Transformers are essential electrical devices that transfer energy between alternating current (AC) circuits. Among the various types, dry-type transformers are widely used in applications that require voltage reduction in conventional power circuits. In use since the 1960s, dry-type transformers remain a popular choice across the commercial, industrial, and utility sectors. These industries depend on them for voltage regulation in systems that operate heavy-duty machinery and large energy loads.
Renowned for their simple design, high efficiency, and cost-effectiveness, dry-type transformers are a go-to solution in areas requiring safety, reliability, and environmental sustainability. Their ability to operate in extreme conditions—such as high humidity, fire-prone environments, or seismic zones—makes them ideal for challenging installations.
A dry-type transformer is an electrical transformer that uses air as the dielectric medium and does not contain any insulating liquid. Its windings and core are air-insulated and housed within sealed enclosures. Thanks to high-temperature insulation systems, these transformers are durable and environmentally friendly.
Unlike oil-filled transformers, dry-type transformers are resistant to short circuits, vibration, and temperature fluctuations, and they require minimal maintenance. They are especially suitable for indoor settings or locations where liquid leaks or fire hazards must be avoided—such as residential complexes, commercial buildings, or flood-prone regions.
Dry-type transformers work on the principle of electromagnetic induction. They have two coils—primary and secondary—wound around a shared magnetic core. When AC current passes through the primary coil, it generates a magnetic field that induces a voltage in the secondary coil. The voltage output depends on the turns ratio between the coils.
Instead of oil, dry-type transformers rely on natural air convection or forced-air cooling. The windings are encapsulated in epoxy resin, which not only insulates them but also protects against dust, moisture, and contaminants. The resin also improves heat dissipation and reduces fire risks.
Manufacturers typically offer two main types of dry-type transformers:
These transformers feature windings fully encapsulated in epoxy resin, providing protection from moisture, corrosion, and dust. CRTs are ideal for humid environments, indoor spaces, and areas with high fire risks. They are non-hygroscopic, non-flammable, and require no routine maintenance.
Their ability to withstand overloads, partial discharges, and low power losses translates to excellent efficiency and a long service life. CRTs range from 25 kVA to 12,500 kVA and typically use Class F insulation (90°C temperature rise). They can also be installed outdoors due to their strong resistance to environmental stress.
VPI transformers use foil or tape windings, which are impregnated under vacuum and pressure using H-class polyester resin. This process eliminates air pockets and improves mechanical strength, thermal stability, and dielectric performance.
VPI transformers are known for their rugged design, moisture resistance, and thermal resilience, making them suitable for outdoor use, earthquake zones, and extreme climates. They come in power ratings ranging from 5 kVA to 30 MVA, with insulation classes F (155°C) or H (180°C), and protection levels up to IP56.
Dry-type transformers offer a wide range of benefits:
The decision between dry-type and oil-immersed transformers largely depends on the installation environment and safety requirements. Consider the following:
Dry-type transformers are often preferred for small to medium-voltage applications in settings that prioritize safety, space efficiency, and low maintenance. Their air-cooled systems make them simpler and safer for a wide range of installations.
Before performing any maintenance, the transformer must be powered off. Here’s how to proceed:
Regular cleaning and visual inspection can prevent premature wear and maintain efficiency.
In addition to cleaning and inspection, periodic testing may be necessary. According to NEC 450.11.11, tests should be performed when signs of damage—such as overheating or insulation issues—are present. Only qualified engineers should perform these tests with proper safety gear and tools.
This test checks the ratio between the primary and secondary windings. If you observe abnormal performance or discoloration, call a professional to conduct this test using appropriate equipment.
This test measures insulation resistance between windings and ground over time. The Polarization Index (PI) is calculated by dividing the 10-minute reading by the 1-minute reading. A PI value greater than 2.0 indicates acceptable insulation.
Dry-type transformers offer a safe, efficient, and eco-friendly solution for power distribution. With their versatile applications, minimal maintenance needs, and superior safety profile, they are becoming an increasingly popular choice in modern power systems. Whether for indoor commercial use or outdoor utility applications, dry-type transformers provide reliable performance in even the harshest environments.