To get a fully understand the insulation materials of dry-type transformers.

First of all, we need to know what parts the structure of dry-type transformer is composed of?
Dry-type transformer is a power transformer whose cores and coils are not impregnated in insulating liquid (insulating oil). The main components are: iron core, low voltage winding, high voltage winding, high voltage lead, iron yoke, insulation material, radiator and so on.

The insulation of dry-type transformers includes winding insulation, lead and tap changer insulation.
Wind Insulation: Main insulation, insulation between high voltage windings and low voltage windings, alternating phase and ground insulation.
Longitudinal insulation, different turn-to-turn, inter-layer and inter-segment insulation in the same winding.

Lead and tap changer insulation:
Main insulation, insulationof the leads or tap-changer to ground or to other windings.
Longitudinal insulation, insulation between leads of the same winding and between parts of a tap-changer.

The function of transformer insulation material is to block undesired current paths.
The insulating materials of Dry-type transformers mainly include electrical laminated, insulating paper, cardboard, crepe paper, dispensing paper, cable paper, ash, white cloth tape, compression tape,and so on.

Introduction of specific insulation materials:
1. Fiberglass Laminated Sheets like 3240 epoxy sheets are often used for iron core insulation. Epoxy resin is a non-flammable, flame retardant and self-extinguishing solid insulating material.

2. Insulation films such as PET film and polyimide film are widely used for interturn insulation and the insulation cylinders.

3. Prepreg DMD, made of electrical grade non-woven cloth bonded with PET film then impregnated with epoxy resin, often used for low voltage layer insulation.

4. Glass fiber mesh is used for the reinforcement of winding or layer insulation.

5. Alkali-free fiberglass cloth is used for layer insulation in transformers.

6. Alkali-free fiberglass unidirectional binding tapes are used for binding transformer iron core and fixating winding as well as interturn insulation.

7. NHN insulation paper, a composite paper made of two-layer Nomex plus polyimide film in between, most used for interturn or layer insulation.

8. Sheet Molding Compound (SMC), made of assembled rovings impregnated with unsaturated polyester resin, used for non-encapsulated high-voltage coil pads.

Insulation grade of transformer
The insulation grade refers to the heat resistance grade of the insulation material used in the winding of the motor (or transformer).

Class A Class E Class B Class F Class H
Maximum allowable temperature (℃) 105 120 130 155 180
Winding temperature rise limit (K) 60 75 80 100 125
Performance reference temperature(℃) 80 95 100 120 145