1. What is a Dry Type Transformer? Structure and Operating Principles
Dry type transformers are transformers that do not use any liquid insulation. Instead, their windings and cores are immersed in a closed container pressurized with air. Dry-type transformers do not have moving parts and are completely static devices. In normal operation conditions, they ensure long service life and no malfunctions. Unlike liquid-immersed transformers cooled by oil or non-flammable liquid dielectric, dry-type transformers use only high-grade insulation systems recognized as safe for the environment. Dry-type transformers provide safe and reliable power without the need for fire-resistant vaults, oil containment tanks, or toxic gas venting. These important safety factors allow dry-type transformers to be installed inside buildings close to the loads, improving overall system efficiency and reducing secondary wire losses.
2. Structure of Dry Type Transformer
A dry-type transformer has a general structure consisting of three main components: the steel core, winding, and transformer casing.
2.1 Steel Core (Magnetic Circuit)
The steel core of a dry-type transformer is made of good conductor material with high oxidation resistance, typically electrical steel sheets such as Silic. The function of the steel core is to conduct the magnetic flux while also serving as a frame to hold the windings. The steel core consists of multiple thin steel sheets with insulating varnish on the outer surface, with a thickness ranging from 0.3 to 0.5mm, stacked together to form a closed magnetic circuit. The purpose of stacking thin steel sheets together is to reduce energy losses that occur during the transformer’s operation due to eddy currents. The steel core consists of two main components: the limb and the yoke. The limb is where the winding is placed, while the yoke connects the limbs together to form a closed magnetic circuit.
2.2 Winding (Coil Winding)
The winding of a dry-type transformer is typically made of pure copper wire, with a circular cross-section and an outer insulation layer to enhance electrical conductivity, prevent oxidation, and increase the transformer’s service life. The winding functions to receive input energy and transmit output energy. The winding of a dry-type transformer must meet technical specifications such as flexibility, adhesion, electrical strength, elongation, elasticity, and resistance to transformer oil to reduce the occurrence of Foucault eddy currents during operation.
2.3 Transformer Casing
The casing of a dry-type transformer is usually made of steel and designed depending on the type of transformer, serving to protect the internal elements of the transformer. Unlike oil-immersed transformers, the casing of dry-type transformers does not contain oil.
3. Operating Principles of Dry Type Transformer
A dry-type transformer performs the task of voltage transformation based on the principle of electromagnetic induction. There are two types of transformers: step-up transformers and step-down transformers. The working principle of a transformer is based on the phenomenon of electromagnetic induction. Examining a simple transformer consisting of two coil windings wound on a core. If the secondary voltage is lower than the primary voltage, we have a step-down transformer; if the secondary voltage is higher than the primary voltage, it is called a step-up transformer. In a three-winding transformer, besides the primary and secondary windings, there is also a third winding with intermediate voltage. Transformers transform AC power system, called single-phase transformers, or three-phase AC power system, called three-phase transformers. Immersed transformers in oil are called oil-immersed transformers, transformers not immersed in oil are called dry-type transformers, transformers with three columns lying in one plane are called flat-core transformers, transformers with three columns lying in space are called three-dimensional transformers.
4. Classification of Dry Type Transformer
Dry-type transformers include two main types: Cast Resin Dry Type Transformer (CRT) Vacuum Pressure Impregnated Transformer (VPI)
4.1 Cast Resin Dry Type Transformer (CRT):
Cast Resin Dry Type Transformer (CRT) Image of Cast Resin Dry Type Transformer (CRT) Cast resin dry transformers are used in areas with high humidity. This is because their primary and secondary windings are wrapped in an epoxy resin layer. This layer prevents moisture from entering to affect the winding material and provides excellent insulation. Thus, the operation of the transformer is smooth and efficient. This type of transformer is available on the market with capacities ranging from 25KVA to 12,500KVA, with an insulation class of F (90oC Temp. Rise).
4.2 Vacuum Pressure Impregnated Transformer (VPI):
Vacuum Pressure Impregnated Transformer (VPI) Image of Vacuum Pressure Impregnated Transformer (VPI) This type of transformer has winding conductors made of fire-resistant insulation materials. The winding of the transformer is designed as a disk, connected in series or parallel for higher voltages, depending on the corresponding power level with the appropriate voltage level. The insulation of the winding is made from H-grade polyester plastic material to prevent infiltration. The vacuum box protects the primary, secondary windings, and core from moisture and prevents any impact due to moisture. The capacity of this type is only from 5KVA to 30KVA, while the protection level is up to IP 56. Dry-type vacuum pressure transformers have the following outstanding advantages: The machine has high mechanical strength No insulation limitations No temperature fluctuations Safe, very low fire hazard The machine design is very convenient for maintenance
5. Advantages and Disadvantages of Dry Type Transformers
5.1 Advantages of Dry Type Transformers
Using dry-type transformers is very safe for people and electrical equipment. Maintenance and maintenance of the machine are easy and non-polluting. Dry-type transformers are also easier to install than some other transformers, helping to save a lot of costs. Occupies relatively little space Since the machine does not use oil for cooling, it is very environmentally friendly Perfect machine capacity, good overload resistance Dry-type transformers seem to have no risk of fire or explosion during operation Good short-circuit current resistance of the machine The average service life of dry-type transformers is high, about 25 – 30 years Dry-type transformers can be installed and used indoors, outdoors, in harsh climates, or high humidity areas.
5.2 Disadvantages of Dry Type Transformers:
Dry-type transformers have a long service life and are less prone to winding failures. However, in the worst-case scenario during operation, if overload or explosion occurs, the possibility of utilizing the components in the core and winding of dry-type transformers is impossible (meaning dry-type transformers will not have the opportunity for repair but must be replaced with new ones). For the same capacity and voltage, using a dry-type transformer will be much more expensive than an oil-cooled transformer.
6. Dry Type Transformer Testing
6.1 Definition:
To determine the quality of dry-type transformers during manufacturing, delivery, and during annual operation monitoring purposes to reduce the probability of malfunctions during system operation.
6.2. Classification of Dry Type Transformer Testing:
Insulation quality determination test Determination test of technical specifications, structural parameters Determination test of expanded technical parameters
7. Applications of Dry Type Transformers
Due to the outstanding advantages mentioned above, dry-type transformers are widely used in harsh weather conditions or require very long-term stability such as:
- Chemical industry, metallurgy, petroleum industry
- High humidity areas such as swamps, water source protection areas
- High-risk fire areas such as mountains and forests
- Transformer substations in cities
- Underground and indoor transformer substations
- Renewable energy sources such as offshore wind turbines.
