Application of DMD in Dry-Type Transformers

Time: 08-25 2025 Auther: Ztelec group

What is DMD? DMD refers to a composite insulation material made of polyester film + polyester fiber non-woven fabric...

What is DMD?

DMD refers to a composite insulation material made of polyester film + polyester fiber non-woven fabric, typically in a three-layer structure:

Outer layer: polyester non-woven fabric (D)

Middle layer: polyester film (M)

Inner layer: polyester non-woven fabric (D)

This combination provides both the dielectric strength of the polyester film and the impregnation ability and flexibility of the non-woven fabric.

Applications of DMD in Dry-Type Transformers:

Interlayer Insulation

Used between winding layers to withstand inter-turn voltages and prevent breakdown.

Excellent flexibility allows it to fit tightly to coil surfaces.

Turn and End Insulation

Applied at winding ends to provide cushioning and insulation, reducing partial discharge risks.

Filling and Mechanical Support

Due to its strong resin absorption capacity, DMD can be well impregnated during the Vacuum Pressure Impregnation (VPI) process, enhancing mechanical strength and thermal resistance after curing.

Insulation Cylinders or Liners

Commonly used as insulation cylinders in low-voltage windings of dry-type transformers, providing both electrical insulation and mechanical strength.

Key Advantages:

High dielectric strength (thanks to the polyester film layer).

Flexible and easy to wrap around windings.

Excellent resin compatibility, with improved insulation performance after impregnation.

Cost-effective, widely used as Class B (130 °C) or Class F (155 °C) insulation material in dry-type transformers.

Application Scenarios & Selection Guide

Application Scenario	Typical Position in Transformer	Selection Focus	Recommended DMD Grade
Interlayer Insulation	Between low-voltage or high-voltage winding layers	High dielectric strength and good flexibility for tight wrapping	Class B (130℃) or Class F (155℃) DMD
Turn & End Insulation	Coil ends, turn-to-turn areas	Cushioning + electrical insulation, reduce partial discharge	Class F (155℃) DMD, thickness 0.20–0.30 mm
Insulation Cylinders / Liners	Inner and outer layers of low-voltage windings	Mechanical strength + resin absorption, withstands thermal stress	Class F DMD, thickness 0.25–0.50 mm
Filling & Mechanical Support	Slot wedges, spacers, auxiliary insulation	Resin compatibility and rigidity after VPI impregnation	Class B or Class F DMD depending on design
General Electrical Insulation	Pads, wrapping layers, auxiliary insulation parts	Balanced performance and cost	Economical Class B DMD

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