Types of Dry-Type Transformers
According to different structures, they can be divided into open type, enclosed type, and cast type.
(1) Open type. The open type dry-type test transformer is a common type, with its core and windings directly exposed to air, suitable for relatively dry and clean indoor environments (when the ambient temperature is 20 degrees Celsius, relative humidity does not exceed 80%). Generally, there are two cooling methods: natural air cooling and forced air cooling.
(2) Enclosed type. The core and windings are enclosed in a sealed casing, not directly in contact with external air, and can be used in harsher environments. Due to poor sealing and heat dissipation conditions, it is mainly used in places such as mines. The enclosed type can also be filled with sulfur hexafluoride gas at 0.2~0.3 MPa, with forced circulation, making the transformer's insulation and heat dissipation capabilities equivalent to oil-immersed type, suitable for users with high voltage levels.
(3) Cast type. Epoxy resin or other resins are cast as the main insulation, with a simple structure and small size, suitable for users with smaller capacities.
Characteristics of Dry-Type Transformers
Since the insulation level and heat dissipation performance of air are inferior to oil, dry-type transformers using air as insulation consume more effective materials than oil-immersed ones. Therefore, they are only used in places with high fire protection requirements, such as subways, high-rise buildings, factory interiors, or power stations.
Dry-type transformers with air insulation have lower ability to withstand impulse voltage compared to oil-immersed ones. The winding insulation of dry-type transformers adopts heat resistance classes A, E, B, F, H, C, with temperature limits of 60, 75, 80, 100, 125, 150 degrees Celsius respectively (all measured by resistance method). The surface of the iron core and structural parts must not exceed the allowable temperature rise of the contacted insulating materials (measured by thermometer method). Since the core and windings are exposed to air, the impregnation treatment of windings is very important. In addition to impregnating with insulating varnish, the windings must also be coated with moisture-proof covering paint.
