GuideTransformers10 min read
Types of dry-type transformers
A dry-type transformer cools its windings with air instead of oil. There are four constructions, in order of how well they resist moisture, dust and corrosive air: open-wound (ventilated), vacuum pressure impregnated (VPI), vacuum pressure encapsulated (VPE) and cast resin (cast coil). For 600 V-class units up to about 1,000 kVA, VPI is the usual choice; for medium-voltage units indoors, and anywhere the air is damp or dirty, cast coil is.
The short version
- Open-wound: coils dipped in varnish and baked; the cheapest; for clean, dry indoor air and the small sizes.
- VPI: varnish forced into the coil under vacuum and pressure; the standard for 600 V-class units and for medium-voltage up to about 15 kV.
- VPE: coils encapsulated in silicone or epoxy resin; for high humidity, salt air and chemical plants.
- Cast resin (cast coil): coils cast solid in epoxy under vacuum; for medium voltage (5 to 35 kV class), harsh or wet sites, and units that sit idle and must energize without drying out.
- Ratings to know: temperature rise (80, 115 or 150 °C), AA/FA (fan-cooled rating, usually +33%), K-factor for harmonic loads, and the DOE efficiency level.
- Dry-type for indoors and near people; liquid-filled for outdoors, above about 2,500 kVA, and where cost per kVA matters most.
What makes a transformer dry-type
Every transformer has to get rid of its losses as heat: a few kilowatts in a 500 kVA unit, tens in a large one. A liquid-filled transformer does it with oil, which also insulates the windings. A dry-type does it with air, moving by convection or fans through ducts in and around the coils, and the windings are insulated by what’s on them: varnish, resin and the conductor’s own insulation.
That has consequences. Air insulates less well than oil, so a dry-type’s clearances are larger and its impulse (BIL) ratings lower for the same voltage class. Air cools less well, so the windings run hotter, and the insulation is rated for it (class 220 °C materials in most modern units). And air carries whatever is in the room, so a dry-type’s life depends on what settles on its coils. The four constructions differ in how well they keep it out.
The four constructions
| Construction | How the coils are insulated | Resists | Typical use | Relative cost |
|---|---|---|---|---|
| Open-wound (ventilated, dip-and-bake) | Coils dipped in varnish and baked | Least: clean, dry, indoor air only | Small 600 V-class units, 15 to 300 kVA; general-purpose and lighting | Lowest |
| VPI (vacuum pressure impregnated) | The whole coil assembly under vacuum, then flooded with varnish under pressure and baked, so the varnish fills every void | Moisture and dust well; corrosive air moderately | 600 V class, 15 to 1,000+ kVA, the standard building transformer; medium-voltage to about 15 kV | Low to middle |
| VPE (vacuum pressure encapsulated) | As VPI, with a silicone or epoxy resin that coats and seals the coil surfaces rather than a thin varnish | High humidity, salt air, chemical fumes, washdown | Coastal and marine sites, pulp and paper, chemical plants, wastewater | Middle |
| Cast resin (cast coil) | Each coil cast solid in epoxy under vacuum, often with glass-fiber reinforcement | Moisture, dust, corrosive air, mechanical shock and short-circuit forces; can sit idle and energize without drying | Medium voltage, 5 to 35 kV class, 300 kVA to 30 MVA; data centers, high-rise, hospitals, mines, tunnels, offshore | Highest |
- Open-wound
- The original dry-type. Cheap and easy to repair, and fine in an electrical room that stays dry. The varnish coat is thin; in a damp basement the coils absorb moisture and the insulation resistance drops, so a unit that has sat unpowered may need drying out before it’s energized.
- VPI
- The workhorse. The vacuum pulls air out of the coil and the pressure forces varnish in, so there are no voids to hold moisture or to start partial discharge. Medium-voltage VPI units (5 and 15 kV class) are common and cost less than cast coil, with a little less tolerance of a harsh room.
- VPE
- A VPI process finished with a heavier resin, so the coil surface is sealed. The usual choice where the air is wet or chemically active but the voltage is low enough that cast coil would be an unnecessary expense.
- Cast resin
- The coil is a solid epoxy block, so there is nothing for moisture to get into and nothing for dust to lodge in beyond the surface. Cast coils take the mechanical forces of a short circuit better than any other dry construction, which is why large and medium-voltage dry-types are mostly cast. The epoxy can crack from thermal shock if a cold unit is loaded hard, so makers specify a warm-up, and a cracked coil is replaced, not repaired.
Enclosures and cooling
The enclosure decides where the unit can live. NEMA 1 (ventilated, indoor) is the standard for an electrical room; NEMA 2 adds drip shields; NEMA 3R is the outdoor version, with louvers that shed rain, for units mounted on a pad or a wall outside; and a totally enclosed, non-ventilated (TENV) housing seals the unit for dusty, dirty or corrosive rooms, at the cost of a larger, heavier unit for the same kVA since it cools through its skin.
Cooling is by natural convection (AA) or with fans (FA). Fans are usually an option on units above 500 kVA and raise the rating by a third: a 1,500/2,000 kVA AA/FA unit. On a used unit the fans, their controller and the winding temperature sensors that switch them on are part of what’s inspected, since a fan that doesn’t run means the FA rating doesn’t exist.
Temperature rise, insulation class and efficiency
A dry-type’s nameplate gives its average winding temperature rise above a 40 °C ambient: 150 °C is the standard, 115 °C and 80 °C run cooler. The insulation system is class 220 °C in all three, so a lower-rise unit is simply bigger, with more copper and iron, and the margin becomes overload capacity: a 115 °C-rise unit can carry about 15% over its nameplate continuously and an 80 °C-rise unit about 30%, and both run cooler, quieter and longer at rated load. Lower-rise units also lose less energy, which is why the efficiency rules pushed makers toward them.
Those rules matter when you buy. The U.S. Department of Energy’s efficiency standard for low-voltage dry-type distribution transformers (10 CFR 431, the 2016 level) sets a minimum efficiency at 35% load for every size, and a further step takes effect in 2029. A new or new-surplus unit built since 2016 carries a label saying it complies; a 1990s unit is a few points less efficient, which over twenty years of 24-hour service is real money. The rule applies to new units sold, not to used ones installed, so a used pre-2016 transformer is legal to buy and run.
Dry-type or liquid-filled?
| Dry-type | Liquid-filled | |
|---|---|---|
| Indoors | Yes: no vault needed; the NEC’s dry-type rules (450.21) are lighter | Needs a vault or less-flammable fluid and the NEC’s liquid-filled rules (450.23 to 450.27) |
| Outdoors | Possible in NEMA 3R, but not its strength | The standard: pad-mounts and substations |
| Size | Economical to about 2,500 kVA; built to 30 MVA cast | Economical at every size; the only choice above 10 MVA |
| Cost per kVA | Higher, especially above 1,000 kVA | Lower |
| Losses and noise | Higher losses and noisier for the same kVA | Lower losses, quieter |
| Overload | Limited; the windings heat fast | Good: the oil’s thermal mass absorbs peaks |
| Maintenance | Clean and check connections | Oil sampling and the tests in the oil guide |
| Fire and environment | No fluid to leak or burn | Containment for mineral oil; FR3 and silicone are less flammable |
The choice is usually made by location. Indoors, near people or sensitive equipment, and up to about 2,500 kVA, a dry-type is the ordinary answer. Outdoors on a pad, or anywhere large, a liquid-filled unit is cheaper and better. The two overlap at a plant’s unit substation, where a 1,500 to 3,000 kVA transformer can be either; there the room (a vault or not), the fluid (FR3 is allowed indoors with conditions) and the price settle it.
Medium-voltage dry-type transformers
Dry-types at 4.16, 12.47, 13.8 and up to 34.5 kV primary voltage are built cast resin or VPI, from about 300 kVA to 30 MVA, and stand in a plant’s electrical room or a unit substation where a liquid-filled unit would need a vault. They’re the transformers in high-rise buildings (a 13.8 kV riser with a cast-coil unit on every few floors), data centers, hospitals, mines and ships. They need more clearance than a liquid-filled unit of the same class (a 15 kV-class dry-type has a 60 kV BIL as standard and 95 kV as an option, against 95 kV standard in oil), so surge arresters are usually fitted, and a medium-voltage dry-type in a damp or dirty room should be cast coil.
On the used market these are good value: a cast-coil unit that has run indoors for fifteen years has usually seen nothing that could hurt it, and the inspection and tests tell you so. The dry-type listings show what’s available, mostly cast coil and VPI units from 1,000 to 3,750 kVA.
Buying a used dry-type transformer
A used dry-type has no oil to sample, so its history is read from the coils and the tests. Before buying, look for:
- The coils. Cracks, chips or discoloration in cast resin; dark or crazed varnish on VPI coils, which means they have run hot; dust packed in the ducts; signs of water or rust in the enclosure.
- The connections and taps. Discoloration at a bolted joint means it has been hot. Tap links should be on the nameplate’s rated tap.
- Fans, controller and sensors if it has an FA rating, and the winding temperature indicator.
- The electrical tests: insulation resistance against the factory values, turns ratio on every tap, winding resistance and power factor. Our testing guide covers each.
- The nameplate: kVA, voltages and connection, temperature rise, the impulse level, the insulation class and, since 2016, the DOE efficiency marking.
When you know the size and voltages, describe the unit to Magic Search with the construction you want, and it finds the ones for sale and tells you which are cast coil and which are VPI.
Questions people ask
What is the most common type of dry-type transformer?
Vacuum pressure impregnated (VPI) construction, for 600 V-class units from 15 to about 1,000 kVA, which covers the 480 V to 208Y/120 V transformer in nearly every commercial building. For medium-voltage dry-types, cast resin is the most common in new installations, with VPI a close second.
What is the difference between VPI and cast coil?
In a VPI unit the coils are wound with insulated conductor, then varnish is drawn into every gap under vacuum and pressure and baked hard. The coils are sealed but not solid; air still reaches the surfaces. In a cast coil unit each coil is placed in a mold and epoxy resin is cast around and through it under vacuum, so the finished coil is a solid block. Cast coils resist moisture, dust and corrosive air better, withstand short-circuit forces better, and can be energized after months idle without drying; they cost more and a damaged coil is replaced rather than repaired.
Can a dry-type transformer be installed outdoors?
Yes, in a NEMA 3R enclosure, which sheds rain and keeps the windings out of the weather, with cast resin or VPE construction preferred where the air is damp or salty. Most dry-types are indoor (NEMA 1 or 2) units, and that’s where they belong: outdoors, a liquid-filled pad-mount usually costs less and lasts longer.
How long does a dry-type transformer last?
Twenty to thirty years, and longer in a clean, cool room at moderate load. Life is set by the insulation’s temperature: every 8 to 10 °C above its rated hot-spot temperature roughly halves the insulation’s life, so an 80 °C-rise unit, or a 150 °C-rise unit run at two-thirds load, outlasts a fully loaded 150 °C-rise unit.
Do dry-type transformers need maintenance?
Less than liquid-filled units, but not none. Once a year: de-energize, vacuum and blow out the dust between the coils and in the ducts, check the connections for tightness and discoloration, check the fans and their controls if fitted, and take insulation resistance readings. Dust is the enemy: it blocks cooling and holds moisture.
What do AA and FA mean on a dry-type transformer?
AA is the self-cooled rating, with air moving by convection alone; FA is the forced-air rating with fans running. A unit marked 1,000/1,333 kVA AA/FA delivers 1,000 kVA on its own and 1,333 kVA with the fans on, a 33% increase. The fans are usually switched by a winding temperature controller.
What is the difference between a dry-type and an oil-filled transformer?
A dry-type is cooled by air and insulated with varnish or resin; an oil-filled (liquid-filled) transformer is cooled and insulated by a fluid in a sealed tank. Dry-types can go indoors and near people without a vault, need no oil containment or testing, and are lighter per kVA at small sizes. Liquid-filled units cost less per kVA at large sizes, run quieter and cooler, take overloads better, and are the standard for outdoor pad-mounts and substations.
Sources: IEEE C57.12.01 (dry-type general requirements); IEEE C57.12.91 (dry-type test code); IEEE C57.94 (installation and maintenance of dry-type units); 10 CFR 431 (DOE efficiency standards); NEC Article 450.