The choice between a dry-type (cast-resin) and an oil-immersed transformer is usually made on safety, fire risk, and siting. But it also dictates a completely different insulation system. The materials that keep a dry-type unit alive are not the same as those inside an oil unit — and specifying for one while thinking of the other is a common, costly mistake. This guide contrasts the two insulation systems so you can specify the right materials for each.
Two Technologies, Two Cooling Media
A dry-type transformer cools and insulates with air and solid resin; an oil-immersed unit cools and insulates with mineral or ester oil. That single difference reshapes every insulation material in the active stack.
Dry-Type (Cast-Resin) Insulation System
Dry-type units rely on solid, air-cooled insulation — no flammable liquid.
- Slot and phase insulation: flexible composites — DMD for standard Class B/F duty, NMN where thermal and tear margins matter, and NHN for the hottest, hardest-cycling designs.
- Coil interlayer and end insulation: DMD prepreg — a resin-preimpregnated composite that bonds the LV coil layers during cure, eliminating voids.
- Encapsulation: the whole active part is cast in epoxy resin, which also serves as the primary dielectric. No oil means no leak risk, so dry-types suit indoors, basements, and high-fire-risk sites.
The system is “solid” end to end: composites for structure, prepreg for coil bonding, resin for encapsulation.
Oil-Immersed Insulation System
Oil units insulate and cool with the oil itself, so the solid materials are paper-based and designed to be saturated.
- Coil conductor insulation: cable paper — high-purity cellulose wound directly on the conductor, impregnated by oil to form the turn-to-turn dielectric.
- Lead and connection wrapping: crepe paper — stretchable, conformable, ideal for terminations and leads leaving the tank.
- Coil interlayer bonding: diamond dotted paper (DDP) — resin-dotted paper that cures to glue adjacent layers together once the coil is dried and oil-impregnated.
- The oil is the dielectric: it fills every void, carries heat, and provides the bulk insulation between live parts and ground.
Here the system is “paper plus oil”: the cellulose does the structural insulating, the oil does the cooling and void-filling.
Side-by-Side
| Aspect | Dry-type (cast-resin) | Oil-immersed |
|---|---|---|
| Cooling / dielectric medium | Air + solid resin | Oil (mineral or ester) |
| Main solid insulation | Flexible composites (DMD/NMN/NHN), prepreg | Cellulose paper (cable, crepe, DDP) |
| Coil bonding | DMD prepreg, epoxy cast | Diamond dotted paper, oil impregnation |
| Fire / leak risk | Low (no liquid) | Higher (contains oil) |
| Typical thermal class | B/F/H by composite | Class A (105°C) paper, raised by oil cooling |
| Common siting | Indoor, urban, fire-sensitive | Substations, outdoor, high-power |
How to Choose the Materials
- The transformer type is decided first — by fire risk, siting, and power level, not by the insulation. Once the type is set, the material system follows.
- For dry-type, build from composites. Pick DMD/NMN/NHN by the hottest-spot temperature, and use DMD prepreg for coil interlayer and end insulation. See the dry-type selection guide for the full mapping.
- For oil-immersed, build from paper. Specify cable paper for the conductor, crepe for leads, and DDP for interlayer bonding — all saturated by the oil. Browse the Insulating Paper category for grades.
Common Mistakes
- Using paper in a dry-type unit. Cellulose paper is designed to be oil-saturated; in air it loses its dielectric role and the coil is left under-insulated.
- Using composites unimpregnated in oil. A dry-type composite is not the right turn insulation for an oil unit, where cable paper plus oil is the proven system.
- Mixing thermal classes across one assembly. A Class H composite behind a Class A paper sleeve creates the weak link. Keep the whole insulation system at one verified class.
Conclusion
Dry-type and oil-immersed transformers solve the same problem with different chemistry: a dry-type unit is a solid system of composites, prepreg, and resin; an oil unit is a cellulose-and-oil system. Specify to the type, then to the temperature class, and the materials fall into place. For the dry-type side, start from our Flexible Composite Insulation category; for the oil side, from Insulating Paper.
Planning a transformer build and not sure which system — or which materials — fit? Contact our team with your type, rating, and duty, and we will map the insulation scheme to your spec.
Related reading: Insulation Materials for Dry-Type Transformers: A Complete Selection Guide
Browse flexible composites: Flexible Composite Insulation category
Browse insulating paper: Insulating Paper category