, viscosity, vacuum conditions, curing time, and insulation thickness all need to be controlled carefully.

Good casting helps reduce internal air pockets and other defects. That is important because even small imperfections inside high-voltage insulation can become a problem after years of electrical and thermal stress.

Resin Winding

In a resin winding, resin becomes an important part of the winding insulation system. Copper or aluminum conductors may be combined with resin, glass fiber, insulating paper, or insulating film.

The winding needs to handle both electrical and mechanical stress. When the transformer is heavily loaded, the winding heats up. When the load drops, it cools down. This cycle happens again and again during normal operation.

For that reason, the insulation system needs good electrical performance as well as enough mechanical strength to withstand thermal expansion and contraction.

Resin Vacuum Pressure Impregnation

Resin vacuum pressure impregnation (VPI) is another resin insulation technology used in dry type transformers.

During VPI, the winding is exposed to resin under controlled vacuum and pressure conditions. This allows the resin to penetrate the insulation structure more effectively while helping reduce trapped air.

After curing, the winding becomes mechanically stronger and better protected from moisture and environmental influences.

VPI and resin casting are sometimes mentioned together, but they are not the same manufacturing process. The winding structure and production method are different, so the choice depends on the transformer's intended application.

Polyarylamine Insulated Transformers

Polyarylamine insulated transformers use polyarylamine-based polymer materials as part of the insulation system.

Polyarylamine is a high-performance polymer with useful electrical and thermal properties. In transformer construction, advanced polymer insulation can provide additional flexibility when designing compact winding structures and insulation systems.

This type of transformer may be considered where good electrical insulation, thermal performance, and compact construction are important.

Still, it would be a mistake to judge a transformer by its insulation material alone. Core design, conductor selection, winding arrangement, cooling, manufacturing quality, and operating conditions all work together.

In real-world projects, the complete insulation system is what matters.

SF₆ Gas Insulated Transformers

SF₆ gas insulated transformers use sulfur hexafluoride (SF₆) gas instead of conventional liquid insulation.

SF₆ has high dielectric strength and has been used for many years in high-voltage electrical equipment. Under suitable pressure and within a properly designed enclosure, it can provide effective insulation while supporting compact equipment designs.

However, SF₆ systems also come with some special requirements.

Insulation Properties of SF₆ Gas

One of the main advantages of SF₆ is its strong dielectric performance. It can provide effective electrical insulation between energized components when the gas pressure and equipment design are appropriate.

But it is not simply a case of putting SF₆ inside a metal enclosure and calling it done. Gas pressure, temperature, electrical field distribution, enclosure design, and gas tightness all need careful attention.

Heat Dissipation of SF₆ Gas and Transformer Cooling

Transformers generate heat regardless of whether the insulation medium is oil, air, resin, or gas.

In an SF₆ gas insulated transformer, heat from the core and windings moves into the gas and then needs to be transferred through the enclosure and cooling system.

Depending on the transformer design, cooling may use natural gas circulation, forced gas circulation, or an external cooling arrangement.

Thermal design is therefore just as important as dielectric design. A transformer may have excellent insulation but still face problems if heat cannot be removed effectively.

SF₆ Gas Insulated Transformer Components

The exact configuration depends on the manufacturer and application, but an SF₆ gas insulated transformer can include the core, windings, gas-filled enclosure, terminals or bushings, gas monitoring equipment, pressure-control devices, cooling equipment, and protection systems.

Gas pressure monitoring is particularly important. If gas leaks and pressure falls below the required level, insulation performance may be affected.

SF₆ Greenhouse Effect

There is also an environmental issue that should not be overlooked.

SF₆ is a very potent greenhouse gas and can remain in the atmosphere for a long time. As a result, manufacturers and operators need to pay close attention to leakage prevention, gas recovery, recycling, and end-of-life handling.

Environmental regulations are also becoming more important in electrical equipment procurement. For projects considering SF₆ technology, it makes sense to evaluate both its technical benefits and its environmental requirements.

Cable-Type Dry-Type Transformers

Cable-type dry-type transformers use insulated cable or cable-like conductors as part of the winding system.

Instead of using conventional bare or strip conductors, insulated cable technology is integrated into the winding. This can help achieve particular insulation arrangements and, in some designs, more compact construction.

Cable-type dry transformers are generally more specialized than standard resin-cast transformers. They may be considered when a project has specific voltage, insulation, space, or winding requirements.

The cable insulation must still deal with electrical stress, heat, mechanical forces, and repeated temperature changes. So again, thermal design and insulation quality are both important.

How to Choose the Right Dry Type Transformer

Choosing the right dry type transformer starts with the electrical system, not the product brochure.

Rated capacity, primary and secondary voltage, frequency, impedance, vector group, insulation level, temperature rise, cooling method, ambient temperature, installation altitude, enclosure rating, and applicable standards all need to be checked.

The environment is just as important.

A factory may expose the transformer to dust, moisture, chemicals, vibration, or changing loads. A data center may have continuous high loading, harmonics, high power density, and a strong need for future expansion.

For indoor installations, ventilation is especially easy to overlook. A transformer can be properly designed and still run too hot if the electrical room cannot remove enough heat.

Dry Type Transformer Cooling

Cooling is one of the key factors in dry transformer design.

Many dry type transformers use AN cooling, meaning air natural cooling. Heat generated in the core and windings is transferred through the transformer structure and released into the surrounding air.

For larger ratings or heavier loads, AF cooling, or air forced cooling, may be used. Fans increase airflow and help remove heat more effectively.

The cooling method affects temperature rise and continuous load capability. But the room ventilation matters too. A poorly ventilated transformer room can reduce the practical performance of even a well-designed transformer.

Applications of Dry Type Transformers

Dry type transformers are used in a wide range of applications, including commercial buildings, factories, hospitals, data centers, shopping centers, high-rise buildings, tunnels, rail systems, renewable energy projects, and infrastructure facilities.

They are particularly attractive where avoiding transformer oil can simplify fire-safety planning or reduce concerns about liquid leakage.

Solar and wind projects may also use dry type transformers for certain power conversion and distribution applications. The final choice depends on voltage, capacity, load profile, environmental conditions, and project specifications.

Dry Type Transformer vs. Oil-Immersed Transformer

Neither technology is automatically better.

A dry type transformer is often a good choice for indoor electrical rooms, commercial facilities, tunnels, hospitals, and data centers where fire safety and the absence of liquid insulation are important.

Oil-immersed transformers remain widely used in outdoor substations, power plants, utilities, and high-capacity transmission and distribution systems. Transformer oil provides both electrical insulation and efficient heat transfer, which can be a significant advantage for large power transformers.

In the end, the decision should be based on the complete project-not just the transformer type.

Standards and Manufacturing Quality

A reliable dry type transformer depends on much more than the insulation material.

Core construction, conductor quality, winding design, resin processing, mechanical assembly, cooling, and factory testing all influence transformer performance and service life.

Depending on the application, dry type transformers may be designed and tested according to IEC 60076-11, together with applicable national standards and project specifications.

Typical factory tests may include winding resistance, voltage ratio, insulation, no-load loss, load loss, dielectric, temperature rise, sound-level, and partial discharge tests where applicable.

For EPC contractors and procurement teams, it is worth comparing more than price and kVA rating. Two transformers with the same rated capacity can have very different insulation systems, manufacturing quality, cooling performance, and expected service life.

Conclusion

The dry type transformer is an important solution for modern electrical distribution, particularly where indoor installation, fire safety, environmental protection, and reduced oil-related maintenance are priorities.

Resin-type transformers use technologies such as resin casting, resin winding, and resin vacuum pressure impregnation. Polyarylamine insulated transformers use advanced polymer materials for specialized insulation systems. SF₆ gas insulated transformers provide strong dielectric performance but require careful gas management because of the SF₆ greenhouse effect. Cable-type dry-type transformers use insulated cable technology for more specialized winding applications.

The most complicated technology is not always the best choice. Sometimes a conventional resin-cast transformer is exactly what a project needs. In other situations, a specialized insulation or winding system may be a better fit.

For Yawei Transformer, the practical approach is to start with the actual electrical system and working environment. Voltage, capacity, cooling, insulation, installation conditions, and long-term operating requirements all need to work together.

That is what turns a dry type transformer from a product on a specification sheet into reliable equipment that can keep an electrical system running day after day.

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