Diamond Dotted Paper for Oil-Immersed Transformers

Diamond Dotted Paper (DDP) is a specialized insulation material designed for oil-immersed transformers, featuring B-s...

Diamond Dotted Paper (DDP) is a specialized insulation material designed for oil-immersed transformers, featuring B-stage modified epoxy resin uniformly coated in a diamond pattern on both sides of high-performance electrical kraft paper. This unique dotted design maintains open gaps to ensure full oil impregnation and gas evacuation for optimal heat dissipation and insulation, while the resin melts and cures during the winding bake cycle to bond layers into a rigid unit, significantly enhancing the transformer’s mechanical strength and short-circuit resistance.

1. Physical and Dimensional Specifications

Base Paper Thickness:

Common Range: 0.08mm to 0.5mm
Standard Sizes: 0.13mm,0.18mm,0.25mm,0.38mm (Selected based on layer voltage and space constraints).

Roll Width:

Standard Widths: 960mm,1000mm,1020mm,1220mm
Slitting Service: Custom widths (narrow tapes) are available for automatic winding machines (e.g., 25mm-1250mm).

Apparent Density:

Typically between 0.85-1.10g/cm3 High density ensures better mechanical support and puncture resistance.

2. Coating and Curing Properties

Diamond Pattern:

Dot Size: Standard diamond dots are usually 9.5mm*9.5mm
Spacing: Center-to-center distance of approx. 15.9mm.This ensures the resin coverage is 25%-35% of the total area to balance bonding strength and oil flow.
Adhesive Layer Thickness:
Single-side resin thickness is typically 10-15 µm(0.010mm-0.015mm).

3. Electrical and Thermal Indicators

Thermal Class:
Standard: Class A (105°C).
High-temp options: Class F (155°C) or Class H (180°C) using modified resins.
Moisture Content:
Factory standard is strictly controlled between 4%-8%.
Degree of Polymerization (DP):
High-grade DDP requires an initial DP≥ 1200 (some reach1400 ), which is critical for the long-term aging life of the transformer.

4.DDP (Diamond Dotted Paper) Thickness Selection in Oil-Immersed Transformer Design

In the design of oil-immersed transformers, the selection of DDP (Diamond Dotted Paper) thickness is primarily determined by the voltage level (inter-layer potential difference) and mechanical stress (electromagnetic forces related to transformer capacity).

The following are typical recommended DDP thickness ranges based on transformer capacity (kVA):

Small Distribution Transformers (30 kVA – 500 kVA)

These transformers typically adopt multi-layer cylindrical windings with relatively low inter-layer voltage.
Recommended thickness: 0.08 mm – 0.13 mm
Application focus:
Mainly used for turn-to-turn or layer insulation in low-voltage windings.
Due to compact size and higher cooling requirements, ultra-thin DDP helps reduce winding dimensions while ensuring proper oil circulation.

Medium Distribution Transformers (630 kVA – 2500 kVA)

As the capacity increases, the short-circuit electromagnetic forces acting on the windings become significantly stronger, requiring higher bonding strength of the DDP.
Recommended thickness: 0.13 mm – 0.25 mm
Application focus:
Commonly used for inter-layer insulation in high-voltage windings (e.g., 10 kV or 35 kV class).
The 0.25 mm thickness is considered a “golden standard” in this range, providing sufficient mechanical margin to prevent axial displacement of the windings.

Large Power Transformers (3150 kVA – 10,000 kVA and above)

For high-voltage and large-capacity transformers, DDP functions not only as insulation but also as a structural reinforcement material.
Recommended thickness: 0.38 mm – 0.50 mm (or multi-layer composite structures)
Application focus:
Used in end insulation of ultra-high-voltage windings or inter-layer insulation of high-current coils.
In addition to electric field strength, the compressibility behavior of DDP under long-term pressure must be considered to maintain winding clamping force.

5.Key Principles for Thickness Selection

Voltage Gradient:

If inter-layer voltage exceeds 500 V, thickness should not be less than 0.13 mm

If it exceeds 2000 V, 0.25 mm or composite layers are generally required

Current Density:
As transformer capacity increases, rated current rises, and short-circuit electromagnetic forces grow exponentially.
Thicker DDP is necessary to ensure that the resin dots provide sufficient shear strength to firmly bond and lock the conductors.

Temperature Rise Control:
Increasing DDP thickness improves mechanical strength but slightly increases thermal resistance.
Therefore, in large-capacity transformers, DDP is often used in combination with oil ducts/spacers to maintain effective cooling performance.