DDP Paper for Inter-Layer Insulation in Oil-Immersed Transformer Windings

1. Scope of Application

Inter-layer insulation of oil-immersed transformer windings
Voltage levels: 10 kV – 220 kV
Winding types: cylindrical, disc (continuous), multi-layer
Base material compatible with oil-impregnated insulation systems (mineral oil or ester oil)

2. Selection Principles

Selection Based on Voltage Level and Insulation Margin

The thickness of DDP insulation paper should ensure that inter-layer insulation strength meets operating voltage plus safety margin.
Recommended thickness values:

Principle: Higher voltage → thicker DDP paper ; also consider mechanical short-circuit force requirements.

Selection Based on Short-Circuit Force and Mechanical Stress

Large-capacity transformers, high short-circuit current systems, and compact windings → choose thicker DDP  insulation paper  with higher adhesion strength.
Uniform and regular resin dot distribution → ensures inter-layer shear strength.
Typical inter-layer adhesion strength ≥ 400 kPa (adjustable according to design requirements).

Selection Based on Oil Channels and Heat Dissipation

Resin dot coverage should be moderate: 15%–30%
Avoid continuous resin layers → ensure oil channels remain open
Ensure uniform oil impregnation → stable insulation performance

Selection Based on Winding Process and Curing Conditions

Resin dot melting temperature must match the winding drying and curing process
Insufficient curing → inadequate inter-layer bonding
Over-curing → paper embrittlement and accelerated thermal aging

Selection Based on Thermal Performance and Aging

Compatible with oil-immersed transformer insulation class A / F
Stable performance at long-term oil temperatures of 90–110 °C
Short-term thermal resistance ≥ 120 °C
Resistant to thermal aging: no embrittlement, no delamination, no powdering
Selection Based on Oil Compatibility
Compatible with mineral oil or ester oil
Does not affect oil dielectric loss (tanδ)
Good oil absorption → prevents dry spots that could cause breakdown

Selection Based on Winding Structure

Cylindrical winding → ensures radial stability
Disc (continuous) winding → ensures axial stability
Compact winding → improves mechanical strength and reduces the need for additional reinforcement

3. Selection Summary

The selection of DDP insulation paper  should comprehensively consider voltage level, short-circuit force, inter-layer adhesion, oil impregnation, thermal stability, and process compatibility. Proper selection ensures reliable, safe, and durable inter-layer insulation in oil-immersed transformer windings.