Dry Type Transformer & Oil-Immersed Transformer Definition
What is a dry type transformer? Dry type transformer whose core and windings are not immersed in insulating oil, but are insulated by natural air or filling materials(solid insulation such as epoxy or cast resin.
There are several different types of dry type transformers on the market, including:
- Vacuum Pressure Impregnated (VPI) Transformers
- Vacuum Pressure Encapsulated (VPE) Transformers
- Open Wound Transformers
- Cast Coil Transformers
Oil-filled or oil-immersed transformers are a power transformer that immerses the core and windings in special transformer oil, relying on the excellent insulation properties and thermal conductivity of the oil to achieve the insulation and cooling functions of the equipment.
Brief History of Transformer Development
The transformer was invented in 1886. At that time, all transformers were dry type transformers, sometimes referred to as “dry transformers”. Due to the level of insulation materials at that time, it was difficult for dry transformers to achieve high voltage and large capacity. Since the end of the 19th century, people have found that the use of transformer oil can greatly improve the insulation and cooling performance of transformers, so oil-immersed transformers have gradually replaced dry transformers. After World War II, the world economy was restored and rebuilt. With the continuous growth of urban power supply loads, the density of residential buildings, and the increase in high-rise and underground buildings, people urgently need a transformer that is both in-depth and fireproof, explosion-proof, and has excellent environmental protection performance. Therefore, dry transformers have been re-valued and adopted, but oil-immersed transformers have lower costs and more stable performance. Its market share is still higher. In 2023, the global oil-immersed transformer market share is about 70%~75%, and about 80% in China. However, many customers have noted that dry type transformers are more profitable and require less after-sales service, as they are more reliable and less prone to failure. As a result, demand for dry type transformers is on the rise.
Difference between dry transformer & oil transformer
In the distribution system, dry transformers and oil-immersed transformers have their own advantages due to their different structures, performance, and application environments. The following systematic comparison of the two from multiple dimensions helps users make reasonable selections in different projects.
Insulation
Dry type and liquid-filled transformers mainly differ in their insulation materials and cooling methods.
For dry type transformers, the insulation system depends on the transformer design.
Ventilated dry type transformers usually use high-temperature insulation materials, such as Nomex aramid paper, varnish, and air insulation.
Cast resin transformers use epoxy resin to fully encapsulate the windings, providing stronger protection against moisture, dust, and contaminants.
The insulation performance of cast resin transformers is generally better in harsh environments because the windings are sealed inside solid insulation. However, air is less effective as a cooling medium compared with liquid. Therefore, dry type transformers require careful thermal design and sufficient ventilation to maintain insulation life and operating efficiency.
Oil transformers use insulating liquids such as mineral oil, natural ester, or synthetic ester. Mineral oil is the traditional and most widely used option, but it has higher fire risk and greater environmental concerns. Natural ester fluids have a higher fire point, usually above 300°C, while mineral oil is around 160°C.
Therefore, oil-filled transformers do not always mean higher fire risks. However, advanced insulating fluids usually increase the overall equipment cost.
Cooling System
The cooling methods of dry type transformers depend on the transformer design. Ventilated dry type transformers usually rely on natural air cooling or forced air cooling with fans. Cast resin medium voltage transformers are also frequently configured with air cooling (AF), while fans can also be added for higher load requirements.
Oil-immersed transformers provide more cooling options. The liquid circulates inside the transformer and transfers heat to radiators or the tank surface. The heat is then released into the surrounding environment. This cooling method provides higher power density, better overload capability, and more uniform winding temperature at higher ratings.
Common liquid transformer cooling methods include:
- OFAF (Oil Forced Air Forced)
- OFWF (Oil Forced Water Forced)
- ODAF (Oil directed circulation air cooling)
- ODWF (Oil directed circulation water cooling)…
For more information about transformer cooling methods, please refer to: Oil Immersed Transformer Cooling Methods
Appearance and structure
Dry type transformers are small in size and compact in structure. Their cores and coils are usually exposed or epoxy-encapsulated, so users can directly see their internal structure; while oil-immersed transformers adopt a fully enclosed structure, and only the metal shell and terminals of the transformer can be seen from the outside. Regarding bushing, dry type transformers commonly use polymeric, epoxy-resin, or composite insulating bushings, offering good electrical insulation and environmental resistance. Oil-immersed transformers traditionally use porcelain bushings, which provide high dielectric strength and mechanical durability.
Capacity and voltage
Efficiency
The efficiency difference between dry type and liquid-filled transformers is not determined only by the insulation medium. It also depends on transformer design quality, load conditions, core materials, winding design, and cooling performance.
Dry type transformers
Because air has lower heat transfer capability than liquid, dry type transformers usually have lower short-term overload capability. However, this does not mean all liquid-filled transformers are always more efficient.
A well-designed dry type transformer can achieve very high efficiency under suitable operating conditions.
Oil-Immersed Transformer
Oil filled transformers generally have advantages in large-capacity applications. Their liquid cooling system provides better heat transfer and allows more compact designs. This can reduce temperature rise and improve performance under heavy loads.
In practical applications:
Dry type transformers are highly competitive in small and medium capacity systems.
Liquid-filled transformers usually provide advantages in large power systems and continuous high-load applications.
Transformer efficiency depends on the complete design rather than only the cooling medium.
Noise Performance
Both dry type and liquid-filled transformers generate operating noise, mainly caused by core magnetostriction, electromagnetic forces, and cooling equipment such as fans or pumps. The noise of the new three-dimensional wound core dry transformer is lower than that of the conventional oil-immersed transformer.
Dry type transformers may produce slightly higher audible noise because the core and windings are surrounded by air. In addition, forced-air cooling systems can further increase operating noise in high-load conditions.
Noise reduction methods include:
- Optimized core design to reduce magnetostriction
- Improved winding clamping to reduce vibration
- Anti-vibration mounting systems
- Low-noise fans
- Acoustic barriers or transformer rooms
Liquid-filled transformers often have lower perceived noise because the insulating liquid helps absorb mechanical vibration. However, actual noise performance depends on transformer design, capacity, and cooling methods. when overloaded (≥2 times the rated load), the fundamental frequency vibration amplitude of the core will still increase by 3~4 times, but the noise increase is <1dB(A).
Recyclability and Environmental Impact
Both dry type and liquid-filled transformers contain valuable recyclable materials, including copper or aluminum windings, electrical steel cores, and steel structures.
Dry type transformers have an environmental advantage because they do not contain insulating oil. This eliminates oil leakage risks and reduces the need for liquid handling during maintenance and end-of-life disposal.
Liquid-filled transformers require additional management because insulating fluids must be properly tested, recovered, recycled, or disposed of according to local regulations.
Overall environmental impact depends on multiple factors, including transformer efficiency, service life, manufacturing materials, maintenance practices, and end-of-life management.
Applicable environment
Dry transformers have good fire resistance, solid insulation is non-flammable, and they are oil-free and explosion-proof. They are mostly used in small-capacity indoor places, especially for small and medium-capacity power distribution needs. Oil-immersed transformers are generally used for outdoor large-capacity power supply, such as substations, and are suitable for spacious and well-ventilated environments. Oil-immersed transformers may have oil spraying or leakage after an “accident”, causing a fire, and there are places where “oil spill containment areas” are dug.
Cost: Initial vs. Lifecycle
Compared with liquid-filled transformers, dry type transformers usually have a higher initial purchase cost. This is mainly due to the use of advanced insulation materials, manufacturing processes, and additional cooling requirements.
However, lifecycle cost depends on more than the purchase price. Important factors include:
- Installation infrastructure
- Fire protection requirements
- Oil containment systems
- Maintenance requirements
- Energy losses during operation
- End-of-life disposal
Dry type transformers generally reduce maintenance and infrastructure costs because they do not require oil containment, oil testing, or liquid management. In indoor and safety-sensitive applications, these savings can help offset the higher initial cost over a 25–30 year service life.
Liquid-filled transformers usually have lower initial costs, especially in large-capacity applications. Their higher efficiency and strong cooling performance can provide long-term economic advantages in utility and industrial power systems.
Installation Environment
Indoor Installation
Dry type transformers are commonly used for indoor installations. Small to medium capacity dry type transformers are also lighter than liquid-filled units, making them practical for buildings with structural limitations or rooftop installations.
However, compared with oil-filled transformers of the same capacity, dry type transformers usually require more installation space. This is because air cooling is less efficient than liquid cooling and requires more ventilation space.
Overall, dry type transformers are the preferred choice for:
- Indoor installations
- Occupied buildings
- Locations with strict fire safety requirements
- Upper floors or weight-limited areas
High fire point ester fluids can expand indoor installation options for liquid transformers.
However, liquid management and containment systems are still important design considerations.
These requirements add infrastructure costs and complexity compared with dry type transformers.
Outdoor Installation
Standard ventilated dry type transformers are not suitable for direct outdoor exposure without proper protection.
Outdoor installation requires suitable enclosures to prevent moisture, dust, and environmental damage.
Ventilated dry type transformers are generally not recommended for outdoor applications.
For outdoor projects, oil-filled transformers usually have advantages.
Their liquid cooling system provides better heat dissipation.
With proper tanks, coatings, and protection systems, they can operate reliably in outdoor environments.
This makes oil-filled transformers widely used in:
- Utility substations
- Power plants
- Industrial outdoor installations
Maintenance
The maintenance cost of oil-immersed transformers is higher than that of dry type transformers. Dry type transformers generally require minimal routine maintenance. Typical maintenance includes visual inspections, cleaning, checking airflow, and periodic insulation resistance testing. Since there is no insulating liquid, there is no need for liquid testing, sampling, or replacement.
Oil-immersed transformers require regular checks of the oil level, oil temperature, oil quality, gas content (DGA), bushing sealing, etc., and filter oil at least once a year. They require sampling procedures and trained personnel or third-party laboratories.
For more information, please refer to: Dry Type Transformer Maintenance Guide
Final Thoughts
Transformers are core equipment in power systems. The technical essence of dry type and oil-immersed transformers is a trade-off between insulation medium and heat dissipation capacity. Dry type transformers have become the inevitable choice for safety-sensitive scenarios such as urban buildings and underground facilities due to their oil-free fire prevention, compact structure and low maintenance cost; while oil-immersed transformers rely on the excellent thermal stability and overload capacity of the oil-paper insulation system to firmly dominate the field of high-voltage and large-capacity power transmission and distribution. The coexistence of the two is not a replacement relationship, but a complementary technical route.
