Dry Type Transformer: Technical Guide
How a Dry Type Transformer Works
A dry type transformer operates on the same electromagnetic induction principle as any other transformer, but without insulating oil. The core-and-coil assembly sits in air, with the windings insulated by solid materials — usually cast epoxy resin around medium-voltage windings and high-temperature impregnating varnish or thermal-class films around low-voltage windings. Heat generated in the core and windings dissipates directly into the surrounding air, either by natural convection (AN cooling) or by forced air from temperature-controlled fans (AF cooling). The absence of oil means no fire risk from a leaking or rupturing tank, no environmental hazard from oil spills, and no oil sampling or testing requirements over the life of the unit.
Cast Resin vs VPI — Two Construction Methods
Dry type transformers come in two main construction types, each with different applications and price points.
Cast resin transformers, sometimes called epoxy cast or cast coil, have each winding completely encapsulated in cured epoxy resin. The winding is wound onto a mandrel, placed in a precision mold, evacuated to remove all air, then injected with degassed epoxy that cures into a solid void-free block around the conductor. This produces a winding that is essentially impervious to moisture, dust, and minor mechanical impact, with partial discharge values low enough to meet IEC 60076-11 medium-voltage requirements. Cast resin is the standard for 10 kV and above.
VPI (Vacuum Pressure Impregnated) transformers use a different approach. The wound coils are placed in a vacuum chamber, evacuated, then flooded with insulating varnish under pressure, forcing varnish into every gap in the winding before curing. The result is a winding sealed against moisture but not encapsulated. VPI is cheaper to produce and well-suited to low-voltage and some medium-voltage applications in mild environments.
EverWins produces the cast resin construction, which is the appropriate choice for the 10 kV to 35 kV voltage class our product range serves.
Indoor vs Outdoor Installation
Standard cast resin dry type transformers ship in either open-type (IP00) or ventilated cubicle (IP21 / IP23 / IP31) construction, both intended for installation inside a protected building room. The room provides weather protection, while the transformer's louvered cubicle keeps personnel safe from contact with energized parts and allows air convection for cooling.
For outdoor installation, the same core-and-coil assembly is housed in a sealed sheet-steel enclosure rated IP54 or higher, with weather-proof inlet and outlet louvers and forced-air cooling fans triggered by winding temperature. This adds roughly 15 to 25 percent to the unit cost but eliminates the need for a dedicated indoor substation room — usually a substantial space and construction saving for the project.
The choice between indoor and outdoor configuration is normally driven by available floor space, local fire codes, and total project economics, not by transformer performance. A dry type transformer in an outdoor enclosure delivers the same electrical performance as the same unit in an indoor ventilated cubicle.
How to Specify a Dry Type Transformer
At the quotation stage, you will need to confirm the following parameters. Anything you cannot specify, our engineering team can derive from your single-line diagram and site data.
1. Rated capacity (kVA). Total connected load plus diversity factor plus future expansion. Common standard sizes: 100, 160, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500 kVA. Larger units up to 20,000 kVA on request.
2. Rated voltages. Primary (typically 10, 11, 15, 20, 22, 33, or 35 kV) and secondary (typically 0.4 kV for distribution, 0.69 kV for industrial, or 11 kV for sub-transmission).
3. Vector group. Dyn11 is the international default for distribution; Yyn0 is also common, particularly in some Asian markets. Specialty groups available on request.
4. IP rating. IP00 for installation inside larger switchboards; IP21 / IP23 / IP31 for standalone indoor; IP54 / IP55 for outdoor enclosure.
5. Climatic, environmental, and fire class. Per IEC 60076-11: C2 (frequent condensation), E2 (heavy pollution and frequent condensation), F1 (limited fire load and no halogens). Most building applications now require all three.
6. Insulation class. F (155 °C) is standard; H (180 °C) for high-ambient or overload-frequent applications.
7. Cooling method. AN (natural air) standard. AF (forced air) provides overload capability up to roughly 40 percent above the AN rating, useful for sites with sharp load peaks or future expansion.
8. Accessories. Temperature monitoring relay, anti-condensation heaters, electromagnetic shielding for IT-sensitive sites, on-load tap changer if voltage regulation is needed under load.
Dry Type vs Oil Immersed Transformer — Which to Choose?
The choice between dry type and oil immersed is driven primarily by installation environment and applicable building codes, not by electrical performance.
Choose dry type when:
• The installation is indoors, especially in occupied buildings (commercial, hospital, hotel, residential, data center).
• Fire codes restrict or prohibit liquid-filled transformers indoors.
• Environmental risk from oil spills must be eliminated (near water sources, on rooftops, in pristine environments).
• Maintenance access for oil testing is difficult or expensive.
• The application is below 5,000 kVA at distribution voltage levels.
Choose oil immersed when:
• Installation is outdoors with proper pad and oil-containment provisions.
• Capacity exceeds 5,000 kVA, where oil immersed cost advantage grows.
• The voltage level is 66 kV or higher, where dry type becomes impractical.
• Hot ambient conditions favor the better heat dissipation of oil-cooled designs.
For the 30 kVA to 2,500 kVA range used in most commercial and industrial buildings, dry type and oil immersed are technically interchangeable; the choice comes down to where the transformer will sit and what fire and environmental codes apply.
Maintenance and Service Life
A cast resin dry type transformer is one of the lowest-maintenance pieces of major electrical equipment in a distribution system. With no oil and no breather, routine maintenance reduces to:
• Visual inspection every 6 to 12 months — check for dust accumulation, discoloration of the resin (a sign of overheating), and physical damage.
• Cleaning as needed — blow out dust with dry compressed air or vacuum at scheduled outages. Heavy dust accumulation insulates the windings thermally and accelerates aging.
• Infrared thermography under load — scan bushings, tap-changer connections, and the resin surface to catch localized hot spots before they fail.
• Temperature sensor verification — confirm PT100 sensors and alarm/trip relay function during annual maintenance.
• Tightness check — verify primary and secondary terminations are torqued to specification.
With this routine, a properly specified cast resin transformer reliably delivers 25 to 30 years of service in a typical indoor environment, and 20 to 25 years in a moderate-pollution outdoor enclosure. The dominant failure modes — overheating from blocked airflow and electrical damage from external faults — are preventable through facility-level controls rather than transformer maintenance.
