Transformers used in electrical transmission and distribution systems are divided into two groups based on their structural characteristics: oil-filled and dry-type. The advantages and disadvantages of these two types vary depending on their application areas. Dry-type transformers stand out in terms of safety, especially in enclosed spaces where many people are present.
Oil-filled transformers, which contain insulating oil, may pose safety risks such as fire or explosion. However, dry-type transformers do not present such a danger. Therefore, dry-type transformers are preferred in indoor applications such as hospitals, schools, shopping malls, and similar buildings. However, dry-type transformers are more sensitive to external environmental conditions compared to oil-filled models.
Dry Type Transformer Structure and Manufacturing Characteristics
In dry-type transformers, the high-voltage windings are coated with epoxy resin, usually cast in a vacuum. This coating provides strong resistance to moisture, chemical effects, and electrical stresses. The low-voltage windings are insulated with epoxy resin or prepreg technology. In this type of transformer, the windings are not protected by any oil; they operate directly exposed to environmental conditions.
Depending on the environment in which the transformer operates, it can be used in cabinets with different protection classes such as IP00 (unprotected), IP21, IP23, or IP31. This makes it possible to customize dry-type transformers according to the installation environment.
Power Ratings and Dimensions in Dry Type Transformers
Dry-type transformer power ratings generally start from 250 kVA and can reach up to 8 MVA in special applications. One of the most common models is the 1600 kVA and 2500 kVA dry-type transformers. However, transformers of this power rating do not fit into standard concrete substations; therefore, physical space requirements must be carefully calculated when preparing layout projects.
In TEDAŞ (Turkish Electricity Distribution Company) projects, the maximum dry-type transformer power rating to be placed inside a building is generally limited to 1250 kVA. This limit is standardized for both safety and layout reasons.
Cooling Methods in Dry Type Transformers
In terms of cooling method, oil-filled transformers dissipate their heat through circulating transformer oil, while dry-type transformers are cooled by natural airflow. These methods are divided into two categories:
AN (Air Natural): Cooling by natural airflow
AF (Air Forced): Fan-assisted cooling
Fans are usually located in the lower section of the transformer and are controlled by temperature sensors. Thanks to this structure, the transformer can operate safely even under an additional load of up to 40% depending on demand. However, dry-type transformers without cabinets and with an IP00 rating should only be used indoors; for outdoor applications, they must be protected by a cabinet with the appropriate IP protection rating.
Insulation Material Temperature Classes
Winding materials used in dry-type transformers are selected according to temperature classes that determine thermal resistance:
Class F: Resistant to temperature increases up to 155°C.
Class H: Can operate up to 180°C.
In transformer technical documentation, the winding class is indicated as “F/F” or “H/H”. These classes are important technical details to consider when selecting a transformer.
Environmental, Climate and Fire Resistance According to IEC 60076-11 Standard
The environmental resistance of dry-type transformers is determined according to the international IEC 60076-11 standard. The most commonly used classes are summarized below:
Environmental Class:
E0: Can operate in clean and dry environments.
E1: Tolerant to light pollution and low condensation.
E2: Can operate in heavily polluted and humid environments.
Climate Class:
C1: Cannot operate below -5°C, but can withstand temperatures down to -25°C.
C2: Can operate without problems down to -25°C.
Fire Resistance Class:
F0: Used in environments with low fire risk.
F1: Preferred in locations requiring high fire safety.
Most transformers are manufactured to have E2 – C2 – F1 classes. This makes them suitable for safe use even in harsh environmental conditions.
Dry-Type Transformers of the TEDAŞ Type in Turkey
Dry-type transformers to be transferred to public ownership in Turkey must be manufactured in accordance with the TEDAŞ-MLZ/99-031.B technical specifications. These transformers are notable for their overload resistance, lack of explosion risk, seismic resistance, and the possibility of use in enclosed cabinets.
Why Choose a Dry Type Transformer?
Dry-type transformers are preferred because of their low maintenance requirements, lack of explosion risk, fire safety, quieter operation, environmentally friendly structure, compact design, and space-saving features. Many manufacturers offer products in different power capacities such as 400 kVA, 630 kVA, 1600 kVA, and 2500 kVA. Dry-type transformer prices vary depending on the model’s power, insulation class, whether or not it has a protective cabinet, and the type of tap changer. Today, dry-type transformer solutions have become the preferred choice in many projects due to both their technical performance and the safety advantages they offer.