Dry-type transformers, whose core and coils are not immersed in any insulating liquid, are generally used in locations with high safety and fire protection requirements. Small-capacity, low-voltage specialty transformers are also often manufactured dry-type for ease of manufacture and maintenance. There are several types of dry-type transformers:
1. Open Type
Open-type transformers are a common type, with their body open to the atmosphere, and are suitable for relatively dry and clean indoor environments.
Because air has lower dielectric strength and heat dissipation performance than oil, air-insulated dry-type transformers consume more effective material than oil-immersed transformers. Therefore, dry-type transformers are only used in power transformers in locations with high fire protection requirements, such as subways, public buildings, and workshop interiors.
Air-insulated dry-type transformers have lower impulse voltage resistance than oil-immersed transformers. Their use is limited to locations not directly connected to overhead lines and protected from atmospheric overvoltages (otherwise, special lightning protection should be added to ensure that the atmospheric overvoltage amplitude does not exceed the power frequency test voltage amplitude). Therefore, apart from power frequency withstand voltage, no separate impulse strength requirements are specified.
2. Sealed Type
The sealed type is not connected to the outside atmosphere and can be used in harsher environments. Due to its sealing, heat dissipation is poor and it is primarily used in flameproof transformers for mining. It can also be filled with other gases with higher insulation strength and heat dissipation capacity than air. For example, if sulfur hexafluoride is filled at 2-3 atmospheres and forced to circulate, the transformer's insulation and heat dissipation capabilities can be comparable to those of oil-immersed types, making it suitable for high-voltage products.
3. Cast Type
The cast type uses epoxy resin or other resins as the main insulation. It has a simple structure and compact size, making it suitable for smaller capacity products.