How to Select DDP Insulation Paper for Oil-Immersed Transformers

Diamond Dotted Paper (DDP) is an advanced electrical insulation material made by coating specially modified epoxy resin onto high-performance electrical insulating paper, such as high-purity electrical cable paper, in a diamond-shaped pattern. DDP plays an important role in the manufacturing of oil-immersed transformers.

This guide explains the key selection criteria, technical specifications, and processing requirements to help transformer designers and manufacturers choose the right DDP insulation paper for their applications.

1. Key Selection Criteria and Decision Process

When selecting DDP, we recommend following this sequence:

Electrical design (paper thickness) → Mechanical requirements (resin coating weight and pattern) → Process compatibility (heating and curing) → Material compatibility.

2. Detailed DDP Selection Criteria and Technical Requirements

2.1 Base Paper Thickness Selection

The thickness of DDP directly affects the insulation margin between turns and layers, as well as the space occupied within the transformer winding window.

Common thicknesses: 0.05 mm, 0.08 mm, 0.10 mm, 0.13 mm, 0.18 mm, and 0.25 mm.

Selection guidelines:

• Small distribution transformers and turn-to-turn insulation: Thinner grades, such as 0.05–0.08 mm, can be considered to optimize space utilization and reduce winding dimensions.
• Medium and large power transformers and layer insulation: Thicknesses of 0.10–0.18 mm are commonly considered, depending on the insulation design.
• Enhanced insulation or inter-section insulation: A thickness of 0.25 mm or multiple layers may be selected according to the required electrical and mechanical performance.

The final thickness should be determined based on the transformer’s electrical design, winding structure, and applicable technical specifications.

2.2 Resin Coating Weight and Diamond Pattern Distribution

The resin coating weight directly influences the bonding strength after curing and the oil impregnation characteristics of the insulation system.

Selection considerations:

A higher resin coating weight does not necessarily mean better performance. Excessive resin may flow out during heating and curing, potentially obstructing oil ducts, reducing heat dissipation, or causing defects in the insulation structure.

Insufficient resin, on the other hand, may result in inadequate bonding strength after curing, increasing the risk of winding displacement or deformation under short-circuit electrodynamic forces.

The optimal coating weight should balance bonding strength, oil impregnation, heat dissipation, and the transformer’s short-circuit withstand requirements.

2.3 Curing Temperature and Baking Process Compatibility

The B-stage modified epoxy resin used in DDP has a specific temperature-time curing profile. The required conditions may vary depending on the manufacturer and resin formulation.

Typical curing process reference:

• Resin melting and flow stage: Approximately 90°C ± 5°C for 2–4 hours to allow the resin to soften, flow, and bond with adjacent insulation layers.
• Cross-linking and curing stage: Approximately 120–130°C for 6–8 hours to develop the required cured resin structure.

Selection guidelines:

Before selecting DDP, confirm that the temperature control accuracy, heating rate, and operating conditions of your vacuum drying oven or curing chamber are compatible with the supplier’s recommended curing schedule.

Insufficient curing may result in inadequate bonding, while excessive heating may accelerate paper aging or negatively affect insulation performance.

Important: The temperatures and holding times above are general reference values, not universal processing requirements. Always follow the specific DDP manufacturer’s technical data sheet and validate the curing process under actual production conditions.

2.4 Mechanical Properties and Base Paper Purity

DDP must withstand the tension applied during winding while maintaining reliable insulation performance throughout the transformer’s service life.

Key parameters to evaluate:

• Tensile strength and elongation: Ensure that the paper can withstand the tension settings of the winding machine without breaking or tearing during processing.
• Dielectric strength: Verify that the material meets the electrical insulation requirements of the intended transformer application.
• Ash content and pH value: High-purity electrical insulating paper made from suitable pulp is generally preferred to help maintain the cleanliness and reliability of the transformer insulation system.

As preliminary procurement targets, the following values may be considered, subject to the applicable product specification:

• pH of aqueous extract: 6.5–8.5.
• Ash content: Generally below 0.8%.

These values should be verified against the supplier’s test reports and the relevant material standards. Low ionic contamination and good compatibility with transformer oil are important for minimizing the risk of insulation degradation and partial discharge (PD).

3. DDP Insulation Paper Manufacturer and Supply Options

Xujue Electrical, a company under ZTelec Group, specializes in manufacturing DDP insulation paper for oil-immersed transformers. We supply both full-width rolls and slit DDP paper to meet different transformer manufacturing and winding requirements.

Common DDP Insulation Paper Specifications

Specifications may vary depending on the product configuration and manufacturing requirements. Please contact us to confirm availability for your application.

4. Request a DDP Insulation Paper Quote

If you are looking for a reliable DDP insulation paper supplier for oil-immersed transformers, Xujue Electrical can help you identify a suitable material solution based on your technical and production requirements.

To help us recommend the right DDP product, please provide the following information:

• Required width: Full-width roll or slit width.
• Required thickness: Your target paper thickness.
• Application: Such as turn-to-turn insulation, layer insulation, or inter-section insulation.
• Estimated quantity: Your expected purchasing volume, if available.

Our team will evaluate your requirements and recommend a suitable DDP insulation paper solution for your transformer manufacturing process.

Contact us today to discuss your DDP specifications and request a quotation.