Dry-Type Transformer: Types, Cooling Methods & Key Advantages

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Looking for a dry-type transformer that is safe, maintenance-free, and suitable for indoor installations? Unlike oil-immersed transformers, dry-type transformers eliminate fire and leakage risks, making them the preferred choice for commercial buildings, hospitals, data centers, and industrial facilities with strict safety requirements.

1. Structural Forms of Dry-Type Transformers

1.1 Open Type (Most Common)

The core and windings are directly exposed to the surrounding air. Designed for clean, dry rooms (ambient temperature 20°C, relative humidity ≤85%). Cooling methods: AN (Air Natural, self-cooling) or AF (Air Forced, fan cooling).

1.2 Closed Type

Fully enclosed in a sealed housing, isolated from the atmosphere. Mainly used in mining as explosion-proof transformers. However, sealing limits heat dissipation, so we rarely use this type for general applications.

1.3 Cast Resin Type

Uses epoxy resin or other resins as the main insulation. Simple structure, compact size. Suitable for small-capacity transformers (e.g., indoor distribution).

2. Cooling Methods of Dry-Type Transformers

Cooling Method	Operation	Capacity
AN (Air Natural)	Natural convection, no fans	Rated capacity (100%) – continuous operation
AF (Air Forced)	Forced air cooling with fans	Up to 150% of rated capacity – for intermittent overload or emergency use

⚠️ Important: Under AF overload operation, load loss and impedance voltage increase significantly, making it less economical. Therefore, do not use forced air cooling for long-term continuous overload.

3. Main Types of Dry-Type Transformers

Impregnated Air-Insulated Dry-Type Transformer
Epoxy Resin Cast Dry-Type Transformer (most popular for indoor applications)
Wrapped Insulation Dry-Type Transformer
Composite Insulating Dry-Type Transformer

4. Advantages Over Oil-Immersed Transformers

Advantage	Benefit
No Fire or Explosion Risk	Dry-type transformers use flame-retardant insulation materials. Even if a failure occurs or an external fire source exists, the fire will not spread.
No Oil Leakage & No Oil Aging	Eliminates environmental contamination risks and the need for oil sampling, filtering, or replacement.
Low Maintenance – Almost Maintenance-Free	Greatly reduces operation, inspection, and maintenance workload compared to oil-immersed units.
Space-Saving Indoor Design	Typically designed for indoor installation. Can be placed in the same room as switchgear, reducing the overall installation footprint.
Fewer Accessories, No Sealing Issues	No oil conservator, no safety airway, no large number of valves – fewer potential failure points and lower long-term costs.