Transformers are the heart of any power distribution network, stepping voltage up for transmission or down for safe plant use. The two most common construction methods are dry-type and oil-immersed. Dry-type transformers use air or cast resin as the cooling and insulation medium, while oil-immersed transformers use mineral oil to both insulate and carry heat away. The best choice depends on fire risk, indoor or outdoor placement, maintenance capacity, efficiency target, and first cost.
What Is a Transformer?
A transformer is a static electromagnetic device that transfers electrical energy between two or more circuits through magnetic induction. It changes voltage and current levels while keeping frequency unchanged. The core components are a laminated magnetic core, primary and secondary windings, insulation system, cooling medium, and enclosure or tank.
Power transformers for distribution and industrial use are governed by IEC 60076 (Power Transformers) internationally, IEEE C57.12.00 and IEEE C57.12.01 in North America, and GB/T 1094 in China. These standards define winding temperature rise limits, short-circuit withstand, insulation levels, and test routines.
How a Transformer Works
When alternating current flows in the primary winding, it creates a changing magnetic field in the core. That field induces a voltage in the secondary winding. The ratio of primary to secondary turns determines whether the transformer steps voltage up or down. Real transformers have losses — core losses (hysteresis and eddy currents) and winding losses (I²R) — which appear as heat. Managing that heat is what drives the dry-type versus oil-immersed decision.
What Is a Dry-Type Transformer?
A dry-type transformer has no liquid coolant. Heat is removed by natural or forced air circulation over the windings and core. The windings are insulated with materials such as vacuum pressure impregnated (VPI) epoxy resin or cast resin. Cast resin transformers encapsulate each winding in epoxy, giving excellent moisture, dust, and pollution resistance.
Because there is no oil, dry-type transformers present a much lower fire risk and do not require oil containment, firewalls, or oil sampling. They are common in buildings, data centers, hospitals, offshore platforms, and any indoor location where fire safety is critical.
What Is an Oil-Immersed Transformer?
An oil-immersed transformer places the core and windings inside a tank filled with insulating mineral oil. The oil both insulates and transfers heat to the tank walls and radiators, where it is dissipated to ambient air. Larger units add pumps and fans for forced oil and forced air cooling, designated OFAF or ONAF depending on the cooling code under IEC 60076-2.
Oil-immersed transformers are widely used outdoors in substations, utility distribution, heavy industry, and large renewable energy plants. They offer higher efficiency, lower material cost for a given kVA, and better overload capability than dry-type units of the same rating. The trade-off is fire risk, environmental containment, and regular oil testing.
Dry-Type vs Oil-Immersed Transformer: Key Differences
| Feature | Dry-Type Transformer | Oil-Immersed Transformer |
|---|---|---|
| Cooling medium | Air / cast resin / epoxy | Mineral insulating oil |
| Fire risk | Very low; self-extinguishing resin | Higher; oil is combustible |
| Typical location | Indoors, buildings, data centers | Outdoors, substations, heavy industry |
| Efficiency at same kVA | Good; slightly higher losses | Excellent; lower core and winding losses |
| Initial cost | Higher for medium and large kVA | Lower for medium and large kVA |
| Maintenance | Visual inspection, cleaning, fan checks | Oil sampling, DGA, filtration, leak checks |
| Overload capability | Moderate; limited by winding hotspot | Strong; oil buffers thermal swings |
| Environmental risk | No oil spill risk | Requires containment and spill plan |
| Noise level | Often higher due to magnetostriction in core | Tank dampens sound; typically quieter |
| Applicable standards | IEC 60076-11, IEEE C57.12.01 | IEC 60076-2, IEEE C57.12.00 |
| Best for | Indoor, fire-sensitive, limited space | Outdoor, high-efficiency, high-kVA substations |
Types of Transformers by Insulation and Cooling
Vacuum Pressure Impregnated (VPI) Dry-Type
VPI transformers are wound, dried, and impregnated with polyester resin under vacuum pressure. They are lighter than cast resin and cost less, but they are more sensitive to moisture and pollution. Common in commercial buildings and light industrial plants.
Cast Resin Dry-Type
Each high-voltage coil is cast in epoxy resin under vacuum. The result is a solid, moisture-proof block with excellent partial discharge and pollution performance. Cast resin is the preferred choice for data centers, marine, offshore, and environments with salt spray or chemical vapors.
Mineral Oil-Immersed
The traditional distribution transformer. Mineral oil provides insulation and cooling. Standard for pad-mounted and pole-mounted distribution, utility substations, and industrial plants with outdoor switchyards.
Natural and Forced Cooling Variants
Both dry-type and oil-immersed units are available in natural cooling (AN or ONAN) or with fans and pumps (AF, AAF, ONAF, OFAF). Forced cooling increases the continuous rating but adds fans or pumps that need maintenance and power.
Typical Applications
Commercial and High-Rise Buildings
Dry-type transformers dominate because they can be installed indoors near load centers without oil containment or fire separation. They fit in basement electrical rooms and rooftop plant rooms.
Data Centers
Cast resin dry-type units are preferred for their low fire risk and ability to handle non-linear loads. They are often paired with UPS systems and busway distribution to deliver clean power to racks.
Industrial Substations
Oil-immersed transformers are common at the utility interface, where ratings reach several MVA and efficiency over decades of operation matters more than indoor footprint. Indoor dry-type units then step down further to feed switchgear and MCCs.
Renewable Energy and Heavy Industry
Solar farms, wind plants, and smelters frequently use oil-immersed step-up or step-down transformers because of high efficiency, high overload tolerance, and proven outdoor reliability.
How to Select the Right Transformer
- Confirm the kVA rating. Sum the connected load and apply a diversity factor. Verify inrush and motor starting currents.
- Define primary and secondary voltage. Include tap range requirements if the grid voltage fluctuates.
- Choose vector group. Dyn11 is the most common choice for distribution, but verify compatibility with the switchgear and protection settings.
- Specify impedance voltage. Higher impedance limits fault current but increases voltage drop; balance protection coordination and motor starting.
- Select cooling class. AN/ONAN is simplest; add fans or pumps only when space limits the physical size.
- Assess fire and environmental rules. In buildings, dry-type often wins. Outdoors, oil-immersed usually wins on cost and efficiency.
- Check access and maintenance. Oil-immersed units need oil sampling, filtration, and eventual replacement. Dry-type units need cleaning and fan replacement.
Quality Inspection Points Before Acceptance
- Verify routine test reports: winding resistance, turns ratio, no-load and load losses, impedance voltage, and applied/induced voltage tests under IEC 60076 or IEEE C57.12.90.
- Inspect core laminations for alignment and clamping; loose laminations increase noise and losses.
- Check insulation resistance and polarization index before energization.
- For oil-immersed units, review dielectric strength and moisture content of the oil; request dissolved gas analysis (DGA) if available.
- Confirm nameplate data matches the specification: kVA, voltage ratio, vector group, impedance, cooling class, temperature rise, and insulation class.
- Inspect the tank, bushings, and gasket joints for leaks or transit damage.
Supplier Evaluation Checklist
- Can the supplier provide third-party type-test reports for the exact kVA and voltage class?
- Is the design aligned with IEC 60076, IEEE C57, or GB/T 1094 as required by the project location?
- Does the factory follow documented winding, core assembly, and vacuum pressure impregnation or resin casting procedures?
- Can the supplier share routine test data, including no-load and load loss values?
- What is the delivery program, and can the transformer be shipped as a complete unit or only in parts for on-site assembly?
- Is after-sales support available for commissioning, maintenance training, and spare parts?
Frequently Asked Questions
1. Can a dry-type transformer be installed outdoors?
Only if it is in a weatherproof enclosure rated for the location. Cast resin units tolerate moisture better than VPI units, but direct sun, rain, and salt spray still require an IP-rated housing. Oil-immersed transformers are the default for outdoor substations.
2. How often should transformer oil be tested?
For oil-immersed units, oil dielectric strength and moisture are typically checked annually. Dissolved gas analysis is often performed every one to three years, or immediately after a fault or overload event. Dry-type transformers do not require oil testing.
3. Which transformer type has lower losses?
Oil-immersed transformers generally achieve lower no-load and load losses for the same kVA because oil cooling allows a more compact winding and core design. However, modern amorphous-core dry-type units can also reach very low no-load losses.
4. What does vector group Dyn11 mean?
Dyn11 means the primary winding is delta-connected, the secondary is star-connected with neutral brought out, and the secondary voltage lags the primary by 30 electrical degrees. It is the most common vector group for distribution transformers.
5. Do transformers need overload protection?
Yes. Transformers are protected against overcurrent, short circuit, earth fault, and excessive temperature by fuses, circuit breakers, and protective relays sized to the transformer rating and impedance. Buchholz relays protect oil-immersed units from internal faults.
6. What is temperature rise class?
Temperature rise class defines the maximum allowable average winding temperature rise above ambient. Common classes are 105 K (class A), 120 K (class E), 155 K (class F), and 180 K (class H). The class must match insulation materials and expected load cycle.
7. Can a transformer feed a busway directly?
Yes. Many substations connect the transformer secondary directly to a low-voltage switchgear assembly or busway riser. This eliminates cable runs and reduces impedance between the transformer and the distribution system. BANGE Electric supplies busway systems designed for direct transformer coupling.
Source Transformers and Distribution Equipment from BANGE Electric
BANGE Electric manufactures and supplies low-voltage switchgear, distribution boxes, withdrawable switchgear, and busway systems that connect transformer secondary outputs to the loads. Whether your project uses dry-type units in a building or oil-immersed units in an outdoor substation, our engineering team can review your single-line diagram and recommend the right combination of distribution equipment.
View our busway systems or contact our team with your project details for a layout proposal.
Requirements may vary by application, market, and applicable standard. Final specification should be confirmed with the engineering team.