DMD & Prepreg DMD Paper | Dry-Type Transformer Insulation

Time: 07-03 2026 Auther: Xujue Electrical

DMD insulation paper is a three-layer flexible composit […]

DMD insulation paper is a three-layer flexible composite material made of polyester film (Mylar, represented by “M”) and polyester fiber non-woven fabric (represented by “D”) bonded together with adhesive.

In dry-type transformers, it is primarily used for interlayer insulation, turn-to-turn insulation, section insulation, and lead-out wire insulation within the high and low-voltage windings to prevent short circuits and block moisture.

1. Applications of DMD Insulation Paper in Dry-Type Transformers

Interlayer Insulation: Placed between the conductive layers of the transformer coils to provide electrical isolation under high voltage.
Section Insulation (High/Low-Voltage Isolation): Used to separate high-voltage and low-voltage coils, enduring extremely high operating voltages.
Lead-out and End-Insulation: Wrapped around transformer leads or coil ends to prevent edge discharge.
Epoxy Prepreg Applications (DMD Prepreg): In dry-type transformers, DMD paper coated with epoxy resin (diamond-dotted or pre-impregnated DMD) is widely used. After winding, the coil is baked and cured, allowing the resin to bond the entire coil into a solid block. This significantly enhances vibration resistance and moisture proofing.

DMD insulation film

2. Common Specifications and Classifications

DMD insulation paper is mainly classified into Class B (130°C) and Class F (155°C) based on thermal resistance ratings.

A. Common Thickness Specifications
Thickness refers to the total thickness of the composite material. Common specs include (in mm):0.15,0.18,0.20,0.23,0.25,0.30,0.35,0.40
B. Standard Sizes:
Rolls: The standard width is typically 1,000 mm.
Tapes: It can be slit into specific narrow widths based on transformer sizes for automatic or manual machine winding.
C. Common Structure Codes
The structural code of DMD represents the thickness ratio of each layer (calculated in mils, such as 2-2-2, 3-3-3, etc.). Common configurations include 2-2-2 and 3-5-3. Its breakdown voltage usually ranges from 6 kV to 12 kV, depending on the material thickness.

3. Performance Characteristics

Excellent Dielectric Strength: High insulation capacity and breakdown resistance under high frequency or continuous high voltage.
Superb Mechanical Properties: Great tear resistance, high tensile strength, and excellent flexibility.
Good Processing Performance: Soft texture makes it highly compatible with coil winding machines and automatic slot-insertion equipment.

4.DMD Insulation Paper VS Epoxy Prepreg DMD

The fundamental difference between Standard DMD and Epoxy Prepreg DMD in dry-type transformers lies in the presence of pre-impregnated epoxy resin and the resulting difference in structural bonding strength:

Dimension	Standard DMD Insulation Paper	Epoxy Prepreg DMD
Surface Status	Dry, non-sticky, and relatively flexible.	Slightly tacky (contains B-stage semi-cured epoxy resin) and relatively rigid.
Working Principle	Acts purely as a physical insulation barrier with no bonding strength.	Resin melts and flows under heat, curing to bond the coil into a solid mass.
Main Function	Pure electrical insulation, end-insulation, and lead wire wrapping.	Insulation + Structural Fixation (greatly improves resistance to short-circuit electromagnetic forces).
Subsequent Process	Typically requires Vacuum Pressure Impregnation (VPI) after winding.	Baked and cured directly under pressure after foil winding; no extra varnish dipping required.
Partial Discharge (PD) Control	Micro-air gaps can easily remain, making PD control more challenging.	Resin flows to fill all voids before curing, resulting in extremely low PD levels.

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