Dry-type transformer specifications
The dry-type transformer is an electrical device that uses air or gas as the insulating and cooling medium instead of dielectric liquids. Its windings and core are housed in a pressurized and sealed tank, eliminating the need for mineral or synthetic oil that could leak or ignite.
Types of Dry-type Transformers
Section titled “Types of Dry-type Transformers”The two main types are the cast resin dry-type transformer (CRT) and the vacuum pressure impregnated transformer (VPI).
| Type | Power Range | Insulation Class | Typical Temperature Rise | Distinguishing Feature |
|---|---|---|---|---|
| CRT (Cast Resin) | 25 kVA to 12 500 kVA (33.5 hp to 16 750 hp) | F | 90 °C / 194 °F | Encapsulated in epoxy resin, non-hygroscopic, maintenance-free. |
| VPI (Vacuum Pressure Impregnated) | 5 kVA to 30 MVA (6.7 hp to 40 200 hp) | F (155 °C / 311 °F) or H (180 °C / 356 °F) | Varies by design | Impregnated with class H polyester resin, high mechanical and dielectric strength. |
Technical Specifications
Section titled “Technical Specifications”Heat losses constitute a fundamental parameter for sizing ventilation and cooling systems. The following table lists characteristic values as a function of rated power.
| Power Range (kVA) | Heat Loss | Approximate Efficiency (%) |
|---|---|---|
| ≤ 150 | 50 W/kVA / 170.6 BTU/h·kVA | 95 |
| 150 – 500 | 30 W/kVA / 102.4 BTU/h·kVA | 97 |
| 500 – 1000 | 25 W/kVA / 85.3 BTU/h·kVA | 97.5 |
| 1000 – 2500 | 20 W/kVA / 68.2 BTU/h·kVA | 98 |
| > 2500 | 15 W/kVA / 51.2 BTU/h·kVA | 98.5 |
Advantages and Disadvantages
Section titled “Advantages and Disadvantages”| Advantages | Disadvantages |
|---|---|
| They contain no flammable or toxic liquids, so the fire risk is minimal. | Higher initial cost than oil-immersed transformers of equal power. |
| Simplified installation: no containment pits, firewalls, or extinguishing systems required. | Larger volume and weight for the same power due to greater insulation thicknesses. |
| Virtually maintenance-free: no oil changes or oil analysis needed. | Greater sensitivity to dust, dirt, and pests if ventilation is open. |
| High overload capacity thanks to better heat dissipation and insulation quality. | Higher noise level than oil-immersed transformers, caused by magnetostriction and vibrations. |
| Outstanding seismic performance due to rigid and lightweight structure. | |
| Suitable for humid, contaminated, or fire-risk environments (IP56 protection in VPI versions). |
Applications
Section titled “Applications”Dry-type transformers are used in environments where safety and environmental protection are priorities. Some representative applications are:
- Chemical, petrochemical plants and refineries with potentially explosive atmospheres.
- Indoor installations in commercial buildings, hospitals, and data centers.
- Substations in seismic zones or exposed to vibrations.
- Spaces with high humidity or presence of dust, such as tunnels or water treatment plants.
- Distribution systems that require locating the transformer close to the load, reducing low-voltage losses.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the power range of a CRT dry-type transformer?
Section titled “What is the power range of a CRT dry-type transformer?”The cast resin transformer (CRT) is manufactured in powers from 25 kVA (33.5 hp) up to 12 500 kVA (16 750 hp), covering most low and medium voltage distribution needs.
What insulation temperature does a class H VPI transformer handle?
Section titled “What insulation temperature does a class H VPI transformer handle?”VPI models using class H insulation support a maximum operating temperature of 180 °C (356 °F), giving them greater thermal margin in demanding applications.
How much heat does a 500 kVA transformer dissipate?
Section titled “How much heat does a 500 kVA transformer dissipate?”A transformer in the 150 kVA to 500 kVA range dissipates about 30 W/kVA (102.4 BTU/h·kVA); thus, a 500 kVA unit would emit approximately 15 000 W (51 180 BTU/h) to the environment.
What is the typical efficiency of a dry-type transformer above 2500 kVA?
Section titled “What is the typical efficiency of a dry-type transformer above 2500 kVA?”For powers greater than 2500 kVA, the loss is reduced to 15 W/kVA (51.2 BTU/h·kVA), which corresponds to an efficiency of approximately 98.5%, and can exceed 99% in large units.
What IP protection degree can a VPI transformer achieve?
Section titled “What IP protection degree can a VPI transformer achieve?”VPI transformers can be supplied with enclosures offering IP56 protection, ensuring tightness against dust and powerful water jets.
How much does heat loss decrease when going from a 150 kVA transformer to one over 2500 kVA?
Section titled “How much does heat loss decrease when going from a 150 kVA transformer to one over 2500 kVA?”The specific loss decreases from 50 W/kVA (170.6 BTU/h·kVA) for equipment up to 150 kVA to 15 W/kVA (51.2 BTU/h·kVA) for those over 2500 kVA, representing a 70% reduction.
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/heat-gain-equipment-d_1668.html
- electrical4u.com: https://www.electrical4u.com/dry-type-transformer/