DIAMOND DOTTED PAPER
Diamond Dotted Paper for Oil Immersed Transformer Winding Insulation
Diamond Dotted Paper (DDP) – Complete Technical Guide for Transformer Insulation
Introduction of DDP
Diamond Dotted Paper, commonly abbreviated as DDP, is a specialized self-bonding electrical insulation material widely adopted by manufacturers of oil-immersed distribution transformers, power transformers, reactors and instrument transformers. Also known as diamond pattern paper or epoxy dotted insulating paper, DDP is manufactured by coating high-purity electrical-grade kraft paper with B-stage thermosetting epoxy resin arranged in a regular diamond dot matrix.
Unlike plain kraft insulation paper that only delivers dielectric separation, diamond dotted paper provides dual functions: reliable interlayer electrical insulation and permanent mechanical bonding after thermal curing. Under standard heating cycles during transformer vacuum drying, the semi-cured epoxy dots melt and cross-link into a rigid solid. This process fuses adjacent winding conductors and insulation layers into an integrated winding block. The uncoated gaps between diamond-shaped resin dots are a critical design feature, allowing smooth escape of moisture and gas during drying while enabling full penetration of mineral insulating oil for effective cooling and dielectric impregnation.
For decades, diamond dotted paper has become the preferred interlayer insulation material for distribution transformers globally. It effectively mitigates risks caused by electromagnetic vibration, winding displacement and partial discharge under short-circuit fault conditions. Engineers and procurement teams select DDP when insulation systems require both dielectric performance and enhanced mechanical stability without blocking oil flow inside compact transformer windings.
Material Structure & Working Principle
Diamond dotted paper consists of two core components: electrical kraft base paper and patterned B-stage epoxy adhesive dots.
- Base Substrate: High-density cellulose kraft paper made from pure sulphate wood pulp. It offers consistent tensile strength, tear resistance and stable dielectric performance in oil-filled environments. Crepe kraft base versions are also available for applications requiring improved flexibility.
- B-Stage Epoxy Resin Dots: Thermosetting adhesive printed in repeating diamond layouts. At ambient temperature, the resin remains dry, non-sticky and stable. When heated to 90°C–130°C during coil drying procedures, the dots soften, flow and permanently cure into C-stage solid adhesive. Once cured, bonding strength remains stable under continuous operating temperature cycles.
The diamond dot layout is precisely engineered to balance bonding area and fluid permeability. Fully coated adhesive paper would block oil impregnation and trap moisture inside windings, leading to partial discharge and insulation aging. The spaced diamond pattern avoids this defect, creating reliable bonding points while maintaining open channels for vacuum drying and oil filling.
Key Advantages of Diamond Dotted Paper
| Benefit | Technical Explanation |
|---|---|
| Improved Short-Circuit Withstand Capacity | Cured epoxy bonds lock winding layers together, preventing conductor shift and coil deformation under huge electromagnetic forces during power system short-circuit faults. |
| Suppression of Electromagnetic Vibration | Eliminates relative movement between copper conductors during continuous operation, reducing insulation abrasion and lowering transformer noise level. |
| Optimized Oil Impregnation & Drying Performance | Gaps between diamond dots allow free movement of transformer oil and moisture vapor; reduces trapped air and partial discharge risks. |
| Stable Dielectric Performance | Base kraft paper maintains consistent dielectric strength after curing; compatible with mineral oil, natural ester and synthetic dielectric fluids. |
| Controlled, Permanent Bonding | Bonding only activates during factory heating cycles; no premature sticking during storage, cutting and winding processing. |
| Flexible Product Selection | Available in single-sided coating and double-sided coating variants to match different winding structure designs. |
| Compliance with International Standards | Formulated and tested to meet IEC 60554, IEC 60641 and related transformer insulation specifications. |
Standard Technical Specifications
| Item | Typical Parameter Range |
|---|---|
| Base Material | Electrical kraft paper / electrical crepe kraft paper |
| Coating Type | Single-sided diamond dotted epoxy; Double-sided diamond dotted epoxy |
| Common Thickness | 0.08 mm, 0.13 mm, 0.18 mm, 0.20 mm, 0.25 mm, 0.30 mm, 0.38 mm, 0.50 mm |
| Standard Width | 500 mm – 1300 mm; custom slit widths available |
| Thermal Class | Class A (105°C) standard; thermally upgraded grades optional |
| Activation Temperature | Starting melting: ~90°C; complete curing: 120°C – 130°C |
| Bonding Strength (After Curing) | ≥ 450 kPa under standardized test conditions |
| Moisture Content | 4.0 % – 8.0 % |
| Storage Condition | Dry, ventilated warehouse, below 30°C, away from direct sunlight and heat sources |
| Supply Form | Master rolls, slit narrow rolls, custom cut sheets |
Main Application Scenarios
Diamond dotted paper is exclusively designed for oil-immersed electrical equipment insulation systems. Typical applications include:
- Interlayer insulation and bonding for LV and HV windings of distribution transformers (10kV, 20kV, 33kV class)
- Winding interlayer insulation for medium voltage power transformers
- Coil insulation for shunt reactors and series reactors
- Winding insulation of current transformers (CT) and voltage transformers (VT)
- Inter-turn and inter-layer insulation for furnace transformers and special industrial transformers
- Retrofit and rewinding projects for existing oil-type transformers
Single-sided DDP is widely used for basic interlayer bonding where only one surface needs adhesion. Double-sided diamond dotted paper is selected for heavy-duty transformers, high-fault-current applications and reactor coils requiring rigid winding block structure on both sides of insulation layers.
Diamond Dotted Paper vs. Ordinary Kraft Insulation Paper
| Comparison Item | Diamond Dotted Paper (DDP) | Standard Plain Kraft Paper |
|---|---|---|
| Bonding Function | Self-bonding after thermal curing | No adhesive property |
| Winding Stability | Forms rigid winding block; anti-vibration & anti-displacement | Layers remain loose; prone to shift under short-circuit stress |
| Vacuum Drying & Oil Impregnation | Excellent, thanks to spaced dot pattern | Good, but no bonding capability |
| Partial Discharge Risk | Lower; fewer internal air gaps after curing | Higher risk of micro-movement and air gap formation over time |
| Typical Usage | Distribution transformers, reactors requiring fixed windings | General insulation barriers, cylinder liners, non-bonded spacing layers |
Processing & Curing Guidelines
To achieve optimal bonding performance, transformer manufacturers must follow recommended thermal treatment cycles:
- Wind coils with diamond dotted paper placed between winding layers; maintain uniform pressure during winding.
- Conduct vacuum drying with staged temperature rise. Hold temperature between 90°C and 130°C for sufficient duration to ensure epoxy dots fully melt and cross-link.
- Avoid rapid temperature spikes that cause uneven resin flow.
- After curing, allow coils to cool gradually before oil filling.
Improper curing temperature or insufficient holding time will lead to incomplete resin cross-linking, resulting in insufficient bonding strength and reduced long-term reliability.
Storage Recommendations
- Store diamond dotted paper in clean, dry indoor environments with relative humidity below 65%.
- Keep products away from radiators, boilers and direct solar radiation. High temperature during storage may cause premature resin aging.
- Keep rolls sealed inside original packaging until use to prevent moisture absorption.
Rotate stock following first-in first-out practice to maintain consistent adhesive activity.
Frequently Asked Questions
Q1: Can diamond dotted paper be used in dry-type transformers?
A1: Diamond dotted paper is optimized for oil-immersed transformer insulation systems. It is not generally recommended for air-cooled dry-type transformers unless material compatibility testing is completed.
Q2: What is the difference between diamond dotted paper and diamond dotted crepe paper?
A2: Diamond dotted crepe paper uses stretchable crepe kraft as base material, offering higher flexibility for tightly wound compact coils. Flat diamond dotted kraft paper delivers higher mechanical strength for standard layer insulation.
Q3: Does cured epoxy resin contaminate transformer mineral oil?
A3: Fully cured C-stage epoxy resin features stable chemical properties and excellent compatibility with mineral insulating oil. When processed under standard curing cycles, DDP will not release contaminants that degrade oil dielectric properties.
Q4: Can I customize thickness and roll width?
A4: Most suppliers support customized thickness, slit width and dot pattern specifications for large-volume transformer manufacturing projects.
PREVIOUS：DIAMOND DOTTED PAPER NEXT：DIAMOND DOTTED PAPER
Categories
Contact Us
Contact: Tianjin Delda Trading Co., Ltd
WhatsApp: +86 13662056855
Tel:
E-mail: solomon20210701@tjxddq.com
Add: Unit‑101, Workshop 3, Bozhong Automobile Industrial Park, No.5 Xianghui Road, Intersection of Xianghui Road and Gaoying Road, Beizhakou Town, Jinnan District, Tianjin, China