What is the difference between dry-type transformers and oil-immersed transformers? How to choose?

Transformers are common and critical equipment in power systems, used to alter the magnitude of alternating voltage for the transmission and distribution of electrical energy in power transmission, distribution, and other applications. Dry-type transformers and oil-immersed transformers are two prevalent types, exhibiting significant differences in insulation media, heat dissipation methods, and environmental adaptability. This article explores the distinctions between dry-type and oil-immersed transformers and provides recommendations for selecting the appropriate transformer type.

The most significant difference between oil-immersed and dry-type transformers lies in the presence or absence of “oil.” Since oil is a liquid with fluidity, oil-immersed transformers necessarily feature an enclosure. Inside this enclosure is transformer oil, which submerges the transformer windings, making them invisible from the outside. Dry-type transformers, lacking oil, do not require an enclosure, allowing direct visibility of the transformer windings. Another characteristic is the oil reservoir on oil-immersed transformers, which stores transformer oil. However, newer oil-immersed transformers may not include an oil reservoir.

Both are power transformers, featuring an iron core for the magnetic circuit and windings for the electrical circuit. The primary distinction lies in the “oil-immersed” versus “dry-type” design. Specifically, their cooling media differ: the former uses transformer oil (or other oils like β oil) for both cooling and insulation, while the latter employs air or gases like SF6 as the cooling medium. Oil-immersed transformers submerge the core-and-winding assembly within an oil tank filled with transformer oil. Dry-type transformers typically encapsulate the core and windings with epoxy resin; A more commonly used non-encapsulated type now employs special insulating paper impregnated with dedicated varnish to prevent moisture ingress into the windings or core.

Oil-immersed transformers feature external radiators designed to facilitate heat dissipation—specifically, the circulation of internal insulating oil—functioning like heat sinks. Dry-type transformers lack such radiators, relying instead on fans beneath the transformer coils for cooling.

Oil-immersed transformers, due to fire safety requirements, are typically installed in dedicated transformer rooms or outdoors. Dry-type transformers, however, are always installed indoors, usually within distribution rooms.

In terms of production volume and usage, dry-type transformers currently only reach a voltage rating of 35kV and have a relatively smaller capacity compared to oil-immersed types, typically up to about 2500kVA. Additionally, the manufacturing process for dry-type transformers is more complex than that for oil-immersed transformers of the same voltage rating and capacity, resulting in higher costs. Therefore, oil-filled transformers remain more prevalent in current applications. However, dry-type transformers are frequently used in high-demand indoor power supply and distribution locations—such as hotels, office buildings, and high-rise structures—due to their environmental benefits, flame-retardant properties, and impact resistance.

Clearly, both dry-type and oil-filled transformers have distinct advantages and disadvantages. Oil-filled transformers offer lower costs and easier maintenance but are combustible and explosive. Dry-type transformers, with their excellent fire resistance, can be installed at load centers to minimize voltage drop and energy loss. However, they are more expensive, bulkier, less resistant to moisture and dust, and produce higher noise levels.

Next, let's summarize the differences mentioned above into the following seven points:

1. Appearance

Different encapsulation forms: Dry-type transformers expose the core and windings, while oil-immersed transformers only reveal the outer casing.

2. Lead Configuration

Dry-type transformers predominantly use silicone rubber bushings, while oil-immersed transformers mostly employ porcelain bushings;

3. Capacity and Voltage

Dry-type transformers are generally suitable for distribution applications, typically with capacities below 2000kVA and voltages at or below 10kV, though some can reach 35kV voltage levels. Oil-immersed transformers, however, can achieve any required capacity and cover all voltage levels; China's currently under-construction ultra-high voltage 1000kV transmission lines exclusively employ oil-immersed transformers.

4. Insulation and Heat Dissipation Differences

Dry-type transformers typically utilize resin insulation and rely on natural air cooling, with larger capacities supplemented by forced air cooling. Oil-immersed transformers, however, employ insulating oil for electrical isolation. Heat generated by the windings is transferred through circulating insulating oil within the transformer to the radiator fins for dissipation.

5. Application Environments

Dry-type transformers are predominantly used in locations requiring “fireproofing and explosion-proofing,” making them suitable for large-scale buildings and high-rise structures. Oil-immersed transformers, however, may experience oil spray or leakage in the event of failure, potentially causing fires. Consequently, they are primarily deployed outdoors where sites are equipped with excavated “accident oil pits.”

6. Different Load-Carrying Capacities

Dry-type transformers should generally operate at rated capacity, while oil-immersed transformers exhibit superior overload capability.

7. Varied Cost Structures

For transformers of equivalent capacity, dry-type units command significantly higher purchase prices than oil-immersed counterparts.

How to Choose

1. Safety Considerations: If higher safety and fire resistance are required, choose dry-type transformers as they do not contain flammable insulating oil.

2. Power Requirements: For high-power transformers, oil-immersed transformers are typically more suitable due to their superior heat dissipation capabilities.

3. Environmental Constraints: Consider the characteristics of the transformer's operating environment and select the appropriate transformer type for that setting.

4. Maintenance Costs: Oil-filled transformers generally require periodic inspections and maintenance of insulating oil quality, while dry-type transformers have relatively lower maintenance costs.

Dry-type and oil-filled transformers exhibit distinct differences in insulation media, heat dissipation methods, and environmental adaptability. Selecting the appropriate transformer type depends on specific application requirements and environmental conditions. When formulating transformer selection strategies, factors such as safety, power requirements, environmental constraints, and maintenance costs must be comprehensively considered. For critical facilities and high-safety-requirement locations, such as hospitals and schools, dry-type transformers are typically preferred to maximize safety. Conversely, in large industrial complexes and power systems requiring high-power transformers, oil-immersed transformers are often more suitable due to their superior heat dissipation capabilities and ability to withstand harsh environmental conditions.