Shopping malls, hospitals, data centres, tunnels and wind-turbine towers — what they have in common is that keeping an oil-filled transformer indoors is increasingly hard to accept because of fire and environmental risk. The spread of dry-type transformers in recent years can largely be explained by this picture. So when you remove the oil from a system where it did the cooling and insulating, what do you put in its place?
The Challenge of an Oil-Free Design
In an oil-type transformer the insulation system relies on paper and oil working together. Oil both raises the dielectric strength and carries heat away from the winding. In a dry-type design cooling is done with air and heat transfer is markedly weaker. The result: winding temperatures run higher. Since standard cellulosic paper would age quickly at these temperatures, a different insulation family is needed.
Aramid Papers
This is where aramid-based insulation papers come in. Known in the industry by trade names such as Nomex® and Metastar®, these materials are produced from aromatic polyamide fibres instead of cellulose. The requirements for electrical-purpose non-cellulosic papers are defined in the IEC 60819 series. Aramid papers have three decisive properties:
- Thermal endurance: The long-term service temperature is markedly higher than cellulosic paper; it can be used in Class 220 insulation systems.
- Low moisture sensitivity: It does not take up moisture quickly like cellulose and largely retains its dielectric properties in the presence of moisture.
- Tendency not to sustain flame: When the flame source is removed it does not keep burning; this is decisive in indoor installations.
Comparison with Cellulosic Paper
| Property | Cellulosic kraft paper | Aramid paper |
|---|---|---|
| Typical insulation-system class | 105 – 120 (including thermally upgraded) | 200 – 220 |
| Moisture sensitivity | High; drying mandatory | Low |
| Oil impregnation | Required; part of performance | Not required; works in air |
| Overload tolerance | Limited | High |
| Material cost | Low | High |
| Typical application | Oil-type power and distribution transformers | Dry-type transformers, traction transformers, high-temperature motors |
Overload Capacity: The Invisible Advantage
The practical benefit of aramid insulation is not only fire safety. Its high thermal endurance means the transformer can run under short-term overloads without markedly shortening its life. In facilities with a variable load profile — such as charging infrastructure, rail traction systems or wind farms — this tolerance allows the equipment to be sized smaller.
Cast Resin or VPI?
There are two common manufacturing approaches in dry-type transformers, and the conductor choice is directly linked to this preference. In the cast-resin design the winding is embedded in epoxy resin and cast under vacuum; the result is a mechanically very robust block, sealed against moisture and contamination. In the vacuum-pressure impregnation (VPI) approach the winding is impregnated with varnish and a more flexible, more repairable structure is obtained.
In cast-resin production, the conductor insulation's compatibility with the resin and its resistance to the difference in thermal expansion gain importance; stresses between resin and winding can form cracks over time. In VPI designs, the penetration of varnish into the winding is decisive; the porous structure of aramid paper is an advantage here.
In both approaches the final thermal class is set over the whole system. Using a Class 220 conductor is meaningless on its own if the resin or varnish is of a lower class.
Notes on the Production Side
There are a few points to watch in production when working with aramid-insulated conductor:
- Winding tension: Aramid paper is mechanically durable, but if the overlap arrangement is disturbed a weak point forms; the winding angle must be controlled.
- Varnish compatibility: The compatibility of the impregnating varnish and cast resin with the aramid surface must be verified; the system's thermal class is set by the weakest component.
- Cooling channels: In air cooling the channel geometry is part of the design; conductor dimension and channel width must be planned together.
- Cost balance: Aramid is expensive compared with cellulosic paper; but in a dry-type design the alternative is usually a larger machine.
For manufacturers planning to move into dry-type transformer production, the choice of aramid-insulated conductor is not a material decision alone; it is a design decision affecting winding geometry, cooling and the impregnation process together.
Summary
- In dry-type transformers cooling is done with air; winding temperatures are higher than in oil-type.
- Aramid-based insulation papers offer high thermal endurance, low moisture sensitivity and the property of not sustaining flame.
- The requirements for non-cellulosic electrical papers fall under IEC 60819.
- The overload tolerance provided by aramid insulation allows the equipment to be sized smaller under variable load profiles.
- The final thermal class is set together with the resin, varnish and support materials, not the conductor alone.
You can consult our technical team about the choice and supply planning of aramid-insulated conductor in dry-type transformer production.