A transformer is only as reliable as its insulation. The copper and core rarely wear out. What ages, cracks or breaks down is the insulation between turns, layers and windings, and once it goes, so does the transformer.
The right transformer insulation materials depend mainly on one question: is the transformer oil-filled or dry-type? This guide walks through the main materials used in each, what each one does, and how to choose.
Why Transformer Insulation Is Different
Transformer insulation has to do several jobs at once. It must withstand the working voltage and lightning or switching surges. It has to survive years of heat from load current. And it must stay mechanically strong enough to hold the windings together when a short circuit tries to tear them apart.
That is why a transformer uses a system of materials rather than one. Each part, from turn insulation to barriers between windings, is chosen for a specific job.
Insulation Materials for Oil-Filled Transformers
In oil-filled transformers, the paper and board are soaked in transformer oil. The two work together: the oil fills every gap and carries heat away, while the cellulose gives the structure.
Kraft Insulation Paper
Kraft paper made from wood pulp is the traditional turn and layer insulation. Impregnated with oil, it has good dielectric strength and is easy to wind. It is usually used within Class A limits, so the hot-spot temperature has to be controlled. See our insulation kraft papers.
Diamond Dotted Paper
Diamond dotted paper (DDP) is kraft paper printed with a diamond pattern of epoxy resin. During curing, the dots bond the layers of the winding together, which makes the coil much stronger against short-circuit forces. It is widely used as layer insulation in distribution transformers. See our diamond dotted paper.
Crepe Paper
Crepe paper is creased so it can stretch. That lets it wrap leads, bends and irregular shapes tightly. Our insulation crepe papers are used for lead insulation and taping.
Pressboard
Pressboard is thick, dense cellulose board. It is used for cylinders between windings, end rings, spacers, barriers and blocks. It gives the main insulation between high-voltage and low-voltage windings and between windings and the core. Our insulation papers and pressboard range covers these grades.
Insulation Materials for Dry-Type Transformers
Dry-type transformers have no oil, so the solid insulation carries the whole load and runs hotter. Most are built to Class F or Class H, which calls for materials that stay stable at 155 to 180°C.
Aramid (Nomex) Paper
Aramid paper keeps its electrical and mechanical strength at high temperatures and does not melt. It is the standard choice for layer, turn and barrier insulation in Class H dry-type transformers. See our aramid papers and Nomex 410 paper.
Flexible Laminates
Laminates bond a film to one or two paper layers. DMD (polyester film with polyester fleece) is common in Class F, while NMN (aramid, polyester film, aramid) is used in Class H. The film adds dielectric strength and the outer layers add toughness and resin absorption. See our laminated insulation paper composites.
Polyester and Polyimide Films
Thin films add high dielectric strength in very little space. Polyester film suits Class B and F layer insulation, while polyimide film handles much higher temperatures.
Varnish and Cast Resin
Dry-type windings are either impregnated with insulating varnish, often by vacuum pressure impregnation (VPI), or fully encapsulated in epoxy (cast resin transformers). Varnish fills voids, bonds the turns together and keeps out moisture. See our Elantas varnishes and thinner.
Supporting Materials Used in Both Types
- Insulating tapes such as woven polyester, glass cloth and aramid tapes, used to bind coils and insulate leads. See our insulation tapes.
- Fiberglass sleeves to insulate and protect leads and connections. See our fiberglass insulation sleeves.
- Rigid sheets and components such as glass epoxy and FRP for spacers, terminal boards and supports. See our insulation sheets and components.
Quick Comparison
| Material | Main use | Transformer type |
| Kraft paper | Turn and layer insulation | Oil-filled |
| Diamond dotted paper | Layer insulation that bonds the coil | Oil-filled |
| Crepe paper | Lead and irregular shape insulation | Oil-filled |
| Pressboard | Barriers, cylinders, spacers, end rings | Mainly oil-filled |
| Aramid paper | High-temperature layer and barrier insulation | Dry-type (also some oil-filled designs) |
| DMD and NMN laminates | Layer and slot insulation, Class F and H | Dry-type |
| Polyester and polyimide film | Thin, high-dielectric layers | Dry-type |
| Varnish and resin | Impregnation and bonding | Dry-type |
How to Choose Transformer Insulation Materials
- Start with the cooling type. Oil-filled designs use cellulose that works with oil. Dry-type designs need high-temperature synthetic materials.
- Match the insulation class. Every material in the winding must handle the design hot-spot temperature. One low-rated component limits the whole system.
- Check compatibility. Papers, films, tapes and adhesives must suit the oil or the varnish and resin used.
- Think about short-circuit strength. Bonded materials such as diamond dotted paper, or resin-impregnated windings, keep coils rigid under fault forces.
- Allow for moisture. Cellulose absorbs water, so it must be dried thoroughly before oil filling. Dry-type units need varnish or resin to seal out humidity.
For a deeper look at papers alone, read our guides on insulation paper types and transformer insulation paper.
Final Word
Oil-filled transformers rely on kraft paper, diamond dotted paper, crepe paper and pressboard working with oil. Dry-type transformers rely on aramid paper, laminates, films and varnish built for Class F and H temperatures. Choose by cooling type and insulation class first, then by mechanical and compatibility needs.
Harnawa Insulations supplies the full range of transformer insulation materials from one source. Contact us with your transformer rating, cooling type and insulation class, and our team will suggest a complete material set.
Frequently Asked Questions
Which insulation material is used in transformers?
Oil-filled transformers mainly use kraft paper, diamond dotted paper, crepe paper and pressboard with transformer oil. Dry-type transformers use aramid paper, DMD and NMN laminates, polyester or polyimide film and insulating varnish or cast resin.
Why is diamond dotted paper used in transformers?
The epoxy dots on diamond dotted paper bond the winding layers together during curing. This makes the coil stronger against the mechanical forces of a short circuit.
What insulation is used in dry-type transformers?
Dry-type transformers use high-temperature materials such as aramid (Nomex) paper, NMN and DMD laminates, polyester and polyimide films, and varnish or epoxy resin, usually rated Class F or Class H.
What is pressboard used for in a transformer?
Pressboard is used for cylinders between windings, barriers, spacers, end rings and blocks. It provides the main insulation between windings and between windings and the core.
What is the insulation class of a dry-type transformer?
Most dry-type transformers are designed to Class F (155°C) or Class H (180°C), and their insulation materials must be rated for those temperatures.
