How to Choose Between Dry-Type and Oil-Immersed Transformers

01  Differences between Dry-Type and Oil-Immersed Transformers

▲ Characteristics of Dry-Type Transformers

Dry-type transformers do not use insulating oil for their cores and windings, and they have the ability for natural or forced air cooling. Their structural types include fixed insulation encasement (SCB type) and unencased winding structure. The cooling methods can be divided into natural air cooling (AN) and forced air cooling (PF) types. Dry-type transformers are often used in densely populated and space-constrained areas due to their unique properties and are widely adopted.

▲ Characteristics of Oil-immersed Transformers

Unlike dry-type transformers, oil-immersed transformers use insulating oil for cooling, offering good cost-effectiveness and being suitable for high-capacity applications and standalone substations. Their structure includes oil reservoirs and heat dissipation fins, which effectively dissipate heat through the circulation of oil.

02  Detailed Analysis of Dry-Type Transformers

▲ Specifications and Technical Parameters of Dry-Type Transformers

The model specifications of dry-type transformers are composed of multiple letters and numbers. Taking the SCB-11—1250KVA/10KV/0.4KV dry-type transformer as an example, S represents a three-phase power transformer, C indicates that the transformer windings are solid and made of resin cast, and G refers to the air as the insulation medium outside the windings. B represents the bobbin winding, and R indicates the winding type of winding. Additionally, there are technical parameters such as a frequency of 50Hz, a need to control the no-load current below 4%, and a low-voltage withstand strength that needs to reach the standard of no breakdown at 2KV/min.

▲ Requirements for Operating Environment of Dry-Type Transformers

During operation, dry-type transformers require a temperature range of -25℃ to +40℃ and a relative humidity of no more than 85%. Sunlight exposure should be avoided, and good ventilation is necessary to ensure stable operation. An environment without corrosive gases or dust is conducive to extending the lifespan of dry-type transformers.

Comparison of Advantages and Disadvantages and Selection Suggestions

▲ Advantages and Disadvantages of Dry-Type and Oil-immersed Transformers

Dry-type transformers are often used in places with high population density and limited space, such as underground shopping malls, due to their excellent fire and explosion prevention performance; however, they are more expensive. They perform well in these specific scenarios. Oil-immersed transformers, on the other hand, are recommended for larger capacity requirements and humid environments due to their high cost-effectiveness.

▲ Comparison of Specific Transformer Types

The SCB type and SGB type dry-type transformers have unique features in coil winding and heat dissipation performance. The SCB type uses foil winding technology, with the low-voltage coils mainly wound with a single layer of copper foil, while the SGB type uses line winding technology, with the low-voltage coils in a cylindrical structure. Their price differences are not significant, making the SCB type and SGB type popular choices in the market. The S13 type oil-immersed transformer also has significant advantages in performance and cost.

03  Guidelines for Selecting Power Transformers and Examples

When selecting power transformers, one should follow the standards of GB/T17468—2008 “Selection Guidelines for Power Transformers” and GB4208—-2008 “Exterior Protection Class (IP Code)”. This ensures that the selected transformer can adapt to the environmental requirements of the specific place. Operating mode and technical economic indicators are also important factors to consider during selection.

In a newly-built cold-rolled strip steel plant project on an industrial platform, originally three SCB-type dry-type transformers were designed, but after detailed investigation, oil-immersed power transformers were chosen. This choice not only saved funds but also met the high-performance production requirements with its excellent short-circuit resistance and ability to work in high-temperature and high-humidity environments.