Dry-type Transformer

What Is a Dry-type Transformer

A Dry-type Transformer uses air, epoxy resin, or a similar solid insulating material to cool and insulate its windings, instead of the mineral oil found in oil-immersed transformers. Because there is no liquid dielectric involved, this type of transformer avoids the fire and leakage risks associated with oil, which is why it is widely selected for substations located inside occupied buildings, hospitals, shopping centers, high-rise towers, and other sites where flammable liquids are restricted or undesirable in the electrical room.

Ningbo Tianshun Electric Co., Ltd., a company with production based in Xiangshan Coastal Industrial Park and a technical team averaging over 20 years of industry experience, manufactures dry-type transformers as part of its broader range of power transmission and distribution equipment.

Key Advantages of a Dry-type Transformer

• Improved fire safety — without oil as fuel, the transformer significantly reduces the risk of an oil fire, allowing safer indoor placement close to occupied areas.
• No oil leakage or spill risk — this removes the need for oil containment pits and simplifies environmental compliance at the installation site.
• Lower maintenance requirement — there is no oil to sample, filter, or replace over the transformer's service life, reducing routine maintenance tasks.
• Better humidity and dust tolerance — encapsulated or cast-resin winding designs can withstand humid coastal climates and dusty industrial environments better than some open winding types.
• Shorter installation time indoors — since fire-rated oil barriers and containment structures are not required, installation inside a building's electrical room is generally more straightforward.

How a Dry-type Transformer Works

Like any power transformer, a dry-type unit transfers electrical energy between two voltage levels through electromagnetic induction: an alternating current in the primary winding creates a changing magnetic field in the iron core, which induces a corresponding voltage in the secondary winding at a different turns ratio. The key difference lies in how the heat generated by this process is removed. Instead of circulating oil around the windings, a dry-type transformer relies on natural or forced air circulation, aided by ventilation ducts built into the winding structure, to carry heat away to the surrounding air.

Cast-Resin and Vacuum-Pressure Impregnated Designs

Two common winding designs are used in dry-type transformers. Cast-resin windings are fully encapsulated in epoxy resin under vacuum, forming a solid, moisture-resistant block around the conductor. Vacuum-pressure impregnated (VPI) windings are instead coated with resin and cured, leaving a thinner insulating layer while still protecting against moisture and contamination. Both approaches aim to combine reliable insulation with efficient air-cooling paths through the winding.

Temperature Monitoring

Because there is no oil temperature to reference, dry-type transformers typically rely on embedded winding temperature sensors connected to a control relay. If winding temperature rises above a preset threshold, the relay can trigger a cooling fan, an alarm, or, in extreme cases, a trip signal to protect the transformer from thermal damage.

Dry-type vs. Oil-Immersed Transformer

The table below highlights the practical differences that typically guide the choice between a dry-type and an oil-immersed transformer for a given project.

Selection and Installation Guidance

1. Confirm the rated capacity and voltage ratio required by the load, allowing headroom for planned future expansion.
2. Check the local fire code and building regulations, since many jurisdictions specifically require dry-type transformers for installation inside occupied buildings above a certain floor level.
3. Ensure adequate ventilation clearance around the unit, as insufficient airflow can raise operating temperature and shorten insulation life.
4. Plan for professional commissioning, including insulation resistance testing and temperature sensor calibration, before the transformer is energized.

Working with a supplier able to provide installation guidance, equipment commissioning, and after-sales fault handling can help ensure the transformer performs reliably from the first day of operation through its full service life.

Frequently Asked Questions

Q1: Why are dry-type transformers preferred for indoor installations?

Since there is no oil involved, the risk of fire and leakage is significantly reduced, which allows the transformer to be placed inside occupied buildings without the fire-rated barriers and oil containment structures that oil-immersed units typically require.

Q2: Do dry-type transformers need forced-air cooling fans?

Many units rely on natural air circulation for normal loads, but cooling fans can be added to increase capacity temporarily or to keep winding temperature within limits during periods of higher-than-normal load.

Q3: How does humidity affect a dry-type transformer's performance?

Cast-resin and vacuum-pressure impregnated winding designs are specifically intended to resist moisture absorption, which helps the transformer maintain stable insulation performance even in humid coastal or tropical environments.

Q4: What certifications indicate a reliable dry-type transformer manufacturer?

Buyers can look for recognized certifications such as CCC and ISO 9001, which confirm that the transformer has passed relevant national safety testing and is produced under a consistent quality management system.