As an indispensable key equipment in power systems, transformers undertake the critical functions of voltage transformation, power distribution, and transmission. According to differences in cooling methods and insulation media, transformers are mainly classified into two categories: dry-type transformers and oil-immersed transformers. These two types of transformers have significant differences in structure, performance, application scenarios, etc. Understanding their differences is of great significance for the design, selection, operation, and maintenance of power systems.
Dry-type transformers use air as the cooling medium, and their core structure includes:
- Iron Core: Usually made of high-quality cold-rolled silicon steel sheets laminated together.
- Windings: Impregnated or cast with epoxy resin or other solid insulation materials.
- Insulation System: Relies on solid insulation materials (such as Nomex paper, epoxy resin, etc.) to achieve insulation between windings and between windings and the ground.
- Enclosure: Generally a metal or non-metallic enclosure with a protection level of IP20 or higher.
- Cooling System: Natural air cooling or forced air cooling (with fans installed).
Oil-immersed transformers use insulating oil as the cooling and insulation medium, and their main structure includes:
- Iron Core: Also made of laminated silicon steel sheets, but designed to adapt to the oil-immersed environment.
- Windings: Usually copper or aluminum conductors directly immersed in insulating oil.
- Insulation System: Oil-paper composite insulation system (insulating oil + insulating cardboard).
- Oil Tank: A sealed metal oil tank equipped with accessories such as an oil conservator and a breather.
- Cooling System: Natural oil circulation cooling or forced oil circulation cooling (with oil pumps installed).
Dry-type transformers adopt solid insulation materials, such as:
- Epoxy resin casting
- Vacuum Pressure Impregnation (VPI) process
- Solid insulation tape winding
Oil-immersed transformers use a composite insulation system composed of liquid insulating oil (usually mineral oil) and solid insulation materials (insulating cardboard). The insulating oil not only provides insulation but also undertakes the heat dissipation function.
Cooling methods for dry-type transformers:
- Air Natural Cooling (AN): Relies on natural air convection for heat dissipation.
- Air Forced Cooling (AF): Equipped with fans to force air flow, which can increase capacity by 30-50%.
Cooling methods for oil-immersed transformers:
- Oil Natural Air Natural Cooling (ONAN): Natural oil circulation, relying on the oil tank for heat dissipation.
- Oil Natural Air Forced Cooling (ONAF): Equipped with fans to cool the radiator.
- Oil Forced Air Forced Cooling (OFAF) or Oil Forced Water Forced Cooling (OFWF): Uses oil pumps to force oil circulation.
- Dry-Type Transformers: Usually used for small and medium capacities, generally not exceeding 10 MVA.
- Oil-Immersed Transformers: Wide capacity range, from tens of kVA to hundreds of MVA.
- Dry-Type Transformers: Generally not exceeding 35 kV.
- Oil-Immersed Transformers: Can reach ultra-high voltage levels (500 kV and above).
- Dry-Type Transformers: Relatively high no-load losses, but load losses are equivalent to those of oil-immersed transformers.
- Oil-Immersed Transformers: Insulating oil has good heat dissipation effect, and the overall efficiency is slightly higher.
- Dry-Type Transformers: Limited overload capacity, usually not exceeding 20% short-term overload.
- Oil-Immersed Transformers: Insulating oil has a large heat capacity, and the short-term overload capacity can reach more than 50%.
- Dry-Type Transformers: Relatively high noise (55-75 dB).
- Oil-Immersed Transformers: Lower noise (50-70 dB), as the oil tank has a sound insulation effect.
- Dry-Type Transformers: No flammable liquids, good fire resistance, and can be used in high-rise buildings, subways, and other places.
- Oil-Immersed Transformers: Insulating oil is flammable, so fire prevention distances and explosion-proof measures need to be considered.
- Dry-Type Transformers: No risk of oil leakage, more environmentally friendly.
- Oil-Immersed Transformers: Risk of oil leakage and pollution, requiring regular oil quality testing.
- Dry-Type Transformers: Basically maintenance-free, only requiring regular cleaning and inspection.
- Oil-Immersed Transformers: Need regular inspection of oil level and oil quality, as well as oil treatment or replacement.
- Power distribution systems in high-rise buildings and commercial centers.
- Places with high fire protection requirements such as subways, tunnels, and airports.
- Flammable and explosive environments such as petrochemical industry.
- Places with high requirements for power supply reliability such as data centers and hospitals.
- Indoor substations or places with limited space.
- Outdoor substations and power plants.
- Hub substations in power grid transmission and distribution systems.
- Main transformers of large industrial enterprises.
- Occasions requiring large-capacity transformation.
- Power distribution systems sensitive to cost such as rural power grids.
- Dry-Type Transformers: The price is usually 30-50% higher than that of oil-immersed transformers of the same capacity.
- Oil-Immersed Transformers: Lower initial investment.
- Dry-Type Transformers: Low maintenance cost, but slightly lower efficiency may increase electricity fees.
- Oil-Immersed Transformers: Higher maintenance cost (oil treatment, testing, etc.), but high efficiency.
- Dry-Type Transformers: The design life is usually 20-30 years.
- Oil-Immersed Transformers: The design life can reach 30-40 years.
With technological progress and increasing environmental protection requirements, both types of transformers are constantly developing:
Development trends of dry-type transformers:
- Development towards larger capacity and higher voltage levels.
- Application of new insulation materials (such as aerogel).
- Integration of intelligent monitoring technology.
Development trends of oil-immersed transformers:
- Application of environmentally friendly insulating oils (vegetable ester oil, silicone oil, etc.).
- Fully sealed technology to reduce maintenance needs.
- Digital and intelligent monitoring systems.
Dry-type transformers and oil-immersed transformers each have their own advantages and disadvantages. The following factors should be comprehensively considered when selecting:
- Installation environment (indoor/outdoor, space constraints)
- Fire safety requirements
- Capacity and voltage level requirements
- Initial investment and long-term operating costs
- Maintenance capabilities and resources
- Environmental protection requirements
In practical applications, the two types of transformers are not completely substitute relationships but complementary and coexisting, each exerting their advantages in suitable fields. With technological progress, the performance gap between the two is narrowing, but the core differences will exist for a long time. Power system designers and users should select the most suitable transformer type according to specific needs to achieve safe, reliable, and economical power supply.
