Dry-type transformers are a specialized type of power transformer characterized by the fact that their core and windings are not immersed in insulating oil. This raises the question: how do dry-type transformers achieve cooling and insulation without the use of insulating oil, which is commonly used in oil-immersed transformers?
【 Cooling Methods 】
Dry-type transformers typically employ two primary cooling methods:
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Natural Air Cooling (AN):
- In this method, the transformer relies on natural air convection to dissipate heat when operating at or below its rated capacity. The design allows for adequate airflow around the transformer, enabling it to operate continuously without the need for additional cooling equipment. This method is simple, cost-effective, and suitable for standard operating conditions.
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Forced Air Cooling (AF):
- When the ambient temperature rises or the transformer operates under overload conditions, a temperature control system automatically activates fans to provide forced air cooling. These fans enhance air circulation, significantly improving the transformer's cooling efficiency. This allows the transformer to maintain normal operation even in high-temperature environments or during periods of increased load. Essentially, the transformer uses air flow for cooling, with fans providing auxiliary cooling if the temperature exceeds a set threshold (typically around 80°C).
【 Insulation Methods 】
Since dry-type transformers do not use oil for insulation, they rely on other materials and techniques to ensure electrical isolation and protection:
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Solid Insulation:
- Dry-type transformers often use solid insulating materials such as resin-impregnated paper, glass fiber, or epoxy resins. These materials provide excellent dielectric strength and mechanical support, ensuring reliable insulation between the windings and the core.
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Vacuum Pressure Impregnation (VPI):
- In some cases, the windings are treated using a process called Vacuum Pressure Impregnation (VPI). This involves immersing the windings in a liquid resin under vacuum conditions, followed by applying pressure to ensure complete impregnation. VPI enhances the insulation properties, improves thermal conductivity, and increases the transformer's resistance to moisture and contaminants.
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Class F and Class H Insulation Systems:
- Dry-type transformers often utilize Class F (155°C) or Class H (180°C) insulation systems, which are designed to withstand higher temperatures. These systems allow the transformer to operate safely at elevated temperatures, extending its lifespan and improving overall performance.
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Encapsulation:
- Some dry-type transformers are fully encapsulated, meaning the windings are encased in a solid insulating material. This provides additional protection against environmental factors such as dust, humidity, and corrosive gases, making them suitable for harsh industrial environments.
【 For insulation 】
dry-type transformers primarily use epoxy resin as the insulating material, employing various manufacturing processes to ensure optimal insulation performance. The specific types include:
- Epoxy and Quartz Sand Mixture Vacuum Cast Type: This method involves mixing epoxy resin with quartz sand and then vacuum casting it around the windings to provide robust insulation.
- Epoxy and E-Glass Fiber Reinforced Vacuum Pressure Impregnation (VPI) Type: In this process, epoxy resin is combined with alkali-free glass fibers and impregnated under vacuum pressure, enhancing both mechanical strength and dielectric properties.
- E-Glass Fiber Wound and Impregnated Type: This type uses alkali-free glass fibers that are wound around the windings and then impregnated with epoxy resin to create a durable and highly insulating structure.
【 Advantages of Dry-Type Transformers 】
Dry-type transformers do not contain flammable or explosive oils, significantly reducing the risk of fire or explosion during operation. This enhances overall safety. Additionally, they offer excellent electrical insulation, effectively preventing leakage and short-circuit incidents.
- Environmental Friendliness:
Since dry-type transformers do not use oil for cooling or insulation, they eliminate the risk of oil pollution, making them environmentally friendly. They are ideal for applications where environmental impact is a concern.
Dry-type transformers are designed and manufactured with a focus on improving energy efficiency and reducing losses. By optimizing coil structures, using high-efficiency insulating materials, and employing advanced manufacturing techniques, they achieve lower energy consumption and higher operational efficiency.
Utilizing advanced manufacturing processes and high-quality materials, dry-type transformers have a longer lifespan and higher reliability. Under normal operating conditions, they have a low failure rate, ensuring stable power system performance.
【 Disadvantages of Dry-Type Transformers 】
- Limited Moisture and Dust Protection:
Dry-type transformers have relatively weaker protection against moisture and dust. In environments with high humidity or significant dust levels, additional protective measures may be necessary, such as installing protective covers or constructing dedicated switchgear rooms to maintain insulation integrity.
- Lower Tolerance for Harsh Environments:
Compared to oil-immersed transformers, dry-type transformers are generally better suited for indoor installations. Outdoor use requires special attention to protection and cooling to ensure proper operation and extend their lifespan.
- High Requirements for Insulation Materials:
Dry-type transformers demand high-performance insulation materials that offer good thermal conductivity and electrical insulation, such as specially treated epoxy resins. This can increase manufacturing costs.
- Difficulty in Coil Repair:
If the coils of a dry-type transformer are damaged, local repairs are often difficult, and in many cases, the entire unit must be replaced. This increases the complexity and cost of maintenance.
【 Differences Between Dry-Type and Oil-Immersed Transformer 】
Visually, dry-type and oil-immersed transformers are easily distinguishable. Oil-immersed transformers are encased in a metal tank filled with oil, while dry-type transformers allow a direct view of the core and windings.

As previously mentioned, oil-immersed transformers utilize insulating oil for both cooling and insulation. In contrast, dry-type transformers rely on air for cooling and solid insulating materials for electrical isolation. This fundamental difference in cooling and insulation methods is one of the key distinctions between the two types of transformers.
Dry-type transformers lack the insulating oil used in oil-immersed transformers, which results in a more compact design with reduced volume and weight, making them easier to transport and install. The windings and core of dry-type transformers are not immersed in oil; instead, they are typically wrapped in multiple layers of insulating materials, such as epoxy resin, glass fiber, or other high-performance dielectrics. An additional protective layer is often added over the insulating materials to enhance durability and environmental resistance. The core is constructed from stacked laminated silicon steel sheets, which minimize magnetic flux losses and improve efficiency.
Dry-type transformers are particularly well-suited for indoor installations due to their fire-resistant and oil-free characteristics. They are commonly used in environments where fire safety and environmental concerns are paramount, such as in commercial buildings, hospitals, data centers, and industrial facilities that require "fireproof" and "explosion-proof" equipment. Their compact size and low maintenance requirements make them ideal for space-limited areas.
Oil-immersed transformers, on the other hand, are more commonly installed outdoors and are widely used in large-scale power systems, such as grid transmission, substations, and industrial applications. They excel in outdoor environments due to their superior cooling performance, ability to handle higher loads and voltages, and longer lifespan under heavy-duty conditions. The use of insulating oil provides excellent thermal management and dielectric properties, making oil-immersed transformers suitable for high-power applications where reliability and capacity are critical.