In the power transmission and distribution system, dry-type transformers, as core equipment parallel to oil-immersed transformers, have become the preferred power distribution equipment for many special scenarios due to their outstanding advantages such as oil-free design, fire and explosion prevention, small size, and convenient maintenance. Unlike oil-immersed transformers that use insulating oil as the insulation and cooling medium, dry-type transformers use air or inert gas as the cooling medium, and their windings are encapsulated with solid insulating materials such as epoxy resin and Nomex paper, eliminating the need for a sealed oil tank and fundamentally eradicating potential safety hazards such as oil leakage and fire. Since their gradual promotion and application in the mid-20th century, dry-type transformers have undergone decades of technological iteration, forming a complete product series, which are widely used in high-rise buildings, data centers, hospitals, subways, chemical explosion-proof areas and other scenarios with high requirements for safety and environmental protection. This article comprehensively analyzes the definition, working principle, structural composition, classification characteristics, application scenarios and operation and maintenance points of dry-type transformers, combining the latest industry standards and practical application experience to help you deeply understand this efficient and safe power distribution equipment.

I. Core Definition and Working Principle of Dry-Type Transformers

A dry-type transformer is a power equipment that completes voltage conversion through the principle of electromagnetic induction, without the need for insulating oil as the cooling and insulation medium. It adopts natural air cooling (AN) or forced air cooling (AF) as the heat dissipation method, and relies on solid insulating materials to achieve winding insulation. Its core feature is that the windings and iron core are exposed to the air (or enclosed in an inert gas environment), without the need for a sealed oil tank. The simple structure and oil-free design fundamentally solve the safety and environmental hazards of oil-immersed transformers.

The working principle of dry-type transformers is essentially the same as that of oil-immersed transformers, both based on the law of electromagnetic induction. The core process can be summarized as "electromagnetic coupling for voltage transformation, air for heat dissipation and insulation": first, when alternating current is passed into the primary side (high-voltage side) coil, an alternating magnetic field is generated; second, the alternating magnetic field is coupled and transmitted through the high-permeability iron core to act on the secondary side (low-voltage side) coil; finally, an alternating voltage is induced in the secondary side coil under the action of the alternating magnetic field. The voltage magnitude is proportional to the turns ratio of the coils on both sides, thereby realizing the conversion between high voltage and low voltage to meet the voltage needs of different power consumption scenarios.

The biggest difference from oil-immersed transformers is that the core of heat dissipation of dry-type transformers is "air convection" — during operation, the heat generated by the iron core and windings due to current loss is directly dissipated into the surrounding air through natural convection (natural air cooling) or forced air blowing by fans (forced air cooling); at the same time, the windings are encapsulated with solid insulating materials, without relying on insulating oil for insulation. Its insulation temperature resistance level is higher (common F-class and H-class), and the long-term operating temperature can reach 155℃ (F-class) and 180℃ (H-class), which is suitable for high-temperature environment operation. In addition, the oil-free design of dry-type transformers eliminates problems such as oil deterioration and oil leakage, and has more advantages in operational safety and environmental protection.

II. Core Structural Composition of Dry-Type Transformers

The structural design of dry-type transformers focuses on the four core requirements of "oil-free, insulation, heat dissipation and safety". The overall structure is simpler than that of oil-immersed transformers, mainly consisting of core components and auxiliary components. All components work together to ensure efficient and stable operation of the equipment. Its core components have undergone strict insulation treatment, such as windings adopting vacuum casting process and iron core adopting high-quality silicon steel sheet lamination, to maximize insulation performance and heat dissipation efficiency.

(I) Core Components

1. Iron Core: As the core of the magnetic circuit, similar to oil-immersed transformers, it is made of high-permeability cold-rolled grain-oriented silicon steel sheets laminated to reduce eddy current loss and hysteresis loss, and improve magnetic energy transmission efficiency. Different from oil-immersed transformers, the surface of the iron core of dry-type transformers is usually painted with insulating paint or epoxy resin coating to prevent iron core corrosion and excessive eddy current loss, and avoid insulation breakdown between the iron core and windings. Some large-capacity dry-type transformers are equipped with ventilation air ducts in the iron core to enhance air convection heat dissipation effect.

2. Windings: The core component of dry-type transformers and the key to their oil-free insulation. They are made of insulated copper or aluminum wires, divided into high-voltage windings and low-voltage windings. According to different insulation methods, the windings are encapsulated by processes such as epoxy resin vacuum casting and Nomex paper winding to form a solid and firm solid insulation structure, which can effectively isolate air, dust and moisture and prevent winding discharge and short circuit. The high-voltage winding has more turns and thinner wire diameter, while the low-voltage winding has fewer turns and thicker wire diameter. The turns ratio determines the voltage conversion ratio, and the insulation process of the windings directly affects the insulation level and service life of the equipment.

3. Insulating Materials: The core insulation carrier of dry-type transformers, replacing the insulating oil of oil-immersed transformers, mainly divided into two categories: one is winding encapsulation materials, such as epoxy resin and unsaturated resin, which are used to cast the windings as a whole, with excellent insulation performance, mechanical strength and heat resistance; the other is auxiliary insulating materials, such as Nomex paper, insulating tape and insulating spacers, which are used for insulation isolation inside the windings and between the windings and the iron core to ensure insulation reliability.

4. Frame and Shell: As the mechanical support and protection structure of the equipment, the frame is made of section steel welding to fix the iron core, windings and auxiliary components; the shell is usually made of metal with ventilation shutters, which can not only protect internal components from mechanical collision and dust intrusion, but also ensure air circulation to meet heat dissipation needs. The shell of some explosion-proof dry-type transformers adopts a sealed design and is filled with inert gas to adapt to flammable and explosive scenarios.

(II) Auxiliary Components

1. Cooling System: Divided into two types: natural air cooling (AN) and forced air cooling (AF). Natural air cooling does not require additional equipment, relying on natural air convection for heat dissipation, suitable for small-capacity transformers; forced air cooling is equipped with cooling fans around the shell or windings, accelerating air flow through forced air blowing to improve heat dissipation efficiency, suitable for medium and large-capacity transformers, which can increase the short-term overload capacity of the equipment by about 30%.

2. Bushings: Same function as the bushings of oil-immersed transformers, installed on the shell, it is an insulation device that leads the high-voltage and low-voltage winding leads from the inside of the equipment to the external circuit. It needs to withstand the erosion of external line voltage and air environment. Its insulation performance and sealing performance are directly related to the safe operation of the equipment, usually made of porcelain or epoxy resin.

3. Temperature Measurement and Protection Device: Including temperature sensors, thermostats, thermal relays, etc., used to monitor the temperature of windings and iron core in real time. When the temperature exceeds the set threshold, the thermostat will automatically start the cooling fan (for forced air cooling type). If the temperature continues to rise, it will send an alarm signal or even cut off the power supply to prevent the equipment from being damaged due to overheating. Some high-end products are also equipped with online temperature measurement systems to realize real-time upload and remote monitoring of temperature data.

4. Other Auxiliary Components: Including terminal blocks, grounding devices, nameplates, etc. Terminal blocks are used to connect external circuits, grounding devices are used to ensure the safety of equipment and personnel, and nameplates mark key parameters such as rated capacity, rated voltage, insulation level and cooling method of the equipment, facilitating selection and operation and maintenance.

III. Classification and Application Scenarios of Dry-Type Transformers

According to insulation method, cooling method, purpose and phase number, dry-type transformers can be divided into various types. Different types of products are suitable for different use environments and power needs. Combined with industry application experience, the specific classification and application scenarios are as follows, and the selection points for export projects are supplemented:

(I) Classification by Insulation Method

1. Epoxy Resin Cast Dry-Type Transformers (SCB Series): The most common type. The windings are formed by epoxy resin vacuum casting, with excellent insulation performance, high mechanical strength, moisture and dust resistance. They are suitable for most dry-type transformer application scenarios, such as high-rise buildings, data centers, hospitals, etc., and are the mainstream products in the market. Its insulation level is mostly F-class, and some can reach H-class, with a rated capacity range of 30kVA-2500kVA and voltage levels covering 10kV, 35kV, etc.

2. Open-Type Dry-Type Transformers (SG Series): The windings are wound with Nomex paper or other solid insulating materials without casting encapsulation. They have simple structure, low cost and good heat dissipation effect, but poor moisture and dust resistance. They are mainly suitable for dry and clean indoor scenarios, such as ordinary factories and warehouses, and their application scope is gradually narrowing.

3. Encapsulated Dry-Type Transformers (SGB Series): The windings are encapsulated by glass fiber tape winding, which has both the insulation reliability of epoxy resin cast type and the heat dissipation advantage of open type. Its moisture and dust resistance is better than that of open type, and the cost is lower than that of cast type. It is suitable for indoor scenarios with medium humidity and cleanliness, such as office buildings and shopping malls.

(II) Classification by Cooling Method

1. Natural Air Cooling (AN): Relying on natural air convection for heat dissipation, it has simple structure, no noise, convenient maintenance and no additional energy consumption. It is suitable for small-capacity transformers with capacity ≤1000kVA, mostly used in terminal power distribution scenarios, such as power distribution rooms of small office buildings and residential buildings.

2. Forced Air Cooling (AF): Cooling fans are installed on the equipment to accelerate heat dissipation through forced air blowing. The heat dissipation efficiency is more than 30% higher than that of natural air cooling. It is suitable for medium and large-capacity transformers with capacity ≥1000kVA, widely used in scenarios with large loads such as data centers, large shopping malls and industrial parks, and can adapt to working conditions with large load fluctuations.

(III) Classification by Purpose

1. Distribution Dry-Type Transformers: Mainly used to reduce the 10(6)kV grid voltage to the 230/400V bus voltage used by users. Suitable for AC 50(60)Hz, the maximum rated capacity of three-phase is 2500kVA, and the maximum rated capacity of single-phase is 833kVA. It is mainly used in indoor power distribution scenarios, such as high-rise buildings, hospitals, schools, data centers, etc., and is the most important application type of dry-type transformers. The capacity selection should be determined according to the calculated load, and the load rate should be controlled at about 85% to ensure operation efficiency.

2. Special Dry-Type Transformers: Including rectifier dry-type transformers, frequency conversion dry-type transformers, explosion-proof dry-type transformers, etc., which are respectively suitable for different special scenarios. Among them, rectifier dry-type transformers are used for industrial rectification equipment, such as electroplating and electrolysis; frequency conversion dry-type transformers are used for frequency converter supporting, such as new energy, chemical industry and other industries; explosion-proof dry-type transformers are used for flammable and explosive scenarios, such as coal mines, petrochemical explosion-proof areas, etc.

3. Test Dry-Type Transformers: Used for power testing, electrical equipment testing and other scenarios, with the characteristics of wide voltage adjustment range, high precision and small size, suitable for laboratories, power testing institutions, etc.

(IV) Classification by Phase Number

1. Three-Phase Dry-Type Transformers: The mainstream type, with compact structure and high efficiency, can meet the needs of most industrial and civil power consumption, widely used in various indoor power distribution scenarios, with a rated capacity range of 30kVA-2500kVA.

2. Single-Phase Dry-Type Transformers: Mainly used for single-phase power supply scenarios, such as small lighting, household electricity, small laboratories, etc., with small capacity, usually between 10kVA-100kVA.

(V) Summary of Typical Application Fields

The application scenarios of dry-type transformers are mainly concentrated in indoor and special environments with strict requirements on safety, environmental protection and space, including: power distribution rooms of high-rise buildings (office buildings, apartments, hotels) to avoid fire caused by oil leakage; data centers and computer rooms, which require equipment to be noise-free and oil-free to ensure the safe operation of precision equipment; crowded places such as hospitals, schools and shopping malls, where fire and explosion prevention performance is given priority; flammable and explosive areas such as chemical industry and coal mines, where explosion-proof dry-type transformers are selected to eliminate potential safety hazards; indoor power distribution systems of large infrastructure such as subways, airports and ports, which adapt to space-constrained scenarios; and supporting test equipment in laboratories and power testing institutions. From the perspective of export projects, in developed countries such as Europe and North America, dry-type transformers are generally used in data centers and high-rise buildings, requiring CE, UL and other certifications; in regions such as Southeast Asia and Africa, dry-type transformers are also gradually promoted in indoor power distribution and crowded places, and moisture-proof products are preferred in some high-temperature and high-humidity areas.