A dry type transformer carries its heat away through air rather than through oil. That single choice is what everything else follows from, including where the unit can be installed, what happens to it during a fault, and how much upkeep it needs over twenty years.
What “dry type” actually describes
The windings and core sit in a normal air atmosphere instead of an oil tank. The insulation system is solid, applied as varnish, resin, or an encapsulation, and cooling happens by air moving over the surfaces. There is no tank, no conservator, no oil to sample or replace, and nothing on the unit that can burn.
That absence is the whole point. A unit that contains no flammable liquid does not need a fire-rated vault, does not need bunding to contain a spill, and does not become a fire load inside a building where people are working.
The dry type family
The name covers several constructions that differ mainly in how the winding is protected.
- Open wound. The bare winding structure, insulated with varnish or a temperature-rated tape system, open to the air. Suitable for clean, dry indoor locations.
- Vacuum pressure impregnated (VPI). The wound assembly is dried in vacuum, then flooded with resin so the insulating material penetrates the winding insulation. It cures into a bonded, void-free mass that resists moisture and vibration.
- Vacuum pressure encapsulated (VPE). A shell of resin is formed around the winding, so the resin becomes both the insulation and the mechanical containment.
- Cast resin. Each winding is cast in an epoxy mould, giving a rigid, moisture-tight coil with a controlled surface that discharges across cleanly. This is the variant most often specified for indoor HV distribution and for wet or dusty plants.
How the transformer works
The working principle is the same as any other transformer. Alternating current in the primary winding sets up an alternating flux in the laminated core, and that flux induces a voltage in the secondary. The voltage ratio follows the turns ratio, which is why a dry type unit can be built to any ratio the system needs rather than only to a standard one.
What changes from the oil-filled case is the thermal path. In an oil transformer the oil carries heat from the conductor to the tank wall and then to the air. Air alone is a weaker coolant, so a dry type unit of the same rating tends to be larger and to run at a higher temperature rise for the same loss. Insulation has to be rated for that, which is why solid insulation classes matter so much in dry type design.
Where dry type wins over oil
- Installation inside occupied buildings, in basements, or on floors above ground.
- Fire-rated shafts, tunnels, and rooms with strict evacuation requirements.
- Coastal and high-humidity sites, where the encapsulated versions are not affected by condensation cycling.
- Dusty or chemically aggressive plants where an oil tank would need regular sampling.
- Installations where a spill would contaminate a process or a watercourse.
- Routine maintenance teams who do not want an oil-handling programme in their schedule.
Where oil still makes more sense
Air is a poorer dielectric and a poorer coolant than oil, so the advantage inverts as ratings climb. Very high ratings or very high voltage levels are normally oil-filled, because the air clearances and the physical size required by a dry type unit become impractical. Noise is also handled differently: the oil tank and its mass absorb vibration, and an open-wound dry type unit can measure louder at the same rating.
For outdoor installations in clean air at moderate ratings, an oil-immersed unit is usually the more economical and the more compact answer.
Common applications
- Commercial buildings and shopping centres, where the substation sits inside the building.
- Hospitals, where the transformer may be adjacent to patient areas.
- Data centres and telecom rooms, on the floor rather than in a basement vault.
- Industrial plants, including steel, chemical, and electrochemical loads.
- Renewable energy, where the transformer is inside the nacelle, at the inverter station, or on a battery enclosure.
- Rail, port, and marine installations with constrained fire compartments.
Specifying one
The information that determines the design, and therefore the price, is short: primary and secondary voltage, rated capacity, the load profile including any non-linear content, the installation environment with altitude and ambient, the cooling method required, the impedance or fault level the system can tolerate, and any enclosure or IP requirement.
Get those onto paper and the rest is engineering. See the dry type transformer range, the cast resin transformer range, or send us the requirements.
Dry Type Transformer Types: VPI and Cast Resin
Two insulation systems dominate the dry type market, and the choice is mostly about where the unit will live. In a vacuum pressure impregnated (VPI) winding, the coil is dried and impregnated with resin under vacuum, leaving a thin, flexible insulation layer that follows the conductor. It is light for its rating, tolerates thermal cycling well and is easier to repair. In a cast resin winding the coil is encapsulated in epoxy under vacuum, giving a solid block that resists moisture, dust and condensation — which is why cast resin is the usual answer for coastal sites, tunnels, mines and any room that may be washed down.
- VPI — lower weight, better thermal cycling behaviour, repairable; the default for dry, clean indoor rooms.
- Cast resin — sealed against moisture and contamination, higher mechanical strength, no ventilation requirement inside the coil; the default for humid, dusty or corrosive locations.
Both are rated and tested to IEC 60076-11 for dry type transformers. The decision rarely turns on efficiency — it turns on what the air in the room will do to the winding over twenty years.
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Related equipment
If you are specifying this as part of a larger distribution scheme, the surrounding scope is covered on the commercial building distribution, compact substations and switchgear control and protection pages.
Frequently Asked Questions
How do I approach specifying a dry type transformer?
Start from the load and the environment, then let the equipment follow. Working from a catalogue rating backwards is how projects end up with either a marginal unit or one that costs money sitting under-loaded.
When should I choose dry type over oil immersed?
Indoors, and wherever fire risk, oil containment or maintenance access is a concern. Commercial buildings, hospitals, data centres and enclosed industrial spaces are the usual cases. Oil immersed remains common outdoors and at higher ratings, where cost and cooling favour it.
What cooling options are available?
Air natural is the baseline. Where the load profile or the enclosure temperature demands it, forced air cooling with fans raises the usable rating. The choice is made against your ambient range and load profile, not against a catalogue default.
Which standard applies, and is it certified?
The applicable standard and clause set are agreed against your project specification before production starts. Where a project names IEC 60076 for the transformer, we work to those clauses and the routine test report ships with the unit.
What do you need from us to quote?
The single-line diagram or a description of the application, the voltages, the load with its duty, the site conditions, and any standard the project specifies. Send what you have – if something is missing, it can be worked out together rather than delaying the enquiry.